130PDS Coma with MPCC III

[Bibabutzemann]

Hello, i just recently got my 130 PDS and a Baader MPCC3.

I use a EOS1200Da for imaging.

My problem is, that i get stretched stars on the edges. Here an example, the heart nebula (unfinished, only 2 hours exposure time)

Looks pretty bad to me.

I read a few other threads on this topic and as far as i understand, the spacing between corrector and chip should be 55mm, which is achieved by using a common T2 adapter for EOS. 

So spacing shouldnt be the cause of the problem, right? Any ideas? Collimination should be about right, i used the barlow method with a laser.

Also i get D shaped stars when out of focus, but thats because of the focuser...But i dont think it affects the focused image that much.

CS Patrick

 

Edited September 22, 2020 by Bibabutzemann

[alacant]

Hi

The correct spacing from the shoulder of the cc is 58mm. 

The second image shows out of focus d shaped stars. If you can tolerate that, just leave it. Otherwise it's a 5 minute job with a hacksaw to fix it.

HTH

Edited September 22, 2020 by alacant

[michael8554]

I've never understood how a Brand X CC will correct a Brand Y OTA, if the amount of coma on Brand Y isn't what the CC was designed to correct ?

On my old F6.3 SCT I found 58mm to work with the MPCC

Also the MPCC instructions say 55mm with the M48 "Lock Ring" on, 57.5mm with it off.

Michael

[edarter]

The 55mm isn't an exact number, it will require some fine tuning depending on individual manufacturing tolerances of the kit you have. I ended up buying an old camera filter off ebay, removing the glass and then repeated sessions of sanding it down and trying it again until the coma was about right. 

Sanding it accurately was fairly straight forward, wet & dry paper on a flat surface and rub one side of the filter on that applying even pressure around it. 

I've also seen a diagram somewhere that shows what the different shapes of stars mean, spacing too small, too large or whether there is camera tilt. I will see if I can find it. 

Here you go, diagram at the bottom of the article.

https://altaircameras.com/howto-adjust-sensor-reducer-flattener-spacing-for-better-star-shapes/

Edited September 23, 2020 by edarter

[michael8554]

Windows 10 is giving me a full screen red warning message about that site.

If its the image I'm thinking of, there has been conjecture that the Too Far and Too Close labels are reversed !

But if you use the chart and the coma gets better, no problemo !

Michael

[Bibabutzemann]

Thank you so much for your replies!

If the graph from the link is correct, the mpcc is too close to the sensor.(if it would be too far, i would have to buy a thinner T2 Adapter)

I think i will try different distances by unscrewing the corrector a tiny bit.

If i get better results i may buy an old camera filter to fix it, thanks edarter.

Will post any results here.

CS Patrick

 

[michael8554]

2 hours ago, Bibabutzemann said:

I think i will try different distances by unscrewing the corrector a tiny bit.

Clever !

Don't use a stack to evaluate, to avoid guiding errors.

Just max ISO and stretch if necessary.

Michael

[runway77]

Hi, have you managed to solve this problem? If yes what would be your recommendation? I have the Baader MPCC and I use it with a 150pds and a Sony mirrorless camera, distance of the MPCC from the camera sensor is ca. 55mm. I can observe a similar effect on my photos but I would say it's not so noticeable. I have just bought some fine tune T2 rings, 0.3mm, 0.5mm and 1mm but they are yet on the way.

[Bibabutzemann]

Unfortunately it was very rainy for the last few days here in Germany...

Today i had only 15 minutes for testing and it wasnt very accurate, so i will try properly before posting any results.

But one thing i immediatly noticed is that you have to be careful with refocusing when you change the distances.

Because when youre slightly out of focus, you also get strong distortions in the corners.

So be sure to use a Bahtinov mask and take your time.

[runway77]

I bought the Baader fine-tune T2 rings. Instead of the recommended 55mm distance from the CC to the sensor I tried 56mm. The stars in the corners are noticeably better corrected now. Perhaps the optimal distance could even be more than 56mm in my case. That is yet to be confirmed.

[Spaced Out]

Hi

I've wrestled with the Baader MPCC Mk3 and the 130 PDS before, I found the MPCC was quite fickle about spacing but good once it is spaced properly. I solved my coma problems by carefully fine tuning using Baader Delrin spacers.

Make sure the camera is firmly seated against the focuser before starting (to minimise any tilt) - wind the focuser right in and push the camera/CC quite firmly against the focuser while tightening up the thumb screws.

Use a single frame to assess coma and then compare between frames as you add or remove spacers, it can be a bit faffy but it’s worth spending the time and once you've got it good you don't have to touch it again.

Here's the fine tuning spacers I used

https://www.firstlightoptics.com/adapters/baader-t2-delrin-spacer-ring-set.html

I had to snip these plastic spacers with scissors and stretch them slightly to fit but they worked well and are fairly cheap.

You could also try a free trial of CCD inspector to help you work out if you are over or under spaced. I did this and it did help me head in the right direction with the spacing but I’m not sure how accurate the software actually is ?  

https://www.ccdware.com/products/ccdinspector/

Edited November 15, 2020 by Spaced Out

[Bibabutzemann]

Thanks for all your helpful replies!

I finally really took time to sort out the source of the problem (on a bright moon night), so im prepared for the big summer targets 

There were two main steps that solved the issue:

Step 1: Better collimation:

After doing laser collimation, i continued to do some finer adjustments with star collimation and the dslr:

1a)  I used a bright star in the middle for faster focusing with the Bathinov mask. Then i made short exposures to see in which corner the stars were stretched the most

1b)  Then in liveview i collimated the primary mirror in way, so that the bright star moved towards the worst corner a little bit

I repeated 1a and 1b three times and i got fairly symmetrical coma.

 

Step 2: Distancing of the Coma corrector

After Step 1 i noticed the stars where stretched towards the centre of the image on all corners.

This means, i had to increase the distance between chip and corrector.

With a distancing ring, i increased by about 1.5mm

 

Now im finally happy with the star shape :=)

 

Further notes:

- I had to make sure to press the camera against the focuser when tightening the clamping screws. Otherwise i had asymmetric elongated stars on different corners. 

- I also tested possible flex problems. But i didnt notice any movement of the collimation laserpoint when i pointed my telescope in diffierent directions.

- I think filters change the required distancing between chip and corrector, since i noticed stronger coma with the cls-ccd Clip Filter. 

 

 

Now i  hope it will stay like this 😅