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  #1  
Old 29-04-2024, 12:51 AM
MarkInSpace (Mark)
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weird star shapes - why?

Any ideas on why both my reflectors have these weird long stars?
F/4 250mm gso reflector with explore scientific flattener+ zwo filter wheel+asi2600mm
and
F/5 250mm saxon reflector with explore scientific flattener+ zwo filter wheel+asi2600mm

I haven't always had this problem...

The optic system is working fine on my RC8 (using a different flattener), so it suggests the issue is with the explore scientific flattener??
Any thoughts? Help Please!!
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  #2  
Old 29-04-2024, 08:03 AM
glend (Glen)
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Flattener spacing is likely the issue. And frankly one flattener is not a solution for all scopes.
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  #3  
Old 29-04-2024, 08:05 AM
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Possible explanations:

1. Tilt. If it is only on one side or corner of the image.

2. Out of focus.

3. Not thermally stabilised.

4. Wrong spacing for your flattener. Here is Roland Christen's advice on spacing a flattener:

Using Roland's method which states that if you move the focuser in and corner stars get better... you need more spacing and if you move the focuser out and corner stars get better you need to reduce your spacing.

Have you changed the spacing from when you had good stars?

5. Collimation is off, have you moved the scope before you noticed it or bumped into it etc?

Hard to tell from the small photo, a wider field shot with lots of stars would help.

So this same setup only just started doing this? Have you removed the scope and remounted it or banged it somehow?

I suppose a flattener could be banged or dropped and go out of collimation but have never seen that myself.

Greg.
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Old 29-04-2024, 08:07 AM
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Nikolas (Nik)
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Seems more like a tracking issue as the stars are elongated along the same plane.
you need to provide more info and larger images.
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  #5  
Old 29-04-2024, 08:50 AM
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Ryderscope (Rodney)
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I think that Nik is on the money by identifying tracking as a likely suspect. Looking closely at the cropped image with the bright star in the centre, you can see that all the stars in the image have a horizontal elongation to them. This is indicative of a tracking/guiding issue (assuming all subs are affected the same) or if this is a single sub it could be that the scope was bumped during the exposure.

Looking closely at the bright star in the centre of the frame, I can see that it is extended a small amount to the right which indicates something has shifted during the exposure.

It might be worthwhile as well to check the polar alignment. If you are using PHD then try running the guiding assistant which will report on polar alignment error and the RA peak to peak performance of the mount.

And lastly, check that there are no cables snagging as the mount is tracking.
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  #6  
Old 29-04-2024, 10:51 AM
JA
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Hi Mark,

Looking at it there's certainly evidence of elongated stars / transverse star trailing, suggesting tracking issues, BUT you said you had problems with the f4 and f5 and NOT the RC8, which brings that conclusion in to question. A number of things might be different: you mentioned suspecting the flattener, but perhaps there were also differences in how the mount handled these loads, long newtonians versus shorter RC8, a blip on one night using the other scopes, guiding on /off, who knows?

Some more info would help delve in to the issue:
1. Is the first image a FULL field of view? Any cropping and if so how much?
2. If it is FULL field, which scope was used?
3. Is the first image a single sub exposure?
4. What was the sub exposure duration of the first image?
5. Provide a higher resolution image.

You need to do some research to find out about the possibility of it relating to tracking. To potentially rule out tracking, using stellarium (or similar) go to that exact portion of the sky as your first image at the same date and time as the original exposure and set it to the same field of view and orientation. NOW ... index the time forward how ever many seconds you exposed for. Is the change in star position equal to the length of the star trail in your image? Is the direction of your star movement as per your image. If it is you have part of the answer.

The RC8, with its longer focal length not exhibiting this trailing is a conundrum in terms of tracking being at fault and the answer may lie elsewhere, but at least you will have potentially ruled out one possibility.

PS... I take it only one mount was used in all 3 cases: f4, f5 and RC8. ???

