What is possibly going wrong? (With answer from ChatGPT)

[AstroRookie]

Hello,

I'm using a Orion F3/9 Astrograph since 3 years and it has given decent recults, but now for months (only natural, taking into account the few number of clear nights we have over here in this windy [removed word]-hole by the North sea) since I switched to a dedicated astro-camera I have been struggling with the shapes of the stars in my picture (I switched back to my DSLR and I get the same problem, so I exclude the camera's). I considered every thing from bad collimation, over pinched optics, dirty primary mirror, bad guiding to camera tilting in the focuser. Took mesures for all of this:

• replaced the springs for the primary mirror with more sturdy ones
• cleaned the optics (to discover a tinny scratch of about 4mm long - but can this really have such dramatic effects?)
• installed a 3rd screw to hold the camera in the focuser
• tightened 2 screws of the focuser base plate that were not completely tight
• tried to have my polar alignment as good as possible (last night I had total RMS reported by PHD2 of 0.13 (0.49") - see screenshot
• tried to have my collimation as good as possible (looking in the corners of the attached pictures, my collimation looks ok)

I am about to dump every thing in the ditch at the back of our garden ... so any suggestion really appreciated by,

AstroRookie

Atttached files:

test_mf.jpg: 5 minute exposure after manual focus using the diffraction spikes as a reference

test_nina_af.jpg: 5 minute exposure after nina's automatic focus procedure, that's by trying to get round stars is completely out of focus

probleem.jpg: screenshot of the imaging tab in nina

 

Edited March 8 by AstroRookie

[AstroRookie]

Here's the answer of chatgpt to the following question:

In astrophotography can a 4mm scratch in the 203mm primary mirror of a newtonian have an effect on all the stars in the picture taking with it?

 

ChatGPT

In astrophotography, the quality of the optical components, including the primary mirror in a Newtonian telescope, plays a crucial role in determining the final image quality. However, the impact of a defect, such as a 4mm scratch on a 203mm primary mirror, can vary depending on several factors including the nature of the scratch (depth, width), its location on the mirror, and the type of astrophotography you are engaging in.

For visual observations, a 4mm scratch on a large mirror (203mm in diameter) is unlikely to be noticeable. The reason is that the overall surface area of the scratch is small compared to the total area of the mirror, and the light-gathering capability of the mirror remains largely unaffected. The diffraction effects caused by such a scratch are usually minimal, especially for visual astronomy.

In astrophotography, however, the situation might be slightly different. Cameras used for astrophotography are more sensitive and can pick up finer details and imperfections that the human eye might miss. That said, the impact of a 4mm scratch on the overall image quality could still be considered minor for several reasons:

1. Diffraction: The scratch might cause some diffraction effects, potentially leading to slight artifacts in images, especially around bright stars. However, these effects are often subtle and may not be noticeable unless you are looking for them specifically or are imaging objects with high dynamic range.

2. Image Processing: Modern image processing techniques can mitigate many of the imperfections present in raw astrophotography data. Tools available in software used for astrophotography can help reduce or eliminate the visual impact of a scratch.

3. Scope of Impact: The scratch's effect is more likely to be uniform across the field rather than causing a distinct or localized problem on all stars in an image. Depending on its location and the nature of the scratch, its impact might be dispersed or diffused throughout the image.

4. Adaptive Optics and Stacking: For more advanced setups, the use of adaptive optics or image stacking techniques (where multiple exposures are combined to improve signal-to-noise ratio) can further reduce the visibility of any imperfections caused by a scratch.

In conclusion, while a 4mm scratch on a 203mm primary mirror is not ideal and could theoretically introduce some level of diffraction or artifacts, its actual impact on astrophotography is likely to be minimal. The overall image quality would still be determined by a combination of factors including the optical design of the telescope, the quality of the camera, seeing conditions, and post-processing techniques.

Edited March 8 by AstroRookie

[Clarkey]

Have you tried some short exposures and looked at the shape of the stars? If they are round it shows it is not an opitcal issue, but probably a guiding related problem. Are you using an OAG or guidescope?

 

Edit - probably unrelated, but your focus curve is very odd...

Edited March 8 by Clarkey

[AstroRookie]

Hi Clarkey,

I tried 5 sec exposures and the stars are also not round; I'v been using a 60mm SVBONY guidescope without any issues so far; also the PHD2 graph looks fine to me.

AstroRookie

[Clarkey]

9 minutes ago, AstroRookie said:

also the PHD2 graph looks fine to me

If there is any flex between the guide scope image and the scope image the guide graph would be irrelevant. However, if 5 second exposures are also showing poor star shapes the culprit appears to be the optical train. Have you tried rotating the camera and coma corrector to see if the aberration follows?

I'm not trying to be patronising - just trying to rule out the obvious.

Edited March 8 by Clarkey

[AstroRookie]

Hi Clarkey,

no sweat, I don't feel patronized at all, thanks you're willing to look into the problem!

