Skywatcher f4 coma corrector issue

[Craig a]

Ok I’ve got a problem with the f4 skywatcher coma corrector particularly it’s length, when the cam is in focus the corrector protrudes into the tube by about 15mm with the ir/uv filter screwed into the end of it and it’s causing me big problems in my images, I’m getting some horrendous rings in my images, so my flats just dont work, it isn’t protruding far enough in to take chunks out the stars but I’m positive it’s giving me the rings problem, I’m sure the coma corrector is just too long, I have a moonlite focuser fitted to my Quattro 10cf, now is there anything I can do to combat this or is there a different coma corrector designed for f4 that isn’t 100mm long like the sw is? At the moment imaging just isn’t worth it with this issue, I deleted the last image session in a rage so I carnt show the rings I’m getting so I hope you can understand what I’m saying 

[vlaiv]

I'm fairly positive that rings won't be produced by CC protruding into light path.

Rings can be reflection issues if they appear around bright stars (those are in fact same stars - just out of focus). They can also be due to dust particles somewhere in the optical train.

What sort of rings are we talking about here?

[Craig a]

These rings arnt around stars or dust bunny’s these are big rings that emanate out from the center of the image gradually getting less further apart towards the outer part of the image, the scope is flocked and I’ve painted all screws and bolts with Matt black 

[Craig a]

I forgot to add I’m using a led Tracing pad to take my flats, with a t shirt over the aperture 

[vlaiv]

13 minutes ago, Craig a said:

These rings arnt around stars or dust bunny’s these are big rings that emanate out from the center of the image gradually getting less further apart towards the outer part of the image, the scope is flocked and I’ve painted all screws and bolts with Matt black 

So, it's vignetting like this?

could you post your master flat and result of calibration?

Sometimes there is residual vignetting in the image if you don't do proper calibration or calibration fails for some reason (like mismatched temperature or light leak).

Again - not related to the fact that CC protrudes into light path - but it does have to do with model of coma corrector. Some coma correctors are optimized for certain sensor size and if you use them on larger sensors - you get vignetting and poor edge correction.

What camera do you use and what is the model of coma corrector (is it Aplanatic F4 model? it says that it will cover APS-C but does not say much about vignetting)

[Craig a]

I’m using asi533 mc pro, it’s not vignette, it’s very similar to this.........

https://www.cloudynights.com/topic/557931-concentric-coloured-rings-caused-by-flats/

[Craig a]

But it was in my stacked image which I’ve now binned in a rage 

[vlaiv]

9 minutes ago, Craig a said:

I’m using asi533 mc pro, it’s not vignette, it’s very similar to this.........

https://www.cloudynights.com/topic/557931-concentric-coloured-rings-caused-by-flats/

Yep, small sensor will exclude all problems with vignetting.

Maybe some sort of Newtonian rings? That happens when two optical elements are almost touching. One is highly curved and other flat for example.

Only place in your optical train that I can see this happening is use of filter at the end of coma corrector.

Coma corrector probably has highly curved "field lens" (lens facing the telescope) and it is positioned very close to filter thread?

This is only image that I found online that shows CC from that angle - and I can't really tell due to good AR coatings:

Maybe try to move filter between CC and camera instead of it being on the far side of CC.

[Craig a]

Ok yes the front lens of the cc is highly curved and obviously the filter is flat, so Newton rings are possible, it’s strange because I’ve used this scope for years and never had an issue like this so was wondering  if the led pad maybe was causing the issue because that’s a recent thing I’ve started using 

[Roy Foreman]

I have the same setup - SW 10CF plus F/4 coma corrector. and yes it does protrude into the optical path, but the only effect this has on mine is that the star images have more diffraction spikes on one side. That is - four main spikes plus a few smaller ones on one side. With a full frame camera in produces significant vignetting - even without a filter attached.

The Baader MPCC3 is far smaller - less than 2 inches long and is rated down to F/3.5 but star images are not as pinpoint to the edges like with the SW one.

