27 replies to this topic

[HomerPepsi]

Hi,

 

I received a new Baader MPCC MkIII back in april, and when I got it there was marks on the lens. I wrote the store who sold it to me and they are shipping me a new one. He said in the mean time clean it off and use the one they sent me. I finally got around to using my Schmidt Newtonian with the MPCC properly spaced at 55mm. The results are horrendous. Stars are so much worse across the field! Coma in the center of my image! What the hell? Without using the MPCC coma was only at the edge of my field.

 

My optical train is: Schmidt corrector, primary, secondary, MPCC, SX Slim OAG, SX Filterwheel, Sbig ST-8300 m camera

 

It is properly spaced.

The lens is clean.

Focus was as critical as it could get (bant. mask, 1x1 binning, zoomed when focusing).

 

The image below is nearly as raw as it can be, except for a pixinsight auto stretch and saved in png.

https://drive.google...iew?usp=sharing

[HomerPepsi]

These pics are from when I received it. When looking through it from the Cameras side, it looks like it definitely would make more coma. Is it possible that the lens is installed wrong?

 

https://drive.google...iew?usp=sharing

https://drive.google...iew?usp=sharing

https://drive.google...iew?usp=sharing

https://drive.google...iew?usp=sharing

[starcanoe]

Let's see.

 

Coma CORRECTOR  makes coma much much worse.

[HomerPepsi]

As the subject states.... Baader MPCC (multi purpose coma CORRECTOR) makes coma worse, much worse.

[JimMo]

I may be wrong but I don't think you can use a coma corrector with a Schmitt/Newtonian.

[HomerPepsi]

The schmidt corrector does a good job, but does not remove all of it. Others have used the same combintaion and have had good results.  Coma is around the edges, but its still there

 

I remember reading back in the day from baader themselves when Meade was still releasing the SN that the MPCC worked well, and on opt mpcc sales page it states that it works well with SN's which I believe is written by Baader as it is on other sales pages as well.

 

http://www.optcorp.c...c-mark-iii.html

[turnerjs085]

First, is the scope collimated well, including the secondary? A coma correcror makes the tolerances much tighter.

Second, the spacing will probably be different than the 55mm spec. Your scope has less coma than a newt at the same focal ratio, and I would expect the mpcc to overcorrect at 55mm spacing. It actually kinda looks like what is happening as in your pic the head of the coma points away from the optical axis. Hard to tell if it is collimated though.

Also, make sure the shoulder of the mpcc sits flush with the top of your focuser drawtube. It has a tendency to get crooked when clamped down because of the weird undercuts.

Jeremy

Edited by turnerjs085, 14 August 2015 - 06:41 PM.

[gdd]

I can find no information anywhere suggesting the MPCC spacing should be different for the SN6. I recall the spec saying there was a plus/minus 1 or 2mm tolerance from the recommended 55mm. I found Astrobin images for MPCC/SN6/DSLR that looked OK. The same imager also had an MPCC/AT8IN/DSLR image that looked the same so far as the corner stars are concerned. The OP is using a CCD, is it possible the spacing measurements are from the wrong part of the MPCC?

[HomerPepsi]

Sensor plane to the "shoulder" of the mpcc is 55mm, possibly 56mm. But pretty spot on. Hence why I am perplexed.

https://drive.google...sp=docslist_api

This is an image from the night before using the mpcc. You can see the coma on the edge of the field.

Collimation was decent for that night, so I did collimate before going out with the mpcc. Concentric circles (except for the SN secondary is elongated due to its shape, I follow this guide most of the time, check page 14), so I was happy. Have a look at the pics in post 2. Look at the way the light is reflecting through the mpcc. Does it not look as if the element should be flipped?? I'm really thinking this is a faulty mpcc. Coma radiates from the center towards the edge.

Edited by HomerPepsi, 14 August 2015 - 08:44 PM.

[turnerjs085]

It is always possible that it is put together wrong I suppose. I really have no idea what effect that would have... It's good you have another mpcc on the way.

 

Your coma is opposite of a normal newt (and the m31 pic) and not centered in the veil nebula pic you linked which indicates that it is overcorrecting for coma and that something is tilted/miscollimated.  

 

Jeremy

[RAC]

I can see the same problems in your images that I had in mine when I was trying to get the Mk3 MPCC to work with my Royce 10" f4 scope.

 

The problem was inconsistent star shapes and the stars had a bright spot in the middle and a defocused area around it. This drove me mad! The problem was the that the MPCC was way too fussy with tiny amounts of tilt and it also added bad spherical aberration. I tried for months to get good results with it and tried all sorts of spacings.

The solution is return the MPCC and get a Televue Paracorr type 2 and you'll be happy for ever.