About rotating the camera, no I didn't think of that - to be honest I was so disappointed yesterday evening, that the only thing I thought of was giving up this hobby ...

As for "flex between the guide scope and the scope image", I'm sorry but I don't really understand (my native language isn't English)

AstroRookie

[Clarkey]

3 minutes ago, AstroRookie said:

As for "flex between the guide scope and the scope image", I'm sorry but I don't really understand (my native language isn't English)

If the image in the main camera and the guide camera are changing relative to each other due to movement of mirror / focuser / guide scope - 'flex'.

 

Your English is certainly better than my attempt at any European languages. Smattering of French is my limit - much to my shame.

[AstroRookie]

Hi Clarkey,

Maybe stupid question, but would that not be reflected in the graph of the guide software?

AstroRookie

[Kon]

I am not a DSO imager but I recently got a second hand 12" Dob and I was getting flaring in Jupiter's moons. I thought it was collimation but in the end I had some pinched optics; is it likely to be after you cleaned the mirror that you may have tightened the clips around the mirror a bit tighter? Just a thought.

[ollypenrice]

A small amount of matt black paint fully covering the scratch and of a soft, rounded shape, will entirely eliminate it as a cause.

13 minutes ago, AstroRookie said:

Hi Clarkey,

Maybe stupid question, but would that not be reflected in the graph of the guide software?

AstroRookie

No, the guide camera might be following the star perfectly but the imaging scope (or just its mirror) might be moving relative to the guidescope. An off axis guider removes this possibility by guiding on the imaging light cone itself. So called 'Mirror flop' has essentially the same consequences as flexure.

However, I'd put money on tilt, in your case. (Not a lot of money but a bit... ) From what I can see, the elongations all go the same way, meaning they are parallel with each other. Optical defects rarely produce this effect but tilt does. Since you see the same thing in two cameras, it is unlikely to be a tilted chip. (These are not uncommon and some cameras have chip tilt adjustment built in.) I would look for sag or slack in the focuser. Are all your attachments screw-fit rather than push fit? Screw fit is best. Are you placing your camera in this orientation, parallel with the counterweight bar? It's the best.

Tilt can spring up suddenly because some mechanical component becomes loose or a bearing breaks.

Olly

[michael8554]

Two possibilities:

1. You have guiding with RA and Dec errors very similar  RA = 0.33arcsecs, Dec =  0.36arcsecs.

That should yield round stars.

But I notice a 1 arcsec spike in RA and Dec, so what are your RA and Dec Peak errors ?

2. Diff Flex occurs when you have good guiding figures - so the guidescope is being well guided.

But if the guidescope is wobbling, such as due to soft-tipped screws in the guidescope rings, or a wobbly Finder used as a guidescope, or cables dragging.

Then the imaging scope is not being guided the same as the guidescope.

Remember your imaging camera has pixels with dimensions in um.

A wobble less than the diameter of a human hair will elongate a star image.

Michael

[AstroRookie]

@ollypenrice

Don't know if I could orientate the camera as you showed, I think my guide scope and camera would somewhere be in the way - I'll check. I have considered upgrading the focuser, but if that is not the problem then this is yet another "useless" upgrade, and a "30 days money back guarantee" is taking into account my location not really helpful 😉

@michael8554

Diff flex: that's interesting, as I have to say up to now I've always been imaging near the zenith and this is the first time I have been imaging at +- 40° altitude south (rosette nebula) - I'll have a look on how the cables and guide scope behave at that position

Thank you both!

AstroRookie

 

 

[ollypenrice]

57 minutes ago, AstroRookie said:

@ollypenrice

Don't know if I could orientate the camera as you showed, I think my guide scope and camera would somewhere be in the way - I'll check. I have considered upgrading the focuser, but if that is not the problem then this is yet another "useless" upgrade, and a "30 days money back guarantee" is taking into account my location not really helpful 😉

@michael8554

Diff flex: that's interesting, as I have to say up to now I've always been imaging near the zenith and this is the first time I have been imaging at +- 40° altitude south (rosette nebula) - I'll have a look on how the cables and guide scope behave at that position

Thank you both!

AstroRookie

 

 

This change in orientation might also introduce tilt.

Olly

[AstroRookie]

4 hours ago, Clarkey said:

Have you tried some short exposures and looked at the shape of the stars? If they are round it shows it is not an opitcal issue, but probably a guiding related problem. Are you using an OAG or guidescope?

 

Edit - probably unrelated, but your focus curve is very odd...

That's why I posted an image with manual focus by me, where the diffraction spikes served as reference and an autofocus by nina, which has indeed a "funky" focus curve; that's imho because the af routine tries to get round stars but then one can see that the image is not in focus looking at the diffraction spikes. On previous occasions when investigating this problem, nina was not even able  to focus and even with a manual focus plate solving did not work, so in fact I'm making progress (the last remark is with a lot of irony)