David's Astronomy - Notes 2021-03-14 (S0870)

As part of an attempt to better understand and improve autoguiding performance issues (see Investigation - Guiding Issues (West of Meridian), 2021-01-16) the SBIG power cable and the ZWO 178MC USB 3 cable were rerouted during the daytime ahead of session S870 (2021-03-14) . Instead of passing from the back of the imaging train to the right hand fork & then indirectly to floor the cables were rerouted to take them back up to the OTA and then down to the right hand fork & then indirectly to the floor.

A daytime test was performed to ensure that cables positions would cause any holdups or other issue slewing to various parts of the sky.

The cables have to have enough slack between camera and their respective tieoff points on either side of the Dovetail for the Secondary Scope's to allow Focusers to move the cameras in/out of focus. The cables then have to have another slack or easy movement to allow the OTA to be move 90 deg in Declination/Altitude without straining the cables, and then have another slack to allow the telelescope +/-180 deg in azimuth without the cables straining.

The cables also have to allow minimal impact on small declination movements made during autoguiding, or at least less impact that the previous routing. In theory the scope shouldn't feel a force from the cables bending , but may feels a greater force from the weight of the cables hanging off the scope. Any force in the new set up will lie closer to the decination axis and therefore should be smaller. (maximum force on the Dec Gears is hypothesized to occur when cables hang at the large distance from the Dec Axis which they would do in the original routing.

Although the power cable is tied off at the / close to the camera and there should be minimal movement in the cable section between SBIG camera and the tieoff point on the OTA, there is a risk of SBIG camera power disconnections due to movement of the power adapter pins in the SBIG Camera's power port.

Pictures illustrating the cable routing before/after the change are shown below:

S870 was the first night time test of the new cable arrangement but it the benefits or otherwise of the rerouting could not be established because poor weather (wind/cloud) severely limited the observing opportunities during the session. No data collected on clear sky guiding a target west of Meridian, so unclear if the rerouting helps resolve guiding issues, but at leasy no adverse impact was seen on 2 targets guided east of the Meridian. Importantly there were no hang-ups or serious issues with the new cable arrangement during the session.
( It is uncertain whether the one instance of 'CCD Image [0000] has not saved to disk" during the session was caused by a power/communication glitch with the SBIG camera (and a side-effect of new cable routing) or not)

ASTAP - Plate Solving Software

Downloaded and installed ASTAP 64 bit version beta 0.9.512a (dated 2021-03-09) today (2021-03-08), along with H18 star database and demonstrated that it could plate solve a FITS image from SBIG camera, with starting coordinates lying 3 degrees from its true position.

Found that Flip Vertical needed to show image with same orientation as orginal (N Down)

Found that

- Take CCD Image in CCDSoft of starfield containing reasonable number of round stars, and save to disk.
- Load FITS image into ASTAP
- Solve image asstrometric (plate solve)
- Copy Ra and Dec fields into Ra/Dec fields on AstroMain's Observing/TheSky Tab & click on 'Centre in TheSky' button
- Go to Centre

Live Trial (Session S870)

- 5s exposure (3x3) taken with the scope believed to a few degrees off it's true position (this was due to a
- Tried to Image Link using CCDSoft/TheSky6, but failed
- Opened image in ASTAP, Image showing RA 06 44 02.655, Dec +13 13 55.49
- Plate Solved. Solution took 7.7s and reported RA 06 43 26.900, Dec +17 13 50.00 (ie an offset 4 deg)
- Copy Ra and Dec fields into Ra/Dec fields on AstroMain's Observing/TheSky Tab & click on 'Centre in TheSky' button
- Checked that stars in TheSky matched those in image.
- Clicked on TheSky's Virtual Field Centre and synced scope
- Began a short mapping job.
- Saved TheSky Document & commenced live session

Conclusion : Process worked and was fairly quick.

Streamlined Method
An alternate method where tried out whereby after platesolving in ASTAP the ime was saved with new header, then opened in CCDSoft and an image link performed hoping that it would then successfully link in TheSky without having to go through process of copying and pasting RA/Dec values, however this didn't work. Although an WCS is inserted in the FITs file header, OBJCTRA & OBJCTDEC keywords are left with their original values, and as these are the values used by CCDSoft Image Link, the linking doesn't work any better than the originally acquired file.

