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

Elongated stars... Collimation? Backfocus? Tilt? No clue

  • Please log in to reply
24 replies to this topic

#1 Tuant

Tuant

    Sputnik

  • -----
  • topic starter
  • Posts: 28
  • Joined: 26 Apr 2018
  • Loc: Zamora, Spain

Posted 20 September 2021 - 10:29 PM

Good day to everyone.

 

I’ve recently purchased an ASI 294 MM, for imaging with my C8 HD, with the 0.7x focal reducer, Celestron OAG and ZWO filter wheel 7x36.

 

Tonight I’ve been setting up everything. Did the maths to reach the right backfocus (105 mm with the focal reducer):

SCT Adapter: 25.3 + OAG Body: 29 + Male M42 adapter: 12.5 + Filter wheel: 20 + Camera backfocus: 6.5 = 93.3 mm.

 

So I added the (included with the OAG) Medium T-thread Spacer Ring: 11.55mm

93.3 + 11.55 = 104.85 mm

 

After attaching everything, I took the first images to have a glance. What I saw shocked me: awful, elongated stars, and what’s more: some of them showed the central obstruction of the optical tube.

 

I assumed the backfocus distance was wrong (sometimes reality differs from paper) and added and substracted amounts o backfocus (+/- 3 mm +/- 6 mm, +/- 10 mm…). Elongated stars everywhere, repeating outcomes. I’ve discarded tracking or guiding as the problem, since I was taking 2 – 5 seconds frames, with a good PA (made with Polemaster), and also the stars, yet oblong and awful, didn’t show trails or switched positions. Also, focus was made with Bahtinov mask everytime, carefully, so I don't think it's a focus issue.

 

Intentionally, every time I added or substracted backfocus, I’ve placed the camera with the same orientation. And what I’ve found is that depending on the backfocus distance, stars elongation seem to rotate (with no camera rotation). The optical gear seems solid to my eyes, with no sagging.

 

I’ve downloaded the trial versión of CCD Inspector to check if something is wrong with my camera’s sensor, but to be honest, I don’t know how to use it and get lectures from it.

 

As I don’t know what to think about this, any advice woul be welcome. Might be collimation? My camera is damaged with a non ortogonal sensor? Am I doing wrong with backfocus (never saw such a critical difference with +/- 3 mm)?

 

Tomorrow I might try removing the focal reducer or any other part from the optical gear, but if I could get any piece of advice before doing any operation, would be great.

I’m sharing link to the images (in fit) in case someone wants to have a look and / or inspect them.

 

https://easyupload.io/m/goednh

 

Thanks in advance.


Edited by Tuant, 20 September 2021 - 10:33 PM.


#2 bobzeq25

bobzeq25

    ISS

  • *****
  • Posts: 26,004
  • Joined: 27 Oct 2014

Posted 20 September 2021 - 11:29 PM

Add a frame to a post here, showing us the problem.

 

A star showing the central obstruction is a star out of focus.

 

Have you successfully imaged DSOs with that scope, and another camera?


Edited by bobzeq25, 20 September 2021 - 11:32 PM.


#3 cozzmic

cozzmic

    Lift Off

  • -----
  • Posts: 3
  • Joined: 04 Oct 2020

Posted 20 September 2021 - 11:35 PM

Not much of an expert  on all of this as I just got a ASI533 a month or so ago switching form DSLR.

 

Was having problems with elongated stars as well but this image below helped me understand.

 

My stars were all pointing in to the center (camera is too close) so added a few small spacers and its pretty good now, however it was on a Z73 refactor.

 

Also found out that its the camera back focus that needs to be the correct amount. Believe that's 55mm for most ASI's. I first tried 73 mm for the scopes focal plane and it was a complete disaster. 55mm worked great after the few thin spacers were added to tweak the elongation. That was 55mm for the Z73 (with flatterer) and a RedCat 51(no flatterer) that have different focal planes.

 

On a C8 HD, well this could be totally different... but try 55mm if its not workin what the heck!

 

Hope this helps.

