David's Astronomy Pages Notes - Session 357 (2009-03-12 to 2009-03-20) Notes (S356)   Notes (Main)   Home Page   Notes (S358)

Goto Images from 2009-03-15

Restart from Park Position (2009-03-15)

The LX200R telescope was turned-on in the position it was Parked during the previous session - this is a custom set Park Horizon (Due West, Altitude 0 deg). This position means that the telescope tube is horizontal with the left hand fork - with GPS sensor - at it's highest position. 

GPS Fix achieved after 5 asterisks (first light session and morning session on 2009-03-15 took 3 asterisks for scope to get a fix).  

RA PEC was set on (even though RA PEC is zeroed/empty) during the previous session and this prevented the scope slewing slightly west upon start-up.

The scope was slewed to 0 mag star Arcturus and an image taken. The star lay close to the centre of the image (just 1.2 arc min off-centre), indicating start-up process was working well.  

Arcturus  (Bootes)

CCD Image (50% scale size) 0.12s, 3x3 binning, R Filter 2009-03-15 (#357063)  12" LX200R  (at f/5.8) + ST-10XME

On the next night the scope was again restarted from Park. GPS Fix achieved after 6 asterisks.  Slew to Procyon, but star was just outside field of view.

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Focal Ratio / Field of View / Image Scale

Field of View Difference
My normal imaging setup uses a f/6.3 focal reducer with my 8" f/10 scope.  The observational focal length, focal ratio and field of view for my previous and new focuser setups are shown in the table below. 

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Critical Focus Zone

To obtain a sharp image with best possible focus, the CCD detector should sit within a zone known as the 'critical focus zone' (CFZ).  The size of the critical focus zone is based on the focal ratio of the telescope.  The faster the focal ratio the shorter the zone of critical focus is.

The theoretical size of the critical focus zone is computed using the equation

    CFZ (mm)  = (focal ratio)^2 * 2.2 / 1000

 For my new imaging setup (with 12" f/10 scope, f/6.3 focal reducer with TCF-S focuser), the effective focal ratio is f/5.7 (actually 5.66).  Therefore

    CFZ = 5.66^2 * 2.2 /1000  = 0.07 mm 


For real world situations the CFZ value is approximately 10-30% greater than the theoretical value. Thus for my setup 

    CFZ =  0.08 to 0.09 mm

Based on the 0.000085" (0.00216mm) step size for the Optec TCF-S focuser 

    CFZ = 33 steps  (theoretical)  or 36-42 steps (real-world) 

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Collimation

A re-collimation of the scope was performed after observations during the previous 'First Light' session with new 12" LX200R (S356) had shown that the scope was not correctly collimated (Collimation Issues / M13).   

Measurement of stellar FWHM showed that the re-collimation had improved sharpness of fully focused images from 4.7 arc secs (before) to 3.6 arc secs (after). 

An attempt to refine collimation further will be performed in due course.

Collimation Sequence on +7th mag star (altitude 50 deg)

Montage of CCD Images of unfocussed star 10s, 1x1 binning, C Filter (#357064-76)  12" LX200R  (at f/5.8) + ST-10XME

Measurement of peak FWHM before and after re-collimation

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Focus Offsets by Filter (2009-03-15)

An examination of the focus offset for R and Spectral filters was made after observations during the previous 'First Light' session for my new CFW10 filter wheel, had shown appreciable differences in focus position for RGB filters compared to C and BRVI filters.  (see Focus Differences between Filters).   

The offset in focus positions between different filters is required in order to automate acquisition of images taken with different filters. The offset can be used to set an temperature and filter compensated position for the Optec TCF-S focuser prior to taking each image.  

The examination was only partially successful as the Optec-TCF-S ran out of focusing range and there was no time before dawn to repeat the tests. The data does shown that the Red and Spectra filters both have an offset of around -2000 steps compared to C Filter.     Previously an offset of -1550 steps was measured for Spectra filter when used with 8" LX200 Classic at f7.6.

12" LX200R  (at f/5.8) + ST-10XME

Further measurement of focus offsets will be conducted during a future session after adjusting the coarse telescope focus for the C Filter to a position corresponding to c.. 4500 steps. In the meantime value of -2000 will be used for Red, Green, Blue and Spectra filters. 

The following figure shows newly created fields for inputting the Focus Offset with my Program Settings.

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Autoguider - Drive Calibration (2009-03-15)

An autoguider drive calibration was performing using the main imaging CCD, with 20 sec calibration time on both X and Y Axis.

