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

Star shapes... back focus distance or something else?

  • Please log in to reply
25 replies to this topic

#1 bazookaman

bazookaman

    Messenger

  • *****
  • topic starter
  • Posts: 466
  • Joined: 06 Nov 2020
  • Loc: Houston, TX

Posted 18 June 2021 - 11:52 AM

Maybe I'm being too picky, but these stars are bothering me. It's also bothering me that the most round ones are not in the middle, but toward the upper right of the frame. Here is a link to the fits:

https://www.dropbox....__0013.fit?dl=0

 

Tight crops around the edge and center:

corners.jpg

 

Here's the star shape I'm seeing:

stars.jpg

 

Using an ES ED80 with flattener and now a UV/IR cut filter although I was seeing this before.



#2 Tapio

Tapio

    Skylab

  • -----
  • Posts: 4,470
  • Joined: 24 Sep 2006
  • Loc: Tampere, Finland

Posted 18 June 2021 - 01:14 PM

I see elongated stars all over the frame which suggests tracking/guiding problem.

Are you guiding ?



#3 bazookaman

bazookaman

    Messenger

  • *****
  • topic starter
  • Posts: 466
  • Joined: 06 Nov 2020
  • Loc: Houston, TX

Posted 18 June 2021 - 01:25 PM

I see elongated stars all over the frame which suggests tracking/guiding problem.

Are you guiding ?

Yes. How can I tell the difference between optical and guiding issues? Take a shorter test exposure?



#4 xiando

xiando

    Fly Me to the Moon

  • *****
  • Posts: 6,726
  • Joined: 27 May 2015
  • Loc: Cloudy NEOhio

Posted 18 June 2021 - 02:00 PM

 

Yes. How can I tell the difference between optical and guiding issues? Take a shorter test exposure?

 

My unrequested two cents... If there was, for instance, a collimation issue, one might see star trails on the outer peripheries (on a newt, for instance, star trails in the upper left quad would seem to point diagonally toward/away-from the center, upper right-quadrant to the right diagonal, lower left to the lower left, and lower right to the lower right) but relatively "artifact free" stars in the central area and the degree of streaking in each star would reduce as you near the central area of the image (there are also "unbalanced" miscollimations too, where it's worse on one side or another, so I'm not fully treating the idea)

 

When you see streaking in one direction throughout a field, it's likely (imo/e) tracking issues, as already noted by Tapio, although for a newt, alignment issues can contribute (especially, at least in my *personal experience, from the secondary mirror...I had a really sticky secondary mount on my first scope that would tend to **** (edit: idk, another word, skew? is that ok? roll? rotate?) itself slightly off perpendicular to the intended alignment.

 

And yes, shorter exposures can help to some degree when tracking is the issue. On my first scope, I used exposure lengths of 1-10 seconds to minimize streaking from tracking errors (with all concomitant thresholds on what I could image, even with large # frames stacking)

 

Sorry I misread. Maybe., You're using a refractor, right? you could use ether a shorter or longer exposure, depending on what the base comparison time is being used. For instance, if you were presently shooting with a 30 sec exposure, you could just fire up a 2 minute exposure and if tracking is the issue, you'll know it for sure. But if you're already shooting at 300 sec (5 minutes) , probably a short exposure would be easier. 


Edited by xiando, 18 June 2021 - 02:09 PM.


#5 bazookaman

bazookaman

    Messenger

  • *****
  • topic starter
  • Posts: 466
  • Joined: 06 Nov 2020
  • Loc: Houston, TX

Posted 18 June 2021 - 02:42 PM

I forgot to mention, this is a 120s exposure. I have a 30 second, it looks pretty much the same.



#6 xiando

xiando

    Fly Me to the Moon

  • *****
  • Posts: 6,726
  • Joined: 27 May 2015
  • Loc: Cloudy NEOhio

Posted 18 June 2021 - 07:37 PM

since it's a star field, try some at 10 seconds and 5 seconds, and maybe try a few at 300 or even 600 seconds. The longer exposures will show any tracking error better imo, since it *should produce larger streaking. 

 

If it still looks the same, maybe something's going on with your flattener. (the ED80 is a refactor, isn't it?)  idk, refractors aren't my "thang", so I'm not sure of its failure points.  


Edited by xiando, 18 June 2021 - 07:38 PM.

  • Lonkero and bazookaman like this

#7 bazookaman

bazookaman

    Messenger

  • *****
  • topic starter
  • Posts: 466
  • Joined: 06 Nov 2020
  • Loc: Houston, TX

Posted 19 June 2021 - 09:05 AM

Last night I took a 1s and 10s along with my 120s. Compared to each other, the stars look almost identical. This is from the far right edge of the image, lower third line.

