Paracorr and OAG spacings

[discardedastro]

I'm now the happy owner of a Paracorr and OAG, which means guaranteed hours of realising I have the wrong bits/not quite the right spacer/etc!

Slightly blurry photo below of the overall setup. Basically I've got an ASI183MM-PRO as the imaging camera, an ASI 174MM Mini as a guide camera, the ZWO OAG and helical focuser, and a Mini EFW.

The measurements are hard to make precise because - in rather un-Televue fashion - the measurement offsets for the Paracorr's T2 adapter aren't specified. However I've estimated at about 4mm of air between the front flange and the top of the uppermost Paracorr lens.

The imaging train in theory looks like 11mm for the flange-to-sensor, 20mm for the EFW, 16.5mm for the OAG, and I need to achieve 56mm backfocus from the lens (or 52mm from the T2 flange) meaning I think I need a 4.5mm spacer - which is hard to find! I've put a ~7mm spacer in for now as the closest approximate thing I own.

I could just buy a set of 1mm/0.5mm aluminium spacers and insert these between the EFW and OAG and OAG and Paracorr but wondered what approach others might have used in similar situations. I also need to adjust the rotation of the OAG insert, so will need a spacer to offset this so the pick-off prism is at the top of the long edge of the sensor in any case.

The guiding side of things I'm much less worried about given I've got the focuser and a range of different spacers - bit harder to work out but I think I have it set fairly close and can in any case experimentally arrive at the right answer there! Clouds have rolled in so no testing to be done just yet...

 

[discardedastro]

I'm a plonker, I think - the 11mm number I'm not sure where it came from but I need 6.5 from flange to sensor. Which means that I'm off by 4.5mm - the amount which I thought I needed to offset by. So the answer is to entirely remove the excess spacer and all will be well, in theory! I've found a tiny 3mm spacer from TS which I'll nab along with some 1.5 to 0.5 steel spacers just in case the Paracorr demands further adjustment on the lens-to-flange front, but I think I'm sorted. Just need some clear skies! https://www.teleskop-express.de/shop/product_info.php/info/p5731_TS-Optics-T2-extension-M42x0-75---3-mm-length.html

[discardedastro]

OK, further staring at things led me to add in my 11mm spacer from the camera so I now have Paracorr - spacer - OAG - EFW - ASI183MM which gives me 55mm backfocus mechanically. Which also tallies with the ZWO guidance I found on the topic.

However, I've now set it all up outside and can't get any stars to appear in any focus position. I've tried fully inward focus, fully out, everything inbetween, and even offset further back (not that that should improve matters). I normally have the focuser 20mm or so from the inward position (using the MPCC) and have s een some reports the Paracorr requires about 20mm of inward travel to get focused, but as far in as I can go I can't get any stars to appear.

Losing my mind slightly and would welcome any suggestions or thoughts - I'm probably doing something stupid.

[discardedastro]

... and finally, after a lot of mucking around and reassembling bits by the side of the scope in the cold, this works (in the spirit of documenting my failures!):

So - long story short, I didn't need to chase for inward focuser travel compared to my old set-up but actually outward travel.

I put the tunable top back on the Paracorr and used my TeleVue Delite 17mm to give me a visual way to look at focus rapidly and just moved it till it looked right with the focuser racked out halfway and locked it off there, then re-fitted the imaging train and confirmed I could achieve focus - which I could! Light at last.

Then I realised I needed the OAG to be the absolute first thing in the train, so that got switched around, putting my spacer in the middle of the EFW and the OAG.

Then I realised (!) that the ZWO helical focuser was a bit marginal for inward travel - I think removing the included 3mm spacer would've done it but I couldn't get it to shift so replaced it with the standard adapter and again adjusted by hand till it looked right. Ran autofocus on the main camera, then adjusted the guide camera by hand to bring it parfocal.

I've now got to do some adjustments in PHD2 to get the OAG doing its thing correctly, but it's all coming together now. Just wish it were a little less windy tonight!