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How to diagnose stars?

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#1 JP50515

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Posted 23 July 2019 - 08:20 PM

Hey guys so I'm on a new to me scope setup (C925)

I've had it out for two imaging sessions so far.

 

In both instances I'm seeing similar star issues. 



My question is, how do you go about diagnosing what's causing out of round stars?



Is this backspacing, guiding, tilt...etc...and how does one determine between them?

 

 

Ignore the poor processing jobs I am still in the shakedown phase over here and just doing dirty stacks to determine what needs dialing in. 

 

Cave Nebula.jpg

 

Pacman_Nebula-V1-Resize.jpg



#2 CharlesW

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Posted 23 July 2019 - 08:59 PM

It’s a lot easier to see star issues if you don’t stack and integrate. Hard to see through the colors. Since you have an Edge and are using a guide scope, I assume you are using the Celestron Edge T Adapter? If your aren’t you should, then attach the camera and filter wheel so you hit 55mm back focus. 



#3 JP50515

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Posted 23 July 2019 - 09:07 PM

It’s a lot easier to see star issues if you don’t stack and integrate. Hard to see through the colors. Since you have an Edge and are using a guide scope, I assume you are using the Celestron Edge T Adapter? If your aren’t you should, then attach the camera and filter wheel so you hit 55mm back focus. 

yes I am. i'll attach a few subs here as soon as my kid goes down 



#4 Gipht

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Posted 23 July 2019 - 09:37 PM

These look like the makings of some excellent images.  Especially considering you are working at an aggressive image scale.


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#5 JP50515

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Posted 23 July 2019 - 09:38 PM

 

Here's a pretty extreme example sub

 

Ha_Sub.jpg



#6 JP50515

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Posted 23 July 2019 - 09:41 PM

These look like the makings of some excellent images.  Especially considering you are working at an aggressive image scale.

 

Thanks Mate! Yeah I'm definitely nit-picking a bit here, but for my own sake I'm trying to understand how to diagnose these stars. 

Next night out I will be giving some shorter subs a shot just to see if perhaps I'm pushing the limits of the mount at 300s but they are all kinda heading off in one direction so I really don't know what the deal is or where to start looking. 

I know some people here have a really good understanding of what different star shapes indicate so I'm hoping someone can give me a bit of a crash course. 


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#7 Gipht

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Posted 23 July 2019 - 10:03 PM

When you look at the stacking offsets from each  frame sequentially in time,  do they drift in a somewhat predictable pattern during the session?  How much drift per frame?

 

There seems to be a color band across some of the larger blue and green stars.   This may be  isolated to a particular filter.



#8 RazvanUnderStars

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Posted 23 July 2019 - 10:26 PM

I'm not very experienced but to my eyes, they seem to be elongated along the same direction and by the same amount so my suspicion is tracking/guiding errors. 


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#9 Eric Horton

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Posted 24 July 2019 - 04:54 AM

Same here. Not overly experienced but looks like drift down. What's the polar alignment error look like? If guiding what's the total error

#10 james7ca

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Posted 24 July 2019 - 08:18 AM

Are you using an Off Axis Guider (OAG)? If not this could be mirror flop. That said, if you have an EdgeHD and use the mirror locks with some care then it shouldn't be mirror flop (but, YMMV).

 

Also, the backspacing requirement for the EdgeHD isn't 55mm, but it depends upon where you are taking the measurement and what camera adapter you are using and Celestron does provide the necessary instructions to achieve the needed spacing. Celestron gives the backfocus for the 9.25" EdgeHD as 146.05mm +/-0.5mm. But, yes, Charles is right and if you are using Celeston's T-Adapter for the 9.25" EdgeHD you need to use an additional 55mm of spacing between their adapter and the sensor. Note, the camera adapter for the 8" EdgeHD and their other non-EdgeHD SCTs isn't the same as that for the 9.25".

 

From Celestron's EdgeHD whitepaper:

The EdgeHD 925, 1100, and 1400 form their best images 5.75 inches (146.05mm) behind the telescope’s rear cell 3.290×16 tpi threaded baffle tube lock nut. For best results, the image sensor should be located within ±0.5mm of this back-focus distance.

 

It is easy to place a digital SLR (DSLR) camera at the proper distance using the Small T-Adapter (item #93644) for the EdgeHD 800, or the Large T-Adapter (item #93646) for the EdgeHD 925, 1100, and 1400. The small adapter is 78.35mm long while the large adapter adds 91.05mm, in both cases placing the best focus 55mm behind the T-Adapter.


Edited by james7ca, 24 July 2019 - 08:32 AM.


