David's Astronomy Pages
Setting Up Computer Control Automated Control Remote Control Closing Up CCD Temperature Regulation & Setpoint Light Frames Dark Frames Flat Frames Image Sets Locate Images T-Point T-Point Mapping & Telescope Modelling Focusing AutoFocusing Polar Alignment Tuning PEC Training Collimation Autoguiding Mosaics Spectroscopic Images Image Reduction
The process of setting up for an observing session is summarised below.
Check Weather Forecast / Current Weather Conditions
Define Observing Objectives, Targets and Rough Observing Plan (at least mentally)
Switch on House Computer (for later remote control). Note IP address.
Prepare Clothing, Laptop and Observatory Keys
Unlock Observatory / Turn On Light
Put laptop on desktop & open lid
Connect necessary cables to laptop
- Connect USB cable from USB hub (Telescope, CCD Camera and Focuser control)
- Connect USB Mouse
- Connect LAN cable (Ethernet)
- Connect DC line
Switch on Master Power Supply for equipment and PowerLine Network.
Switch on Laptop, Focuser and CCD Camera
Unlock Observatory Roof.
Remove Telescope Cap and fit Dew Shield if required. Set Dew Heater.
Log onto Laptop. Open 'CCDSoft', 'TheSky 6' and 'AIS' Programs
Create New Session e.g. 2009-12-21 (S00404)
Check CCDSoft is set to use SBIG Camera, Filter Wheel and Optec Focuser
Insert Starting Number for CCDSoft images
e.g. 404001 (where 404 is the session number and 001 is the first image of the session)
Connect to Camera from 'CCDSoft'/'AIS' and Set Temperature (eg -25 degC)
Update Time on Laptop using 'Dimension 4' software product (gets time from internet)
Ensure 'TheSky' application is set to use computer time, and that it will use 'LX200 GPS' as the scope.
Switch off Observatory light and dim Laptop Screen
Open Roll Off Roof
Switch on LX200 telescope and check initialisation & GPS Fix occur normally
Check progress towards CCD Set Temperature,
Connect to Telescope from 'TheSky'/'AIS'. Check/Confirm position of scope in virtual sky
Slew to star rich area and take a check image.
Perform image link to confirm position, synchronise scope and check image of acceptable focus and absence of frost slugs or other problem
Open Planner and either copy in Job Queue from Last Sesssion and edit queue / add new additions or build up new job queue from pre-defined targets
If necessary refocus scope by performing a focus profile
Start the job queue
Check first target correctly acquired. Check seeing conditions, wind shake and focus quality
Check VNC Server is running and note IP number (specifically the 4th set of digits)
Close laptop lid
Note : Dark Frames and/or Flat Frames may be taken before main imaging
sessions commences, dependant on status of current master reduction frames and the
need for new ones and on dusk conditions.
>> Observatory Use
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A laptop computer is used to operate my Meade LX200 telescope, SBIG CCD camera and Optec Temperature Compensating Focuser.
The telescope is connected to the laptop via a RS232 serial cable and a USB-Serial Adapter. Slewing of the scope to targets is controlled using TheSky6 planetarium & telescope control package (Software Bisque) either directly or more usually indirectly using my AIS Control Program.
My current ST10-XME CCD camera is connected to the laptop by USB cable (in contrast by earlier ST-7e camera connected by means of a parallel cable). Control of the camera operations is performed using the CCDSoft V5 software package (Software Bisque) again either directly or more usually using my AIS Control Program. A SBIG Filter Wheel is controlled via the CCD Camera.
Since 2008 I have used an Optec Temperature Compensating Focuser. which is connected to the laptop using a serial cable and USB- Serial Adapter. Whilst focusing can be operated by means of the standalone control box which can perform automated focusing dependant on temperature, I instead perform high-level focusing by means of my AIS program which can then adjust focus position for not only temperature changes but also changes in filter. Low level control of the focuser is performed using CCDSoft as an intermediary.
To reduce the number of cables that need to be fitted to the laptop, I've added a USB hub to the observatory in 2012, to which telescope, ccd camera and focuser USB cabling is left attached, making it only necessary to connect / unconnect a single USB cable to the laptop at the start /end of the observing session. Testing and subsequent experience has shown that all 3 devices work ok together through the hub, with no issues noted to date.
