To all the RASA8 + ASI2600MC-Pro users - I have a question regarding tilt and collimation

I have not managed to get perfect stars in the corners with an APS-C chip and your image is about as good as I managed to get them. They can be fixed in processing to an acceptable level (for me at least).

With regard to collimation I had to do it on one of my two RASA 8 while the other was fine from the factory. I did it by attaching a guide camera (ASI290MM in my case) so I could reach the screws (the RASA comes with a c-mount adapter for that type of camera). Worked fine. But I would only attempt to do it if defocused stars are not symmetrical (so don't fix something that  is ok).

It looks like you have a bit more vignetting than I have - I use 2" filters, maybe you use something smaller?

Cheers, Göran

Hello Sektor,

interestingly I face the same issue at the moment and already did some investigating on that issue - more or less systematically because the issues I experienced were of random nature.

In the past I operated my RASA8 with an ASI1600mm Pro and always had pinpoint stars in all corners with an average FWHM of 2.2x
Last Year I purchased the Starizona (RASA8) Filter Holder and the Artesky Filter Holder - both were back-ordered for a long time at the dealership and suddenly were shipped at the same time...

Anyways, since the weather didn't cooperate all winter, I was only able to test with may ASI071MC Pro (17,5mm back-focus).
The pictures looked terrible - very bad seeing, star elongation and vignetting all over. 

With the introduction of the new ASI2600MM Pro in March, I upgraded my equipment with this exact camera and additionally it's older color sibling, the ASI2600MC Pro.

Right from the start I noticed elongation with both cameras on the RASA but none of it with a 70mm Refractor (with full FOV)
So I ruled out camera tilt.

Here are some other lessons learned, since I started to work on the issue more systematically:


1. wiggle-room 
The APS-C sensor size more or less covers the whole FOV of the RASA8 with some minor vignetting to be expected due to the RASA specification page.
But since elongation sometimes is very bad in my pictures, this FOV-limitation might play a role, but certainly is not the only factor because it is not consistent.
The retaining ring that holds the camera/Filter Holder leaves some wiggle-room for the Filter holder to shift a few mm in all directions.
Unfortunately the ring does not force the Filter Holder to sit centered but allows for a slightly off-centered assembly.
This certainly affects the focus (in my opinion) - so I try to "manoeuvre" the FilterHolder centered when tightening the retaining ring.

I don't know if any off-center placement plays a factor here, but it certainly makes sense.
Celestron/Starizona/Artesky should provide a retaining ring that ensures a centered Filter Holder...


2. back-focus when using filters
The RASA8 has very tight  back-focus, 29mm from the lens group and 25mm from the original camera adapter.
Note that the protective window (clear and light pollution) with their 2mm thickness contribute around 1mm to the back-focus. (F divided by 3)
If for example you remove the protective window, this leaves 26mm back-focus from the camera adaptor.
Both, the Starizona and the Artesky Filterholder are designed to leave room for a camera with 17.5mm back-focus.
The Starizona additionally requires to remove the protective window, otherwise it would not fit - The Artesky brand does not come with such information.

So there already is room for error when for instance
- using the Starizona without filter and without protective window. The back-focus would be about 1mm off at 28mm/24mm.
- using the Artesky with filter and with protective Window. The back-focus would be about 1mm off at 30mm/26mm.

I'm using Baader f/2 Filter - based on the manufacturers specification page, their glass is 2mm thick, which should perfectly compensate for a removed protective window.
The Baader f/2 filter come with and additional .5mm plastic ring (compared to regular LRGB, which don't)-  I never paid much attention to this ring - but now since I am investigating this, I wonder if that .5mm ring has some deeper meaning for the RASA8. Maybe the filter glass is not exactly 2mm thick.

I have attached a picture I shot last night to show my vignetting on a single 180second sub from the ASI2600MM.
Now that I look at this example, I also notice some elongation in the middle but still worse star shapes in each corner.

sadr_elongated.jpg

(interestingly) I also noticed that "Sadr" is not engulfed in  this "Rasa"-halo. That' another issue I kept working on for months now. I wonder If my back-focus experiments have something to do with it. <-- one hot tip here: If using the shiny ASI camera in (bight) light-conditions, it is possible that the red aluminum reflects some light to bounce around, even when using  a large dew shield. I used black velour to cover the reflective camera sides. This did some good but never eliminated the bad "Rasa"-Halo.