 

This is the difference between them and this was at the center of the image! That was also the best I could ever get the MPCC working. I am also not the only one to find these results with the MPCC Mk3 at f4.

Corrector test by Raymond Collecutt, on Flickr

[gdd]

These pics are from when I received it. When looking through it from the Cameras side, it looks like it definitely would make more coma. Is it possible that the lens is installed wrong?

 

https://drive.google...iew?usp=sharing

https://drive.google...iew?usp=sharing

https://drive.google...iew?usp=sharing

https://drive.google...iew?usp=sharing

In the 1st and 3rd views one of the reflections in large and out of focus. My reflections of light source are small.

 

In the 2nd and 4th views all of your reflections are small. One of my reflections is large and out of focus.

 

You may be right, the element is flipped.

Edited by gdd, 15 August 2015 - 11:38 PM.

[Pierre Lemay]

I am also not the only one to find these results with the MPCC Mk3 at f4.

I use a Baader MPCC Mk2 on my 20 inch f/3.9 and images are very good to the edge of the field of an ES 20mm 100 deg AFOV eyepiece. The corrector is screwed directly to the 20mm eyepiece barrel with the proper spacer.

[RAC]

 

I am also not the only one to find these results with the MPCC Mk3 at f4.

I use a Baader MPCC Mk2 on my 20 inch f/3.9 and images are very good to the edge of the field of an ES 20mm 100 deg AFOV eyepiece. The corrector is screwed directly to the 20mm eyepiece barrel with the proper spacer.

 

 

Visual may be ok but imaging can bring a whole new level of annoyance if the stars aren't perfect and perfect they must be if you're fussy like me.

 

Have you compared a Paracorr to the MPCC.

[wh48gs]

My optical train is: Schmidt corrector, primary, secondary, MPCC, SX Slim OAG, SX Filterwheel, Sbig ST-8300 m camera

 

For given focal ratio, SN has less than half the coma of a paraboloid. Hence coma corrector designed for the latter will, if at the same distance from focal plane, induce negative (tail down) coma of opposite sign. To offset SN coma, it should be pulled closer to the image, approximately half the distance needed for a paraboloid.

 

Vla

[Pierre Lemay]

Visual may be ok but imaging can bring a whole new level of annoyance if the stars aren't perfect and perfect they must be if you're fussy like me.

 

Have you compared a Paracorr to the MPCC.

Not directly with my telescope. But I've frequently observed in ~16 to 30 inch f/4 newtonians that had Paracorr 2's. The difference in coma, if any, was slight enough that I did not notice it, especially at low power. An MPCC does a fine job at f/4 and slower ...at half the price of a Paracorr II (when I bought the Baader mk2 it was 1/3 the price of the Paracorr II !). Of course the paracorr has many advantages ( like tunable top) and with a faster scope I would not hesitate to using one.  But for the moment I'm satisfied with my current setup: a fixed MPCC for the widest eyepiece I use and no coma corrector for the higher power eyepieces ( note: my mount tracks). 

 

I've never used the MPCC for atrophotography but many of  the pictures I've seen taken with it trough f/4ish newtonians were stunning. 

[HomerPepsi]

Corrector works fine in my friends 203 f4 Newtonian. Perhaps there is some issue with my collimation I am not seeing.

[Nils Olof Carlin]

Corrector works fine in my friends 203 f4 Newtonian. Perhaps there is some issue with my collimation I am not seeing.

I would say the answer lies in posts #7 and #15. Try re-read them. An S-N is *not* a Newtonian. The corrector works by creating coma of the opposite sense, and the amount can be tuned by setting the right distance corrector-focal plane (the longer, the more anti-coma). OPT claims: "The MPCC Mark III has been tested and found to work well on Meade SN Schmidt-Newtonians." However, as far as I see, the Baader Planetarium makes no such claim - and I suspect the OPT claim is not reasonable at face value. If the correction for Newtonians is optimum at the recommended distance to the focal plane, in an S-N where about half the coma is removed by the corrector (plate - edit), you should expect bad overcorrection (as described in post #7!).
With the distance corrector-focal plane about halved, you might tune in good correction, but I have no idea if this is mechanically possible.

Nils Olof

(Edit: PS A Paracorr works in much the same way, but I believe there is no non-destructive way to shorten the distance)

Edited by Nils Olof Carlin, 29 August 2015 - 03:55 AM.

[Starman1]

The MPCC usually employs spacers between it and eyepiece or camera.  Either using no spacers or reduced spacers should allow you to experiment with placement.

[ed_turco]

I can hardly imagine anyone going out and buying a Schmidt-Newtonian, which corrects for coma pretty well and then buying a coma corrector to get rid of that little "coma' at the edge of the field of an eyepiece.

