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Old 19-10-2019, 09:07 PM
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h0ughy (David)
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RASA 8 and attaching a asi 294mc

Can't quite get my head around this backfocus distance for the camera the manual says that the adapter is 17.5mm and the camera sensor is 6.5+4 =28
So do I need a spacer to get 29 or doesn't it need it?

From the celestron manual

Backfocus– For all flat-field astrographs, including the RASA 8, it is important that the camera’s sensor is positioned at the proper distance from where it mounts to the telescope. Otherwise, performance at the edge of the sensor, especially larger sensors, may suffer. For the RASA 8, this location is 29 mm from the top surface of the lens group cell. A camera’s backfocus, 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 backfocus cannot be used with the RASA 8 as the sensor cannot be placed at the proper location. DSLR cameras cannot be used with the RASA 8 due to their large obstruction and long backfocus. Installing a Camera If using a camera that mounts with M42 threads, the supplied M42 camera adapter can be used. This will likely require utilizing optional M42 extension rings to place the camera sensor at the proper location. When using the M42 camera adapter, the required backfocus distance is 25 mm. So, for example, if the backfocus of a camera is 17 mm, then 8 mm of M42 extension ring(s) between the camera and M42 camera adapter will be needed. For best results with larger sensors, get the backfocus within 1 mm of the required distance. The supplied C-thread camera adapter can be used to attach any camera which mounts with a standard C-thread. If the camera’s sensor spacing meets the C-thread standard (i.e. 17.5 mm backfocus), no additional adapters are required.
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Last edited by h0ughy; 19-10-2019 at 09:41 PM.
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  #2  
Old 19-10-2019, 09:45 PM
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And the diagrams plus what's in the box when I got the camera
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Last edited by h0ughy; 20-10-2019 at 07:23 AM.
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Old 20-10-2019, 12:52 AM
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I think you will be OK at 28mm (1mm short), as the sensor on the ASI294 is 23mm diag so there should not be any aberrations as you would get with a full frame sensor.

If not it would be easy to make 1mm washer cut from thin cardboard with scissors or a scalpel.
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Old 20-10-2019, 07:14 AM
Stefan Buda
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The way I read it, you need a 7.5mm M42 extender ring.
It is confusing though because the pdf drawing shows a custom camera adapter that is different to what your pictures show.
If your custom camera adapter was like the one recommended on the pdf link, then you probably would not need an extender.
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Old 20-10-2019, 07:30 AM
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Quote:
Originally Posted by Stefan Buda View Post
The way I read it, you need a 7.5mm M42 extender ring.
It is confusing though because the pdf drawing shows a custom camera adapter that is different to what your pictures show.
If your custom camera adapter was like the one recommended on the pdf link, then you probably would not need an extender.
That's the reason I am asking as it did confuse me. I also added the stuff I got in the box with the camera but I am still confused with the spacer from celestron. I can't really confirm for myself what it should be. So you get 7.5mm? Carefully can you explain to a simpleton how you got that?
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Old 20-10-2019, 07:31 AM
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Quote:
Originally Posted by billdan View Post
I think you will be OK at 28mm (1mm short), as the sensor on the ASI294 is 23mm diag so there should not be any aberrations as you would get with a full frame sensor.

If not it would be easy to make 1mm washer cut from thin cardboard with scissors or a scalpel.
There is a spacer in the package but I am still confused with the setup with the distance? Something isn't right
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Old 20-10-2019, 07:47 AM
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It appears that the T2 ring screws down flush OVER the 4mm "nose", so essentially negating it. Therefore, the total camera distance to sensor is 6.5 + 11 = 17.5.

Therefore to get to the 29 you need an 11.5mm extension insert.
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Old 20-10-2019, 08:03 AM
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Originally Posted by LewisM View Post
It appears that the T2 ring screws down flush OVER the 4mm "nose", so essentially negating it. Therefore, the total camera distance to sensor is 6.5 + 11 = 17.5.

Therefore to get to the 29 you need an 11.5mm extension insert.
Lightbulb moment..... Oh yeah that makes sense. So I am stuffed anyway at the moment as I will have to get one.

