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Baader MPCC question

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#1 Ettu

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Posted 10 April 2013 - 07:20 PM

I'm getting a Baader MPCC (multi-purpose coma corrector) to use with my 3 Pentax XW and a 36mm Hyperion eyepieces on a f4.5 16" dobsonian. My question is in regards to the recommended '...55mm spacing from the back of the MPCC to feild stop of the EP's... +/- 3mm'. I was looking on the AlpineAstro website at this recommendation and their table of examples of EP's and recomended spacers. They don't add up? For example the 36mm Hyperion has a 40mm space already from the EP fld stp to where a spacer would be screwed on. Yet the recommend is to add a 28mm more giving a total of 68mm spacing? And my XW pentax's vary as much as 15mm between them from the feild stop and where an extension would be screwed on, Yet the recommendation is for the same spacer for all. Not exactly a +/- 3mm result.

Since all of these EP's are parfocal, I'm beginning to wonder if the proper spacing isn't better described as 55 mm into (closer toward the mirror from) the focal point of the main mirror? Or where is it?

Somehow it makes some sort of intuitive sense to me that in a parfocal set, the MPCC spacing would be the same for all somewhere in regards to the EP holder's stop?

Would appreciate any insight into this.

Keith (Ettu)

#2 shams42

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Posted 10 April 2013 - 09:37 PM

I'm sorry that I can't answer your question, but I wanted to make sure that you know that the MPCC imparts a substantial amount of spherical aberration. I'm not sure how satisfying the visual experience will be.

#3 Ettu

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Posted 11 April 2013 - 11:10 AM

Thanks for your reply Shams. I did not know that. I have just recently gotten into visual observation. I've been restoring a 16" LightBridge for that purpose. What I do know is that when I put my older MPCC (I see they now offer an newer improved version) on any of my EP's, even though it was far from the recommended spacing, it noticably improved the sharpness of the stars. On that basis I was ready to order the proper spacers, and perhaps even get the newer version of the MPCC too.

Maybe I should ask at this point, what is the consensus recomendation here (or from you at least..) of a good corrector for an f4.5 dob? I suppose it depends too on what EP's one uses. In that regard, I primarily use Pentax XW's.

You mentioned spherical aberation issues. I presume that means that although coma may be improved, that stars won't be in focus across the FOV?

Keith (Ettu)
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#4 Ettu

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Posted 11 April 2013 - 11:26 AM

Followitg up on my original question.
Bob Luffel from AlpineAstro did get in touch with me last evening and helped clear up a few things.

First, in my newbie-ness, I incorrectly understood where the feild stop point is. Once that was cleared up the example spacing tables on his website Do add up.

Next, as I suspected, the distance from The focuser collar or EP seat, to an MPCC would be the same for all of a set of parfocal EP's. In other words, instead of a set of spacers for each EP, and having to either move the MPCC from EP to EP, or get multiple MPCC's, one could permanently mount the MPCC in the focuser barrel at the ideal spacing for one EP, and it would then be perfectly placed and usable for all. So, that's what I'm going to do.

#5 Starman1

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Posted 11 April 2013 - 04:50 PM

Additionally, you can experiment to find the right combination of spacers.
Here is what you do:
Since the coma corrector has to be at a particular place in the light cone of the scope in order to correct coma and not vignette heavily, if one eyepiece is properly spaced from the lens and the scope brought into focus, you can simply freeze the focuser in place (i.e. not move the knobs at all), then attach the coma corrector to another eyepiece and slide the ensemble into the focuser.
If the eyepiece is not now in focus, you have an error in spacing.
If the eyepiece comes into focus when pulling it out of the focuser, try thinning the spacing a bit and slide the ensemble into the focuser again.
If the eyepiece needs to go in more, you should start with that eyepiece for your spacing adjustment.
You have the correct spacing when the eyepiece + coma corrector is in focus at the same point as the previous eyepiece and its spacer(s).
Yes, this means that, properly spaced, all your eyepieces will focus at the same point. By changing the spacers, you have parfocalized your eyepiece set.
Any new eyepiece can be adjusted in the same way.