#11 JP50515

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Posted 24 July 2019 - 09:58 AM

When you look at the stacking offsets from each  frame sequentially in time,  do they drift in a somewhat predictable pattern during the session?  How much drift per frame?

 

There seems to be a color band across some of the larger blue and green stars.   This may be  isolated to a particular filter.

I will have to check and see if there's a pattern.  In regards to the color band, I'm not exactly sure what you're referencing. I get a reflection halo on large stars which I know to be from my filters. 

 

Same here. Not overly experienced but looks like drift down. What's the polar alignment error look like? If guiding what's the total error

PA should be spot on, since I'm on a pier. My guiding was really good (some of the best I've had) at < 0.30 arc-sec/pixel. However I suppose it's possible I bumped stuff as I just built a roll away obsy around it. However, the pacman was the night before that, so should have for sure had good PA. (excellent according to sharpcap) 

 

Are you using an Off Axis Guider (OAG)? If not this could be mirror flop. That said, if you have an EdgeHD and use the mirror locks with some care then it shouldn't be mirror flop (but, YMMV).

 

Also, the backspacing requirement for the EdgeHD isn't 55mm, but it depends upon where you are taking the measurement and what camera adapter you are using and Celestron does provide the necessary instructions to achieve the needed spacing. Celestron gives the backfocus for the 9.25" EdgeHD as 146.05mm +/-0.5mm. But, yes, Charles is right and if you are using Celeston's T-Adapter for the 9.25" EdgeHD you need to use an additional 55mm of spacing between their adapter and the sensor. Note, the camera adapter for the 8" EdgeHD and their other non-EdgeHD SCTs isn't the same as that for the 9.25".

 

From Celestron's EdgeHD whitepaper:

I am using a guidescope but always use the mirror locks. I use the celestron T-Adapter + zwo's recommended adapters for 56mm backspacing with the EFW + ASI1600 (can't actually get right on 55mm with provided spacers) 

 

00000PORTRAIT_00000_BURST20190717140315235.jpg



#12 Gipht

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Posted 24 July 2019 - 05:32 PM

These very well could be double stars of equal size, but that would be unusual.

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#13 kathyastro

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Posted 24 July 2019 - 05:40 PM

When diagnosing out-of-round stars, the most important diagnostic is the direction of the long axis.  Are they stretched north-south, east-west, radially or tangentially?  North-south indicates a polar alignment problem.  East-west indicates a tracking problem.  Radially indicates coma or an un-flat field.  Tangentially indicates field rotation.


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#14 JP50515

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Posted 25 July 2019 - 02:52 PM

When diagnosing out-of-round stars, the most important diagnostic is the direction of the long axis.  Are they stretched north-south, east-west, radially or tangentially?  North-south indicates a polar alignment problem.  East-west indicates a tracking problem.  Radially indicates coma or an un-flat field.  Tangentially indicates field rotation.

Thanks Kathy,

 

This is very useful information. So I did a bit of testing last night. Dialed in my PA to 0.02 in both axis. Never have I had a better alignment according to sharpcap. So I can pretty confidently rule that one one. 

  • I re-aligned the guidescope to imaging scope on one star. Based on my setup I really don't think its differential flex. My guiding was also fantastic last night due to the good PA averaging 0.18-0.22 arc-sec/pixel so I'm ruling out guiding as the issue. 

Star drift does not appear to be the problem here. 

 

  • I checked collimation, which on a de-focused star appears to be spot on. 

 

  • Interestingly enough, the stars are also identical across all filters and exposure lengths. 1 second exposures give the same stars problems as 5 minute exposures. 

 

  • Most interesting to me though was what I realized after the meridian flip....stars still elongated in the same direction. NE to SE if referencing the image cardinally. 


This pretty much points me to the OTA and/or imaging train. Seeing as I'm fully threaded through the whole train and using the suggested manufacturer adapters I'd be hard pressed to suspect tilt as the issue...but it is a quite long train with it's backfocus at 146mm behind the focal reducer. But I know plenty of others image with this exact train setup so I'm not sure how I'd even remove tilt if there were any. 


At this point...I'm a bit stumped on where to look/adjust. Could I have tilt in my corrector plate somehow, or something?

Where would others be looking given this knowledge? 



#15 RazvanUnderStars

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Posted 25 July 2019 - 05:35 PM

OP, that was a good test and a very relevant finding that the exposure length or flip do not matter.

 

I don't have enough experience with distortions to comment, but at least I can point to two resources. Perhaps the reference book is Star Testing Astronomical Telescopes: A Manual for Optical Evaluation and Adjustment (the first edition, which I have, is 10x cheaper than the second).

 

Also see detailed page on optical aberrations.


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#16 JP50515

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Posted 25 July 2019 - 07:12 PM

OP, that was a good test and a very relevant finding that the exposure length or flip do not matter.