 

Here are the 1s, 10s, 120s:

Untitled-1.jpg



#8 bazookaman

bazookaman

    Messenger

  • *****
  • topic starter
  • Posts: 466
  • Joined: 06 Nov 2020
  • Loc: Houston, TX

Posted 19 June 2021 - 09:09 AM

Here is a star closer to center. Definitely more round, but still eccentric.

 

Untitled-1.jpg



#9 xiando

xiando

    Fly Me to the Moon

  • *****
  • Posts: 6,726
  • Joined: 27 May 2015
  • Loc: Cloudy NEOhio

Posted 19 June 2021 - 09:25 AM

So...then maybe check your spacing on the flattener.

 

Afaik, if the spacing is off, (too close or too far) you can get ?coma? artifacts similar to a miscollimation in a newt when using a refractor. I don't use one (flattener) myself, but a friend who did (back when we used to do astronomy together) spent a bit of time getting his spacing to its ideal point to minimize spherical aberration. Its ideal spacing could be as little as 1mm either way to dial it in.

 

that's about all I have to offer. I'm not an expert. I gave up on my own flattener due to frustration dialing it in, as well as naturally imparted star bloat from its use, which I was persnickety about (not the best camera for imaging  at the time I first got my newt, which itself is ok but no "prize", but/and I wanted pinpoint-ier stars)


Edited by xiando, 19 June 2021 - 09:29 AM.

  • bazookaman likes this

#10 bazookaman

bazookaman

    Messenger

  • *****
  • topic starter
  • Posts: 466
  • Joined: 06 Nov 2020
  • Loc: Houston, TX

Posted 19 June 2021 - 11:22 AM

Ugly! Could flattener spacing really cause this?

 

Untitled-1.jpg



#11 KTAZ

KTAZ

    Surveyor 1

  • *****
  • Posts: 1,930
  • Joined: 09 Apr 2020
  • Loc: Scottsdale, AZ

Posted 19 June 2021 - 11:40 AM

There seem to be a number of potential issues here. You have three shapes in the first aberration inspector mosaic.

 

There are stars with a directional angle from bottom left to top right. That seems like classic tilt.

 

There are stars in the far upper left corner that seem to point towards the center...that could be back spacing off by just a hair.

 

There are stars in the lower right that have kind of cross pattern like in your post #7. That could be an indication of pinched optics. Is this a new scope?



#12 bazookaman

bazookaman

    Messenger

  • *****
  • topic starter
  • Posts: 466
  • Joined: 06 Nov 2020
  • Loc: Houston, TX

Posted 19 June 2021 - 01:13 PM

I bought it used from an experienced ap'er, I think he bought it new and had used it very little.

Here is a 10s sub from last night, if that helps...

https://www.dropbox....ocus_2.fit?dl=0


Edited by bazookaman, 19 June 2021 - 01:17 PM.


#13 KTAZ

KTAZ

    Surveyor 1

  • *****
  • Posts: 1,930
  • Joined: 09 Apr 2020
  • Loc: Scottsdale, AZ

Posted 19 June 2021 - 03:04 PM

I bought it used from an experienced ap'er, I think he bought it new and had used it very little.

Here is a 10s sub from last night, if that helps...

https://www.dropbox....ocus_2.fit?dl=0

Hard to tell on one frame. Not many stars to be seen even after a debayer and stretch.

 

Perhaps a more star rich field would help.

 

I'm no refractor expert, but I think pinched optics would show across the entire field. Most likely it is a combination of the first 2 that I mentioned. See if minor adjustments to back spacing help, then check if you are seeing any tilt.

 

Taking an exposure that is longer so you can capture some reasonable detail would help. Say 30 seconds. Then rotate the camera 90 degrees, take another, and see if the pattern follows the rotation.


Edited by KTAZ, 19 June 2021 - 03:05 PM.

  • bazookaman and AstroStock like this

#14 bazookaman

bazookaman

    Messenger

  • *****
  • topic starter
  • Posts: 466
  • Joined: 06 Nov 2020
  • Loc: Houston, TX

Posted 19 June 2021 - 03:45 PM

 

 

Taking an exposure that is longer so you can capture some reasonable detail would help. Say 30 seconds. Then rotate the camera 90 degrees, take another, and see if the pattern follows the rotation.

That's a good place to start, thanks!



#15 bazookaman

bazookaman

    Messenger

  • *****
  • topic starter
  • Posts: 466
  • Joined: 06 Nov 2020
  • Loc: Houston, TX

Posted 19 June 2021 - 04:23 PM

Also, here's a link to a stacked and somewhat processed version... I think all 4 corners show some warp effect so I am pretty confident that I need a bit more spacing. Other than the star shape, I'd say this one is coming out pretty decent for a newbie!