Now, with the MPCC, you could do it another way:
With a set of parfocalizing rings (available from many sources).
Attach the MPCC to a short 2" extension tube. Make sure you have a 2" eyepiece that fits the extension tube and that is held the right distance away from the MPCC lens, either with spacers between the MPCC and extension tube, or with a parfocalizing ring on the eyepiece. Insert each of your other eyepieces in turn into the extension tube and focus by sliding them in and out and lock the position with the parfocalizing rings. Now, in the future, merely inserting each eyepiece until the ring hits will bring them close to focus. You may have to tweak focus a bit, but all of your eyepieces will focus at pretty much exactly the same place. You will have parfocalized them all at the correct distance from the lens.

if you want to use a Barlow, use a 2" barlow and insert the MPCC/extension tube/eyepiece stack into the barlow.

All that being said, though this will yield significantly better star correction over most of the outer field, it will reduce optical sharpness slightly in the very center of the field. The MPCC was designed, originally, for imaging work. The TeleVue Paracorr is the visual corrector that is designed to not reduce image quality anywhere in the field of view. It is, though, more expensive, heavier, and has a telescoping top to accomplish the spacing changes you are making using spacers and rings.

#6 olivdeso

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Posted 11 April 2013 - 06:54 PM

I'm sorry that I can't answer your question, but I wanted to make sure that you know that the MPCC imparts a substantial amount of spherical aberration. I'm not sure how satisfying the visual experience will be.


+1. At high magnification with the XW, it would be better to not use the MPCC.

#7 Ettu

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Posted 11 April 2013 - 06:54 PM

Thank you Don for taking the time for your detailed help and descriptions. It boosts my confidence that I now understand how these CC's work. And it sounds like Baader and Televue's CC's work the same way, regarding spacing.
Thanks again,
Keith (Ettu)
N45W90
website
http://www.willowber...et/keithnk_m42/

#8 Ettu

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Posted 11 April 2013 - 07:53 PM

...I incorrectly understood where the feild stop point is.
(snip)


Perhaps it would be usefull to others if I explained a little more where I went wrong measuring '...55mm from the fld stp to the MPCC...'

For the 36mm Hyperion, the feild stop is almost the same diameter as the inside barrel of the EP, and I simply didn't recognize it for what it was. Apparently, when in focus and with a traditional EP, the fld stp is at the fcl pt inteface of the main mirror and EP. The design of the Pentax XW's however is not traditional. The fcl pt interface is inside the EP, beyond what otherwise appears from the outside to be the fld stp. So a measurement from the apparent fld stp gives the wrong spacer answer. It has to be figured out, as Don described above. That's made somewhat easier in my case because I do have one conventional EP, parfocalized with my XW's, that I can use to derive the correct spacer for the XW's

Keith (Ettu)
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#9 Ettu

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Posted 11 April 2013 - 08:08 PM

At high magnification with the XW, it would be better to not use the MPCC.


Could you explain why? I did notice that my 7mm XW didn't seem to have the same improvement as my 20mm did.

Would you say the same if a T.V. paracorr?

Thanks,

Keith (Ettu)
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website
http://www.willowber...et/keithnk_m42/

#10 Starman1

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Posted 11 April 2013 - 11:34 PM

At high magnification with the XW, it would be better to not use the MPCC.


Could you explain why? I did notice that my 7mm XW didn't seem to have the same improvement as my 20mm did.

Would you say the same if a T.V. paracorr?

Thanks,

Keith (Ettu)
N45W90
website
http://www.willowber...et/keithnk_m42/

The MPCC adds spherical aberration to the center of the field because it is a simple lens. The effect is like an under or over-corrected mirror, so the star images on center and planetary/lunar detail aren't as good as without the corrector.
The TeleVue doesn't have that issue (one of the reasons it also magnifies 1.15X). It has 5 elements and is a lot more expensive to make.
Bear in mind, too, that high power eyepieces see a smaller portion of the telescope's focal plane.

#11 Mark9473

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Posted 12 April 2013 - 11:37 AM

I wanted to make sure that you know that the MPCC imparts a substantial amount of spherical aberration. I'm not sure how satisfying the visual experience will be.

I just came across a product announcement for something that claims to solve the SA problem. link

#12 Starman1

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Posted 12 April 2013 - 02:43 PM

Not too surprising. The original TeleVue Paracorr had 4 lenses, and the new one, which corrects shorter f/ratios, has 5.
It's always possible to more completely correct an image with more lenses.
When I owned an SCT, I tried 2, 3, and 4-lens focal reducers. The best results? 4-lens arrangements.
Add the correction of a Paracorr to its ease in use, and it's not too hard to see why it is so popular.






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