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Oddly shaped stars and count numbers

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#1 amajed172

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Posted 15 April 2019 - 05:08 PM

Hi, last night I imaged for the first time using my new EdgeHD 8" but the results wasn't pleasing.

 

my gear:

Sky Watcher EQ6-R + QHY PoleMaster

EdgeHD 8" + Celestron FR 0.7x

ZWO ASI1600MM Cool Pro

ZWO EFW + ZWO LRGB 36mm unmounted filters

ZWO ASI290MM + 80mm Guide Scope

 

Bahtinov Mask (I used Sirius star for focusing, I think now it's a bad choice considering it's apparent size)

 

My back focus was 106mm from the reducer to the sensor. ( the extra 1mm was recommended by ZWO when using filters)

 

everything is screw in type, so everything is tight together.

 

my guide RMS between 0.4px and 0.5px.

 

 

the next image was shot using the following settings:

120sec exp

1x1 binning (I think I should've used 2x2)

Lum Filter

Unity Gain 139

 

 

Qe8Lvrh.jpg

 

the picture looks like a bit out of focus, but the shape of the stars is weird, is that how it supposed to look like if I'm out of focus?

 

I suspected that I need to collimate my SCT, but when I tested it using an artificial star it was a perfect Donut. so it shouldn't be that.

 

 

what do you think?



#2 Janco

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Posted 15 April 2019 - 05:13 PM

Might be tilt. Try rotating the camera and compare results. ZWO does sell a tilting adapter which can be inatalled just infront of the camera.

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#3 OldManSky

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Posted 15 April 2019 - 05:15 PM

Knowing pixel error in your guiding doesn't do much good without knowing the pixel scale of your guider.

Since the elongation in the stars is almost entirely one direction, it looks like tracking error.

But it could be a right-left tilt of your camera with regard to the optical path...which could either be sag or a collimation problem.



#4 amajed172

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Posted 15 April 2019 - 05:18 PM

Might be tilt. Try rotating the camera and compare results. ZWO does sell a tilting adapter which can be inatalled just infront of the camera.

Sent from my SM-G950F using Tapatalk


I'll try tilting the camera and compare the results.

Thanks

#5 amajed172

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Posted 15 April 2019 - 05:22 PM

Knowing pixel error in your guiding doesn't do much good without knowing the pixel scale of your guider.
Since the elongation in the stars is almost entirely one direction, it looks like tracking error.
But it could be a right-left tilt of your camera with regard to the optical path...which could either be sag or a collimation problem.

Sorry, the pixel scale is 1px.

I did polar alignment using qhy polemaster.

And exp was 120sec only. It shouldn't show any sign of bad tracking even if I'm not guiding.

Collimation checked and it was good.

So is it the tilting that causes it?


Thanks

#6 OldManSky

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Posted 15 April 2019 - 06:52 PM

Sorry, the pixel scale is 1px.

I did polar alignment using qhy polemaster.

And exp was 120sec only. It shouldn't show any sign of bad tracking even if I'm not guiding.

Collimation checked and it was good.

So is it the tilting that causes it?


Thanks

If you can rule out the other possibilities, yes -- tilt is probably it.

But "bad tracking" can show up in 120 seconds...:)



#7 Stephen Kennedy

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Posted 15 April 2019 - 08:52 PM

Remember that this time of year you imaging outside the plane of the Milky Way Galaxy into deep space and should not expect to find many stars but a large number of very distant galaxies. 



#8 amajed172

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Posted 16 April 2019 - 12:22 AM

If you can rule out the other possibilities, yes -- tilt is probably it.

But "bad tracking" can show up in 120 seconds...smile.gif

Sorry, when I said the px scale is 1px that was for the guiding camera, I'm assuming you want the imaging camera's scale, right?

imaging scale was 0.52px according to the plate solver. (I was at 1x1 binning, I think I should've gone with 2x2 binning to get smaller stars?)

 

so my RMS is 0.4~0.5px and my imaging scale is 0.52px, what does that tell you? am I tracking poorly?

 

is there a way to check tilting is daylight? or I just have to rotate the camera and see which direction it goes?



