Celestron C8 (Basic) Info Needed

[haitch]

Right getting ready for my holiday this year to my holiday home in France where I have my C8 waiting for me when I get there.

This year I intend to try taking a few shots through it so with that in my mind I'm getting a few things sorted but, of course, it currently sits 650 miles away so I have to figure things out remotely.

First off... Can someone tell me the circumference of the exterior of the tube at the end around the corrector plate? I'm going to build a dew heater with some ni-chrome wire so need the dimensions.

Second... Can someone tell me if the back thread on the Celestron f6.3 reducer are SCT or T thread? (not having used the reducer before just wondering if I need an adaptor of if it'll screw straight onto my T-adaptor?

Thanks for any help

[LukeSkywatcher]

Not sure about your second question but for a dew tape, you sit it behind the corrector plate. So all you need to know id the diameter of the OTA (200mm).

[michael.h.f.wilkinson]

The OTA is wider than 200mm (the mirror is oversized to reduce vignetting). I can measure it and send the dimensions this evening. The F/6.3 reducer/corrector is has a male SCT thread at the rear, so the standard visual back attaches to it. You need an SCT-T2 adapter from Celestron which gives the right spacing to the chip of a DSLR to attach it to your (D)SLR.

[Dunkster]

I've only got a metal tape, which doesn't bend very well but kinks... it reads 29" around the end, 28" around the tube just behind. Paul's right, you should heat just behind the thick part at the end for better heat transfer.

The Celestron reducer has a male SCT thread on it - so you screw on whatever you normally would on the end.

[haitch]

Thanks everyone.

28" then for the dew tape (I'll use Velcro and allow a bit of adjustment) The wire I've got is 10 ohms per ft so I reckon a double run round the end in series controlled by a rheostat (in my case a 12v dimmer off eBay) should do the trick - oops, moving off into DIY Astronomer territory now.

I'll need an adaptor for the back then. I've only used it visually before and it only has a 1.25" visual back so I'll go for an SCT to T adaptor and probably a 2" pushfit for visual as I now have mostly 2" EPs (& a diagonal) when I'm using it visually. Is there any issue I need to be aware of with spacing from the reducer to the camera sensor (Canon DSLR)? I know that an SCT actually changes its f/l as you shift the primary so does this impact on using a reducer (can't remember whether it's a longer f/l with the focal plane nearer the back of tube or extended away - guessing shorter f/l closer to tube so minimum extension out the back would be an advantage?)

Michael - I see you mention the Celestron T2 adaptor for correct spacing - is that spacing critical? Is the issue vignetting or some other optical issue with element to sensor spacing?

Sorry I'm rambling but I guess someone who knows these things will understand?!?!

Cheers

[michael.h.f.wilkinson]

I added a 2" pushfit adapter to my C8 and it has transformed the instrument. So much better at wide views (1.34 deg max).

[haitch]

I added a 2" pushfit adapter to my C8 and it has transformed the instrument. So much better at wide views (1.34 deg max).

Michael, that's what I was hoping - are you referring to just my thoughts on getting a 2" for visual there or am I better of using my DSLR with a 2" barrel adaptor into a pushfit behind the reducer? Another reason I'm interested in using 2" for imaging is that my old 300D has the filter removed and I use it with a 2" Baader IR filter on my fracs.

I know the reducer is intended for imaging but have you ever tried one visually?

[michael.h.f.wilkinson]

I did use the reducer visually, in combination with 1.25" EPs and that works quite well. It does not work on most 2" EPs due to vignetting, so my visual work is now done without reducer. On the imaging side, I have only used my C8 with a 35mm SLR and focal reducer (and off/axis guider (manually guiding the scope, those were the days (not!))). This set-up ensures the correct distance to the reducer. I am not sure a 2" push-fit + push-fit to T2 will give the same distance.

[haitch]

Thanks for that Michael - I think it's better if I look into the spacing in a topic all of its own!