My headcold is impeding my brain
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Old 20-10-2019, 08:20 AM
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https://www.bintel.com.au/product/bi...=6cc98ba2045fi hope this may work although I may have to look for a specific 11.5mm spacer. Would like to know how the variable adapter fits whether I could get the distance from that as it looks bigger than the 10mm
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Old 20-10-2019, 09:06 AM
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Quote:
Originally Posted by h0ughy View Post
Can't quite get my head around this backfocus distance for the camera the manual says that the adapter is 17.5mm and the camera sensor is 6.5+4 =28
So do I need a spacer to get 29 or doesn't it need it?
Yes you will need a spacer or new adapter to get the sensor in to the correct location. The correct location is defined variously as :

= 29mm (28.73mm?) from the TILT COLLAR SURFACE (shown in RED on the exploded view in the detailed drawing you provided) OR

= 25mm from the front butting surface of the M42 camera adapter mounted on the scope (according to the EXCERPT form the Celestron manual you provided below)
Quote:
Originally Posted by h0ughy View Post
From the celestron manual

Backfocus– For all flat-field astrographs, including the RASA 8, it is important that the camera’s sensor is positioned at the proper distance from where it mounts to the telescope. Otherwise, performance at the edge of the sensor, especially larger sensors, may suffer. For the RASA 8, this location is 29 mm from the top surface of the lens group cell. A camera’s backfocus, 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 backfocus cannot be used with the RASA 8 as the sensor cannot be placed at the proper location. DSLR cameras cannot be used with the RASA 8 due to their large obstruction and long backfocus. Installing a Camera If using a camera that mounts with M42 threads, the supplied M42 camera adapter can be used. This will likely require utilizing optional M42 extension rings to place the camera sensor at the proper location. When using the M42 camera adapter, the required backfocus distance is 25 mm. So, for example, if the backfocus of a camera is 17 mm, then 8 mm of M42 extension ring(s) between the camera and M42 camera adapter will be needed. For best results with larger sensors, get the backfocus within 1 mm of the required distance. The supplied C-thread camera adapter can be used to attach any camera which mounts with a standard C-thread. If the camera’s sensor spacing meets the C-thread standard (i.e. 17.5 mm backfocus), no additional adapters are required.
All that being the case, and IF the adapter shown in your picture of your RASA 8 is indeed the Celestron M42 Camera adapter, then you will need to put the sensor 25mm away from its front abutting surface. Since the sensor is recessed 6.5mm in to the body of your camera, then the distance between the camera's front abuttment surface (the red aluminium surface at the base of the front male M42 thread) and the Celestron M42 camera adapter mounting surface requires a spacer/adapter of total length 25-6.5mm = 18.5mm. You can do this in one of two ways since you already have an 11mm M42 spacer ring (black T2 ring) that came with the camera:

1. Use an additional 7.5mm extender spacer ring with the 11mm spacer ring you already have to get a total length of 18.5mm OR ....

2. Use an 18.5mm (probably custom made) one piece spacer ring on its own and
NOT use the 11mm spacer ring supplied with the camera.


Best
JA

Last edited by JA; 20-10-2019 at 09:41 AM.
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  #11  
Old 20-10-2019, 09:33 AM
Stefan Buda
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Quote:
Originally Posted by h0ughy View Post
That's the reason I am asking as it did confuse me. I also added the stuff I got in the box with the camera but I am still confused with the spacer from celestron. I can't really confirm for myself what it should be. So you get 7.5mm? Carefully can you explain to a simpleton how you got that?
JA explained it well, so I don't need to.
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Old 20-10-2019, 11:47 AM
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Thanks for the reply guys. getting my head around this wasn't easy

i took the adapter off the front to look at it and it looks flush to the screw ring which roughly measures 7.5mm from the lenses

14 using the adapter and the 11mm zwo adapter
6.5 back focal to ccd
20.5 sub total
28.73 desired total
8.23bit left to find - maybe a 3mm and a 5mm or a custom 8.23mm
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Last edited by h0ughy; 20-10-2019 at 12:55 PM.
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Old 20-10-2019, 01:06 PM
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if someone can fill me in on how the variable spacer from bintel works from their practical experience I would be grateful.

i could just go the 7.5mmcelestron adapter and the camera 6.5focal plane plus the rest to make the 28.73mm which would be 14.73mm
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Old 20-10-2019, 03:30 PM
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The variable spacer is just a spacer with a normal female M42 thread and a long male thread with a lock ring. It might get you the desired spacing to test but I doubt it will be reliably square enough to use long term with how tiny the focus zone is at F2.