Some of the useful aspects of computer control are the ability to slew to target based on its name, ability to take an image of any exposure/filter and have the computer compare the positions of stars in the image to expected positions and determine an offset or required correcting jog Also the ability to record the ambient temperature with time, measure a star properties and automatically select best focus position.
The laptop and associated installed software provide a means for automated control of the telescope & camera, whilst the laptop and a network connection to the house provide a means for remote monitoring / remote control (see sections below)
A Belkin Serial to USB Connector was formally used for used several years but it began to get intermittent, but annoying Driver related crashes in April 2006, which couldn't be resolved and was eventually replaced with a NewLink USB Serial Adapter in August 2006 ]
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With the main observatory equipment under computerised control it readily possible to automate the acquisition of a sequence of target. I do this by means of my AIS Control Program which reads through and implements a Job Queue containing the night's target imaging requirements.
Three levels of safeguard are maintained to ensure the self operation of the telescope. These are i) Limits held by the Telescope, ii) limits held by TheSky programme and iii) limits/position checking within my AIS control program. Coupled to this is the requirement for appropriate cabling and freedom of movement when slewing., offline software testing, and correct alignment of the scope. Despite this there remains a small residual risk of equipment damage when there is no one watching over the scope.
Whilst there are other automated queue programs or scripting tools, I like to work with my own software which I have been able to customize over the years to meet specific imaging/project challenges and in a way that I find comfortable with. Key features of my job queue management is the ability to insert, add, skip, move up/move down tasks, and to stay on a target to take additional frames if required.
One of the techniques I've perfected is utilising CCDSoft/TheSky Image Link to
assess telescope position after a slew and make a correcting jog(s),
Other useful routines include guiding between frames and mosaic generations., and
realtime sky condition/image quality reporting via graphs.
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The Clair Observatory is setup to permit remote control of the telescope and imaging from inside my house using a LAN connection.
Remote control is mainly used for monitoring status of automated image queue,
addition/insertion of new targets into the job queue, monitoring sky conditions/image
quality with any consequential re-prioristiation of targets in the job queue.
Since the remote control is over the entire laptop (not just a particular
package) it allows any function or program to operated on the observatory laptop
exactly as if I was sitting in the observatory myself. This is a particular
handy ability especially in the cold depths of winter.
Of course these operations requires reliable performance by the network and associated remote access software. It took a number of years before the Remote Control technique was perfected. The current setup comprises
A laptop in the observatory (Observatory Laptop) connected to Telescope and CCD camera
A desktop computer or second laptop in the house (House Computer)
Both computers operating Network software (TCP/IP) on Windows Vista
Both computers using UltraVNC remote access software (see below) with observatory laptop running VNC server, with the house computer running VNC Viewer
A Power-Line networking connection operating at ~100 Mbps connecting observatory to household router (LAN) with a 54g wireless connection operating in house.
Remote control connectivity is generally reliable, though on old occasions Microsoft/Network/Firewall
problems may interfere with smooth operation.
A key requirement before leaving the observatory and retreating inside is to ensure that VNC Server is live on the laptop and the IP address has been noted (it can occasionally change if laptop hasn't been used for a few days)
Wireless Connection (2004-2012)
From 2004 to mid 2012 I used a wireless (WI-FI) connection to the house's LAN network. This provided sufficiently good connection up until Feb 2009, when I upgraded to a larger scope and built a new run off roof, afterwhich I began to get increased problems from lost connectivity during remote operation. This required making unwanted trips out to the observatory to restart the wireless connection, but in some cases the connection couldn't be restored without rebooting the laptop. This made remote control somewhat frustrating and has been one factor in reducing my use of the observatory in 2011-2012. Trying alternative WIFI settings & channels and alternative VNC parameters hasn't really improved the situation, and it is assumed that the problem is somehow associated with the much larger telescope/ metal work and possibly the rolled off roof lying between house and observatory.
