Strange effect on edges

[04Stefan07]

OK, here is what I did. I have attached a photo of the spacing but there is a slight problem.

ASI1600MM (6.5mm) - EFW (20mm) - M42 Spacer (5mm) - Field Flat
= 31.5mm (depsite ES, ZWO and Altair Astro saying their back focus requirement is 55-56mm)

Now the biggest problem with this is the damn focus distance for this scope! The second photo shows that my focus is all the way out and I can't go any further. Is there a way I can adjust it so the focus can be further in??

With the photos, its a full moon tonight so a bit washed out and had a few clouds. No guiding as I am waiting on my new mount and guidescope so I kept the exposures under 10 seconds.

Photos (there is a weird artifact on them, not sure what it could be)
- 9 second exposure
- 4 second exposure
- 1 second exposure

I have uploaded the photos here (2x2 bin) around 8mb each. Are you able to check the corners? The stars do look more round and pin-point on all edges which may look promising but would like a second opinion.

Thanks!

1second_frame_and_focus_3.fit 4seconds_frame_and_focus_4.fit 9seconds_frame_and_focus_1.fit

[symmetal]

The stars do look good in the corners but being binned that will hide any issues to some degree. As to why it seems to work best at a spacing way below the recommended distance I can't say.

Your scope only has around 55mm of focus travel and as it was supplied with two 2" extension pieces implies they would need to be used in some situations to achieve focus. You've run out of focus as the 24mm reduction you made in the flattener spacing distance has to be made up by the focuser just to achieve focus.

Maybe the FF is not compatible with the scope, although it falls within its requirement specification range. Your scope being f7 is nearer the slower end of the range and I would have thought slower scopes would be less critical on spacing distance than faster scopes.

Take some test images unbinned and if they are still very good then you could just buy a longer 2" extension to replace one of yours to give some focus travel to spare. Maybe you could email AltairAstro to see what they say, or maybe someone on the forum can give an explanation as to why it works best at such a reduced spacing distance. 🤔

Alan

[04Stefan07]

Got a bit of progress.

At 53mm and 56mm (need to purchase some smaller spacers for 55mm), looks to be better but now the stars look a bit like seagulls.

 

Below is 53mm, 56mm had the same effect.

[symmetal]

I have had stars turning into seagulls when the spacing is too far away, rather than the classic sausage stars rotating around the centre. There is still a part of the star image pointing towards the centre too which may imply the FF is perhaps not compatible as it's showing too far and too near effects at the same time.

What did your unbinned images look like when the spacing was way under that recommended, as you posted previously when they looked good binned.

Alan

[04Stefan07]

Yeah they looked better and the stars more pin point when the spacing was way under recommendation specs.

I have been threading the flat directly onto my focus extension tube which I think is better than using the push fit adapter it came with.

[symmetal]

I would leave it at the spacing that gives the best star shapes even if it's way outside specification. It looks as if the flattener isn't that compatible with your scope for some reason, but by a strange quirk it works as it's supposed to at a much reduced distance, when covering your sensor size. 🙂

Yes, threading all the pieces together is the best option to help minimize any tilt issues, as long as there's still a way to rotate the camera orientation as required.

Alan