Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

PHD2 OAG and crescent stars

  • Please log in to reply
5 replies to this topic

#1 Lord Beowulf

Lord Beowulf

    Vendor - Orion Ranch Observatory

  • *****
  • Vendors
  • topic starter
  • Posts: 2158
  • Joined: 13 Oct 2008
  • Loc: Cedar Park, TX

Posted 17 February 2019 - 10:33 AM

Hi All:

 

As I sit here dreaming of a new moon phase without clouds and 90+% humidity, I'd like to resolve one of my latest niggling issues with my astrophotography rig, and I'm hoping someone here has a solution.  I use a Taurus Tracker III OAG with a QHY5III178M guide camera configured as shown here:

 

4871.jpg link31.png

 

The problem I'm seeing is that despite having used a Bahtinov mask several times to get the two cameras to the same focus point, it would appear that there's enough difference in path length and materials that the focus of the two cameras drift relative to each other as a function of temperature.  The result is that while I can focus the main camera very well using SGP and my focus motor, the guide camera is not always in good focus.  This probably wouldn't be a problem if it wasn't for the small pixel size of the camera and the fact that since I'm picking off the edge of the SCT, an out of focus condition results in a slightly crescent shaped star instead of round.  The net result is that since PHD2 is apparently picking the brightest spot in the crescent rather than centering on the whole object, and since that fluctuates with atmospheric conditions, I end up with poor guiding along the direction of the crescent (RA in this case) even though it's excellent in the other direction (DEC).  This of course results in slightly elongated stars in the RA direction in my image.

 

So, is there anyway to alter the star detection algorithm settings in PHD2 to better center on the out of focus star it's getting?  I don't think there's much of a mechanical fix to be had.  I've debated trying to design a second focuser specifically for the guide camera, but I'm not sure the pitch there is very compatible with my 3D printing abilities.  I'm also not sure how I'd automate focusing the guide camera.

 

Any thoughts/suggestions are appreciated.

 

Thanks,

 

Beo    



#2 StephenW

StephenW

    Viking 1

  • -----
  • Posts: 554
  • Joined: 04 Feb 2005
  • Loc: San Jose, CA

Posted 17 February 2019 - 11:21 AM

Are you sure the differences in focus is due to temperature changes and not something else?  Given both main and guide camera are very close to each other and plugged into the same OAG body, it seems unlikely that there would be a significant change in focal distance?

 

Is the pick-off prism/mirror adjustable on these OAGs?  If so, have you tried moving it in as much as possible (without intruding on the main sensor) to try move as far from the edge of the light path as possible?

 

The only other suggestion I would have is to try a field flattener to hopefully get better stars at the edge of the field.



#3 rockstarbill

rockstarbill

    Fly Me to the Moon

  • *****
  • Posts: 5448
  • Joined: 16 Jul 2013
  • Loc: Snohomish, WA

Posted 17 February 2019 - 01:00 PM

The Edge telescope are already flat. No need for a field flattener. It sounds to me like the guide camera is sliding out or in over the course of the imaging session. Have you confirmed that the guide camera is well secured?

#4 Lord Beowulf

Lord Beowulf

    Vendor - Orion Ranch Observatory

  • *****
  • Vendors
  • topic starter
  • Posts: 2158
  • Joined: 13 Oct 2008
  • Loc: Cedar Park, TX

Posted 17 February 2019 - 07:51 PM

I'm not talking about variation over the course of an imaging session.  I'm talking about day-to-day variation.  I live in Texas.  One day I may be imaging at 85-100 and the next at 20-35 F.  (Friday it was 85, Saturday it was 30!)  We're talking about a very small amount of variation here too, but with the small pixel size and high magnification combination, it doesn't take much.  The pixels on the guide camera are about half the size of the pixels on the imaging camera (2.4 vs 3.91 um).  Even with binning, a star that spreads a bit causes PHD to wobble unacceptably.  I'd just like to be able to get PHD to do a better job of following the entire star rather than trying to follow a peak.  As far as adjusting the pick-off mirror, it's already at the point where a small amount of vignetting is sometimes visible in the camera frame.  As far as anything being loose, if that was the case, the guiding would wobble all over the place as the cameras moved relative to each other.  The problem is solely a focus problem and the resulting star shape due to the edge of field.

 

Thanks,

 

Beo    



#5 StephenW

StephenW

    Viking 1

  • -----
  • Posts: 554
  • Joined: 04 Feb 2005
  • Loc: San Jose, CA

Posted 17 February 2019 - 09:53 PM

Ok - so you have two distinct problems:

 

- Your OAG can't maintain focus for your guide camera with big temperature swings

- You are getting crescent shaped stars when slightly out of focus.

 

This old post from Frank may help with the misshaped star issue: https://www.cloudyni...-oag/?p=5497040

 

Basically he suggests directly looking at which part of the light cone the pick-off mirror is actually getting and trying to ensure it's not, for example, getting some of the secondary shadow.  If you can fix the star shape then PHD2 should still work well, even with a slightly defocused star.

 

Your original request to change how PHD2 calculates star centroids to handle crescent shaped stars isn't likely to happen, but you could always post on the PHD2 forum and see what they say.



#6 nateman_doo

nateman_doo

    Viking 1

  • **---
  • Posts: 542
  • Joined: 17 Sep 2017
  • Loc: Dirty Jersey

Posted 17 February 2019 - 10:13 PM

I get them too, but I am using a focal reducer.  wonder if its a sign my collimation is poor?




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics