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

Long exposure images not showing round stars..

  • Please log in to reply
24 replies to this topic

#1 DRNIK1987

DRNIK1987

    Lift Off

  • -----
  • topic starter
  • Posts: 14
  • Joined: 05 Oct 2021

Posted 25 November 2021 - 11:45 AM

My guiding figures are pretty good, the guiding seems to be Bumpy for a bit of times. but The RMS errors are under 1 always.

Even after this when i take any image of 400s, it shows mild trails. the trails happen only in one direction- North south direction.

 

Attaching the guiding images, Captures image and Guiding assistant..

Pl help..!!!

Attached Thumbnails

  • 2021-11-25 (2)-2-1.jpg
  • 2021-11-25 (6)-1-6.jpg
  • 2021-11-25 (5)-5-4.jpg
  • 2021-11-25 (3)-1-2.jpg


#2 Dwight J

Dwight J

    Soyuz

  • *****
  • Posts: 3,944
  • Joined: 14 May 2009
  • Loc: Lethbridge, Alberta, Canada

Posted 25 November 2021 - 11:52 AM

If you are using a guidescope differential flexure could be the culprit.


  • happylimpet likes this

#3 Tapio

Tapio

    Fly Me to the Moon

  • -----
  • Posts: 5,017
  • Joined: 24 Sep 2006
  • Loc: Tampere, Finland

Posted 25 November 2021 - 12:06 PM

Please provide information of your gear - telescope, mount, camera, guiding equipment.



#4 DRNIK1987

DRNIK1987

    Lift Off

  • -----
  • topic starter
  • Posts: 14
  • Joined: 05 Oct 2021

Posted 25 November 2021 - 12:08 PM

Please provide information of your gear - telescope, mount, camera, guiding equipment.

Eq6 r pro, Tamron 70-200mm G2 lens, Asi 533mc pro, ZWO mini guidescope, ZWO asi 462mc for autoguiding



#5 DRNIK1987

DRNIK1987

    Lift Off

  • -----
  • topic starter
  • Posts: 14
  • Joined: 05 Oct 2021

Posted 25 November 2021 - 12:08 PM

If you are using a guidescope differential flexure could be the culprit.

I just dont understand what u mean by that...! any more details?



#6 f430

f430

    Viking 1

  • *****
  • Posts: 724
  • Joined: 29 Sep 2015
  • Loc: La Mesa, CA.

Posted 25 November 2021 - 12:17 PM

The 6.9 Arc-Min polar alignment error is a good bit high.

With a bit of care you should be able to get that down to less than 1.

You're using a 1.5 sec exposure time. Try 3 secs or so, after you fix the polar alignment error.

 

if you're using a guide scope, make sure it is solidly mounted, and focuser locked down tight.

Also, try to improve the focus on your guide camera. 

 

It's fixable, just work at it.


  • Glass Eye likes this

#7 Dwight J

Dwight J

    Soyuz

  • *****
  • Posts: 3,944
  • Joined: 14 May 2009
  • Loc: Lethbridge, Alberta, Canada

Posted 25 November 2021 - 12:28 PM

I just dont understand what u mean by that...! any more details?

 

It means that your guidescope moves during the long exposure compared to your imaging scope/lens.  It doesn’t show in your guiding but does on your long exposure.  It could be a set screw not tight enough or guide scope mounting moving slightly.  It doesn’t have to move much to mess up an exposure.  Make sure everything is tightened.  If it persists, try shorter exposures.


  • happylimpet likes this

#8 Tapio

Tapio

    Fly Me to the Moon

  • -----
  • Posts: 5,017
  • Joined: 24 Sep 2006
  • Loc: Tampere, Finland

Posted 25 November 2021 - 02:10 PM

With Tamron 70-200mm lens PA error doesn't easily show in images and same applies to guide scope differential flexure.

 

I think the zoom lens is the biggest culprit. They just are not so good with stars (with few exceptions).

Also the attachment to astro camera could be problem. And guide scope attachment to imaging setup - how is this done ?



#9 michael8554

michael8554

    Surveyor 1

  • -----
  • Posts: 1,535
  • Joined: 19 Aug 2020
  • Loc: Wiltshire UK

Posted 26 November 2021 - 04:53 AM

Your 6.9arcmin PA error is a bit larger than the 5arcmin max that PHD2 recommends, but it's not unguidable.

 

Your RA and Dec guide errors in the second screengrab are similar RA = 0.72 Dec =  0.65,  so stars should be round.

 

Not so good in the first grab, RA =1.78,  Dec =  0.75, that means elongated stars in the RA direction.

 

So if you think stars are elongated in the Dec direction, which has good guiding, then the guidescope is moving up and down while guiding, which is Differential Flexture.

 

And guidescope focus is way out, HFD = 7.45.

 

But it's 5.56 in the second grab, so is your guidescope focus loose ?

 

Hard to judge star quality from your lo-res grab, and it's only part of the image.



#10 alphatripleplus

alphatripleplus

    World Controller

  • *****
  • Moderators
  • Posts: 126,701
  • Joined: 09 Mar 2012
  • Loc: Georgia

Posted 26 November 2021 - 08:26 AM

As the OP's issues may have a number of causes, moving from Mounts to Beginning Deep Sky Imaging for a potentially better fit.



#11 DRNIK1987

DRNIK1987

    Lift Off

  • -----
  • topic starter
  • Posts: 14
  • Joined: 05 Oct 2021

Posted 28 November 2021 - 12:51 PM

last night i tried with 10 inch Newt scopeon EQ6 r pro, ZWO 461mc and ZWo mini guidescope.

guiding figures were consistently around 0.5 RMS errors.

the Guidescope also appears to be reasonably fixed, still i had 4 out of 5 images where trails were present in image at 400 s.

guiding didnt show any problems.

any fixes?

 

Your 6.9arcmin PA error is a bit larger than the 5arcmin max that PHD2 recommends, but it's not unguidable.

 

Your RA and Dec guide errors in the second screengrab are similar RA = 0.72 Dec =  0.65,  so stars should be round.

 

Not so good in the first grab, RA =1.78,  Dec =  0.75, that means elongated stars in the RA direction.

 

So if you think stars are elongated in the Dec direction, which has good guiding, then the guidescope is moving up and down while guiding, which is Differential Flexture.

 

And guidescope focus is way out, HFD = 7.45.

 

But it's 5.56 in the second grab, so is your guidescope focus loose ?

 

Hard to judge star quality from your lo-res grab, and it's only part of the image.

 

 

It means that your guidescope moves during the long exposure compared to your imaging scope/lens.  It doesn’t show in your guiding but does on your long exposure.  It could be a set screw not tight enough or guide scope mounting moving slightly.  It doesn’t have to move much to mess up an exposure.  Make sure everything is tightened.  If it persists, try shorter exposures.

 

 

The 6.9 Arc-Min polar alignment error is a good bit high.

With a bit of care you should be able to get that down to less than 1.

You're using a 1.5 sec exposure time. Try 3 secs or so, after you fix the polar alignment error.

 

if you're using a guide scope, make sure it is solidly mounted, and focuser locked down tight.

Also, try to improve the focus on your guide camera. 

 

It's fixable, just work at it.



#12 licho52

licho52

    Mariner 2

  • -----
  • Posts: 218
  • Joined: 15 Dec 2020

Posted 28 November 2021 - 09:24 PM

The 6.9 Arc-Min polar alignment error is a good bit high.

With a bit of care you should be able to get that down to less than 1.

You're using a 1.5 sec exposure time. Try 3 secs or so, after you fix the polar alignment error.

 

if you're using a guide scope, make sure it is solidly mounted, and focuser locked down tight.

Also, try to improve the focus on your guide camera. 

 

It's fixable, just work at it.

6.9 is just fine with autoguiding, No issues will come from this at OP's scale.

 

3s will fail even more if 1.5 shows trailing

 

Any reasoning behind such recommendations?  Just curious.


  • happylimpet likes this

#13 michael8554

michael8554

    Surveyor 1

  • -----
  • Posts: 1,535
  • Joined: 19 Aug 2020
  • Loc: Wiltshire UK

Posted Yesterday, 06:57 AM

guiding figures were consistently around 0.5 RMS errors.

the Guidescope also appears to be reasonably fixed, still i had 4 out of 5 images where trails were present in image at 400 s.

guiding didnt show any problems.

 

The overall figure of 0.5 RMS is fine, it's differing RA and Dec figures that will lead to trailing. 

 

For example 0.5 may be the average of 0.35 and 0.65.

 

Or the guidescope is loose - we're talking microns of movement, often because of soft-tipped adjusting screws on guidescope rings, or a wobbly finderscope mounting.

 

And the PHD2 guys say that 5arcmins PA is easily guided, no need to waste imaging time getting tiny figures.



#14 f430

f430

    Viking 1

  • *****
  • Posts: 724
  • Joined: 29 Sep 2015
  • Loc: La Mesa, CA.

Posted Yesterday, 12:37 PM

Any reasoning behind such recommendations?  Just curious.

It's simply what seems to work for me. This one is using 5 sec, but I usually use 3.

 

This is using my TEC 140, and ES 80 guide scope with ZWO 174m camera.

Attached Thumbnails

  • Works for me 1.jpg


#15 DRNIK1987

DRNIK1987

    Lift Off

  • -----
  • topic starter
  • Posts: 14
  • Joined: 05 Oct 2021

Posted Today, 10:35 AM

guiding figures were consistently around 0.5 RMS errors.

the Guidescope also appears to be reasonably fixed, still i had 4 out of 5 images where trails were present in image at 400 s.

guiding didnt show any problems.

 

The overall figure of 0.5 RMS is fine, it's differing RA and Dec figures that will lead to trailing. 

 

For example 0.5 may be the average of 0.35 and 0.65.

 

Or the guidescope is loose - we're talking microns of movement, often because of soft-tipped adjusting screws on guidescope rings, or a wobbly finderscope mounting.

 

And the PHD2 guys say that 5arcmins PA is easily guided, no need to waste imaging time getting tiny figures

Guidescope seems to be fixed enough. Still m finding the images to be trailing. My guiding many time shows two lines of RA and DEC lines not aligned properly, i don't know how to fix this..



#16 kathyastro

kathyastro

    Soyuz

  • *****
  • Posts: 3,809
  • Joined: 23 Dec 2016
  • Loc: Nova Scotia

Posted Today, 11:07 AM

Your sample image in the first post showed what appears to be coma, not star trails.  The presence of scroll bars alongside the image suggests we are not getting the full image.  It looks like it is near the bottom of the image, right of centre.  Which means the "heads" of the comas are pointing towards the centre, as they often do.

 

If you were using a Newtonian, I would suggest getting a coma corrector.  However, it appears you are using a camera zoom lens coupled to an astro camera.  So the obvious thing to check would be the adapters connecting the two.  Do you have the camera sensor at the correct distance from the flange of the lens?  Can you add or remove spacers to see if that makes a difference.

 

I don't believe that this is caused by guiding, but don't dismiss the suggestions of differential flexure out of hand.  The characteristic symptom of flexure is when good guiding leaves star trails anyway.  "The guidescope seems to be fixed enough" is not evidence.  Mine seemed to be fixed enough too, until I remounted it and my images got better.



#17 DRNIK1987

DRNIK1987

    Lift Off

  • -----
  • topic starter
  • Posts: 14
  • Joined: 05 Oct 2021

Posted Today, 11:20 AM

Your sample image in the first post showed what appears to be coma, not star trails.  The presence of scroll bars alongside the image suggests we are not getting the full image.  It looks like it is near the bottom of the image, right of centre.  Which means the "heads" of the comas are pointing towards the centre, as they often do.

 

If you were using a Newtonian, I would suggest getting a coma corrector.  However, it appears you are using a camera zoom lens coupled to an astro camera.  So the obvious thing to check would be the adapters connecting the two.  Do you have the camera sensor at the correct distance from the flange of the lens?  Can you add or remove spacers to see if that makes a difference.

 

I don't believe that this is caused by guiding, but don't dismiss the suggestions of differential flexure out of hand.  The characteristic symptom of flexure is when good guiding leaves star trails anyway.  "The guidescope seems to be fixed enough" is not evidence.  Mine seemed to be fixed enough too, until I remounted it and my images got better.

there were no coma in images as the porton which i showed was of mid periphery, not extreme periphery...

even in newt with coma corrector i m getting trails in most of the session.

 

attached image is of last night.

any inputs as to why the guiding lines of two axes are not aligned???

Attached Thumbnails

  • 2021-11-30.jpg


#18 DRNIK1987

DRNIK1987

    Lift Off

  • -----
  • topic starter
  • Posts: 14
  • Joined: 05 Oct 2021

Posted Today, 11:27 AM

If you are using a guidescope differential flexure could be the culprit.

hello Dwight, attached image is of last night's session,

Question: Is is normal to have two lines of RA and DEC to be out of alignment like this every now and then ? 

they sometimes get aligned and sometimes get dealigned. 

and sometimes i get good figures in same setup and during same session the RMS jumps considerably, any inputs?

Attached Thumbnails

  • 2021-11-30 (1) (1).jpg

Edited by DRNIK1987, Today, 11:29 AM.


#19 Tapio

Tapio

    Fly Me to the Moon

  • -----
  • Posts: 5,017
  • Joined: 24 Sep 2006
  • Loc: Tampere, Finland

Posted Today, 11:33 AM

The trendlines are mostly used for drift align purposes.

 

https://openphdguidi...sualization.htm

"The other checkbox, labelled 'Trendlines', will superimpose trend lines in both axes to show if there is a consistent overall drift in the star position.  This is primarily useful for drift aligning where the declination trendline is used extensively.  But the RA trendline can show if your mount is tracking systematically slow or fast (or is seeing the effects of flexure) and can help if you are trying to set up custom tracking rates.  The trend line for Dec is also useful if you're trying to choose the correct direction for uni-directional Dec guiding."



#20 kathyastro

kathyastro

    Soyuz

  • *****
  • Posts: 3,809
  • Joined: 23 Dec 2016
  • Loc: Nova Scotia

Posted Today, 12:08 PM

PHD2 is not doing enough.  There are very few corrections shown on either axis. 

 

Each axis drifts for different reasons.  Declination drifts due to an inaccurate polar alignment.  RA drifts due to tracking errors.  Drift isn't a problem, but lack of correction is.  You need to do some tuning of PHD2 so it makes bigger corrections sooner.  If you find it over-controlling, then you have gone too far with the adjustments, but right now, it is not doing enough.



#21 Steve_M_M

Steve_M_M

    Gemini

  • *****
  • Posts: 3,335
  • Joined: 09 Jun 2004
  • Loc: Austin, TX

Posted Today, 12:16 PM

I am new to all of this, but had issues similar to yours when I started a few months ago.  My guidng was awesome and m stars looked wonky.  For me, it was flexure.  I had it all over....the guidescope was shifting ever so slightly, the mounting plate for the scope was not 100% secure, the focuser had slop, the imaging train depended on compression holders rather than direct threaded connections, and the big one of me was the telescope/giuidescope was not balanced well (I was using a side by side plate).