Powerline Connection (2012+)
Frustration with Wireless Connection dropout became so bad in 2012 that I began to consider other options for gaining a computer connection between observatory and house. Retrofitting a wired ethernet cable between observatory and household router was considered but quickly dropped to bottom of the list due to relative difficulty of retrofitting a cable to the observatory and to household router indoors. The use of Power-line networking to extend the household LAN across the existing household power circuit and power cable to the observatory was a much more appealing idea, but initial lack of knowledge about this technology and concerns about impact of electrical interference and CCD image quality and the use of extensions cables and intervening residual current device (RSD) stopped me from pursuing this for sometime.
Finally (Oct 2012) I took the plunge and bought a pair of TP-Link AV200 Mini Multi-Streaming Powerline Adapters (TL-PA211) online for a cost of £32. The pair of adapters are easily twinned via a 'Pair' Button and data communication’s security is achieved by built-in 128-bit AES encryption. One Powerline was installed in the observatory with Ethernet cable connected to Laptop with another Powerline Adapter installed indoors with Ethernet cable connected to the household router. Initial tests appear promising, with transmission rates of 100-110 Mbps (compared against max 200 Mbps for the system), no visible impact to Flat & Dark Frames when data is being transmitted and good/acceptable cnnectivity to the house computer. The limiting factor on overall transmission becomes the wireless transmission rate between the router and house computer, which could theoretically be raised by means of an additional Powerline adapter and the processing priority on the VNC server on the laptop observatory.
Remote Access Software
Earlier attempts with remote access (pre-2006) had used Microsft NetMeeting and/or Microsoft Remote Assistance. These attempts would sometimes work ok, but where generally less than satisfactory.
From 2006 to 2010 I used RealVNC as my VNC software, and remains as a backup. RealVNC worked far better than NetMeeting and Remote Assistance.
Since 2010 I've used UltraVNC as my VNC software, with good success (wireless connection allowing). UltraVNC has a Viewer that can be set in Listening Mode, meaning that remote connection between the House Computer and the Observatory can be made before leaving the observatory (assuming that the House Computer has been switched on first).
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The process of closing up the observatory at the end of a night session is summarised below.
Command Telescope and CCD Camera to Shut Down.
- Telescope parks in predefined location
- Camera turns off cooling and waits for warmback, then disconnects Unlock Observatory / Turn On Ligjht
Turn Off Focuser. Turn Off Telescope
Close Observatory Roof
Switch on Observatory Light
Turn off CCD Camera
Remove Dew Shield (Check for condensation and remove with hair dryer if necessary) Replace Cap
Lock down observatory roof
If not too tired
- Create Summary List of FITS Files with Header Information
- Command copy of FITS files from New area to Raw area ready for Reduction
- Copy Log files from general log area to session's own log folder
Wipe down telescope/camera if appropriate
Disconnect USB cables and store under desktop
Take laptop, double-check observatory roof locks
Leave and lock-up Observatory
Note : Dark Frames and/or Flat Frames may be taken before shutting down the telescope and camera, dependant on status of current master reduction frames and the need for new ones
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My CCD cameras operates Temperature Regulation whereby the temperature of the
CCD chip can be held at a specific cooled temperature (up to around 30-33 deg below
ambient temperature). I always run the camera with temperature regulation on and
cool the camera to reduce dark (thermal) current.
At the start of observing session I select a suitable temperature set-point based on the initial and expected minimum ambient temperature. I will normally select a temperature that is some multiple of 5 degC (typically either -15, -20, -25, -30 degC) , such that the cooling power of the camera's peltier cooling is between 65 and 85%. ST-10XME has low dark current and it more convenient to chose a temperature that can apply the use Selected Dark Library frames during Image Reduction,, rather than minimize an already low dark current.
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Light frames are typically taken with an exposure time taken from the following
0.12s, 0.2s, 0.5s, 1s, 2s, 3s, 4s, 5s, 8s, 10s, 15s, 20s, 30s, 45s, 60s (1 min), 90s, 120s (2 min), 180s (3 min), 300s (5 min)
This allows the number of Dark Library Frame sets to be reasonably managed, simplifies the reduction process and provides some consistency in reporting exposure times.
