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Old 12-01-2022, 09:59 AM
Stephane
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Coma issues & fingerprints on filters

Hi all,

Last night I set up my rig with the hope of using my asi294mc for the first time on Thor's Helmet. The camera did not disappoint, more nebulosity appeared in the stretched preview than I could have dreamed of with my DSLR! My previous egg-shaped stars issue fixed itself, so I am assuming it was due to poor polar alignment or tilt. Either way, glad that issue disappeared.

However, one issue persisted: coma. The coma is terrible and I took hours trying to fix it. I never got past rotating the image for Thor's Helmet. I tried so many different configurations. Below is the exact setup I used (not my photo though):

L-Extreme filter -> Baader MPCC III -> 16.5mm -> 21mm -> ASI294mc

As you can see on the two images below which I took, I don't think we're talking a millimetre or two of spacers. Is it possible that coma has been over-corrected? Or would coma appear perpendicularly if that were the case? I experimented for hours and gave up. Thanks for helping.

I also accidentally touched the L-Extreme filter and Baader MPCC with my fingers a couple of times. Any tips on cleaning filters would also be much appreciated.

Clear skies,
Stéphane
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Old 12-01-2022, 03:20 PM
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I just use a lint free microfibre cloth . To clean lenses lucky only had to do it once. Just used a brand new one in a light circular motion. Someone else might have a better idea. As for camera it should be 55mm back from the lense. Not to sure about your coma corrector and if you need to deduct it length.
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Old 12-01-2022, 03:45 PM
AdamJL
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I clean my objectives with a microfibre cloth, and sometimes some isopropyl for stubborn stains. You could use meth as well for really baked on stuff.

If it's quite dusty, get something like a lens brush first to get rid of the main particles, then blow air across it with a rocket blower or something like the Hurricane canless air system. Then use the microfibre cloths.

it's very much like doing your car with a foam wash and two bucket method: you want to ensure all the "big" particles are gone first with the foam wash, because rubbing these against your pain will scratch the surface and give you swirl marks. Then you go at it with a microfibre cloth and two buckets to ensure no contamination gets into your clean bucket.

Obviously you can't use two buckets here, but you can use two microfibre cloths
So wipe no more than two times across the objective, then use another side of the cloth to finish off or another cloth.

Pec Pads are also a great thing to have on hand, I have been using them for 20 years on my photography gear.
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Old 12-01-2022, 03:46 PM
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floyd_2 (Dean)
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You might want to use a puffer brush before you wipe lenses / filters just to make sure that there are no particles that could scratch the surface. In my case I use puffer brush, then set aside 3 x small tear drop shaped pieces of pure cotton wool. I put a drop of pure isopropyl alcohol on the first cotton wool, and gently clean the lens / filter. I then switch to cotton #2 (dry) and clean the IPA off the lens / filter before it evaporates and leaves anything behind on the surface. I then switch to cotton #3 and breathe a mist onto the lens / filter and give it a final clean. All cotton gets used once and binned.

With your MPCC setup, my assumption would be that the MPCC is the first thing into your focuser. And then you measure 55mm from the rear most lens of the MPCC to the surface of the camera chip as a starting point.

I did this with my refractor and ASI533MC-PRO and it wasn't even close lol. I ended up putting in extensions from that point until it was right. I guess it might need some playing around, but 55mm from the last lens element as a starting point might be ok as Kuz said. Your camera looks to have 17.5mm of back focus including the mounting ring, so that would be deducted from the 55mm. Looking at your photo, you're about right apart from perhaps where the surface of the final element of the MPCC is. If it's not at the top edge of the MPCC, there may be some extra mm of back focus being added into the equation. If you have some extra spacers, it might be worth doing a quick experiment next time you get dark skies.

Last edited by floyd_2; 12-01-2022 at 03:58 PM.
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Old 12-01-2022, 04:34 PM
Startrek (Martin)
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Camera sensor to Baader MCPP coma corrector should be 55mm to 56mm
I noticed your Coma corrector doesn’t have a collar or bumper ring ( see photo ) mine has
Also photo attached shows 294MC mechanical drawing
Sensor face to face of camera barrel is 6.5mm then add T2 ring at 11mm then add 21mm adapter then add 16.5mm adapter equals 55mm
So spacing set up looks ok but not sure about the collar or bumper ring missing on your Baader Coma Corrector
Did it arrive with the collar ??
It may or may not make a difference ??

As far as cleaning the L Extreme I use one of my microfibre cleaning clothes from my glasses case
Just softly wipe in a circular motion from center to outside of filter
I also use an eye piece hurricane blower to remove find dust

Hope the above helps
Martin
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Old 12-01-2022, 04:35 PM
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Tough one. By the direction of the eggy corner stars it looks like you are still under the 55mm spacing.

Looking at your gear setup photo it looks like the corrector is missing a ring spacer that's about 2mm thick and a t-thread. Like this picture.

