Ad Astra Observatory

Hello Jimbo,

Sorry for the slow reply, just returned from a week trip to China.

That is an ambitious project! (an f/3.3 mirror has to be a real bear to

produce well). It would be a nice alternative to something like takahashi’s
very expensive hyberbolic astrograph.

As you know, the MPCC is designed to null the coma from an f/4.5 paraboloid.
So, at f/3.3 you will still have some coma present. I don’t have access to
the actual design formula of the MPCC, so I can’t be of much help to you in
establishing the optimal correction of your mirror.

I recommend that you contact Mike Palermiti at ITE (www.iteastronomy.com).

He has optically tested the Baader MPCC and has extensive knowledge of coma
correctors. He should be able to give you an educated recommendation.

I would be very interested to hear what you learn, and how your project
works out for you (you are actually the second person to ask this question,
so there are others that are interested in making such fast astrographs).

Best Regards,
Bob Luffel

Alpine Astronomical, LLC

www.alpineastro.com

—–Original Message—–
From: Jimbo S. Harris [mailto:jimbo@jimbo.net]
Sent: Friday, March 17, 2006 2:32 PM
To: service@alpineastro.com
Subject: Baader MPCC and a f/3.3 Newtonian?

Hi,

If you’re not the right person to ask this question, my apologies. I’m

looking for some technical info with respect to the Baader MPCC.

I’m going to be making the primary mirror for (what I plan to be) a 6”
f/3.3 Newtonian astrograph.

I do not plan on using the telescope for visual use, and my main imaging
cameras are DSLR and 35mm film (Going to CCD later would decrease the FOV
and thus make my life a little easier, so I’m not worried there, and I have
no plans to try medium-format film). What I’m looking for, then, is a

“good” photographic field about 44mm in diameter (aren’t we all? (: ).

I’m planning on using a Baader MPCC to correct the coma; this fast scope
will probably have an MPCC as a “permanent fixture” in the focuser.

My questions are two:

1) Do you feel that the MPCC will do a decent job of coma correction with
such a fast objective?
2) Would it do an even better job if I overcorrected my primary? By how
much?

My feeble understanding of the way that coma correctors work is that they
eat coma by adding some spherical aberration, and that if the primary is
overcorrected, the “hyperbolic aberration” (”anti” spherical aberration)

balances the spherical aberration out, thus producing even better images
than possible with a parabolic mirror. With the clear understanding that
overcorrecting the primary will make the MPCC a requirement (”not just a
good idea”), I’m ready, willing, and able to push this mirror over that
edge, since I know I’m going to use it strictly as a camera lens.

I’m still early enough in the mirror-making process that I can choose to

make the mirror a little slower if it’s really not worth trying to push it;
I’ll make the mirror f/3.5 or even f/4 if I have to, but I’d rather not
leave almost a full degree of FOV ( ~5� @f/3.3, ~4.16� @f/4) lying on the
grinding table if I don’t have to.

Can you help me figure out how to plan this telescope around the MPCC?

I appreciate your time and effort on this.

Regards,
Jimbo S. Harris