Bin Mode (1x1, 2x2 or 3x3) will be selected dependant on the observing target (size & magnitude), seeing conditions and requirement or other wise to reduce download time and/or reduce image file size. For planetary imaging and astrometry I will tend to use 1x1 binning, whilst for image large dim objects I will tend to use 3x3 binning. for photometry I will use either 3x3 or 2x2 binning dependant on star brightness]
At 1x1 binning I will have a image with 2148 x 1472 pixels but will take 8.7 secs to download and occupy 6.1 MB of disk space, whilst at 3x3 binning I will have an image 716 x 490 pixels (still very reasonable) but only take 1 sec to download and occupy only 0.7 MB of disk space.
Images are normally taken full frame unless there is reason to reduce download time & reduce image file size (eg planetary imaging) or there is a need to remove edge effects for taking Mosaic Frames (One side of my ST-10XME main chip seems to be impact by shadowing from the 45 deg mirror associated with the guide chip when using F6.3 focal reducer and a workaround of using realtime image cropping has been adopted)
[Method used for earlier ST-7E camera was similar except that I used only 1x1 and 2x2 bin modes and sub-framing was not necessary for getting good Mosaic frames. ]
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I maintain a library of Master Dark Frames which I use during Image Reduction.
To provide frames to make Master Darks, I take Dark frames at 1x1, 2x2 and 3x3 binning at specific temperatures (eg -15, -20, -25 & -30 degC) and specific exposure times: 0.12s, 0.2s, 0.5s, 1s, 2s, 3s, 4s, 5s, 8s, 10s, 15s, 20s, 30s, 45s, 60s (1 min), 90s, 120s (2 min), 180s (3 min), 300s (5 min) . I also take Bias Frames (0s exposure), I will typically take 17 frames for each Bin/Exposure combination. In theory this is more than is needed for short duration exposures but is about right for longer exposures so I keep to this number.
The dark frames for a given temperature setpoint are ideally all taken during
a single session (either one taken during a cloud night or after a main observing
session has been completed)
Additional dark frames may be taken to reduced Flat Frames if Library Darks are not available for the Temperature and Flat Frame exposure time.
(Method for earlier ST-7 camera was similar except that I used only 1x1 and 2x2 binning modes. It was noticed that the Dark Frame characteristics of my earlier ST-7E camera changed with camera age, which required Dark Library to be replaced on an annual basis. Dark current was also noted to not only vary with CCD Chip temperature but also to some degree with temperature of Camera Casing / ie with Ambient Temperature. This introduced additional noise to images taken on warmer days even where operating CCD chip temperature was the same)
Goto the section on creating Master Dark Frames.
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I try to create a Master Flat Frame for reach filter, each time I change the
imaging configuration or when I suspect that flat frame characteristics have somehow
changed. Changes that necessitate taking new flat frames include : change in optical
configuration (eg imaging with/without focal reducer), change in collimination,
change in camera rotation angle. I have wondered whether a change in temperature
and thus focus position might also change the associated flat field (or flat field
moves in line with changing focus), I haven;t read any reports on this and haven't
yet been able to prove or quantify this effect myself.
To provide frames to make Master Flat Frame, I take Flat frames at 1x1, 2x2 and 3x3 binning. I will typically take 15 of 17 frames for each Bin Mode with an exposure that gives pixel values around 75% of saturation level. Unless I have corresponding Master Dark Frame in Dark Library, I will take a set of Dark Frames at the same temperature/exposure as the Flat Frames.
Acquiring really good flat frames is still a task that I'm yet to perfect.
Ideally I can the best flat frames by taking a Dusk or Dawn Sky Flat, however the
available time (not to bright/not too dark) to acquire a set for flat frames for
each filter and each bin mode is too short - even when ordering flats according
to bin size and filter related light sensitivity. The available time can be
stretched out to some extent by dithering frames so that stars are removed by median
or average-median combining, however time to dither frames using jogs of the LX200
GPS/R is longer than with LX200 classic.
[Opportunity here to investigate temporarily turning off the RA drive so that we don't get any stationary bright star points]
In order to collect Flat Frames taken through more (all) filters I will often use the alternative 'Dome' Flat method where by I point the telescope at a large sheet of white paper placed on one wall of the observatory and lit my a light bulb in the observatory, This is done in way that the scope does not cast a shadow on the white sheet. A white teeshirt placed over the front of the scope may be used to help diffuse the light.