 



#4 cozzmic

cozzmic

    Lift Off

  • -----
  • Posts: 3
  • Joined: 04 Oct 2020

Posted 20 September 2021 - 11:39 PM

hmm pic did not upload

 

look at this post

 

https://www.cloudyni...-my-back-focus/



#5 sctbrd

sctbrd

    Explorer 1

  • -----
  • Posts: 72
  • Joined: 17 Nov 2009

Posted 20 September 2021 - 11:44 PM

This is a good start- check your stars across the frame and compare to this guide. If the FWHM are different in the corners probably a tilt issue, maybe collimation.

 

CCDInspector, I have just started messing with it also, isn't too difficult to use. The hardest part is to decipher what to do with the results.

I take a single frame of a star field with not bright DSOs or clusters and open it with CCDInspector. Click measure and then you can check your results and look at graphical representations of your setup. Then you can take another frame, measure, and compare. The frames do not have to be in focus, it says results may be better slightly out of focus...

(Too bad there isn't a guide on how to make adjustments with the results you get-Im working on it but clouds are hampering my efforts).

post-322714-0-86551200-1599509754.png


  • mariemarie and starrycanuck like this

#6 Tuant

Tuant

    Sputnik

  • -----
  • topic starter
  • Posts: 28
  • Joined: 26 Apr 2018
  • Loc: Zamora, Spain

Posted 21 September 2021 - 03:17 AM

It seems I haven't been able to explain the problem with words, so I've converted the fit files into jpg to show what's going on.

 

https://ibb.co/7bxrKZY
https://ibb.co/1mWNVGY
https://ibb.co/k15YRk5

 

@bobzeq25 that's what I was thinking too, but focus was achieved carefully with a Bahtinov mask, and also, with just a slight movement of the focus, the "donut" stars begin to blurr, showing their maximun definition when they're showing the central obstruction.

 

I've imaged before with this scope (with an ASI 294 MC) and with this cam (with a TS 61 EDPH). Managed to do some (in my eyes) decent images.

 

https://ibb.co/m45TsHQ
https://ibb.co/w6XCZhY

 

@cozzmic Celestron says it's 105 mm with the 0.7 FR. 55 mm is too low (the OAG itself consumes more than 60mm).

 

@sctbrd That's exactly what I don't know what to do with CCD Inspector, reaching conclussions based on what it measures. Anyway, as you can see on the images, the elongated pattern has very little to do with the concentric patterns of "too close" or "too far". Had I had a pattern like that, I would have easily identified wheter I need to add or remove backfocus milimeters.



#7 michael8554

michael8554

    Apollo

  • -----
  • Posts: 1,436
  • Joined: 19 Aug 2020
  • Loc: Wiltshire UK

Posted 21 September 2021 - 07:57 AM

I see no signs of spacing issues in your FITs.

 

All I see is elongated stars at the centre and edges of the images, indicating guiding issues.


  • OldManSky likes this

#8 Tuant

Tuant

    Sputnik

  • -----
  • topic starter
  • Posts: 28
  • Joined: 26 Apr 2018
  • Loc: Zamora, Spain

Posted 21 September 2021 - 08:04 AM

I see no signs of spacing issues in your FITs.

 

All I see is elongated stars at the centre and edges of the images, indicating guiding issues.

With 2 and 5 second exposures? Also stars didn't changed position from frame to frame (took several of them during 40+ minutes).

 

Mount is CEM60 EC, well balanced and PA with Polemaster. Elongation changed in shape and orientation as I added or substracted backfocus. And the saturated star doesn't show elongation. Don't get me wrong, you probably are more experienced than me, but I'm not sure it has to do with guiding.



#9 bobzeq25

bobzeq25

    ISS

  • *****
  • Posts: 26,004
  • Joined: 27 Oct 2014

Posted 21 September 2021 - 10:36 AM

Clearly guiding, combined with poor focus.  The elongation is uniform across the frame.  It's all going the same direction.

 

The stars may look better defined with the central obstruction visible.  But you don't measure focus by "better defined", you measure it by the smallest possible width.  That won't be easy to determine with guiding errors.