Can't really be much more help than that I'm afraid

[Craig a]

4 minutes ago, Roy Foreman said:

I have the same setup - SW 10CF plus F/4 coma corrector. and yes it does protrude into the optical path, but the only effect this has on mine is that the star images have more diffraction spikes on one side. That is - four main spikes plus a few smaller ones on one side. With a full frame camera in produces significant vignetting - even without a filter attached.

The Baader MPCC3 is far smaller - less than 2 inches long and is rated down to F/3.5 but star images are not as pinpoint to the edges like with the SW one.

Can't really be much more help than that I'm afraid

I find it incredible that skywatcher produce a coma corrector designed for this scope that protrudes Into the tube, but that may not be the issue at play, I was thinking that the led panel is too bright even with a t shirt on the tube and bouncing light off the IR/UV filter as my exposure for my flats was 0.3 of a second. 

[vlaiv]

7 minutes ago, Craig a said:

I find it incredible that skywatcher produce a coma corrector designed for this scope that protrudes Into the tube, but that may not be the issue at play, I was thinking that the led panel is too bright even with a t shirt on the tube and bouncing light off the IR/UV filter as my exposure for my flats was 0.3 of a second. 

I've read reports that aplanatic CC fits stock focuser without issues.

Maybe your moonlite focuser is lower profile than stock focuser?

[Craig a]

2 minutes ago, vlaiv said:

I've read reports that aplanatic CC fits stock focuser without issues.

Maybe your moonlite focuser is lower profile than stock focuser?

Would that matter? as the focus point would still be the same position? If using the stock focuser it would just be racked in further 

[vlaiv]

2 minutes ago, Craig a said:

Would that matter? as the focus point would still be the same position? If using the stock focuser it would just be racked in further 

I think you are right - it should not matter which focuser you use.

What can matter is distance to camera. Do you have proper spacing there? You might be slightly off and not notice it since you are using ASI533 - it is smaller sensor.

Another way to deal with that is to raise primary mirror up the tube. In fact - collimation can impact focus position as it moves mirror up down.

[alacant]

2 hours ago, Craig a said:

ir/uv filter screwed into the end of it

Hi

We also use the GPU cc on a 208mm f3.9 with and without front threaded filter. No rings, but a few thoughts...

My bet would be on the filter causing the rings. You should have enough space in your allocated 55mm to put the filter at the camera side. Easily tested by taking a flat frame without the filter.

IIRC, the light panel reduces its brightness by varying the refresh rate. Anything less that 1s exposure on our panel and we get bands appearing on the flat frames. Maybe try that too? A sheet of white card works well.

One last thought. as @vlaiv surmises, with the 533, you may get lucky and not need the cc.

Cheers

Edited February 17, 2021 by alacant

[Craig a]

4 hours ago, vlaiv said:

I think you are right - it should not matter which focuser you use.

What can matter is distance to camera. Do you have proper spacing there? You might be slightly off and not notice it since you are using ASI533 - it is smaller sensor.

Another way to deal with that is to raise primary mirror up the tube. In fact - collimation can impact focus position as it moves mirror up down.

I’m pretty sure I have correct spacing as I’m using the zwo OAG and that replaces the one of the spacers that comes with the 533 and the spacer brings me to 55mm back focus so with the OAG in place not sure I can fit the filter behind the coma corrector, regarding the mirror I would have to move it up at least 15mm to take the corrector out the tube and I don’t think the collimating screws are that long. 

[Craig a]

3 hours ago, alacant said:

Hi

We also use the GPU cc on a 208mm f3.9 with and without front threaded filter. No rings, but a few thoughts...

My bet would be on the filter causing the rings. You should have enough space in your allocated 55mm to put the filter at the camera side. Easily tested by taking a flat frame without the filter.

IIRC, the light panel reduces its brightness by varying the refresh rate. Anything less that 1s exposure on our panel and we get bands appearing on the flat frames. Maybe try that too? A sheet of white card works well.

One last thought. as @vlaiv surmises, with the 533, you may get lucky and not need the cc.

Cheers

The point you make about not needing a coma corrector with the 533 is a good one, I may actually try without it the next clear night and check the stars in the corners, also will try without the uv/ir filter, but as the 533 doesn’t have the uv/ir glass in front of the sensor would that be a problem on a Newtonian? 

