RASA 8 backfocus

[AbsolutelyN]

I've been looking at the backfocus distance for my RASA 8 + filter draw + 1600MM and seems to be concluding that my camera back-focus is too much. Would anyone be-able to sense check this for me? 

Rasa backfocus from the M42 adaptor is 25mm.

I have Starizona filter draw which is 19.5mm
Then the 1600MM which is 6.5mm

That's already 26mm so 1mm over.

Then I have filters - 2" Baader which are 2mm thick so I think that adds roughly 1mm more bringing it to 27mm.

My HA (Baader f/2) filter gets nice stars. The RGB ones are 'reasonable' but not perfect. 

Am I right that they are not perfect because the spacing is 1mm to 2mm too much? 

Any thoughts appreciated.
 

Edited August 21, 2020 by AbsolutelyN

[Whirlwind]

Quote

Then I have filters - 2" Baader which are 2mm thick so I think that adds roughly 1mm more bringing it to 27mm.

That's the wrong way round.  1/3 of the filter thickness adds to the telescope's optical backfocus or subtracts from the camera's optical distance of the camera setup.  So in your example your camera's optical backfocus would be 19.5 + 6.5 - (1/3)*2 = 25.33mm.

You might want to check that the adapters are well seated and not protruding by a mm or so

[tomato]

I have just set up my new RASA 8, I noticed the adapter rings have a soft backing material which would compress to some degree upon tightening, maybe half a millimetre or  so?
 

I presume you have removed the optical window as recommended by Celestron if using filters?

[AbsolutelyN]

On 24/08/2020 at 09:14, Whirlwind said:

That's the wrong way round.  1/3 of the filter thickness adds to the telescope's optical backfocus or subtracts from the camera's optical distance of the camera setup.  So in your example your camera's optical backfocus would be 19.5 + 6.5 - (1/3)*2 = 25.33mm.

You might want to check that the adapters are well seated and not protruding by a mm or so

Brilliant thank you -  easy to get this wrong or confused 

[AbsolutelyN]

2 minutes ago, tomato said:

I have just set up my new RASA 8, I noticed the adapter rings have a soft backing material which would compress to some degree upon tightening, maybe half a millimetre or  so?
 

I presume you have removed the optical window as recommended by Celestron if using filters?

Yes filter removed. Image quality is acceptable, I just wish could get it exact. There are some interesting threads on Cloudy Nights about the adaptor - https://www.cloudynights.com/topic/681876-rasa-8-spacer-ring-3d-file-share/  

Look forward to seeing your images with the new RASA, it's an amazing scope.