 

Here's your fundamental problem.  That's an awful setup to try to start out with in DSO AP.  Too big a scope.  That makes issues hard to diagnose, you wind up wasting a _large_ amount of time thinking about (or doing) irrelevant things like collimation.  Your problem is definitely not collimation.  Or backfocus.

 

I have no idea why you're doing 5 second exposure.  Those would be about right (still a bit short) with my F2 RASA in my Bortle 7 skies.

 

For learning DSO AP, a small scope is _far_ better.  For many reasons.  One major one is that it makes it easier to see where your problems are.  If you use a small refractor, you won't be worrying about collimation.  You won't be shooting (inappropriately) 5 second lights.

 

You have major problems.  I downloaded a frame from your post #1, PixInsight can't measure the stars widths.

 

Good mount.  Small refractor.  The ideal setup for learning DSO AP.  Start there.  When you've got that working well, then is the time to think about a larger scope.  Right now, it's just getting in your way.


Edited by bobzeq25, 21 September 2021 - 10:43 AM.


#10 Tuant

Tuant

    Sputnik

  • -----
  • topic starter
  • Posts: 28
  • Joined: 26 Apr 2018
  • Loc: Zamora, Spain

Posted 21 September 2021 - 01:25 PM

I'm not here to brag or argue at all, but I think I haven't made myself clear. I'm not an astrophotography guru or an APOD maker, but I don't think I'm a begginer. I've been into astrophotography and astrometry for some years. I already lived the stage of learning with small and medium refractors, and I still image with 61mm and 120mm aperture APO's. And it is not the first time using the C8. 

 

The equipment is near the same: Same mount (IOptron CEM60 EC), same tube (C8 HD with 0.7x FR), same OAG (Celestron OAG), same guide cam (ASI 290mm mini), and ASI 294 MC Pro (wich I'm replacing with the 294 MM Pro with the ZWO EFW).

 

Examples of subs with no pre or post processing with the equipment listed above. They are not be wonderful perfect subs, backfocus is not perfect and they could be better, but before switching cam (and consequently redoing the backfocus), I didn't have this kind of issues. The difference now is abysmal.

 

 

Messier-1-180sec-1x1-L-0020-1.jpg

 

 

Messier-27-120sec-1x1-L-0001-1.jpg

 

SN-7814-120sec-1x1-L-0080.jpg

 

The reason why I'm taking 2 and 5 seconds exposures was to frame, focus and test image geometry.

 

I respect your opinion, coming from a person with 25k+ posts, yet I'm hesitant that with 2 second exposures a bad tracking would cause such effect. But as I said, I'll give it credit and tonight I'll try some tests at Polaris and surroundings, with even less exposure time. And I'll "focus on focus", taking a picture of the Bahtinov diffraction pattern before every test. Might bring some light into this.



#11 bobzeq25

bobzeq25

    ISS

  • *****
  • Posts: 26,004
  • Joined: 27 Oct 2014

Posted 21 September 2021 - 01:43 PM

_Don't_ use less exposure time.  Use the proper exposure.  Also take flats and dark flats (necessary with a 294, instead of bias) and darks.

 

Try imaging, instead of trying whatever.  See if imaging works.  If not, you'll have much better information to go on.  Your problem is that you don't know what's wrong.  Your guesses about what that was are what made me think you were a beginner.

 

Among other things, it may (and should) be possible to measure star width.  With such short exposures, I could not, even with PixInsight.

 

I also (strongly) recommend a star field in a standard location.  Star motion near Polaris is unique.  You don't want unique motion, you want the usual motion.  One very bright star in the frame is not helpful.

 

The way you're experimenting, in an attempt to figure things out, just makes things harder to figure out  Try imaging, instead.  You'll then have a lot better information.


Edited by bobzeq25, 21 September 2021 - 01:45 PM.


#12 alphajuno

alphajuno

    Lift Off

  • -----
  • Posts: 5
  • Joined: 18 Jul 2009

Posted 21 September 2021 - 04:52 PM

You could consider posting in the ZWO forum.  Their expert can help determine if it is a camera-related problem.  He is very good. 