What I would do would be to use that to work out your correct spacing (While accepting that tilt issues are likely) then carefully measure the total spacer distance between the RASA M42 adapter face and camera face and get on to the Likes of Josh Bunn on here to get a through threaded M42 adapter made to the exact length (Or maybe 1mm or so less) that you need. Slightly less to allow you to use some delrin washers to pack it out perfectly and make it easier to remove things later.


I am interested to see how you go given I have an ASI294 and have looked at the RASA 8 since it was released.
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Old 20-10-2019, 08:26 PM
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I use the Starizona filter slider with the 294. It is 19.5mm plus the 6.5 of the 294 with the 11mm spacer removed . Starizona says that is exactly the back focus required and it works well.
That extra 4mm on the 294 is thread and gets taken up as part of the filter slider dimensions. So if Celestron recommends 25, Starizona combination is 26

For the 224, I use the 12.5 - 17mm variable spacer at 12.5 which takes it to 25mm with the 12.5mm of the 224 camera
There is a lot of confusion out there .25mm, 26mm,27.5mm ,29mm ??. 25 and 26 work so I stick to them.
cheers
Paul

Last edited by lollywater; 21-10-2019 at 12:06 AM.
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Old 21-10-2019, 09:52 AM
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i have ordered a extension set from bintel so hope to get it tomorrow.

whats a few more days.....

i will probably get a custom one in the future
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Old 21-10-2019, 10:40 AM
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Depending on your useage I reckon a custom one is the go. Variable spacer to work out the exact length and a custom one to suit later.

Paul’s use in EAA where he wants detail in the main area and wants it quickly would potentially be slightly more forgiving of any tilt issues than what I would do with a RASA where I would be pushing exposure times as long as feasible and wanting nice stars right to the corners for a heap of subs to integrate. Not sure what your plans are?
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Old 21-10-2019, 12:24 PM
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Paulís use in EAA where he wants detail in the main area and wants it quickly would potentially be slightly more forgiving of any tilt issues than what I would do with a RASA where I would be pushing exposure times as long as feasible and wanting nice stars right to the corners for a heap of subs to integrate.

My requirements are quite different than astrophotographers ,so a lot of my Rasa 8 advice may not be appropriate. I have never taken much notice of what you guys do. I have come to appreciate how much time and effort you put into getting your images perfect. You have to be a perfectionist to be an astrophotographer. I dont have what it takes . I am happy with little donuts in the corner.

I notice ZWO have introduced a tilt adjuster with T2 connections. I have ordered one from Bintel. I dont know how effective it is and what complications in spacing that it may produce. I wont get too carried away with the tilt adjusting but, if it is easy to use then it may be worthwhile
cheers
Paul
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Old 21-10-2019, 02:40 PM
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Quote:
Originally Posted by lollywater View Post
Paulís use in EAA where he wants detail in the main area and wants it quickly would potentially be slightly more forgiving of any tilt issues than what I would do with a RASA where I would be pushing exposure times as long as feasible and wanting nice stars right to the corners for a heap of subs to integrate.

My requirements are quite different than astrophotographers ,so a lot of my Rasa 8 advice may not be appropriate. I have never taken much notice of what you guys do. I have come to appreciate how much time and effort you put into getting your images perfect. You have to be a perfectionist to be an astrophotographer. I dont have what it takes . I am happy with little donuts in the corner.

I notice ZWO have introduced a tilt adjuster with T2 connections. I have ordered one from Bintel. I dont know how effective it is and what complications in spacing that it may produce. I wont get too carried away with the tilt adjusting but, if it is easy to use then it may be worthwhile
cheers
Paul
i have the tilt adjuster already but its not the issue for now or part ofthe problem i have
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Old 22-10-2019, 09:08 AM
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On the mirror tilt issue with the Rasa, I only saved 1 image last night. If you ignore my ngc 6744 in Pavo which I know you guys would ask what the hell is this,and just look at the corners, you can judge for yourself the extent of the tilt.
The only reason I saved this image is because I couldnt visualize it very well on screen and saved it so that I could give it a slight stretch on Astrotoaster.

It was the first time I used my zwo 183 on the Rasa. It was 11s x 65 which is a hell of a long exposure time for me. I prefer the 294 for the wider field and the 290 mono for the small dim galaxies on the Rasa.
cheers
Paul
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