 

https://www.dropbox....hed_NR.tif?dl=0


  • KTAZ likes this

#16 KTAZ

KTAZ

    Surveyor 1

  • *****
  • Posts: 1,930
  • Joined: 09 Apr 2020
  • Loc: Scottsdale, AZ

Posted 19 June 2021 - 04:35 PM

That's a very nice image that you can be proud of. At that FL a casual observer wont even see the star shapes as anything but round.

 

Using aberration inspector on your image, the stars in the very center of the field still show a clear tilt towards the upper right, and the lower left have the most, so tilt may still come into this.


  • bazookaman likes this

#17 VincenzoZito

VincenzoZito

    Sputnik

  • -----
  • Posts: 34
  • Joined: 31 Oct 2019

Posted 19 June 2021 - 05:48 PM

Hi,

The first thing to do would be to remove the flattener and check the stars in the center of the field.
If everything is ok we have found the culprit.


  • xiando, bazookaman and AstroStock like this

#18 bazookaman

bazookaman

    Messenger

  • *****
  • topic starter
  • Posts: 466
  • Joined: 06 Nov 2020
  • Loc: Houston, TX

Posted 20 June 2021 - 10:10 AM

Doh, last night I did my last exposure at 90° rotation but I totally forgot that would mess up my guiding so the stars are somewhat elongated as I had no guiding. I still see significant stretching in the corners so I'm going to tackle that first. I just got a QHY spacer kit so that will get it done

 

Anyway, here's the 90° rotated and stretched... at just a glance I can't tell a significant difference.

https://www.dropbox....GB_VNG.tif?dl=0



#19 bazookaman

bazookaman

    Messenger

  • *****
  • topic starter
  • Posts: 466
  • Joined: 06 Nov 2020
  • Loc: Houston, TX

Posted 04 July 2021 - 11:43 AM

Here's another sub from last night. I tried adding a mm to the backfocus prior to this but then I clearly had "arcing" stars in the corners so I took the mm back out.

 

https://www.dropbox....__0008.fit?dl=0

 

Man, those stars around the right edge are ugly... they look like "x"s.



#20 KTAZ

KTAZ

    Surveyor 1

  • *****
  • Posts: 1,930
  • Joined: 09 Apr 2020
  • Loc: Scottsdale, AZ

Posted 07 July 2021 - 08:58 AM

The bottom sectors and the right hand sectors still look like classic back focus (too close). When you say arcing, do you mean like in the bottom sketch below?. You might need to be using smaller increments than 1MM. Like .20 or even .10. Once I get to the center and the upper left quadrant, it seems like there  may be some tilt in play. So I still think you are seeing both.

2pozzus.png

 

I wish I could help with the cross shaped stars, but the best I can give you is that I've seen similar posts where that was caused by pinched optics. The cell that holds the lenses may be to tight, I'd discuss with Orion. Is it an Orion ED80? Have you contacted them and sent them samples?

 

Understand that with a budget scope, the focuser may not be robust enough to handle the gear without introducing a bit of tilt.


  • charlieb123 and bazookaman like this

#21 unimatrix0

unimatrix0

    Viking 1

  • -----
  • Posts: 565
  • Joined: 03 Jan 2021

Posted 07 July 2021 - 09:16 AM

Cross shaped stars, like the W.O Zenithstar series telescopes.

Your lens are too tight, search the forum for cross shaped stars or the post I made just about a week ago, I had the same issue, which isn't fully resolved yet. 

I did my homework and read into it.  Unfortunately in my case I can't get the right balance, my stars are no longer cross shaped but they are also not perfect either. Looks very similar to your images with a dripping water drop sort of thing going on. 
I'm still working on it 

Read on 
https://www.cloudyni...read/?p=6494121

 

and here
https://www.cloudyni...d#entry11198555

 

and here
https://www.cloudyni...liam-optics-gt/

 

and here
https://www.cloudyni...iangular-stars/

 

and here
https://www.cloudyni...ars-from-ed127/


Edited by unimatrix0, 07 July 2021 - 09:22 AM.

  • bazookaman likes this

#22 06AwzIyI

06AwzIyI

    Mariner 2

  • *****
  • Posts: 208
  • Joined: 14 Jul 2020

Posted 07 July 2021 - 09:52 AM

I just went through a grueling process trying to diagnose my own similar issues. It might not be the solution but here's something to try:

  • Put the camera on a loop and preview the incoming images at high zoom/magnification in the affected corners
  • Then make very minute adjustments to focus and see how it affects the shape

I was floored after spending hours and hours messing with backfocus when I realized it was something this simple. In my case, my focuser backlash wasn't set high enough in my autofocus routine and wasn't hitting optimum focus for the corners.