#9 amajed172

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Posted 16 April 2019 - 12:23 AM

Remember that this time of year you imaging outside the plane of the Milky Way Galaxy into deep space and should not expect to find many stars but a large number of very distant galaxies. 

True, I'm guess the shape of the stars is what making me unable to stack my subs...



#10 OldManSky

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Posted 16 April 2019 - 07:55 AM

Sorry, when I said the px scale is 1px that was for the guiding camera, I'm assuming you want the imaging camera's scale, right?

imaging scale was 0.52px according to the plate solver. (I was at 1x1 binning, I think I should've gone with 2x2 binning to get smaller stars?)

 

so my RMS is 0.4~0.5px and my imaging scale is 0.52px, what does that tell you? am I tracking poorly?

 

is there a way to check tilting is daylight? or I just have to rotate the camera and see which direction it goes?

Your guiding error was about +- .5 arc-secs (based on your guider imaging at 1"/pix).  You were imaging at 0.52"/pix (right?).  So your error was about +- 1 imaging pixel.

That's not poor tracking, but ideally you want to have your error less than 1 imaging pixel.

There looks to be 3-6 pixels of error in the image, so it probably *is* tilt in the system.  Which can be hard to track down.

I don't know of a way to do it in daylight...sorry.  



#11 Janco

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Posted 16 April 2019 - 11:41 AM

Your guiding error was about +- .5 arc-secs (based on your guider imaging at 1"/pix).  You were imaging at 0.52"/pix (right?).  So your error was about +- 1 imaging pixel.

That's not poor tracking, but ideally you want to have your error less than 1 imaging pixel.

There looks to be 3-6 pixels of error in the image, so it probably *is* tilt in the system.  Which can be hard to track down.

I don't know of a way to do it in daylight...sorry.  

A 1"/pix guiding error on a 400mm guidescope with a long focal length won't be good enough to be honest. You might need to check if the trailing is occurring along the RA axis or Dec axis in your image. I am actually now leaning more towards tracking being the problem as the length of the star trails seems to be uniform. Tilt would've caused stars to look like their trailing in one area and in focus or less trailed in another


Edited by Janco, 16 April 2019 - 11:42 AM.

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#12 amajed172

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Posted 16 April 2019 - 12:52 PM

Your guiding error was about +- .5 arc-secs (based on your guider imaging at 1"/pix).  You were imaging at 0.52"/pix (right?).  So your error was about +- 1 imaging pixel.

That's not poor tracking, but ideally you want to have your error less than 1 imaging pixel.

There looks to be 3-6 pixels of error in the image, so it probably *is* tilt in the system.  Which can be hard to track down.

I don't know of a way to do it in daylight...sorry.  

I really don't know, so if I'm using 2x2 binning, will that make my error 0.5 imaging px?

 

anyway, my next try out will be 2x2 binning and OAG. will see how it goes.



#13 amajed172

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Posted 16 April 2019 - 12:54 PM

A 1"/pix guiding error on a 400mm guidescope with a long focal length won't be good enough to be honest. You might need to check if the trailing is occurring along the RA axis or Dec axis in your image. I am actually now leaning more towards tracking being the problem as the length of the star trails seems to be uniform. Tilt would've caused stars to look like their trailing in one area and in focus or less trailed in another

guide scope is 600mm, I don't think it's not good enough, considering my focal length is 1422mm, not that far.

I always see setups with 600mm for imaging scope and 200mm for guide scope. isn't it worse?



#14 Janco

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Posted 16 April 2019 - 01:11 PM

guide scope is 600mm, I don't think it's not good enough, considering my focal length is 1422mm, not that far.
I always see setups with 600mm for imaging scope and 200mm for guide scope. isn't it worse?

Its not about the guidescope being too short but your RMS. I use a 400mm guide ST80 with my 9.25 Edge HD and have been able to get up to 10 min subs with no trails. However my guide error is well below 0.5"/px and anything above I seem to notice trails.

However, based on the fact the stars are trailing almost equally across the field it looks like tracking, so I would suggest looking down that road for sollution albeit OAG, Binning Imaging Camera, east heavy RA all those things.

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#15 amajed172

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Posted 16 April 2019 - 01:24 PM

Its not about the guidescope being too short but your RMS. I use a 400mm guide ST80 with my 9.25 Edge HD and have been able to get up to 10 min subs with no trails. However my guide error is well below 0.5"/px and anything above I seem to notice trails.