Long exposure images taken through Clear Filter, images taken at low declination in direction of nearby city and images taken on moonlit evenings are the images and images where galaxy or nebula is of very low S/N is where deficiencies in the flat field are most observable. In due course I would like to build a proper light box, but I haven't had access to suitable materials, specifically a suitable light diffuser.
[The Method used for earlier ST-7E camera was similar except that I acquired
image at 1x1 and 2x2 bin modes. Before using the heavier duty Optec TCF-S,
the quality of flat fields with JMI focuser was also compromised by camera droop]
See section on creating Master Flat Frames.
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Image data is typically acquired as part of a set of multiple frames taking through
one or more filters. Frames through different filters can use different exposure
times. eg 3 x 60s (B), 3x 30s (V), 3 x 20s (R). Frames
are not acquired in a mode where the filter cycles round for each Frame
My AIS control program has the limitation that all frames within an image set have to adopt the same binning and subframe modes. This limitation does however make it easier to specify most target and besides I can get around the limitation fairly easily by placing an appropriate 'continue without slew' job' - with alternate bin/subframe modes - in the queue directly after the first job.]
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In order to place a target object in the centre of the CCD frame or place it at a specified position offset from the center, my AIS control program will take up to 3 or 4 locate frames (3x3, Clear Filter) and use image linking to determine the precise RA/Dec location of the CCD Image and use this information to jog the scope towards the required position. Smart Exposure option will select an exposure time (5 sec to 30s) from a preloaded table based on distance from the galactic plane and distance from the horizon. (a rich star-field in Cygnus near to Zenith will require a shorter exposure than a sparse star-field in Ursa Major and near to the horizon). The process will then repeat until the target object lies within the preset tolerance (0.5 arc min)
With my 12" LX200 R telescope (with its improved pointing and jog precision) and ST--10XME camera with its larger field of field, a target object can normally be brought to required CCD position with just two locate images.
[The Method used for earlier 8" LX200 classic and ST-7E camera was similar except that I acquired locate images with 2x2 binning. With poorer jog precision it was normal to need 3 locate images to bring a target to the required position in the CCD frame]
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With my earlier 8" LX200 classic (with its less precise pointing) and ST-7E camera (with its smaller field of view) I used T-Point software (Software Bisque) to improve the pointing capability of the scope. This was successful in bringing the target into field of view at the first slew attempt.
With my new 12" LX200 R (with improved pointing and jog performace) and ST-10XME camera (with larger field of view) I have less problem with acquiring targets and bringing them to the required position in the CCD frame., and consequentially I'm not currently using T-Point.
Although the jog operation with LX200 GPS/R is more precise it does see to take longer to perform than with rthe LX200 Classic, and their would be opportunity to reduce overall locate time by improving pointing accuracy using TPoint.
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TPoint Mapping .....
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I have two means of focusing with my setup
- coarse focusing using the LX200's focus knob (which moves the primary mirror back/forward
- fine focusing using Optec TCF-S focuser which is attached to the back of the scope
Unless optical equipment is being changed the coarse focus is not used and the mirror lock is on.
The Optec focuser is a Crayford Style focuser with a stepper model where the minimum step distance of only 0.000085". The focuser has a total travel distance of 0.6" (equivalent to 7000 steps). Attached to the back of the LX200 visual back it gets around the mirror-shift problem that effects the SCT's coarse focusing and can be remotely (via Handbox) or PC controlled (via RS-232 connection)
Once trained the Optec TCF-S focuser is capable of automatically correct focus
position for ambient (Tube) temperature changes completely by itself.
However I actually operate the focuser instead in a fully computer controlled mode.. This allow me to not only adjust focus position for temperature effects, but also adjust focus position according to which filter is being used. Temperature related changes are made based on a pre-determined temperature coefficient (focus steps per degC change) and a reference focus position at a specified 'datum'. This is set based on focus position for the Clear Filter. Filter related changes to focus are made based on a predefined table of focus position offsets for each filter relative to the Clear Filter.
With settings in place a new session can be begun with the scope automatically focused and in the knowledge that appropriate focus changes are made through the remainder of the session as temperature falls (or rises). In practice there appear to be night to night variations which I don't fully understand the origin (is an atmospheric related modification to precise focus or an issue over stablisation of tube/imaging equipment, and therefore my approach is to QC the FWHM measurements from real time images and refocus the scope if focus looks off to generate a focus offset for the session.
