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

Still another person asking for help with elongated stars

Astrophotography Beginner Equipment Imaging
  • Please log in to reply
13 replies to this topic

#1 Northrim

Northrim

    Explorer 1

  • -----
  • topic starter
  • Posts: 54
  • Joined: 20 Jul 2019

Posted 16 May 2021 - 03:12 PM

I've been trying to get the backfocus nailed on my light train.  I had been getting elongated stars in all four corners.  I spent the other night working on backfocus.  In the photo below (quick stretch just to show the star issues) you can see that my stars in the upper half of the image are "pretty good" but in lower left and lower right I'm getting more noticeable elongation.  The image was only cropped a tiny bit all around to get rid of stacking artifacts.  I'm surprised by the difference between upper and lower half of the image.

 

The listed backfocus for my flattener is 55mm.  I've had to extend it to around 66mm to get this good.

 

Explore Scientific ED80

Explore Scientific Field flattener (55mm listed backfocus)

ZWO Filter slider with Baader sky and moon glow

ZWO ASI294 Camera

Orion Star Shooter autoguider and 50mm guide scope

 

AVX  mount Guiding with Phd2

RMS Error: RA 0.75"  DEC 0.42"  Total 0.86"  RA Osc 0.21

 

Bortle 5 area

 

Do I need to keep adding some spacers or am I having some other problem?  I've heard about sensor tilt but don't know anything about it.

 

Any advice would be welcome and much appreciated.

 

M101 bin 1x1 40-180s 120 gain -5C 5-14-21 quick stretch thumb.jpg

 

 

Here are links to a larger version of this photo as well as another taken the same night.

https://drive.google...iew?usp=sharing

https://drive.google...iew?usp=sharing

 

Thanks for any advice.

Clear Skies Everyone!



#2 Tony Bonanno

Tony Bonanno

    Vostok 1

  • -----
  • Posts: 171
  • Joined: 03 Apr 2007
  • Loc: Santa Fe, New Mexico

Posted 16 May 2021 - 03:22 PM

This may not be applicable at all, but could it be the wind at your location?  Amazing how wind can create very small movements that bloat/distort stars.  I had a problem with my 115 refractor.  Turns out that the "sandbag" hanging at the bottom of the mount was swaying due to the wind and that was causing the problem.  After removing the sandbag, I had round stars again smile.gif .  Also, if you have PEC, be sure it is enabled.  You may have to re-program your PEC.


Edited by Tony Bonanno, 16 May 2021 - 03:24 PM.


#3 pyrasanth

pyrasanth

    Skylab

  • *****
  • Posts: 4,215
  • Joined: 08 Jan 2016

Posted 16 May 2021 - 03:23 PM

Sensor tilt is the CMOS or CCD sensor inside your camera not being flat on its mounting board- so when you screw your camera into the imaging train the threads on the camera and other devices are hopefully perpendicular but the sensor is tilted relative to these fixings. The issue can be corected with a tilt corrector which has three sets of screws that can tilt the entire camera to compensate. I believe quite firmly that sensor tilt should not happen. The manufacturers of cameras are more than aware how much it sucks to have sensor tilt but don't seem to do much about it especially with the cheaper Chinese brands. I don't know how much this affects the more permium brands.

 

Other factors such as back focus, which you've noted, can affect the shapes of the stars at the edges of the field as well as polar alignment being out with long exposures. I suggest the first step is to get a varaible spacer and experiment with 1 mm adjustments to see if you can improve the star shapes. If not use a tool like CCD inspector and evaluate for tilt & collimation.


Edited by pyrasanth, 16 May 2021 - 03:25 PM.


#4 robbieg147

robbieg147

    Viking 1

  • -----
  • Posts: 882
  • Joined: 23 Mar 2020
  • Loc: Kent, England

Posted 16 May 2021 - 03:49 PM

To my eyes the trails are generally pointing to the center so I would say your sensor is slightly too close but I think you also have tilt as top half is not as bad?

 

Your flattener's back focus is 55mm and your camera's back focus is 6.5mm so your spacer should be 48.5mm.

 

I would expect quite a large tolerance with your sensor? are you sure the spacing is correct you are using?


  • hornjs likes this

#5 RogeZ

RogeZ

    Surveyor 1

  • *****
  • Posts: 1,788
  • Joined: 21 Nov 2004
  • Loc: Palm Beach Gardens, FL

Posted 16 May 2021 - 03:59 PM

Post an image of your camera train. Backfocus shows you are too close. This is not tilt alone.