The drive calibration results for the main imager and the corresponding details for the Autoguider are shown below. 

The ratio   "+X value Imager/+X value Autoguider"  is 1.216  (from 12.7005/10.4426)
This ratio is somewhat bigger than the ratio of   1 / ( Imager Pixel Size / Autoguider Pixel Size ) = 1 / (6.8/7.4) = 1.088

12" LX200R  (at f/5.6) + ST-10XME

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Ted Agos Universal Focal Reducer Adapter Tube

During the the 'first light' session the new LX200R scope was used with its accompanying standard Meade Microfocuser. Although the standard Microfocuser is a far less capable focuser than my existing temperature compensating Optec TCF-S focuser I was interested to gain some basic experience with the Meade focuser and examine whether there might be any specific imaging application were it would hold an advantage over the TCF-S focuser. 

Because the Microfocuser is shorter than Optec TCF-S focuser it was imagined that the focuser would have the advantage of being able to reach to higher declination positions with 'straight-through' imaging before reaching the limits caused by the fork base.   Whilst is very probably true for imaging at the standard f/10 focal ratio,  it was found that for f/6.3 imaging using my Ted Agos adapter tube (external ID 2") there was a unexpected limit to the distance that the tube could be inserted into the Microfocuser. It was found that the tube would stop 1-3/4" (4.5 cm) short of a fully inserted position.   

Investigations revealed that the clear aperture of the Microfocuser (ID 2") is not the inherent problem, but instead the block was caused by the standard 3" visual back mount on the rear port cell having a clear aperture of only 1.5/8" diameter - a ridiculous small size given the actual opening on the back of the scope (2-1/8").   As a consequence the overall length from the back of scope to the rear end of the CCD Camera when using the Microfocuser & Adpater Tube was 9", rather than expected 7-1/4" to 7-1/2".

The Optec TCF-S uses its own mount accessory (3" female thread with Optec-2400 male dovetail ring) to connect to the 3" male threads on the back of the 12" LX200R. The 3" Mount has a clear aperture of  2.16" (54.9mm) and can therefore take the Ted Agos Adapter Tube fully inserted.   

The Meade Microfocuser was duly swapped out for the Optec TCF-S  and as expected the Adapter Tube was able to be fully inserted into the focuser. Measurements showed the overall length from the back of scope to the rear end of the CCD Camera was just 8-1/2",  a full 3/4" shorter than when using the Meade Microfocuser ! 

[ It is understood that Peterson Engineering EyeOpener can be used to replace the standard visual back opening the optical path to a full 2" diameter and still allow the Meade Microfocuser to be used.  It is supposed that this would then allow a Ted Agos Adapter Tube to be fully inserted, but thus is not personally confirmed ]

f/6.3 Imaging Equipment used during 'First Light' Session  with Meade Zero Image Shift MicroFocuser and Standard Visual Back		f/6.3 Imaging Equipment used for subsequent sessions with Optec TCF-S focuser and Optec 3" Mount
[ Larger image (not annotated) ]		[ Larger image (not annotated) ]
Vignetting Compared Vignetting measurements made using CCD Inspector using 2x2 Binned Flat Frames from session a) S356 (First Light Session) - T.Agos Adapter Tube with Meade Microfocuser a) S358 - T.Agos Adapter Tube with Optec TCF-S Focuser Both images demonstrate the the illumination centre (approx centre of optical path) is offset from CCD Centre by around 3 arc mins.  This show that scope is not correctly colliminated. Nevertheless the images clearly show that the images taken using the Meade Microfocuser setup suffer noticeably more vignetting than those using the Optec TCF-S focuser setup.
Vignetting levels with Meade Microfocuser/Standard Visual Back - 2009-03-09 (S356) Illumination Reduction Levels is the 4 corners are  6%, 7%, 15% & 20%
Flat Analysis Map (made using CCD Inspector)  Master Flat Frame / Night Sky Flats (Reduced Size) 15 x 60s exposure (average median combine), 2x2 binning, C Filter 2009-03-09, 12" LX200R  (at f/5.8) + ST-10XME
Vignetting levels with Optec TCF-S - 2009-03-26 (S358) Illumination Reduction Levels is the 4 corners are  7%, 8%, 12% & 14%
Flat Analysis Map (made using CCD Inspector)  Master Flat Frame / Dawn Sky Flats  (Reduced Size) 19 x 3s exposure (average median combine), 2x2 binning, C Filter 2009-03-26, 12" LX200R  (at f/5.8) + ST-10XME

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Focus Offsets by Filter (2009-03-16)

Following provisional measurements of the focus position offsets of my Red and Spectra filters relative to my Clear filter (2009-03-15,above), a more extensive set of measurements were made the following night (2009-03-16).  Focus profiles were made for U, B, V, R, I, Red, Green Blue and Spectra filters, bounded either side by focus profiles measurements made using the C Filter (this helped to remove the slight temperature effects from 0.2-0.5 deg C changes in temperature).  

