ED80 and field flattener

[luka]

Hi

I recently acquired a 2nd hand Astronomics field flattener (AT2FF (https://www.astronomics.com/astro-tech-2-inch-field-flattener-astro-tech-tmb-refractors_p17393.aspx)) and finally got a chance to try it out on my ED80 last night. The results were more than disappointing as the flattener made everything much, much worse. I don't have photos on hand but the stars in the corners/sides were way more "elongated" than on the ED80 alone. I tried it on Canon 1200D and on Nikon D90, same results.

I am sure I am not using it right. I had the following configuration:
ED80 <-> AT2FF <-> T-ring <-> camera.

Thinking of the distance between the shoulder of AT2FF and camera sensor:
1200D with 10mm thick T-ring gives distance of 44 + 10 = 54mm which is within AT2FF required 57mm +/- 4mm.

D90 with 8mm thick T-ring giving 54.5mm distance. Again within specifications.

Any ideas what I am doing wrong?
Thanks

[traveller]

Sounds like you are doing the right configuration. Can you post a photo of the set up?
Also, stupid question, but did you leave the diagonal in?
Bo

[luka]

Photo of the setup is attached. From left to right:
Canon 1200D, T-ring, AT2FF, ED80.

Also attached are images taken with ED80 only and then with AT2FF.

Thank you

[rustigsmed]

hmmm that is a lot worse, I would be trying to get the spacing closer to the 57mm as a starting point it could be making the difference - it may need more.

I'm not sure if design specifications is correct? doesn't it mean that it will correct somewhere (as best it can) somewhere within the range of +-4mm of 57mm (53mm-61mm)? I don't have a refractor but coma corrector's they have to be pretty close 1-2mm to work properly.

cheers

[glend]

You need one of these:

http://www.bintel.com.au/Astrophotography/Camera-Adapters---T-Rings/Bintel-Camera-Adapter--2--/35/productview.aspx

It will slide right into the 2" end and give you the required Canon backfocus. If needed get the EOS bayonet as well if the one you have does not unscrew from the bayonet fitting.

[rustigsmed]

I probably would have suggested something like this
http://www.teleskop-express.de/shop/product_info.php/info/p5731_TS-Optics---T2-Extension-Tube---Length-3mm.html
or these
http://www.teleskop-express.de/shop/product_info.php/info/p4886_TS-Stainless-Steel-Fine-Tuning-Ring-for-T2-thread---thickness-1-0-mm.html

as it seems to reach focus already it's the distance from sensor to FF

canon -> T-ring -> spacer solution -> AT2FF -> focuser.
44mm - -> 10mm -> 3mm


Then again i'm not a frac guy, perhaps someone could chime in to confirm how critical spacing is?

[luka]

Thanks for your suggestions.

I will 3D-print some spacers (non-threaded) and hold things together by hand to see if increasing the spacing improves things. Will report here again... just need to wait for the clouds to finally go away :-(

[traveller]

If all fails, get the Orion field flattener http://www.bintel.com.au/Astrophotography/Reducers--Correctors--Flatteners/Orion-Field--br-Flattener/679/productview.aspx
I have one and it screws into 10 mm canon t ring and the whole rig slips into the focuser and I get round stars in all corner.
Bo

[luka]

Reporting back, I tried extending the distance between the DSLR and the AT2FF by using 3D-printed spacers and hand-holding the DSLR. Tricky but doable for short exposures.
I gradually increased the distance by up to another 13mm and the images did not show improvement. No idea what is wrong with the AT2FF, luckily I did not pay much for it.

It looks like I will be getting the Orion flattener in the end.