Backfocus help - calling all 8" f/12 CC imagers

[Jonny_H]

Hi all,

I hope you are well.

I need help (don't we all 😉) with a backfocus question re: my 8" f/12 CC and 533mc pro.

I thought I would use tonight as a first test for imaging with the CC (to date I have only done a bit of visual work).  However I couldn't for the life of me achieve focus. I spent about 45 mins trying all sorts of combinations with and without the scopes 2" and 2 x 1" extension tubes. This in conjunction with and without the ZWO 21mm and 16.5mm extenders for the camera but to no avail.

Where did I fail the test? I have had no issues at all with this camera and other scopes I own or have owned, e.g. Esprit 100, 150PDS. The CC however seems to be my nemesis!

Any tips/pointers would be hugely appreciated.

Regards,

Jon

[jacko61]

Hi. Which make of CC do you have?  Just looking at the Orion model, the back focus from the 2" accessory collar is 166mm. Depending on your model, that distance might vary.

Graeme

[Jonny_H]

Thanks Graeme.

I have the StellaLyra version but I think the Orion, TS and SL are more or less the same.

That is at least a starting point I can work with.

I will wait to see if any other owners have resolved similar issues also.

 

Thanks,

Jon

[Jonny_H]

So thinking about this further - even with the 4 inch extension tubes that came with the scope along with the 21 & 16.5mm extenders that came with the camera (taking into account the 6.5mm spacing between the sensor and first ring) - I am still 20mm short.

Just trying to work out if this is correct or not? 🤔

[alacant]

Hi

What is '8" f12 CC'? 

 

[CraigT82]

3 minutes ago, alacant said:

Hi

What is '8" f12 CC'? 

 

8" f/12 Classical Cassegrain

[Jonny_H]

12 minutes ago, CraigT82 said:

8" f/12 Classical Cassegrain

Correct. Sorry I should have elaborated rather than using acronyms! 🙈

[jacko61]

Just a quick point. The ASI533 is 17.5mm from the sensor to the front of the fixed* black ring. 

graeme

* I say fixed - I've tried to move the one on my 533 and it's solid so it's staying there. 

[fifeskies]

7 minutes ago, jacko61 said:

* I say fixed - I've tried to move the one on my 533 and it's solid so it's staying there. 

The black ring supplied attached to my 533 in the box can be easily removed, only took gentle force to unscrew it.

I do have it on the camera all the time as I needed all 3 rings supplied to get the correct back-focus for my GT81 with reducer.

[Jonny_H]

15 minutes ago, jacko61 said:

Just a quick point. The ASI533 is 17.5mm from the sensor to the front of the fixed* black ring. 

graeme

* I say fixed - I've tried to move the one on my 533 and it's solid so it's staying there. 

You are right. I think my 6.5mm must have been referring to the sensor to the back of the fixed ring (mine doesn't seem to want to budge either and I am too scared to apply force 😆)

That means I am down to a 9mm difference then.....