3. additional Equipment attached to the tube
I'm running my Rasa with two additionally attached scopes mounted to the rear cell, using the mount holes.
These scopes are tiny and light weight
- the Celestron ASPA 
- 50mm guide scope
The rear cell is very stiff, so I did never think about bending issues here - and still don't.
But a couple days ago I experimented with a setup to use both ASI2600 at the same time and target.
To do so I attached a 70mm refractor to the Rasa, using an additional Losmandy dove plate on top of the tube.
The refractor, it's heavy 4-lens setup, with focus motor, the camera and some adaptors (filter wheel, rotator) runs at about 5 kg.

Yesterday i ran this setup on NGC 7000. The mount slews the Rasa into a 90degree angle which leaves the refractor "hanging" aside the tube - pulling with it's whole weight. Again I did not pay attention that this might cause problems, since alle equipment always seems very solid.

But in this case, the pictures where "off" on one whole side - the stars on the right side were showing a donut hole already.
When I brought the right side back into focus, the left side was off.
I initially tweaked the setup according to 1. and 2. and lost most of this very clear and precious night over it - that' so rare at my place, just thinking about it... I have to wipe away a tear.
Only after I disassembled the whole setup and removed the refractor on top of the tube, the next sub I took looked much better.
That means, the weight of the additional accessory on top of the Rasa affected the light path while bending the tube (I guess).

Maybe you have attached a zoo of stuff to your tube and it might play a role.

----

OK, I hope this makes sense to you and helps to identify the issue.
I'd appreciate any input into this issue myself, but I guess in the end it's the FOV of the telescope and it's limits to run an APS-C camera sensor 

Oh, two additional quick thoughts about the vignetting/elongation.
- If dew shield is not attached properly and being slightly off or oval shaped, this might affect the light path
- currently at my place, the summer nebulas are visible from around 2am just atop some trees. Similar to a misplaced dew shield, the trees - although not visibly affecting the picture - might obscure parts of the view - something I'll look into as soon as I get the chance...


Take care and clear skies,
Stephan

Tilt can be measured using ASTAP CCD inspector.

Interesting. I have the feeling I see some shifts in how corner star look from night to night. I have sturdy dovetail bars on the top of both my RASA8s, so that should help stabilizing the scopes. I have no astrodarkness up here until late August but then I plan to have a double RASA 8 rig up and running, with a SW72ED guide scope on top of one and a Samyang 135 (or possibly a Canon 200) piggybacking on the other. I may be in for trouble, but who is not in this hobby……

I think you are right about the problem with sideways camera movement. I have two RASA8 now. Celestron has changed the retainer ring from the first to the second one so they apparently reacted to the issue. In the new one it is tapered so it centers the camera when tightened. With the first one I tried to fix the problem with sideways camera movement by putting in a circular piece of 1 mm steel wire behind the threads so the camera could not move sideways. Seems to work.

I do not have an image of the new tapered version and I rather not remove it from the scope right now. But in essence Celestron has filled out the space behind the threads in the retaining ring. I do not think it needs to be tapered.

You find an image of the wire-ring on this thread:

https://stargazerslounge.com/topic/368295-closeup-of-the-tadpoles-from-a-rasa-8-wide-field/?tab=comments#comment-4003131

It is a wire I bought in a garden store and has a thin green plastic cover. That made it a bit too thick so I removed the cover leaving a steel wire with 1.0 mm diameter. That worked better. It has to be perfectly curved and made in a circle that makes it press outwards so no part of the wire comes between the front plate of the scope and the adapter (which will cause tilt). When properly positioned it will fill up that space behind the treads that allows the adapter to slide sideways.

As you see I use an Artesky RASA8 adapter so I can screw in a 2" filter but it should also work with the original adapter.

Cheers, Göran