Best
JA

Last edited by JA; 29-04-2024 at 02:13 PM.
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Old 29-04-2024, 11:26 PM
Startrek (Martin)
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Any reason why you’re using a flattener on a Newtonian Reflector ?? One would assume that flatteners were primarily designed to correct fields from refractors.
A good quality coma corrector is all you should need eg TS GPU on a standard Newt.
I’ve used 8” and 10” Newts for years and just required a coma corrector to eliminate coma from the corners.

I would check your tracking ( guiding ) if using a guide scope and PHD2 on the following -
Image Scale ratio between main scope and guide scope ( stick around 1:4 )
Differential Flexure between main scope and guide scope ( no shoe type guide scope brackets ) Use an OAG where possible
Check settings in PHD2 using the installation wizard
Get a good calibration at Dec = 0 ( utilise Calibration Assistant )
Utilise a Dark Library in PHD2
Check polar alignment and ensure your error is below say 2 or 3 arc min
Carry out Star tests on an area of the sky that has more stars ( brighter stars ) in the field to check all areas of the image.
Check collimation using both a Laser and Cheshire
Check focuser for any slop in the draw tube and also tilt when using grub screws

Martin
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  #8  
Old 01-05-2024, 04:26 PM
MarkInSpace (Mark)
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thanks

Lot of ideas here for me to review - thanks
a few more bits to update:
I am using a OAG
I called my explore scientific thing a flattener - my mistake - it is a coma corrector
I use a CEM70 and my tracking is usually very good - during the night in question I was at 0.5"

last night, I setup to take samples to show in this thread, but my images were back to normal (for now at least)... so it's a mystery
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  #9  
Old 01-05-2024, 07:06 PM
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Quote:
Originally Posted by MarkInSpace View Post
Lot of ideas here for me to review - thanks
a few more bits to update:
I am using a OAG
I called my explore scientific thing a flattener - my mistake - it is a coma corrector
I use a CEM70 and my tracking is usually very good - during the night in question I was at 0.5"

last night, I setup to take samples to show in this thread, but my images were back to normal (for now at least)... so it's a mystery
Interestingly, I tried to get some imaging done on the 28th in Melbourne, as you are, and I had what I would consider identical elongated stars as your images on my C8/AM5. All across the field of view. I thought it might be collimation and check with a Tri-Bahtinov mask and collimation was good. Clouds came over around 11pm so I gave up and never saved any images.

Something atmospheric?

Chris.
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  #10  
Old 02-05-2024, 09:46 PM
MarkInSpace (Mark)
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Quote:
Originally Posted by ChrisD View Post
Interestingly, I tried to get some imaging done on the 28th in Melbourne, as you are, and I had what I would consider identical elongated stars as your images on my C8/AM5. All across the field of view. I thought it might be collimation and check with a Tri-Bahtinov mask and collimation was good. Clouds came over around 11pm so I gave up and never saved any images.

Something atmospheric?

Chris.
Wow! did not expect to read this!! Now I'm really scratching my head!
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  #11  
Old 03-05-2024, 09:25 AM
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Wind?

Greg.
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  #12  
Old 03-05-2024, 12:26 PM
Dave882 (David)
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Quote:
Originally Posted by ChrisD View Post
Interestingly, I tried to get some imaging done on the 28th in Melbourne, as you are, and I had what I would consider identical elongated stars as your images on my C8/AM5. All across the field of view. I thought it might be collimation and check with a Tri-Bahtinov mask and collimation was good. Clouds came over around 11pm so I gave up and never saved any images.

Something atmospheric?

Chris.
ok so that's very weird - cause I had huuuuge dramas on the 28th (and to an extent on the 27th as well) and had to bin almost all of my data I was collecting cause I couldn't get good stars and the EAF was just refusing to give me a good focus point. Ended up using a bhatinov mask to get focus but even then was very poor quality data. In Sydney tho....
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