#22 Dwight J

Dwight J

    Soyuz

  • *****
  • Posts: 3,944
  • Joined: 14 May 2009
  • Loc: Lethbridge, Alberta, Canada

Posted Today, 12:17 PM

As Tapio noted, you can improve Dec guiding by guiding in one direction only thus eliminating any backlash issues with the Dec axis however your guiding accuracy in Dec seems fine.  Odd excursions in RA could be wind gusts or some anomaly in the gear train.  “East heavy” in the RA axis keeps the worm gear in contact with the wheel gear and could help but worm float looks different as in more amplitude in back and forth as the worm vacillates between the teeth of the wheel (main drive gear) and shows oblong stars in the RA axis.  The other situation that could be at play again is flexure as either the guide camera, guide scope, or it’s mounting moves.  If the movement is rare it shouldn’t be much of a problem.  Some mounts have an odd error such as the 4/3 error in older Celestron mounts that showed odd hard to pin down RA errors.  


Edited by Dwight J, Today, 12:18 PM.


#23 Borodog

Borodog

    Gemini

  • *****
  • Posts: 3,181
  • Joined: 26 Oct 2020

Posted Today, 12:19 PM

hello Dwight, attached image is of last night's session,

Question: Is is normal to have two lines of RA and DEC to be out of alignment like this every now and then ? 

they sometimes get aligned and sometimes get dealigned. 

and sometimes i get good figures in same setup and during same session the RMS jumps considerably, any inputs?

 

It's simply what seems to work for me. This one is using 5 sec, but I usually use 3.

 

This is using my TEC 140, and ES 80 guide scope with ZWO 174m camera.

It would probably improve tracking to use longer guide exposures in poorer seeing, although multi-star guiding should probably mitigate this. It might also help if you're using a relatively low sensitivity guide camera, like a color one rather than mono.



#24 DRNIK1987

DRNIK1987

    Lift Off

  • -----
  • topic starter
  • Posts: 14
  • Joined: 05 Oct 2021

Posted Today, 12:51 PM

As Tapio noted, you can improve Dec guiding by guiding in one direction only thus eliminating any backlash issues with the Dec axis however your guiding accuracy in Dec seems fine.  Odd excursions in RA could be wind gusts or some anomaly in the gear train.  “East heavy” in the RA axis keeps the worm gear in contact with the wheel gear and could help but worm float looks different as in more amplitude in back and forth as the worm vacillates between the teeth of the wheel (main drive gear) and shows oblong stars in the RA axis.  The other situation that could be at play again is flexure as either the guide camera, guide scope, or it’s mounting moves.  If the movement is rare it shouldn’t be much of a problem.  Some mounts have an odd error such as the 4/3 error in older Celestron mounts that showed odd hard to pin down RA errors

Infact last night's session had all images with mild oblong stars in RA axis direction, that too were more oblong in the east/west corners of the image...! In some of the images, the stars were trailing in entire image in east/west and in some Centre's stars looked rounder than periphery. 

I m using coma corrector in all images...



#25 Professor2112

Professor2112

    Viking 1

  • *****
  • Posts: 967
  • Joined: 23 May 2020
  • Loc: Massachusetts

Posted Today, 05:38 PM

there were no coma in images as the porton which i showed was of mid periphery, not extreme periphery...

even in newt with coma corrector i m getting trails in most of the session.

 

attached image is of last night.

any inputs as to why the guiding lines of two axes are not aligned???

I think your MnMo might be too high.  With the ZWO mini guide scope you’re at a few arc seconds per pixel, without looking up the pixel scale exactly, I’m going to guess its somewhere around 3-5 arc seconds or so per pixel, maybe more, which means your mount won’t be sent a correction pulse until it’s quite a ways off(for instance, if you were at 5 arc seconds per pixel, that means phd won’t send a correction until the scope has drifted a whole arc second off course with your setting of 0.20), potentially enough to cause trailing.  Then I was using that guide scope, I had MnMo set really low, to 0.05.  I was at over 7 arc seconds per pixel with my 174mm.  It looks as if the mount is only correcting for very large spikes.  Try lowering the MnMo and see if it helps, you never know, it could be as easy as a setting.  If you haven’t done so already, I’ll calculate your pixel scale for your guide setup later when I have some more time, that way you can convert the MnMo number of pixels to arc seconds.  That gives you a much better feel for where it should be set at. You want it to be a bit less than your imaging pixel scale.  




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