RASA 8 and attaching a asi 294mc

[h0ughy]

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.

[h0ughy]

And the diagrams plus what's in the box when I got the camera

[billdan]

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.

[Stefan Buda]

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.

[h0ughy]

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?

[h0ughy]

There is a spacer in the package but I am still confused with the setup with the distance? Something isn't right

[LewisM]

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.

[h0ughy]

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

[h0ughy]

https://www.bintel.com.au/product/bintel-t-thread-spacer-ring-set-2/?v=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

[JA]

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)


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

[Stefan Buda]

JA explained it well, so I don't need to.

[h0ughy]

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

[h0ughy]

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

[The_bluester]

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.

[lollywater]

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

[h0ughy]

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

[The_bluester]

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?

[lollywater]

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.

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

[h0ughy]

i have the tilt adjuster already but its not the issue for now or part ofthe problem i have

[lollywater]

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

[lollywater]

David, back to your original question.
I think you will have trouble with the variable extender on the 294. The extender has female one end and male the other. You need female both ends to attach the 294 and the 183 to the Rasa. The male of the extender attaches to the female of the 290 leaving the female extender to attach to the Rasa. I dont have one, but I think you may need a 5mm female to female as well as the 12mm variable extender for the 294. The Starizona is female/female

Re the image,I have assumed the Starizona is correct spacing and altered the extender accordingly for the 290.
cheers
Paul


sorry, image is upside down

[h0ughy]

Have a 48 to 42mm adapter

[lollywater]

I have a few 42-48 adapters. The dont fit on my Rasa

[h0ughy]

The one from zwo does

[lollywater]

Hmm. I have the one from ZWO as well . I cant get it to fit anywhere on the Rasa but I will take your word for it. The 11mm female/female 42 that comes with the 294 will attach to my Rasa .

On my Rasa , a variable extender needs a double female 42 spacer as well to attach the 294 to it. The 11mm that comes with the 294 plus a variable extender will be too long . I need the 11mm double female, so to get to 26mm I need a number of smaller 42 spacers. I dont have any smaller than 5mm. The Starizona on its own sorts that out for me.

The variable extender attaches my 290, 224 and 178 to the Rasa to a total of 26mm
cheers
Paul

[astrod]

For the ngc 6744 image it doesn't quite look like tilt but slightly worse tracking in one axis. I have a RASA8 where I had tilt and the effect is to have elongated stars in one corner (or side).
I introduced a ZWO tilt adapter 11mm + 10mm spacer + 6.5mm sensor distance and could focus well. I also added a T2 shim which moved the focus about 10% of the focus range.
Now I introduced the ZWO tilt adapter to fix tilt and I found that because it sits on the outside part of the locking collar it fixed the problem (without using its adjustment features) because it sits perfectly square whereas normal (narrow) T2 extensions tend not to sit sufficiently accurately.
I can give more details because I've glossed over things.

[lollywater]

Thanks Rod.
Is that poor tracking in one axis because of poor polar alignment? How should I fix that?
My Rasa used to have significant tilt but a trip back to Celestron improved it heaps.

When I added a 10mm spacer to the 11mm + 6.5 mm camera =27.5, I had no trouble with focus either. The Starizona plus camera is 26mm so I have been keeping all the cameras at 26mm. Frankly, I dont notice any difference.
cheers
Paul

[The_bluester]

Can you try a long single exposure? That would accentuate any tracking issues, at that focal length you should be able to get a minute or more unguided without major issues but if you have a polar alignment issue it should show up reasonably clearly by then, and any mechanical issues would be a lot more apparent.

If you used the ASI294 I would suggest a minute, at 120 gain and -15 degrees (It is a 294MC Pro isn't it? I know you have mentioned it elsewhere before but cant think in which thread) The area covering NGC253 and NGC288 should fit on the sensor of the 294 through the RASA.

[lollywater]

Thanks Paul
I will try that. None of my ZWO's are cooled.
cheers
Paul

[The_bluester]

Ah, I had it in my mind you had a cooled 294.

You should still be able to do a minute or so exposure without too many dramas with hot pixels spoiling the show. I assume that the gain levels are the same as the cooled ones. If not, unity gain is 120 on the 294MC Pro and for a longer exposure I would use the equivalent if it is different on the non cooled version. I use it in the long exposures as for me it provides the best compromise between quick capture and linearity of response.

[lollywater]

Paul
I have a RisingCam 1600 that is cooled but I very rarely use it because I prefer Sharpcap to RisingSky software.
I will try both cameras at 1 minute and see how I go.
Might see you next Saturday night at Snake Valley.
cheers
Paul

[astrod]

Hi Paul

(I'm having trouble getting the hang of how to use this messaging system.)

Because stars are (slightly) elongated in one direction and generally across the whole image then I guess the RA tracking is not as good as Dec. I do get this for some parts of the sky and certain seeing conditions. Poor PA you might see the Dec corrections being more busy but guiding should be ok. It might be slight backlash or tracking near the meridian which expose balancing imperfections.

Glad your significant tilt problem was fixed. What I found were subtle tilt problems when using certain configurations of adapters. When you see these problems you initially don't know whether it is the mirror; the "secondary" collimation; adapters holding your camera or if the sensor in the camera is slightly off-square.

I have removed the clear window because I found a configuration where I use 1.25" filters sitting in the middle of the tilt adapter. I was nervous about forking out money for the Starizona system plus want to buy 2" filters; not knowing if narrowband would work, etc. The short answer is the 1.25" filters work well at F2.

Not having the clear window and adding 1.25" filters downstream changes the distances a bit. I think I inferred (measured indirectly) for full focus travel (30 revolution of the focuser knob) the focal plane moves about 3mm so it can pick up a fair bit of slop (so numbers like 26 or 27.5 work). Having said that, I guessed that the best (design) focus would be somewhere where the focuser is say 30% where 0% in fully in and 100% is fully out. This means focus is achieved when starting from fully-in with 10 revolutions of the focus knob. Or equivalent 1mm of focal plane positioning. I'm seeing very sharp stars across my sensor when I do this (when I have it focussed and the mount is tracking well).

Rod





Thanks Rod.
Is that poor tracking in one axis because of poor polar alignment? How should I fix that?
My Rasa used to have significant tilt but a trip back to Celestron improved it heaps.

When I added a 10mm spacer to the 11mm + 6.5 mm camera =27.5, I had no trouble with focus either. The Starizona plus camera is 26mm so I have been keeping all the cameras at 26mm. Frankly, I dont notice any difference.
cheers
Paul

[lollywater]

Here is ngc 6744 ,58s x 10, zwo 294 uncooled, slight stretch on astrotoaster. I had to increase the min star size on sharpcap because of hot pixels.
The tracking seems ok to me and no donuts.

[The_bluester]

The stars in the top corners (Particularly) suggest that the spacing is not quite right. If it were mine I would say the camera needed to move a small amount further away from the scope, but the RASA seems to reverse that. And perhaps a tiny amont of sensor tilt, both of which I reckon the F2 will make it fiendishly touchy about, we could be talking about a fraction of a mm here.



The stars in the corners appear like little arcs is what I am referring to.

[lollywater]

Thanks Paul
A tilt adjuster has just arrived from Bintel. It is 11mm,so if I add a 10 mm spacer (as Rod suggested) to the 294 that will make 27.5. I will give that a go next clear night.
I am not sure what the problem was with the 283. That was the first time I used it on the Rasa. It wouldnt screw all the way on the Starizona filter holder , so it wasnt 26mm. But I cant see why that would cause a tracking problem
cheers
Paul