#13 freestar8n

freestar8n

    Vendor - MetaGuide

  • *****
  • Vendors
  • Posts: 10,880
  • Joined: 12 Oct 2007
  • Loc: Melbourne, Australia

Posted 21 September 2021 - 05:59 PM

With 2 and 5 second exposures? Also stars didn't changed position from frame to frame (took several of them during 40+ minutes).

 

Mount is CEM60 EC, well balanced and PA with Polemaster. Elongation changed in shape and orientation as I added or substracted backfocus. And the saturated star doesn't show elongation. Don't get me wrong, you probably are more experienced than me, but I'm not sure it has to do with guiding.

If those are short exposures and you are using EdgeHD it is probably just collimation and you need to tweak it.

 

With a normal sct miscollimation will show different stars across the field, but with EdgeHD they may look similarly oblong.

 

Any time you make a change to the setup you will probably need to re-collimate.  And I gather the EdgeHD8 with reducer can be finicky.

 

It isn't tilt because it is uniform across the field.  The only other thing in short exposures would be a fan vibration or very fast gearbox noise, which I doubt.

 

If you haven't already tried re-collimating - give it a shot.  Normally you need to study a star in the center of the field - but if the system is field corrected then ideally once any star is round, they should all be.

 

Frank



#14 AaronH

AaronH

    Explorer 1

  • -----
  • Posts: 86
  • Joined: 20 Apr 2021

Posted 21 September 2021 - 08:01 PM

I really don't think it's a tracking/guiding issue.

 

The central obstruction is clearly visible, showing plenty of contrast, and with a fairly even disk on all sides. If there was movement during the exposure, the central obstruction would be masked to at least some degree by that movement (as the "donut" moves across the sensor), and the line of the disk would be thicker on the axis of movement.

 

Something is weird here. It's not back-focus. It's not tracking. It's not tilt.

 

As Frank says above, re-collimate. That's surely the first step.

 

If that failed, I'd go for the most minimal setup possible (just the scope and the camera), then see if the problem exists. If not, then add the focal reducer/extension tubes and see if it exists. Then add the filter wheel and see if it exists. Then add the OAG and see if it exists. You don't need to strive for perfection when doing so, I think it's going to be pretty obvious if you've got issues, given they're so pronounced.


Edited by AaronH, 21 September 2021 - 09:42 PM.


#15 freestar8n

freestar8n

    Vendor - MetaGuide

  • *****
  • Vendors
  • Posts: 10,880
  • Joined: 12 Oct 2007
  • Loc: Melbourne, Australia

Posted 21 September 2021 - 09:32 PM

I would also make sure the mirror is loaded up against gravity by finishing focus turning the primary knob counter clockwise. Be sure to do that before collimating and when focusing for an exposure. 
 

frank



#16 Tuant

Tuant

    Sputnik

  • -----
  • topic starter
  • Posts: 28
  • Joined: 26 Apr 2018
  • Loc: Zamora, Spain

Posted 21 September 2021 - 09:59 PM

Well, I'm back from the observatory. Thanks for all the replies, I've taken them into consideration and was going to try collimation, but decided to run some tests before.

While I was doing the tests, I was writing it down and saving the frames to illustrate it. Here is what I wrote and did.

 

I’m writing this while I’m at the observatory, so I can recall every step.

 

I remember I had some pictures I took some time ago of the equipment, when the 294 MC was installed. I’m trying to mimic the whole setup, with more faith than theory, since the backfocus measurement is way larger than the recommended 105 mm. In fact, it is:

6.5 (camera) + 11 female- female T2 extender + 18 male T2-female extender + 16 filter drawer + 66.8 OAG = 118.3

 

I have to warn you: the mount is quite unbalanced, since I have both the C8 and the TS61 on the same mount, and the 294 MC was installed with the TS61 previously to this modification. I decide to not waste time on balancing, since I want to do this tests quickly before the upcoming clouds. Also, no guiding. I know what you may be thinking, but I really trust this mount.

 

Focused on a bright star.

 

1.jpg

 

And surprisingly, stars are much round, in my eyes they even seem to ask for a bit more backfocus.