[vlaiv]

2 minutes ago, Craig a said:

The point you make about not needing a coma corrector with the 533 is a good one, I may actually try without it the next clear night and check the stars in the corners, also will try without the uv/ir filter, but as the 533 doesn’t have the uv/ir glass in front of the sensor would that be a problem on a Newtonian? 

I think that you'll have to use coma corrector with 533.

Radius of diffraction limited field is F^3 / 90 = 4^3 / 90 = 0.711mm - or diameter is about 1.42mm. Diagonal of 533 is 15mm or so?

Although we don't need diffraction limited performance for DSO imaging as seeing will swamp it - I'm thinking that coma will easily start to show after about first third, maybe half way to the edge of the frame if you don't use coma corrector.

You can certainly go without UV/IR cut filter and for DSO imaging - that is even better as you will pick up a lot of IR signal - that will boost your SNR.

Only issue that you'll have is with color balance - colors will look a bit strange (a bit washed out) and you'll have trouble color balancing properly.

[alacant]

37 minutes ago, Craig a said:

would that be a problem on a Newtonian? 

Hi

No. No fat refractor stars:) Everything is focused at the same distance. The only refractive stuff is the coma corrector and filter. I think modern sensors cut off reasonably sharply either side of the visible.

Cheers

Edited February 17, 2021 by alacant

[Craig a]

14 minutes ago, alacant said:

refractive stuff is the coma corrector

Would that cause fat stars?

[Craig a]

33 minutes ago, vlaiv said:

Only issue that you'll have is with color balance - colors will look a bit strange (a bit washed out) and you'll have trouble color balancing properly

You say trouble, how much trouble are we talking getting colour balance? 

[alacant]

5 minutes ago, Craig a said:

Would that cause fat stars?

Refractor lenses not focusing all wavelengths at the same point. 

[Craig a]

1 minute ago, alacant said:

Refractor lenses not focusing all wavelengths at the same point. 

I me an would the coma corrector itself cause fat stars not using the filter?, as it’s a 4 element lens basically

[alacant]

29 minutes ago, Craig a said:

it’s a 4 element lens

Yeah, we use the GPU cc too. It's very good, but the small amount of false colour we do get had its source in the cc. Easily eliminated however and certainly no fattening of stars.

Try with and without. You may get away with no cc.

Edited February 17, 2021 by alacant

[vlaiv]

7 minutes ago, Craig a said:

I me an would the coma corrector itself cause fat stars not using the filter?, as it’s a 4 element lens basically

I asked the same question long ago - for focal reducer on RC - and got very nice answer - "Why don't you try it out?"

CCs and FRs work in converging light beam, have small aperture (2" max?) and they bend light only slightly - in fact I think that there is greater chance of FR having issues than CC that only corrects for coma - minimal disturbance of wavefront.

I don't think there will be much issues due to CC - but again, I can't be certain and it would be best to simply try it out.

15 minutes ago, Craig a said:

You say trouble, how much trouble are we talking getting colour balance? 

Quite a bit. After some wavelength - 820nm or so - all three colors have basically the same response to IR.

If target is strong in IR - this means that RGB ratio will be disturbed.

Say you have 4:2:1 RGB ratio and then you add another 2:2:2 - because all three channels have same sensitivity to IR.

instead of 4:2:1 you'll have 6:4:3

Earlier red was twice as strong as green (4:2) and now it is only 50% stronger (6:4), green was earlier twice as strong as blue (2:1) but now it is only 25% stronger (4:3)

Some things in the image will have this problem as they emit in IR part of spectrum (stars, clusters and galaxies) while some things won't - emission nebulae, possibly reflection nebulae.

If you have combination of the things in the image - color correction is almost impossible - as you need to apply correction to those things affected and not apply correction to those not affected.

Whatever you do - some things will have their color be off.

You can still do color balance so that image looks nice - but you can't get accurate color. There will always be something in the image that is either too saturated or not saturated enough. Mind you - using UHC or similar filters also makes color correction impossible - but people use them anyway.