Comet star shapes in corners: is this a backspacing issue to the flatener? (StellaMira 2" FLAT & Askar FMA230)

[AstroPhil]

Hi all,

I've been trying to use an Askar FMA230 at it's native focal length (275mm) by using a StellarMira 2" flattener, coupled to an ASI533MC cemera. But when I use the recommended back spacing of 55mm I get the dreaded comet shape stars in the corners (screenshot-1 below: all 4 corners look the same, 'tails' pointing towards the centre of the FOV, I just use one for illustration purposes below). I note one of the reviews on FLO did suggest that 55mm was too small and 70-75mm is better. I'll try that out once I get clear skies. BUT the odd thing is that when I use the bare OTA and no flattener the same issue is seen (this is screendshot-2). The performance with the matched ASKAR 0.84 reducer/flatener (taking this to 230mm) is fine (screenshot-3). These are all single frames, 120s exposure, using Optolong L-extreme filter, with stretch, debayer, and gradient removed.

So, just curious if anyone has seen/had this before with similar setups/flateners? Or has any pearls of wisdom as to how to calculate/determine backspacing? If not, it's trial-and-error, I guess, assuming this really is a backspacing issue... If not I guess I'll be using Askar's flattener and 230mm FL!

Cheers, Phil

[woldsman]

If you are not using a flattener, then the field won't be flat  - hence the same result could occur if the back spacing wasn't measured correctly when the flattener was in place.

The strange thing is the fact you obtained a flat field when using the reducer - this has the same back distance.

I don't have the Stellamira 2" flattener but it looks like the back spacing is 55mm. For a very fast focal ratio maybe the distance would be longer if you were using an adjustable flattener.

But maybe this is an adjustable model (?). 

Otherwise, all I can suggest is that there was something different about your image train when you switched from the reducer to the flattener. Re-checking the measurements (again!) may resolve the issue. 

Worst case would be if the Stellamira flattener is not optimised for your Askar and you need the native flattener. 

 

Edited July 16 by woldsman

[alan potts]

I bought a flatener .5 I think and had the exact same issue with a 115mm F7 scope. As I live out here in Bulgaria I never bothered to send it back which I should have. I too have an exact 55mm and it don't work. However as to whether it's tad short or the other way I am not sure, better people than me can tell. I never tried to get it right as I do not have any shims to try the correction.

Alan

[Vroobel]

The FMA230 is a great scope, but it's rare to have no problems with the back focus. I use it with the original reducer, so have a 230mm focal length. Unfortunately, the back focus isn't equal to the declared 55mm, presently it's 52.5mm and I'm not sure if it's the end. It worked with 2600MC earlier, now I use 2600MM with it, but the problem still exists. Unlike a typical refractor, the Askar doesn't follow the below-mentioned rule. 

 

 

Edited July 16 by Vroobel

[Elp]

Seems odd someone's suggested +15-20mm, never had that with a refractor.

I've got the SM 0.8x reducer flattener and tried 56.5mm with my Starfield 102 (bearing in mind ive added 1mm to account for a luminence filter), this was too far so next time I need to go closer to 55mm.

[AstroPhil]

Thanks all for the thoughts... yep, if I need to use the reducer then so be it. At this point I am curious to solve the mystery though! I did also see that there is a table of data that goes with the StellarMira adjustable flattener (the one I am using is the fixed type but I assume they are very similar optically) and that suggests shortler FL needs longer back focus (here: https://www.firstlightoptics.com/stellamira-telescopes/stellamira-2-adjustable-field-flattener-with-m48-adapter.html))

So at the very least I'll try, and and see if the stuation improves, I guess.

[AstroPhil]

Quick update. After some playing about with back spacing the other day I *thought* I had cracked it with 69.5mm (would have gone with 70mm, as some posters have been recommending, but if I did that, the focusser could not quite focus the image at all. So 69.5mm it is). But tonight a 5min sub gives the below (taken just 20 mins ago). So, not sure still. The lower right corner is particulary bad... I suspect that the focal length I'm trying with is just a bit too short, and out of spec. Back to the Askar reducer/flattener I think...!

[MonsterMagnet]

I've got a similar issue with my Stellamira 90ED Triplet although I am using the official Stellamira x0.8 flattener.  I followed this guide exactly:

 

 

But I am still getting bad stars in the corners, especially at the top.  It didn't affect my Wizard image as that is a small DSO and in the centre but it's going to affect other things.  Looking at the image posted above showing the shapes of stars depending on the backfocus I think I need to move the camera further away from the sensor than the guide above (possibly due to the 2 inch Antlia filters) but even then it should only be 1 or 2mm which would be surprising if it fixes this:

 

 

Any suggestions greatly appreciated.

MM

Edit:  I've gone back and looked at some images I took with the same scope and flattener with different filters and the setup below which is for a different filter drawer and they don't seem to have the issue.  More investigation needed.

 

Edited Monday at 13:58 by MonsterMagnet

[AstroPhil]

Yep, very interesting (and slightly annoying). I'd be really nice if there was more information/test report sent with flatteners (such as backspace measurements with focal length, or focal plane curvature detail) as they all seem to just state that one back focus of 55mm when often it is somewhat depedent on what FL scope it is paried with... there's surely more info (or a method/formula) out there rather than just trial and error...

[Anne S]

On 22/07/2024 at 14:48, MonsterMagnet said:

I've got a similar issue with my Stellamira 90ED Triplet although I am using the official Stellamira x0.8 flattener.  I followed this guide exactly:

 

 

But I am still getting bad stars in the corners, especially at the top.  It didn't affect my Wizard image as that is a small DSO and in the centre but it's going to affect other things.  Looking at the image posted above showing the shapes of stars depending on the backfocus I think I need to move the camera further away from the sensor than the guide above (possibly due to the 2 inch Antlia filters) but even then it should only be 1 or 2mm which would be surprising if it fixes this:

 

 

Any suggestions greatly appreciated.

MM

Edit:  I've gone back and looked at some images I took with the same scope and flattener with different filters and the setup below which is for a different filter drawer and they don't seem to have the issue.  More investigation needed.

 

I make the back focus of the top example 56mm. That would assume 3mm thick filters. How thick are your filters? I allow 0.7 for Baader. As it's an APS-c camera the bf would have to be very close to the 55mm. Also, have you allowed for the connections between the imaging items? They may not fit together exactly.

The other solution is to get BlurXterminator to correct the stars! Assuming you've also got Pixinsight.....