RC8 - Elongated stars in the corners. Not sure where it comes from... What do you think?

[Doversole]

Hi All,

I got a question for you.

This my equipment: RC8 scope from StellaLyra, ASI071MC-cool camera, EQ6R Pro mount. This photo of M13 was actually the first light of my new (second hand) ASI071MC camera, better suited to the RC8 scope than my previous one, the ASI533MC (larger sensor, bigger pixels).

You may notice the stars are a bit elongated in the corners. I checked before taking this picture a defocused star in the middle and was quite happy with the shape of the donut.

I got a few assumptions on the issue here but would welcome your views:

- The RC is suppose to have no coma. Well that's the theory. Maybe given the large sensor (28mm diagonal) you still get some coma and I just need to accept it?

- Distance between the mirrors: in theory the FL should be 1625mm. Plate-solving indicates it is 1635mm. Should I do something about it?

- The nice donut star shape indicates (I think) that the secondary is well collimated. I never dared checking the Primary. Are the elongated stars a sign of bad Primary collimation?

Thanks and clear skies!

[newbie alert]

I haven't had any experience with RC's but I think your collimation is out a tad looking at the brighter stars

[vlaiv]

4 minutes ago, Doversole said:

- The RC is suppose to have no coma. Well that's the theory. Maybe given the large sensor (28mm diagonal) you still get some coma and I just need to accept it?

RC type scope will not have coma, but they will have astigmatism. Elliptical shaped stars are sign of astigmatism.

Looking at the image above, I would say that you need to do proper collimation of your scope. Round defocused star in center of the field is not enough for good collimation as there could be issue with the primary.

There also could be some sensor tilt as well, but leave that for later if you still have issues after collimation.

Here is a good guide and explanation on collimating RC8 - https://deepspaceplace.com/gso8rccollimate.php

I personally don't like Bahtinov mask for checking defocus in corners and rather rely on FWHM values instead (I also use FWHM for focusing rather than B mask).

8 minutes ago, Doversole said:

- Distance between the mirrors: in theory the FL should be 1625mm. Plate-solving indicates it is 1635mm. Should I do something about it?

Not much of a difference to worry about. Plate solving depends on pixel size as well, and pixel size is often rounded up.

For example - ASI071 is said to have 23.6mm width, 4.78um pixel size and 4944px in width. 23600 / 4944 = 4.77364...

So either pixel is not 4.78 or width is not exactly 23.6mm or both values are just a bit different than quoted and there might be row of pixels that is not used or something.

In any case 1635mm is close enough to design FL of 1625mm so don't worry about that.

13 minutes ago, Doversole said:

- The nice donut star shape indicates (I think) that the secondary is well collimated. I never dared checking the Primary. Are the elongated stars a sign of bad Primary collimation?

Yep, look at explanation on above link to understand how tilt of primary (and focal plane) can lead to different star shapes across the field.

ASI071 is APS-C sized sensor and that is approaching size of corrected field for RC8. You might never get pin point stars in far corners of the sensor unless you use flattener.

Corrected circle is quoted (by TS in Germany) to be 30mm - or rather as they put it: "Astrophotography with camera sensors up to 30 mm diameter possible without corrector". I'm not really convinced this means pin point stars up to that diameter, but rather acceptable star shapes up to that field size (whatever is acceptable to each of us ).

I think it would be best to do proper collimation and then see if your corner stars are good enough or should you crop or consider using flattener.

[FLO]

30 minutes ago, vlaiv said:

ASI071 is APS-C sized sensor and that is approaching size of corrected field for RC8. You might never get pin point stars in far corners of the sensor unless you use flattener.

I think it would be best to do proper collimation and then see if your corner stars are good enough or should you crop or consider using flattener.

Vlaiv is right. He often is 🙂 

Steve

[Adam J]

Its a big sensor for this type of scope, but i cant help but look at your image zoomed in and wonder if you are perfectly focused. It may be that you are best off focusing 1/3rd of the way out from the centre of the image. 

But then again I don't see split diffraction spikes so maybe not. Maybe just crop and live with it as I dont think anything is wrong with the scope as such. 

Adam 

Edited July 4, 2022 by Adam J

[Doversole]

Thanks, that was helpful.

I measured the size of the stars across the photo and stars are bigger on the top of the image (close to 6 pixels) compared to the centre and bottom of the image (closer to 4 pixels).

I will try your collimation method @vlaiv for the Primary.

Cheers!

 

[Clarkey]

FWIW I use the DSI collimation method with a REEGO collimator for the secondary mirror. RC collimation is known to be tricky and it took me a call while to get the best method for my RC8.

[newbie alert]

Looking at the image again , stars look elongated in the same direction on my phone..

[Paul M]

Notwithstanding that being a very nice image as is, I'd agree that the star elongation isn't symetrical across the 4 corners.

It'll be interesting to see the results of your collimation process. I'v got a growing itch to be the owner of a large RC and collimation is often noted as being a bit of a task on them. The only thing holding me off right now is that my EQ6 mounts likley isn't anywhere near adequate for what I have in mind! 😬

[vlaiv]

Here is example of collimation issue on my RC8 with 4/3 sensor (ASI1600):

Top left corner:

Top right:

Bottom left:

Center of the field:

After collimation (although for this one seeing was poor so stars are soft):

Corner:

Center:

When you get good collimation - stars tend to be round all over. Mind you - this is ~22mm diagonal, not ~28mm

[Doversole]

I used a PI tool to get a map of the star size. Confirms it's not fully collimated!

[davies07]

Hi,

Yes, I would agree. Your scope is out of collimation. The asymmetric halos around the stars are a dead give away. That is coma.

Unfortunately, RC scopes need care to collimate well and the RC8 is particularly tricky to collimate because of its mechanical design. In a normal RC scope, eg the RC10 truss,  the focuser is bolted to the back plate of the scope and forms a fixed positional reference for collimating the mirrors, but in the RC8 scope the focuser is attached to the back of the primary mirror. When you move the primary mirror you also move the focuser. So normal collimation methods might not work so well. You can end up in an endless loop of adjustments which can be time consuming and very frustrating. Imagine being out under a lovely sky, collimating your scope and anything you do seems to make matters worse. I've got the T-shirt for that one.

I got Es Reid's help to sort mine out. What we developed was a method to collimate the scope on the bench during daylight. Then do a final check on the stars. Es's method does not involve using the focuser at all - no lasers, no Cheshire eyepiece. Just a piece of card with a hole in it. This thread:

tells the story of how this method unfolded and why.

The end result was a successful technique which I've written up here:

https://www.dropbox.com/s/avpu2vn6s3ynsz5/Collimating GSO Ritchey Chretien with a plastic disc V2.pdf?dl=0

Alternately, I see that you are a couple of hours away from Cambridge and an option would be to contact Es and take your scope to him for collimation and tuning. This would have the additional benefit of getting Es to do a Ronchi test to check focal length and a knife edge test for the quality of the mirror. Es has recently subjected my new RC10 to such tests. Well worth it. You could PM me to discuss more.

David