However, based on the fact the stars are trailing almost equally across the field it looks like tracking, so I would suggest looking down that road for sollution albeit OAG, Binning Imaging Camera, east heavy RA all those things.

Sent from my SM-G950F using Tapatalk

The problem with east heavy or OAG is auto Meridian flip. But will try oag and hope it can find a guide star easily

#16 Stelios

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Posted 16 April 2019 - 02:53 PM

If you are using a guidescope, you very likely are experiencing differential flexure. (And it doesn't matter how tightly fastened the guidescope is). 

 

With differential flexure (very common with SCT's in particular, but also other types of scopes near meridian), your guiding looks perfect but the stars are elongated. 

 

OAG cures this completely.  Also an OAG will help you diagnose if your problem is DF or something else (which is now impossible to tell). 


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#17 RedLionNJ

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Posted 16 April 2019 - 04:04 PM

I would go about determining the root cause of the fuzzy, elliptical stars by doing this:

 

1. Pick a 3rd or 4th magnitude star, somewhere near your target and use the Bhatinov mask for a 2 or 3 second exposure. Repeat a few times until you're really spot-on with the focus. Lock your mirror and repeat. Make sure you are still in focus.

 

2, Take the mask off and do a few more 2-3 second exposures. Does your target star look round? Really "pixel peep" at it. Place the star at various points in the camera's FOV and repeat. Does the star stay as a pinpoint (or at least a circular disk)?  You may just have ruled out tilt as well as focus.

 

3. Move to your intended target and try a 10-second, then 30-second, then 60-second exposure. Check each one. Where does the elongation start to show (or maybe it doesn't, any more)?

 

BTW - seeing an onfocused star as a donut in an SCT is hardly the same as being correctly-collimated. Look up how to properly collimate a SCT - there's a lot more to it than just getting a donut.

 

Those who recommend an OAG for this situation are not wrong, but with mirror lock and a guidescope truly secured (e.g. to the mount, not to the "top" of the SCT), good results can be achieved without an OAG.



#18 amajed172

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Posted 26 April 2019 - 03:57 PM

So I finally got my OAG and tested it, and now I have a new problem.

 

the stars now look like a "light coming out of a torch" I don't know how to describe it, you can see it in this picture:

XPraFtD.jpg

 

the focus is the best I can get, I used bahtinov mask, and used the HFR tool in SGP. can't get any better.

 

also this "star shape" happen even in a very short exp (5 secs) so it's not tracking or guiding issue.

 

I'm not sure what causes this, it shows up even in 1x1 or 4x4 binning. 

 

 

my setup was:

EdgeHD 8" at 2032 F10 so without Focal Reducer. 

backfocus was 133mm

 

 

 

 

help me out please. thanks



#19 jerahian

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Posted 26 April 2019 - 04:07 PM

So I finally got my OAG and tested it, and now I have a new problem.

 

the stars now look like a "light coming out of a torch" I don't know how to describe it, you can see it in this picture:

XPraFtD.jpg

 

the focus is the best I can get, I used bahtinov mask, and used the HFR tool in SGP. can't get any better.

 

also this "star shape" happen even in a very short exp (5 secs) so it's not tracking or guiding issue.

 

I'm not sure what causes this, it shows up even in 1x1 or 4x4 binning. 

 

 

my setup was:

EdgeHD 8" at 2032 F10 so without Focal Reducer. 

backfocus was 133mm

 

 

 

 

help me out please. thanks

That looks like a classic collimation issue to me.  You will need to collimate your SCT.  See here for a description of collimating an SCT.

 

-Ara



#20 amajed172

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Posted 26 April 2019 - 04:29 PM

That looks like a classic collimation issue to me.  You will need to collimate your SCT.  See here for a description of collimating an SCT.

 

-Ara

Thanks for the link, I did try to collimate the scope before, but the shape was a perfect donut so I left it as it is, even after reading the guide in this link, I still don't think I'll do any progress from the last time I tried to collimate my scope, I mean, if it's a perfect donut, what else do I look for? I searched a lot, but everyone keep saying look for the perfect donut...

 

 

Thanks




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