A routine could easily be written that would read the FITS file saved by ASTAP, and write in new values for OBJCTRA & OBJCTDEC based on CRVAL1 (RA deg) & CRVAL2 (Dec deg). The routie could help by saving the FITS file with a specific name/specific location to aid the location of the image in ASTAP.

Command Line Method
ASTAP has a command line intereface, documented at www.hnsky.org/astap.htm#astap_command_line
So it is feasible to have button routine in AstroMain for aligning the scope when its position is only roughly known.

- Button
- Take 10s image   'Snap.1933.52.FIT
- Run Shell
    astap.exe -f   "C:\Data\CCD Imaging\New\2021-03-14 S00870\_FOCUS\Snap.1933.52.FIT"    -r 5
- Read WCS data.

Note: The -update option will update the Image's fits header with the found solution. However when this was tried the resulting .FIT file couldn't be opened in CCDSoft (CCDSoft produces a dialog "Undescribed error. Error code = 701" ). Fortunately the original file is saved with a .bak extension.

Ra/Dec Position
Getting image's RA/Dec from WCS. We need to read CRVAL1 & CRVAL2

CRVAL1 = 2.444617601865E+001 / RA of reference pixel (deg)
CRVAL2 = 5.366454564370E+001 / DEC of reference pixel (deg)

RA = CRVAL1 / 15 (RA in hours)
Dec = CRVAL2 (Dec in degs)

RA = 1.629745
Dec = 53.664

Image Scale
Getting image's pixel scale from WCS or INI file. We need to read CDELT1 & CDELT2.

CDELT1 = 3.882527481525E-004 / X pixel size (in deg)
CDELT2 = 3.885837446741E-004 / Y pixel size (in deg)

Image Scale (in arcsec / px) = ( CDELT1 * 60 * 60 + CDELT2 * 60 * 60) /2
Image Scale = 1.398 arcsec/px

[ Number of stars in object http://www.xparallax.com/ ]

Image Rotation
Getting image's rotation from WCS or INI file. We need to read CROTA2.

CROTA2 = -7.529744000419E-001 / Image twist of Y axis (deg)

Image Rotation = CROTA2 + 180
Image Rotation = 179.2 deg

For a more precise (better) value then could implement (CROTA1 + CROTA2) / 2 + 180

180 is added to CROTA2 in order to get rotation to the numbe equivalent to that consistent with how TheSky6 measures Image Rotation. It seems reasonable given that the Camera Orientation is North Down.

Update 2021-03-16
Following the successful use of ASTAP in session S870 (2021-03-14), a new button routine bas been built in AstroMain 3.38.5 to streamline the process of checking and resetting telescope pointing. This new routine was successfully operated during session S871 (2021-03-16). See AstroMain - New Button Routine to Check/Reset Telescope Pointing


	Session Aims & Highlights
	- Observing Result - Night Summary Plot - Session Event Log

	Operational Issues
	- Critical Issues (0), Major Issues (0), Minor Issues (4), Continuous Improvement (2)

	Images from 2021-03-14 >> [ Local Files >> ]

	Camera Cable Rerouting
	ASTAP - Plate Solving Software

This Web Page:	Notes - Session 870 (2021-03-14)
Last Updated :	2024-02-21
Site Owner :	David Richards
Home Page :	David's Astronomy Web Site

David's Astronomy Pages
Notes - Session 870 (2021-03-14)

Session Aims & Highlights (2021-03-14)

Operational Issues (2021-03-14 S870)

Camera Cable Re-Routing

ASTAP - Plate Solving Software

David's Astronomy Pages Notes - Session 870 (2021-03-14)

Session Aims & Highlights (2021-03-14)

Operational Issues (2021-03-14 S870)

Camera Cable Re-Routing

ASTAP - Plate Solving Software

David's Astronomy Pages
Notes - Session 870 (2021-03-14)