  • bazookaman likes this

#23 bazookaman

bazookaman

    Messenger

  • *****
  • topic starter
  • Posts: 466
  • Joined: 06 Nov 2020
  • Loc: Houston, TX

Posted 07 July 2021 - 11:12 AM

The bottom sectors and the right hand sectors still look like classic back focus (too close). When you say arcing, do you mean like in the bottom sketch below?. You might need to be using smaller increments than 1MM. Like .20 or even .10. Once I get to the center and the upper left quadrant, it seems like there  may be some tilt in play. So I still think you are seeing both.

2pozzus.png

 

I wish I could help with the cross shaped stars, but the best I can give you is that I've seen similar posts where that was caused by pinched optics. The cell that holds the lenses may be to tight, I'd discuss with Orion. Is it an Orion ED80? Have you contacted them and sent them samples?

 

Understand that with a budget scope, the focuser may not be robust enough to handle the gear without introducing a bit of tilt.

Yes, there is an obvious curvature like the above bottom image. When I remove the 1mm addition, it goes away. So I presume I'm within 1mm of correct spacing.

 

It's an ES ED80.

 

I suspect you are correct about the focuser, it's got some wicked backlash too. I've been using 500 step backlash correction for AF in SGP.

 

Cross shaped stars, like the W.O Zenithstar series telescopes.

Your lens are too tight, search the forum for cross shaped stars or the post I made just about a week ago, I had the same issue, which isn't fully resolved yet. 

I did my homework and read into it.  Unfortunately in my case I can't get the right balance, my stars are no longer cross shaped but they are also not perfect either. Looks very similar to your images with a dripping water drop sort of thing going on. 
I'm still working on it 

Read on 
https://www.cloudyni...read/?p=6494121

 

and here
https://www.cloudyni...d#entry11198555

 

and here
https://www.cloudyni...liam-optics-gt/

 

and here
https://www.cloudyni...iangular-stars/

 

and here
https://www.cloudyni...ars-from-ed127/

I'll read those threads when I get a chance, thanks for the links! I didn't realize refractors could get pinched. Would that be a problem from the factory? It is a used scope, but I don't think it was used much at all. I'm not sure if he made any adjustments.



#24 unimatrix0

unimatrix0

    Viking 1

  • -----
  • Posts: 565
  • Joined: 03 Jan 2021

Posted 07 July 2021 - 03:29 PM

Yes, there is an obvious curvature like the above bottom image. When I remove the 1mm addition, it goes away. So I presume I'm within 1mm of correct spacing.

 

It's an ES ED80.

 

I suspect you are correct about the focuser, it's got some wicked backlash too. I've been using 500 step backlash correction for AF in SGP.

 

I'll read those threads when I get a chance, thanks for the links! I didn't realize refractors could get pinched. Would that be a problem from the factory? It is a used scope, but I don't think it was used much at all. I'm not sure if he made any adjustments.

Mine was like that , out of the box. Being a newbie I didn't realize that this was an issue and if I look back at my first images from last year, I see all the cross shaped stars in all my images.  
Some tips were given to loosen the plastic worm screws (Willam Optics) around the lens, being too tight and how the temperature changes. 
I did that, but now the cross shape is gone but I got droopier looking stars. I'm guessing now they are not aligned or too loose, I don't know, but I would probably need some precision tools to fix it, but that I don't have so I'm thinking about making an artifical star with flashlight and a pinhole and experiment with that instead of wasting clear nights messing with this telescope. 
I would suggest to get the back spacing very precise, I use a measuring caliper, I don't trust what the manufacturer says about t rings and spacers so I measure it myself. Once that is taken care of , take some snapshots at stars and see what's up. 
What gives it away that there is a problem still is getting a fairly high HFR, meaning the stars are still too fat, while the scope is perfectly focused and the sky is clear. 


Edited by unimatrix0, 07 July 2021 - 03:34 PM.


#25 sbharrat

sbharrat

    Viking 1

  • -----
  • Posts: 792
  • Joined: 28 Nov 2020
  • Loc: NJ, USA

Posted 08 July 2021 - 08:35 AM

...
I did that, but now the cross shape is gone but I got droopier looking stars. I'm guessing now they are not aligned or too loose, I don't know, but I would probably need some precision tools to fix it, but that I don't have so I'm thinking about making an artifical star with flashlight and a pinhole and experiment with that instead of wasting clear nights messing with this telescope. 
... 

Oof frown.gif Hope you didn't loosen it too much. I had a similar experience https://www.cloudyni...far-is-partial/ and it didn't have a happy ending. Crossed-fingers for you....




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