Will my Refractor Benefit from a Field Flattener?

[PeterC65]

I have a TS-Optics 72mm F6.0 apo refractor which I use for visual and EAA. Natively, it’s quite wide field, but I also have a StellaMira x0.6 reducer / field flattener to give me an even wider field of view.

With my IMX585 camera (12.9mm diagonal), I get some distortion of the corner and edge stars without the reducer / field flattener, and even more distortion with it, even though the back focus is set correctly to 55mm.

Here is an aberration inspection image with the native scope:

And here is one with the x0.6 reducer / field flattener:

I recently bought an Askar FMA135 and it gives a really sharp image, even when zoomed in to the corners.

Here is an aberration inspection image with the FMA135:

The FMA135 has a built in field flattener and that got me wondering whether the StellaMira (x1.0) field flattener might sharpen up the image from the 72mm refractor, but the results using the x0.6 reducer / field flattener are putting me off.

In a recent post about Focal Reducers, @vlaiv mentioned that a scope may not be able to deal with more than a certain amount of reduction, so I am wondering if that might be the case here, and that while the field is not flat with the x0.6 reducer, the x1.0 field flattener would indeed sharpen things up.

What do people think?

 

[Louis D]

I'm no imager, but I use a TSFLAT2 on my 72ED and 90mm APO refractors to good effect.  I can't stand field curvature with ultrawide eyepieces and low powers.

I attach it to the front of my diagonal, replacing the 2" diagonal's nosepiece.  I use 15mm of spacer extension for best correction visually.

Edited October 26, 2023 by Louis D

[vlaiv]

I think that x0.6 reducer might be a bit too much for this optics. Resulting F/ratio is F/3.6 - and that is tall order.

I do know that other FF/FRs do work well with short F/6 optics. For example, I used TSRED279 - which is 2" version of x0.79 reducer by TS on my 80mm F/6 triplet. It works fairly well. Makes stars a bit astigmatic in the corners of 4/3 sensor - but that shows only if seeing is very good.

I'm sure that x1.0 Field flattener will work well - as long as flattener itself is good optically.

I've heard that TSFLAT2 works very good - people tend to use it for visual as well with small scopes as it has plenty of working distance (needed for 2" diagonal and accessories).

I just saw this as well:

https://www.teleskop-express.de/shop/product_info.php/info/p10127_TS-Optics-1-0x-Refractor-Flattener-for-APO---ED-with-70-72-mm-aperture.html

Maybe that would be best option as it is designed for ED/APO scopes of 72mm aperture?

Then there is this as well:

https://www.teleskop-express.de/shop/product_info.php/info/p12208_TS-Optics-REFRACTOR-0-8x-Corrector-for-ED---Apo-with-70-72-mm-Aperture.html

That is FF/FR with x0.8 reduction - maybe worth a try?

[michael.h.f.wilkinson]

I have a 0.6x reducer/flattener which I have tried on my 80mm F/6 triplet, but I did get some distortion at the edges of a crop-sensor  DSLR image. Th TRF-2008 0.8x reducer has no such issues. I never use the flattener for wide field observing.

[vlaiv]

@PeterC65

Are you using UV/IR cut filter with that sensor?

Most of these astronomy OSC cameras come with just AR (Anti-reflex) coatings so they need additional filtration in UV and IR especially if one is using refractive optics with them.

What you are seeing in images might be poor correction in UV/IR part of spectrum.

[Budgie1]

1 hour ago, vlaiv said:

@PeterC65

Are you using UV/IR cut filter with that sensor?

Most of these astronomy OSC cameras come with just AR (Anti-reflex) coatings so they need additional filtration in UV and IR especially if one is using refractive optics with them.

What you are seeing in images might be poor correction in UV/IR part of spectrum.

I'm wondering if the back focus also needs to be extended slightly. The direction of the halos from the larger stars in the second image may indicate the sensor is slightly too close to the reducer lens? 

I know @PeterC65 says the back focus is set to 55mm, but if a UV/IR filter is being use then this may require an extra mm or so, due to the thickness of the filter glass.  

Edited October 26, 2023 by Budgie1

[PeterC65]

@vlaiv, I have a filter wheel between the scope / reducer and the camera. One of the filters is an Astronomik L2 UV/IR cut filter and that was in use when I took all of the images that I posted above.

The filter wheel / camera setup is such that the distance between the M42 thread on the scope side of the filter wheel and the camera sensor is 55mm plus 1/3 of the filter thickness. So the back focus should be spot on, but as @Budgie1 mentions, looking at the star shapes it seems like it needs a bit more. When I next get a clear night sky I'm planning to back the camera out a little to see if this improves matters. Someone on CN has seen the same edge aberration issues with this scope and a different x0.8 reducer though, and adjusting the back focus didn't improve matters, so I'm thinking that I might just be pushing my luck with the scope.

I have considered the TSFLAT2 field flattener but it screws into the focus tube, replacing the 2" clamp, and it has an M48 camera side connection. I use the scope for visual too, and the x0.6 reducer has a 2" nosepiece, so I'd prefer to leave the 2" clamp in place and use a field flattener with a 2" nosepiece. The M48 camera side fitting would also require quite a bit of faff to connect to my filter wheel / camera setup (which is nicely setup for the correct back focus in most instances).

The StellaMira field flattener seems to get good reviews with a variety of smaller aperture F6ish scopes, but I haven't seen a review of it with the TS-Optics 72mm F6.0 apo.

 

[PhilB61]

I think you can't rely too much on the 55mm backspacing, its really just a reasonable starting point and in many instances will be to be adjusted depending on each individual setup. For my SW 72ED with the StellaMira flattener 55mm spacing still gave significant corner elongation, increasing the spacing to 58mm helped but in the end I swapped the flattener for an adjustable one and was able to completely cure the problem.