#6 otocycle

otocycle

    Apollo

  • *****
  • Posts: 1,463
  • Joined: 08 Jun 2005

Posted 16 May 2021 - 04:41 PM

I am iterating through the same kind of issue...AT72EDII and Hotech SCA field flattener with a Canon T6i (and hopeless banding from the AF points).   Starting at the 55mm backfocus specification, I am adding more T2 extension spacers to improve the uncropped edges, with some success.   65mm distance is much better.   More spacers are on the way from ebay !

 

I am not ready to attack any sensor tilt until I get the spacer distance dialed in. Will try another DSLR too (450D) to see if there is any difference.

 

This is a quick M101 stack of 10x60s (+darks/flats/bias) processed in StarTools from last night.

 

 

M101_AT72_T6_HOTECH_10mm.jpg


Edited by otocycle, 16 May 2021 - 04:44 PM.


#7 Northrim

Northrim

    Explorer 1

  • -----
  • topic starter
  • Posts: 54
  • Joined: 20 Jul 2019

Posted 16 May 2021 - 07:56 PM

Thanks for the information and suggestions thus far.

 

Here is the light train as requested by RogeZ:

 

Light train.jpg

 

6.5 mm     Camera native spacing 

11 mm      ring

1 mm         spacer

5 mm        spacer

0.5 +0.25+0.25 mm spacers

21 mm        spacer

21 mm        ZWO Filter Slider 

ES Field Flattener (with lens cap on end)

 

I've just ordered a variable spacer to cut down on the number of connections and do fine tuning.

 

Clear Skies!

 

 

 



#8 RogeZ

RogeZ

    Surveyor 1

  • *****
  • Posts: 1,788
  • Joined: 21 Nov 2004
  • Loc: Palm Beach Gardens, FL

Posted 16 May 2021 - 08:13 PM

Thanks for posting, I would contact ES Support, there might something out of spec in this flattener. How was the field at 55mm backfocus?

#9 Northrim

Northrim

    Explorer 1

  • -----
  • topic starter
  • Posts: 54
  • Joined: 20 Jul 2019

Posted 16 May 2021 - 08:33 PM

Thanks for posting, I would contact ES Support, there might something out of spec in this flattener. How was the field at 55mm backfocus?

At 55mm I had a lot of coma  towards the corners.  66.5mm is definitely an improvement.



#10 RogeZ

RogeZ

    Surveyor 1

  • *****
  • Posts: 1,788
  • Joined: 21 Nov 2004
  • Loc: Palm Beach Gardens, FL

Posted 16 May 2021 - 09:05 PM

I did a quick search because I got curious :)

There are old threads regarding this flattener that say it works best at 45mm backfocus. Do a search and try it, the new one is 55mm but the old one seems to work better at 45mm?

#11 Northrim

Northrim

    Explorer 1

  • -----
  • topic starter
  • Posts: 54
  • Joined: 20 Jul 2019

Posted 16 May 2021 - 09:16 PM

I did a quick search because I got curious smile.gif

There are old threads regarding this flattener that say it works best at 45mm backfocus. Do a search and try it, the new one is 55mm but the old one seems to work better at 45mm?

Thanks.  The slope of the elongation seems to indicate  I need to go farther out but I'll try a test at shorter length.  



#12 robbieg147

robbieg147

    Viking 1

  • -----
  • Posts: 882
  • Joined: 23 Mar 2020
  • Loc: Kent, England

Posted 17 May 2021 - 02:18 PM

Do you have the reducer at the correct working distance? this does not look right to me?



#13 Northrim

Northrim

    Explorer 1

  • -----
  • topic starter
  • Posts: 54
  • Joined: 20 Jul 2019

Posted 17 May 2021 - 03:21 PM

Do you have the reducer at the correct working distance? this does not look right to me?

The photo shows the current configuration which is about 66.5 mm distance from the flattener.  This gives me a much improved image from 55 mm but I think I may need to move back even further.  The photo in the original post is with this configuration.



#14 robbieg147

robbieg147

    Viking 1

  • -----
  • Posts: 882
  • Joined: 23 Mar 2020
  • Loc: Kent, England

Posted 17 May 2021 - 03:45 PM

Sorry my mistake I got thrown by the filter drawer I have a ZWO camera as well!




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





Also tagged with one or more of these keywords: Astrophotography, Beginner, Equipment, Imaging



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics