TAL 200K Optical Aberrations

[tweeb]

Talites,

I've been using my TAL200K Klevtsov-Cassegrain for imaging since acquiring it in 2018, it's the later F8.5 version with curved vanes and Crayford focuser, I use a Canon 1100D DSLR. However ever since I started I have noticed a radial aberration that increases as you go further off-axis. Is this just the expected spherical aberration you would see with a Cassegrain or something different. I presume it is not guiding or polar alignment error because it is a radial aberration. Single 300s image (of M17 Omega) attached. For info Nenad Filipovic's TAL website shows the aberrations of the PSF for Klevtsov (and others) -  http://www.nfilipovic.com/astrophotography/tal200k-ota-review

If it is 'just' spherical aberration can it be corrected by a field flattener? Could I just use say an SCT field flattener - I have a Meade Reducer/Flattener for a Meade 8in SCT - I realise I would need to change the back of the 200K for an SCT back and get a new focuser - https://www.berlebach.de/?bereich=details&id=380&sprache=english#

Any help or advice appreciated.

Thanks

Terry

 

 

[Alkaid]

It's field curvature, nothing to worry about (for visual anyway) as present in many catadioptic designs.  A focal reducer / field flattener will fix it for imaging, but not sure how you would get on with a Meade / Celestron one as these are made to suit F10 scopes.  Hopefully some of the advanced imagers will know what to do...

[cmitran]

Dear Terry,

Did you get any answer or solution to this issue?

I also have a Tal 200k and converted it to 2" using a Essato focuser. I wonder if a field flattener would correct the issue you described and also if a flattener is possible, given the short backfocus the Tal has.

 

Thanks 

Cristian