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Celestron 8" RASA and ZWO ASI 1600MM PRO

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#1 Derry 571

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Posted 16 September 2020 - 03:13 PM

I have a new Celestron 8" RASA and a new ZWO ASI 1600MM PRO camera. People are getting some stunning images using this combination.

 

My problem for now is that I'm unable to get things to focus. The telescope instruction manual for the Celestron RASA says the telescope requires a 25 mm backfocus. I don't currently have the right combination of extension rings for 25 mm. However, because of my enthusiasm I decided to try a few combinations of the extension rings I have on hand thinking there might be a slight "fudge factor". NOPE! They mean 25 mm.

 

Can someone recommend a combination of extension rings with part numbers and a source?

 

I also want to be able to use 2" narrow band filters so I'll be needing a 2" filter drawer. Since everything I need seems to be on back order I'd be willing to buy a set of extension rings for now so at least I can get up and working and place an order for a filter holder and filter and wait for the back order.

 

Any suggestions would be appreciated.

 

I'd like to be able to start getting some astrophotography images in this lifetime.    



#2 audioengr

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Posted 16 September 2020 - 04:22 PM

25mm is really short, just over an inch. Is your camera screwed directly into the front cell adapter? 

 

The filter drawer may have to be the only thing between the camera a scope.  Celestron says there is a clear filter there already that should be removed when using other filters.

 

From Celestron:

 

"Observing programs that require a filter usually require

more than one filter, so the filters are mounted in a filter
rotating wheel. A small motor turns the wheel so that
different filters can be inserted into the light path without
manual intervention. All too often though, the filter wheel
is considerably larger than the camera body itself, and
when placed at the focus of a RASA, the filter wheel
blocks a considerable amount of light. There is currently
no easy solution to the filter-wheel dilemma, although
custom solutions do exist. For example, filters can be
mounted in a sliding drawer and manually inserted into
the light path."

 

Obviously it can be done.  They note that an optimum camera had around 15mm back focus. 

 

They also say that the RASA creates a 43.3mm circle of light, so a sensor diagonal of this dimension is optimum.

 

Also:

 

"For the RASA 8, this location is 29 mm from the top surface of the lens group cell. A camera’s

back focus, also known as its flange focal distance, is the distance between the camera sensor
and where the camera body mechanically connects to a camera lens or telescope. Therefore,
cameras with sensors deeply recessed within their body having more than 29 mm of back focus
cannot be used with the RASA 8 as the sensor cannot be placed at the proper location."

 

Back focus with included camera adapter 25mm (.98")
Back focus from top of threaded collar 29mm (1.14")

 

The ASI1600 has 6.5mm of back focus, so you theoretically have 22.5mm for the filter drawer and adapters or a M42 to M42 extension tube.  ZWO filter drawer is 21mm.

 

Before you get the filter drawer this or this might do the trick if it's M42 threads. The ZWO extension is in stock.  I think this will thread onto the collar and the camera onto it.

 

If you want to be sure to nail-it, I would get this one, also in stock.


Edited by audioengr, 16 September 2020 - 04:44 PM.

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#3 Derry 571

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Posted 16 September 2020 - 05:49 PM

Thank you audioenge for your reply. The flange on the RASA has male M42 threads and the flange on the ASI 1600 has male M42 threads. Looking at the image of the ZWO M42 to M42 extension it appears to have male threads on one end and female threads on the other. Does the term "T ring" imply female threads on both ends? Maybe the image is misleading? I've placed the order for the ZWO extension. Guess I'll find out when it gets here.



#4 audioengr

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Posted 16 September 2020 - 06:58 PM

Thank you audioenge for your reply. The flange on the RASA has male M42 threads and the flange on the ASI 1600 has male M42 threads. Looking at the image of the ZWO M42 to M42 extension it appears to have male threads on one end and female threads on the other. Does the term "T ring" imply female threads on both ends? Maybe the image is misleading? I've placed the order for the ZWO extension. Guess I'll find out when it gets here.

That's wierd that the camera has male threads.  I think most have female threads.

 

It's a bit expensive, like $125, but you can get exactly the right length and threads doing this.  There will be no more accurate adapter than this.  Make sure you get the length dimension nailed.



#5 dswtan

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Posted 16 September 2020 - 09:31 PM

This may be the blind leading the blind, since I still don't have round stars LOL, but respectfully, the above seems to be over-complicating things compared to my RASA-ZWO set-up copied from folks much more expert. 

 

For me, it's just (as adapted from first thread below): 

 

RASA8 >

Celestron LP filter > 
Celestron OEM M42 aka T2 adapter > (comes with the RASA)
7.5mm M42 aka T2 adapter > (Baader or Astromania)

ZWO cooled camera with 17.5mm to sensor (use the 11mm female-female M42 aka T2 that came with the camera on the 1600 for example)

 

The Baader part is officially "Baader T-2 / 7.5 mm Extension Tube (T-2 part #25C)". Astromania one is on AMZ. 

 

I'm currently messing around with the Baader delrin spacer rings to try to improve things. It's not going great, in all honesty. YMMV.

 

Original threads:
https://www.cloudyni.../#entry10039593

https://www.cloudyni...pacing-mystery/
 

Keep this handy too:
https://astronomy-im...tions-55mm.html



#6 Derry 571

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Posted 16 September 2020 - 10:40 PM

Thank you Mariner 2. I think that T2/7.5MM extension tube is exactly what I've been looking for.

 

The male and female threads will be in the right order and the right size and it should bring the spacing right up to 25 mm.

 

I'm still new to this and sometimes I seem to be having a hard time wrapping my mind around some of this hardware. I need one of these things to screw into one of those things and they need to be the right size but if they don't fit do they make an adapter and if so what are these things called and where the heck can I get one? 

 

I just ordered the Baader T-2/7.5 mm from AMZ. Should be here next week. Can hardly wait to give it a try.


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