 

The problem is that "coma'' isn't coma; it is astigmatism.  Schmidt-Newtonians don't do much for correcting astigmatism and a coma corrector isn't going to do anything to correct astigmatism either.

 

That coma-corrector must have set someone back a good sum of money.  So why would someone buy it without asking the many kind folks here about what was needed?  Plenty of knowledgeable people here with time on their hands!

 

What is it these days?  People don't ask questions and go ahead and buy?   So that person buys a coma-corrector and then, it dawns on him to ask questions?

 

The problem of overcorrecting for coma is self evident.  And only one person picked this up.  With coma "fans'' directed to the center of the field?

 

I hope that Baader has a money-back guarantee.  Otherwise, someone is stuck with an unneeded coma corrector at considerable cost.

 

 

ed

[jsmoraes]

I use a GSO Coma corrector. And I had some problems because of mistakes on assemble the lenses after a cleanning operation.

 

Therefore, if the lenses are in wrong order and position, the corrector will be worse.

 

55 mm spaced - This is a theoretical value. You can test others. Badder MPCC works from 50 mm up to 65 mm.

 

 

when I got it there was marks on the lens ... Coma in the center of my image!

It seems that someone damaged and (why not ?) dismount and mount it. And like me, with mistakes.

[leveye]

I bought a used MPCC II awhile back and had issues with it making things worse at prime focus with a DSLR on a 6" Newt. Seemed to be working in reverse. The spacing to my cameras sensor was perfect. Hmm..After lots of head scratching and then examining pictures of other correctors it turned out that the main lens that faces the primary had to be flipped for it to work properly. The lens had a ring that was sealed and un-tampered with right from the factory. It had been assembled wrong.  Someone there not doing their job properly. 

 

This is how it was originally assembled and the side facing the secondary...

 

 

Had to flip it around.

 

 

After that the star field was perfect side to side and corner to corner. Makes one wonder just how many went out like that?

 

 

 

 

 

 

 

 

 

 

 

 

[HomerPepsi]

I can hardly imagine anyone going out and buying a Schmidt-Newtonian, which corrects for coma pretty well and then buying a coma corrector to get rid of that little "coma' at the edge of the field of an eyepiece.

 

The problem is that "coma'' isn't coma; it is astigmatism.  Schmidt-Newtonians don't do much for correcting astigmatism and a coma corrector isn't going to do anything to correct astigmatism either.

 

That coma-corrector must have set someone back a good sum of money.  So why would someone buy it without asking the many kind folks here about what was needed?  Plenty of knowledgeable people here with time on their hands!

 

What is it these days?  People don't ask questions and go ahead and buy?   So that person buys a coma-corrector and then, it dawns on him to ask questions?

 

The problem of overcorrecting for coma is self evident.  And only one person picked this up.  With coma "fans'' directed to the center of the field?

 

I hope that Baader has a money-back guarantee.  Otherwise, someone is stuck with an unneeded coma corrector at considerable cost.

 

 

ed

 

Pretty critical I'd say. If you had a schmidt Newt, then you'd know that the information is sparse when it comes to such matters. If you check my image in post 9 you'll see that coma corrected "pretty well" is not.

 

As for astigmatism, I wasnt aware parabolic mirrors suffered from astigmatism. My RC does, and I can tell in my images. However I do not see any in my SN.

 

Also, there is a grand market for MPCC's if you haven't noticed. So I wouldn't say I am stuck with anything. Please take your smugness elsewhere.

[HomerPepsi]

I use a GSO Coma corrector. And I had some problems because of mistakes on assemble the lenses after a cleanning operation.

 

Therefore, if the lenses are in wrong order and position, the corrector will be worse.

 

55 mm spaced - This is a theoretical value. You can test others. Badder MPCC works from 50 mm up to 65 mm.

 

 

when I got it there was marks on the lens ... Coma in the center of my image!

It seems that someone damaged and (why not ?) dismount and mount it. And like me, with mistakes.

I think messing with my spacing is the key. 55mm for a normal newt. My 8" SN has half the coma a normal 8" Newt does according to starizon. So maybe I need either 1.5 or 0.5 times the spacing?

[ed_turco]

I was being smug?  I have never ever had to say this but take a look at the credits below my name and see what I offered you based on a half century of experience.  I was only offering to help you, for heaven's sake.

 

Even though I don't mention all that I have done, it so happens that I have successfully ground and polished a number of Schmidt-Newtonians, so I do know something about them and when all is said and done about your defective corrector, you will find that what I said was true; you will not need a coma-corrector for your telescope. 

 

I ought to know something about coma-correctors too.  I've ground a few of these in my time.   Also successfully.

 

Watch where you tread;  I am not the only person on here who has big toes to step on.

 

 

ed