Best focus position in terms of Focuser Steps (Optec TCF-S) was derived by a quadratic best fit to each set of profile data.  Focus Offset was then calculated by subtracting the averaged Best Focus Position of the two bounding Clear Filter profiles from the filter's Best Focus Position.  After averaging offsets where more than one measurement was made for a given filter the following table of offsets was generated.

Focus Offsets for the CFW10 filter set at f/5.6 (2009-03-16)

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Asymmetric Focus Profiles (2009-03-16)

More detailed examination of Image Profiles created for defining filter offsets showed that whilst the measured offsets are probably reasonably valid, the precise method used to calculate best focus could be questioned.   Previous focus profiles with 8" LX200 Classic and previous night profiles using 12" LX200R have shown focus profiles that are broadly symmetrical around best focus position (modelled using a quadratic, 2 order polynomial equation)  tonight's profiles seem to display a more asymmetric form which seem to be better fitted by a 3-order polynomial.    

2s exposures, 1x1 binning 12" LX200R  (at f/5.8) + ST-10XME

It's not certain why the focus profiles are seemingly asymmetric. Possible causes considered are: : 

• effect of dew  
  - the dew heater was unintentionally not switched on, and corrector plate at front of scope did eventually dew up during session
  however the asymmetry is evident in profiles from close to start of session.
  

• effect of atmosphere / low altitude  
  - 'focus' star was relatively low altitude - varying from 30 deg to 17 deg during focusing session
  - asymmetry is evident in profiles taken at 30 deg altitude which shouldn't have caused at atmospheric problem
  

• periodic error effects 
  - periodic error might cause some elongation of stars in RA, but this seems unlikely to be significant as exposure times are small (2 sec). and don't explain consistent direction of asymmetry.
  

• primary mirror 'pinched' by mirror lock
  - the primary mirror was adjusted and relocked since previous session
  - not sure this would produce asymmetry, but I suppose it could ?
  

• drift in collimination ssince last session or that scope was mis-corrected during the previous evening's re-collimination attempt.
  - defocussed stars seem to show coma effects / miscollimination 
  - not sure this would produce asymmetry, but I suppose it could ?
  

• feature of Meade's ACF (R) optic design
  - seems unlikely (not evident in profiles from previous night), but I suppose it could ?
  

An enlarged view of the 'focus' star in 1x1 binned frames indicate that the star appears noticeably different either side of focus.

2s exposures, 1x1 binning 12" LX200R  (at f/5.8) + ST-10XME

Profiles from previous session (for comparison)

Note : It was noted that CCDSoft's calculation of stellar FWHM seems to be independent of Aperture Diameter setting (at least no difference was observed when using diameters of 5 and 10 pixels based on 22 separate images)

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Plate Scale Differences / Impact on Colour Combining

A secondary impact of having filters that are not precisely parfocal is that images taken at different focus position have very slightly different plate scales.  This means that images can not be precisely aligned for successful combination without resizing some of the filtered images.    

This impact was examined using images of Comet Lulin, taken through Clear, Red, Green and Blue filters.  The focus position for the Clear filter whilst approximately parfocal with my BVR filters is significantly different from the best focus position for the RGB filters.  The differences in focus position means that the image scale of Clear filter (1.611 "/px) is sufficiently different from the image scale of the RBG filters (1.626 to 1.627"/px), that a simple stack of all four filters means that star away from the image centre are not perfectly aligned and appear as two 'star' profiles - one from C filter and one from RGB filters combined.

Stacked Clear, Red, Green, Blue images of Comet C/2007 N3 (Lulin)  aligned on comet / central star, but with no resizing or allowance for different image scales Notice that stars appear as 'double' points towards edge of image

Annotated Image Scale (50% size reduction with star inserts at normal size) 1 x 30s C, 1x30s Red, 1x30s Green, 1x45s Blue (average combine) 2x2 binning Frames aligned on central star, before stacking, but without image resizing 2009-03-17, 12"" LX200R (at f5.7) + ST-10XME

Resizing the images to a common image scale (1.611"/px) and then, cropping them to a common image size, then allows the images to be properly aligned throughout the full area of the image and allows the star profiles from each filter image to overlay exactly which is obviously vital when colour combining.  The improvement can be clearly seen in this simple average stack.

Stacked Clear, Red, Green, Blue Image of  Comet C/2007 N3 (Lulin) individual frame resized to a common image scale before aligning  and stacking Notice that stars appear as single objects to edge of image

Annotated Image Scale (50% size reduction with star inserts at normal size) 1 x 30s C, 1x30s Red, 1x30s Green, 1x45s Blue (average combine),  2x2 binning Frames resized and aligned before stacking 2009-03-17, 12"" LX200R (at f5.7) + ST-10XME

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Bias Frames / SBIG Power Lead Extension (2009-03-20)

The existing lead from SBIG power supply is a bit too short when using CCD camera with new taller 12" scope, and may cause cable snag for certain sky positions (in particular for automated towards the North).  A custom power extension cable was therefore acquired from a UK supplier (Colburn Electronics, Coventry). The 2m extension cable has the added advantage of being more flexible that the existing very stiff cable and less expensive than the SBIG's own power extension cable.

Power Lead Extension for ST Series CCD camera (Made by Colburn Electronics, Coventry)

In order to test operation with the new extension cable and check for absence of added noise a series of 17 bias frames where recorded at 1x1, 2x2 and 3x3 binning. The resulting master frames were then compared with previous master frames, and confirmed that there were no operational problems with the new extension and that it was caused no added noise.

Master Bias Frame with 2m Power Extension Lead (-20c)

CCD Master Bias Frame (25% size) Median average (17 frames) 1x1 binning 2009-03-20,  ST-10XME

Previous Master Bias Frame with regular SBIG power lead (-25c)

CCD Master Bias Frame (25% size) Median average (17 frames) 1x1 binning 2009-02-19,  ST-10XME

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Daytime observing (2009-03-20)

During a daytime session, an attempt was made to see if images could be recorded in daylight. This was with the principal aim of discovering whether it was going to be possible to perform Drive Training on a distant Landmark using the CCD camera, avoiding the need to remove the camera in order to use eyepieces.

Using full 12" aperture it was noted that CCD saturation was occurring with the shortest possible exposure of 0.12 secs even when using a U filter (the least transmissive filter available) . The telescope was then stopped down to a ~ 1" diameter and it became possible to record images with 0.12 to 2 sec exposure (again with U filter).

Whilst the resulting image(s) were not very pretty on this occasion due to Frost Slugs on the CCD, they did at least demonstrate that daytime Drive Training should be possible.

Distant Fence + Sheep  (taken through Mist and affected by Frost Slugs on CCD)

CCD Image (50% size, rotated) 0.12s exposures, 3x3 binning, U Filter 2009-03-20 daytime image (#357153) 12" LX200R  (at f/5.8) + ST-10XME

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Frost Slugs (2009-03-20)

During a daytime session, ambient temperature 15 degC, the CCD camera was cooled to -20 degC. Subsequent light frames with U filter show frost slugs on the CCD (and possible effects of condensation/frost on CCD window ?), indicating that the desiccant had become 'saturated'

Frost Slugs

CCD Image (50% size) 1s exposures, 3x3 binning, U Filter 2009-03-20 daytime image (#357158) 12" LX200R  (at f/5.8) + ST-10XME

It was surprising to have frost slugs form so soon after acquiring a new camera.  Two possible sources for the water moisture are considered.:

- moisture that entered the camera when front plate was take off for attaching the filter wheel on 2009-03-08
  (performed prior to first light session, case was opened indoors where humidity was low (~ 40%) and left open for some 15 mins)

- moisture that somehow entered the camera during session on 2009-03-16 when dew formed on the telescope/equipment.
 (upon removing the desiccant container it was noted that circular rubber seal was ?missing 
  either a) it was never present, b) it fell into the outer housing when removing or c) it lies squashed flat into the recess ).

The desiccant container was removed and baked in an oven for 3 hours at 180 degC. 
After replacing the 'recharged' desiccant container, the CCD camera was then run for 2 hours at -5 degC. No signs of frost were noted in observatory light frames.

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