 

2.jpg

 

3.jpg

 

Since both cameras share the same distance to the sensor (6.5 mm), I switch to the 294 MM, then refocus with Bahtinov mask. Good sign, the focus seems perfect with no adjustment (as should be), preserving the focus of the 294 MC.

 

4.jpg

 

I take a quick frame, and the outcome is more than disgusting. Judge by yourself.

 

5.jpg

 

Back to the 294 MC. Everything as expected, round stars (with the same non exact backfocus and such, just for the test)

 

6.jpg

 

Reaching this point, I think I can remove guiding and/or tracking from the equation, along with focuser sagging, filter wheel and collimation. Correct me if I’m wrong but if one cam shows round stars and another one doesn’t… the problem may be with the camera, right?

I know both cameras have same mechanical design. But I’m here to try solutions so I decide to play a bit with backfocus, removing 18 mm, and imaging on the few sky without clouds, near 34 Persei

 

7.jpg

 

Later on, another test removing 5 mm (removed 16 mm from filter drawer and added 11 mm T2 female-female to connect the camera with the OAG)

 

8.jpg

 

It seems the more spacing I remove, the better the stars get (yet they are still horrible). But I cannot remove more space. Whatsmore, I shoud add 20mm in order to use the filter wheel.

Correct me if I’m wrong, but shouldn’t both cameras behave the same way in what backfocus means?

 

Finally, just to do the last test, I place the 294 MM with the TS61, (the needed backfocus is 55mm, since I’m using it with its dedicated reducer): 6.5 camera + 20 filter wheel + 12 T2 extender + 16.5 T2 male to M48 female = 55 mm.

This is the image I get (migh not be useful but it’s the only portion of the sky without clouds).

 

9.jpg

 

Sincerely, I don’t know what’s going on, and can’t reach a conclussion. There were like about 40 mins of clear sky after everything, and tested again the 294 MC just to be sure it was not an illusion, and run an autofocus routine on Sequence Generator Pro. Everything smooth.

 

All the images are 30 seconds subs (except the focus frames, 2 secons each) with no calibration, just as they get out of the camera.

 

Sorry for the long post.


Edited by Tuant, 21 September 2021 - 10:04 PM.


#17 cozzmic

cozzmic

    Lift Off

  • -----
  • Posts: 3
  • Joined: 04 Oct 2020

Posted 22 September 2021 - 08:20 PM

OK can somebody tell me how to upload a picture on this txt thing ( I'm a looser), did not work for that last post from my pic directory.

 

Thank you to Explore 1 for putting that pic up that I wanted to.

 

Kinda confused but that's not unusual for me.

 

Cheers



#18 rockstarbill

rockstarbill

    Cosmos

  • *****
  • Posts: 9,529
  • Joined: 16 Jul 2013
  • Loc: Snohomish, WA

Posted 22 September 2021 - 10:38 PM

My guess with the trailing star situation is that a vibration is causing this issue, possibly with the cooling fan on the camera. Can you test with the fan not on at all and no cooling? 

 

As for the donut stars, that would typically be poor focus or poor collimation of the optics. First things first though, the star trails need to get fixed.



#19 Tuant

Tuant

    Sputnik

  • -----
  • topic starter
  • Posts: 28
  • Joined: 26 Apr 2018
  • Loc: Zamora, Spain

Posted 11 October 2021 - 05:56 PM

Well, after some tests, it is vibration caused by the camera fan. It is as obvious as:

- 2 seconds frame and focus with the 12v fan cable plugged: horrible elongation.

- 2 seconds frame and focus without it plugged: nice, round stars.

From the 3 ZWO cams I own (294 MC Pro, 294 MM Pro, 2600 MM Pro) the only one that doesn't behaves this way is the 294 MC. The other 2 models show that vibration when combined with the C8, probably because at this FL, the vibration is more clear. On the TS61, with a really short focal lenght, it is vastly disguished.

Now it's time to replace those fans for something with better quality.

Great testing and quality components from ZWO...

Thanks to everyone for your help. Much appreciated.