But I have the same corrector and had to put in an extra spacer (it's about 1-2mm) to remove coma with an asi071. Used delrin rings. See attached pic

You can try just unscrewing some of your spacers a bit to see if that improves the coma. Will also give you an idea of how much you need. Then you can buy some delrin rings or some thin spacers depending on the amount required
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Last edited by ChrisV; 12-01-2022 at 04:49 PM.
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Old 12-01-2022, 05:19 PM
sunslayr (David)
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Mpcc is 57.5 from m48 and 55 from t2. I use R.O.R cleaner for greasy lenses. Leaves no residue.
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Old 12-01-2022, 06:43 PM
Stephane
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You guys are all so awesome! I’ve managed to clean the filters. Not perfect, but way better. Next time, I will use the isopropyl, but don’t want to be doing this too often, so will wait until really necessary - perhaps in a year or so (unless I smudge them again).

As for coma, I’ve worked out how to get 55-56mm, so I’ll experiment with 55 and 56 and see which performs better. Martin, the Baader ring was missing on mine, and the 16.5 was missing in the Thor’s helmet photo (although I had the 16.5 set up earlier that evening and still had coma). I’ve measured 55mm and am bang on now, so let’s wait and see.

Clear skies and thanks again to everyone,
Stéphane
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Old 12-01-2022, 07:37 PM
RyanJones
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You are missing the M48 to T2 step down adapter from your MPCC. I think this might be the bumper ring that Martin is referring to. Either way the thickness of said ring is 3mm. If you are connecting your MPCC directly to the full aperture M48 thread, you need to add this 3mm into your spacer sizes.

Cheers

Ryan
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Old 13-01-2022, 10:42 AM
AdamJL
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By the way, the topic of filter cleaning came up on the NINA Discord channel. This video was linked to and it’s by Astrodon

https://youtube.com/watch?v=t5NAIbV1l8o

Basically what we already said but they also say you can use running water for the beginning part (I use a lens brush)
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Old 13-01-2022, 10:42 AM
Stephane
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Quote:
Originally Posted by RyanJones View Post
You are missing the M48 to T2 step down adapter from your MPCC. I think this might be the bumper ring that Martin is referring to. Either way the thickness of said ring is 3mm. If you are connecting your MPCC directly to the full aperture M48 thread, you need to add this 3mm into your spacer sizes.

Cheers

Ryan
Ryan, didn’t Martin include those 3mm in the measurements he gave me? I applied exactly what he recommended and landed bang on 55mm.

One thing I have learned with astrophotography is that there are no “get out of jail free” cards. Any mistake will cost you when imaging! Hopefully coma will soon be a thing of the past for me. Also, not sure how the filter imperfections will affect imaging. They look perfectly clean, but when I shine them in the sun at the right angle, I see slight imperfections. Time will tell.

Thanks again,
Stéphane
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Old 13-01-2022, 10:43 AM
Stephane
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Quote:
Originally Posted by AdamJL View Post
By the way, the topic of filter cleaning came up on the NINA Discord channel. This video was linked to and it’s by Astrodon

https://youtube.com/watch?v=t5NAIbV1l8o

Basically what we already said but they also say you can use running water for the beginning part (I use a lens brush)
Great! Thanks Adam! This is what I need for future cleans.
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Old 13-01-2022, 07:40 PM
RyanJones
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Quote:
Originally Posted by Stephane View Post
Ryan, didn’t Martin include those 3mm in the measurements he gave me? I applied exactly what he recommended and landed bang on 55mm.

One thing I have learned with astrophotography is that there are no “get out of jail free” cards. Any mistake will cost you when imaging! Hopefully coma will soon be a thing of the past for me. Also, not sure how the filter imperfections will affect imaging. They look perfectly clean, but when I shine them in the sun at the right angle, I see slight imperfections. Time will tell.

Thanks again,
Stéphane
If you look at the photo of the MPCC that Chris V sent you and compare it to the setup you posted you can see there is a locking ring missing but more importantly the M48 to T tread adaptor. It’s that adaptor that has the thickness you are missing. It’s no big deal if you know it’s not there and can compensate for it. You just need to make up 58mm backspacing instead of 55mm. How you do that can be interesting for someone starting out who hasn’t yet collected the piles of “ used once “ bits lol. If you do have that adaptor and can shuffle around your current spacers to work then that’s great. I doubt your current setup would vignette with the t-ring adaptor in place ( that’s why it’s removable ) so if you’ve got it use it. You’re 100% right there are no short cuts. That’s why I feel learning not just by instruction but by understanding is so important in this hobby ( and real life ). I don’t often communicate that well and sometimes I come across blunt when I say things but everything I say is intended to help share that.

Cheers
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Old 13-01-2022, 08:21 PM
Stephane
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Thanks Ryan, I finally understand. It turns out that I was using the M48 to T thread, but for some silly reason, I screwed it into the 16.5mm adapter so that it would fit onto the coma corrector, rather than on the coma corrector itself

Really appreciate the help. It’s cloudy for the next couple of nights, but hopefully it’ll work now. I have three 1mm spacers if I still get coma, so I can add these one at a time if needed.

Thanks again,
Stéphane
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Old 13-01-2022, 10:01 PM
RyanJones
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Quote:
Originally Posted by Stephane View Post
Thanks Ryan, I finally understand. It turns out that I was using the M48 to T thread, but for some silly reason, I screwed it into the 16.5mm adapter so that it would fit onto the coma corrector, rather than on the coma corrector itself

Really appreciate the help. It’s cloudy for the next couple of nights, but hopefully it’ll work now. I have three 1mm spacers if I still get coma, so I can add these one at a time if needed.

Thanks again,
Stéphane
Excellent. I look forward to seeing the results
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