 

I don't have enough experience with distortions to comment, but at least I can point to two resources. Perhaps the reference book is Star Testing Astronomical Telescopes: A Manual for Optical Evaluation and Adjustment (the first edition, which I have, is 10x cheaper than the second).

 

Also see detailed page on optical aberrations.

It doesn't look much like any of those (although definitely bookmarking that link!) 

They're all trailing in the same direction...



#17 RazvanUnderStars

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Posted 25 July 2019 - 07:24 PM

When you checked the collimation (which you said appeared spot on), was the defocused star still elongated? Also, what exactly did you check then? Just that the Poisson spot was centered or did you also look at the diffraction rings?



#18 JP50515

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Posted 25 July 2019 - 09:22 PM

When you checked the collimation (which you said appeared spot on), was the defocused star still elongated? Also, what exactly did you check then? Just that the Poisson spot was centered or did you also look at the diffraction rings?

I took a screenshot. I'll dig it off the imaging laptop in the morning. 



#19 f300v10

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Posted 26 July 2019 - 10:31 AM

OP, while it probably isn't the cause of your elongated stars, you do have a rather large ratio between your image scale and guide scale, close to 6x.  Long term you could eliminate several possible causes of guide error by switching to an OAG, while also improving your image to guider scale ratio.  You also have plenty of back focus to fit an OAG into your image train.  

 

I use the same ZWO guide scope you have on my MN190, but switch to the Celestron OAG when using my C11.  The OAG took a bit of getting use to but I do get more accurate guiding with it, which I believe is mostly due to giving PHD2 more data to work with at the higher guide scale.


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#20 JP50515

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Posted 26 July 2019 - 11:50 AM

OP, while it probably isn't the cause of your elongated stars, you do have a rather large ratio between your image scale and guide scale, close to 6x.  Long term you could eliminate several possible causes of guide error by switching to an OAG, while also improving your image to guider scale ratio.  You also have plenty of back focus to fit an OAG into your image train.  

 

I use the same ZWO guide scope you have on my MN190, but switch to the Celestron OAG when using my C11.  The OAG took a bit of getting use to but I do get more accurate guiding with it, which I believe is mostly due to giving PHD2 more data to work with at the higher guide scale.

Yeah, the problem is that switch requires a new camera as well I think which makes it a pretty hefty upgrade all at once. 

I am really curious to see if I can't figure out what's causing these star shapes so I can diagnose whether or not I actually "need" an OAG. 



#21 f300v10

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Posted 26 July 2019 - 12:37 PM

Yeah, the problem is that switch requires a new camera as well I think which makes it a pretty hefty upgrade all at once. 

I am really curious to see if I can't figure out what's causing these star shapes so I can diagnose whether or not I actually "need" an OAG.

I assume you mean a new guide camera for use with the OAG?  While not ideal I would think your 120mm would suffice, at least short term.  But as you point out guiding appears to have nothing to do with your star shape issue if you are seeing the same shapes in 1 second subs.  Best of luck tracking it down, I've had similar issues with my MN190 and collimation was the key.


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#22 JP50515

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Posted 26 July 2019 - 12:52 PM

I assume you mean a new guide camera for use with the OAG?  While not ideal I would think your 120mm would suffice, at least short term.  But as you point out guiding appears to have nothing to do with your star shape issue if you are seeing the same shapes in 1 second subs.  Best of luck tracking it down, I've had similar issues with my MN190 and collimation was the key.

Speaking of which. This is what my airy disk looks like...now I'm new to SCT collimation but from my beginner eyes this looks pretty good...maybe I'm wrong tho?

 

Defocused_Star.jpg



#23 JP50515

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Posted 26 July 2019 - 12:53 PM

When you checked the collimation (which you said appeared spot on), was the defocused star still elongated? Also, what exactly did you check then? Just that the Poisson spot was centered or did you also look at the diffraction rings?

Just quoting you so you see the airy disk image.



#24 f300v10

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Posted 26 July 2019 - 01:12 PM

To my eye the doughnut is slightly off center to the right.



#25 Stelios

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Posted 26 July 2019 - 02:36 PM

Speaking of which. This is what my airy disk looks like...now I'm new to SCT collimation but from my beginner eyes this looks pretty good...maybe I'm wrong tho?

 

attachicon.gif Defocused_Star.jpg

You *cannot* collimate accurately when you are that much defocused. Plus you have a heat plume, and even at this amount of defocus the secondary shadow is slightly offset to the right.

 

Attempt a much tighter focus and look again. You may also want to try metaguide--once you figure out how to set the camera exposure properly, it will show very clearly the direction (and roughly the amount) of mis-collimation. 


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