[ At its simplest temperature coefficients are determined from measurements of focus position at two different temperature, ideally separated by several degrees and where both measurements are for from a scope fully equilibrated to ambient temperature. To supplement this the best fit line through focus position from several temperature points could be also be used ]
My current approach to determining best focus position is as follows :
- check star is already roughly focused using LX200's coarse focus and mirror is locked
- center a moderately bright star in CCD frame (best done using a star above 60 deg inclination)
- take a 1 or 2 sec sub-frame exposure of star at 1x1 binning
- roughly focus the fine focus if its not already focused.
- use a focusing routine in my AIS program to step the focuser through a range of focus points +/- about current focus position and take a subframe image at each position, measuring and plotting the FWHM of the star, then finally calculate a 2nd order polynomial through the set of points to determine best focus position and FWHM at best focus position, and move the focuser to the new best focus position.
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Although the Optec TCF-S focuser is capable of automatically correct focus position for ambient (Tube) temperature changes completely by itself (once trained), I actually operate the focuser instead in a fully computer controlled mode.. This allow me to not only adjust focus position for temperature effects, but also adjust focus position according to which filter is being used. Temperature related changes are made based on a pre-determined temperature coefficient (focus steps per degC change) and a reference focus position at a specified 'datum'. This is set based on focus position for the Clear Filter. Filter related changes to focus are made based on a predefined table of focus position offsets for each filter relative to the Clear Filter.
The following link - Autofocusing (2008-05-02) - shows a graphical log of focus position for one particular session and illustrates focus position being adjusted with changing tube temperature and changing filter
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Polar Alignement ...
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Application of PEC (Periodic Error Correction) by the telescope/telescope mount is a vital step to getting good image quality. (other key elements are Good Polar Alignment, Good Collimation, Correct Tracking, Good Focusing and a Stable Mount/Tube (Note : windshake is a particular issue for my roll-off roof observatory)
PEC Training is the process of measuring the periodic error profile and determining
the appropriate correction curve (PEC).
With my previous 8" LX200 Classic the scope was trained directly by setting the scope to Learn mode and autoguiding on a star for an 8 minute period with opportunity to refine using further 8 minute periods of training.
With my 12" LX200 R the my approach is to use the PEMpro package (CCDWare) with PEC Off to measure the periodic error across a 24 minute interval (3 x 8 minutes cycles) and then analyse and model the data to give a PEC Curve which can then be uploaded in Autostar II. I then refine the PEC Curve using PEMpro again to record the residual periodic error with PEC on. The new data is analysed, modelled and the resulting refined PEC curve uploaded into Autostar II
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I collimate my SCT scope by adjusting the 3 knobs/screws in the center of the corrector lens which control the secondary mirror, based on information gleaned from CCD Images. I prefer this to visual/eyepiece as it minimizes the impact on the observing setup.
Two main approaches are used.
1) first approach is to CCD image of a de-focussed moderately brighter star and either use visual observation of th resulting image or analysis by AIS program to quantity the amount (if an of displacement of central dark area relative to bright rings. If dark center is offset then collimation tuning is used to bring the dark center into the center of the bright ring. Adjusting the collimation screen shows up as either improvement of deterioration in the quallifed parameter.
2) second approach is to use CCDWare's' CCD Inspector which work by analysing a CCD image of a dense starfield (200 + stars) and calculates the miscollimation between the optical centre of the image and physical centre, expressed in either pixels or arc seconds. In a perfectly collimated telescope, these two centres will coincide. As much as a few arcseconds mis-collimation can affect image quality.
One issue I find with collimation is that ideally I would like to perform collimation on a star or starfield near to but it this with the position with the tube nearly vertical it is impossible to physically reach the collimation screws. The compromise is to perform collimation on starfield lying at a lower inclination
Collimation needs to be performed on a night of good seeing, and free of windshake or other disturbance, with no risk of clouds interrupting the task.
Where any inprovement in collimation is made the benefits to sharprness of star images is very clear.
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The ST-7 and ST-10XME cameras both have self-guiding CCD chips built in. The size of the guiding chip in the ST-7E camera is quite smaller and often there is a suitable star to guide upon without some repositioning of the target, which may compromise the target position in the CCD frame. Nevertheless Autoguiding was occasionally used.
However autoguiding using the guide chip remains a technique which I under-use. Instead I have tended to simply take several shorter exposures which I then stack together or I employ the alternative approach of autoguiding between frames using the main image chip.
Regardless of whether autoguiding is performed during imageframe exposure using the guide chip or between image frames using the main chip the process of 'guide' calibration is the same. This involves centering a moderately bright star in the centre of the CCD frame and then run CCDSoft Calibrate Guider facility which will guide the scope up/down/left/right for a set number of seconds, take a image and then calculate the pixel movement per second for N, S, E, W guiding. I do this using the Main Imaging Chip at 1x1 binning. Once calibrated CCDsoft then handles the application of the appropriate guiding correction whatever the binning mode and chip (Main Chip or Guide Chip)
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Mosaic frames are acquired by a routine in my AIS control program that reads a list of RA/Dec coordinates and commands the scope to successively slew to each frame position in turn and take images with a specific bin/exposure/filter configuration.
Steps used to define a mosaic frames are as follows:
i) centre the required mosaic region in TheSky's virtual sky and use Mosaic facility
(under Tools/Mosaic) to define the number of rows/columns, percent overlap and Field
ii) hit Apply button to create the planed mosaic frames in TheSky, called Mosaic A1, Mosaic A2, Mosaic A3 etc
iii) review mosaic frames and if neccessary modify number of rows/columns, centre position to get the right mosaic area
iv) run a Define Mosaic routine to read the coordinates of each planned Mosaic frame and write them to a file
v) specify the imaging requirements for each mosaic frame
The mosaic frames coordinates can either be defined sometime before the session or during the actual observing session itself.
When using F6.3 focal reducer one (not yet tested without reducer) one side of my ST-10XME main chip is affected by a shadow from the 45 deg mirror associated with the guide chip and I therefore take Mosaic Frame images with an appropriate sub-frame setting that automatically clips off the offending edge.
Note: It is important that TheSky's Mosaic Window is closed for imaging operations using my AIS program as it interferes with successful image acquisition.
[The Method used for earlier 8" LX200 classic and ST-7E camera was similar except that I acquired full frame images and due to the smaller field of view I had to take more frames to cover the same target area]
See Mosaic Construction
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Slitless spectroscopic images are acquired through Star Analyser 100 which is
held in one of the filter cells in my Filter Wheel. Whilst the spectra are
relatively low resolution, and are more easily contaminated by adjacent star light,
they are relatively easy to acquire and can be interspersed with normal imaging.
Taking a spectra is no more difficult than specifying the 'S' filter/or its filter
wheel position, 1x1 binning (to maximise resolution) and an appropriate exposure
time dependant on the target magnitude (this will be longer than normal as star
light is spread out along a line of pixels).
Star Analyser is not parfocal with my clear filter and I therefore use a routine in my AIS control program to apply a focuser offset when taking Spectra images.
I don't have an easy way of orientating the Star Analyser so resulting spectra are perfectly aligned with one axis of the CCD frames and I therefore rely on rotating the spectra image during later spectra processing.
Since spectra are not taken through a slit there is a risk that spectra of a target is contaminated or even overwhelmed by either the zero or first order image of an adjacent star. For really bright stars the level of contamination by occasional dim star can probably be accepted, but where the target is dim anjd lies in a busy starfield it just may not be possible to acquire an useable spectra with the spectra configuration. Whilst the problem could be reduced by rotating the camera + filter wheel unit, the workaround is inconvenient to general image acquisition. The overhead of taken extra flat fields and later reconfiguration back to standard orientations means that this option is only considered for specific high value targets such as a quasar/red shift measurement or supernova spectra.
s undertaken by acquiring low resolution spectra using Star Analyser
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The process of image reduction is summarised below.
(* To be written )
>> Observatory Use
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|This Web Page:||Observatory/Telescope Methods & Practices|
|Last Updated :||2015-05-15|
|Site Owner :||David Richards|
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