#20 DirtyRod

DirtyRod

    Mariner 2

  • *****
  • Posts: 286
  • Joined: 23 Mar 2021
  • Loc: Arizona

Posted 11 October 2021 - 09:55 PM

Well, after some tests, it is vibration caused by the camera fan. It is as obvious as:

- 2 seconds frame and focus with the 12v fan cable plugged: horrible elongation.

- 2 seconds frame and focus without it plugged: nice, round stars.

From the 3 ZWO cams I own (294 MC Pro, 294 MM Pro, 2600 MM Pro) the only one that doesn't behaves this way is the 294 MC. The other 2 models show that vibration when combined with the C8, probably because at this FL, the vibration is more clear. On the TS61, with a really short focal lenght, it is vastly disguished.

Now it's time to replace those fans for something with better quality.

Great testing and quality components from ZWO...

Thanks to everyone for your help. Much appreciated.

Very interesting. I’ve been following as I have the same issue with a very similar setup. Unfortunately, I have the issue whether I’m using a cooled or uncooled camera so I was kinda hoping for a different root cause. Planning to work on on collimation this weekend.



#21 Tuant

Tuant

    Sputnik

  • -----
  • topic starter
  • Posts: 28
  • Joined: 26 Apr 2018
  • Loc: Zamora, Spain

Posted 13 October 2021 - 01:54 AM

@ DirtyRod - Why don't you post a image of your elongated stars to help the diagnosis?



#22 DirtyRod

DirtyRod

    Mariner 2

  • *****
  • Posts: 286
  • Joined: 23 Mar 2021
  • Loc: Arizona

Posted 13 October 2021 - 08:41 AM

@ DirtyRod - Why don't you post a image of your elongated stars to help the diagnosis?

Sure. Didn't want to hijack your thread but I certainly would appreciate the help. Let me grab one. 



#23 DirtyRod

DirtyRod

    Mariner 2

  • *****
  • Posts: 286
  • Joined: 23 Mar 2021
  • Loc: Arizona

Posted 13 October 2021 - 09:14 AM

Here is a typical image and the story. 

 

EdgeHD-8, focal reducer, and an ASI53MC Pro on an AVX. PHD2 multi-star guiding now using an ASI174mm in a 60mm guide scope although I first started with an OAG and a 120mm. Stars are typically elongated uniformly across the entire image like the first image. Sometimes bloated but last weekend I also got heart shaped stars on the very first target of the night (M13) but then they were elongated on M33 and M42. M13  was the very first target and Orion was the last target of the night.  M13 was the only target shot facing west and has the worst stars. 

 

Initially focused on tracking and got the tracking down to .7 total RMS although RA is still twice DEC. I cant see Polaris so my process is to align, polar align, align and then do a drift alignment in PHD2. Multi-star guiding and I first focus using a B-mask. I also did some adjusting on the AVX to improve balance because the RA nut was so tight I could not balance the mount properly. 

 

This weekend I'm planning to go back to the OAG with the 174mm and work on collimation to get it the best I can get using the star deformation. 

Attached Thumbnails

  • EB683959-1D8C-43CD-935C-BE3CB5BC5F0D.jpeg
  • BCA042C7-6225-471B-96D4-CF49D47AC078.jpeg

Edited by DirtyRod, 13 October 2021 - 09:25 AM.


#24 DirtyRod

DirtyRod

    Mariner 2

  • *****
  • Posts: 286
  • Joined: 23 Mar 2021
  • Loc: Arizona

Posted 13 October 2021 - 09:19 AM

Here is Orion.

Attached Thumbnails

  • 182DA740-F76D-47F2-8C32-475D4A1E3F64.jpeg

Edited by DirtyRod, 13 October 2021 - 09:20 AM.


#25 fewayne

fewayne

    Surveyor 1

  • *****
  • Posts: 1,510
  • Joined: 10 Sep 2017
  • Loc: Madison, WI, USA

Posted 13 October 2021 - 09:30 AM

Good detective story. Thanks -- it  always helps to see how other folks have diagnosed problems.

 

Funny that the fan would cause a vibration along a sharply-defined axis like that.

 

Now you've got me thinking about trying a similar test with my 183MM, which actually got dropped at one point.




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