Ad Astra Observatory

Dear Mr. Harris,

if you decide to reduce your usable field to APS-sized CCD’s it may be ok with 38mm free lens diameter in terms of vignetting and with acceptable spot-size down to f3.7.
So the best compromise would be to reduce your demand down to f3.5-3.8 and to make final star tests with the MPCC in position.
Please excuse, but we have not the possibility to give you the optical parameters of the MPCC or optimizing parameters for shorter focal mirrors. In my opinion it will be the best solution to reduce your planned f ratio a little and make a standard mirror parabolisation.

Best regards,

Martin Rietze
BAADER PLANETARIUM

——– Original Message ——–
Subject: Re: MPCC hyperbolic mirror overcorrection factor (25-Apr-2006 21:40)
From: jimbo@jimbo.net
To: m6ri@baader-planetarium.de

> Mr. Rietze,

>
> Thank you for your prompt response to my inquiry.
>
> I understand your reluctance to tell me that it will work, only to have me be disappointed with the results. As I have neither the glass nor the skill
> to properly figure 6, 8, or more lens surfaces, my effort will depend upon being able to obtain the best results possible with an off-the-shelf coma correction solution.
>
> Perhaps I can word the question in another way; I am still in the early stages of grinding, and can push the focal length a bit longer, if necessary to compensate. My mirror is, as I mentioned, 150mm in diameter.
>
> Given that the MPCC is tuned for f/4.5, and for a focal-plane-to-outer-lens

> distance of 55mm, can you give me an idea of the fastest f/ratio that you would recommend the MPCC be used upon? Suppose that I were to say that I would find a 25-micron spot size acceptable, and that my CCD is 2/3 the size of a 35mm film frame (The diagonal width of the CCD in question is ~28mm)?
>
> I am earnest in my desire to design my amateur astrograph around your MPCC product. Can you help me to design a primary that will work?
>
> Thank you again for your time and effort on this matter.
>
> Regards,
> Jimbo S. Harris

> Newark, CA, USA
>
> At 07:23 AM 4/25/2006 +0000, you wrote:
>
> >Dear Mr. Harris,
> >
> >we are sorry but it is very unlikely that our MPCC will work for your >telescope.

> >At first it is not calculated for f3.3, maybe it is possible to modify the
> >main mirror but a much better approach is to calculate a individual coma >corrector and field flattener for this telescope.
> >Second the free diameter is 38mm, this is enough for f4.5 but not for >f3.3. With 35mm film or full frame CCD you will have severe vignetting. >You will need around 68mm free diameter for the necessary 55mm between the
> >outer lens and the focal plane.
> >There are special correctors for very fast Newtons on the market, they are

> >built with the necessary geometrical and optical sizes. Maybe a Wynne type
> >is that what you need.
> >Please excuse, but our product line doesn’t include such correctors.
> >
> >Best regards,
> >

> >Martin Rietze
> >BAADER PLANETARIUM
> >
> >
> >——– Original Message ——–
> >Subject: MPCC hyperbolic mirror overcorrection factor (21-Apr-2006 6:02)

> >From: jimbo@jimbo.net
> >To: service@baader-planetarium.de
> >
> > > Hello,
> > >
> > > I am an amateur astronomer and amateur telescope maker and wish to design

> > > my primary mirror around your company’s coma corrector. I am currently
> > > grinding a 150mm mirror which I intend to turn into a 500mm focal length
> > > f/3.3 Newtonian astrograph.
> > >

> > > I intend to use the lens primarily with a DSLR, but might also use it for
> > > some 35mm film photos (I will not use it for medium format).
> > >
> > > Such a fast mirror will have a severe amount of off-axis coma, and I have

> > > decided to use the Baader MPCC to correct it. I understand that the > MPCC is
> > >
> > > not tuned for such a fast f/ratio (is it correct that it is best tuned for
> > > f/4.5?). Also, I understand that changing the distances between the focal

> > > plane and MPCC and between the MPCC and the mirror can help the MPCC to
> > > work to best effectiveness. However, from my understanding of how coma
> > > correctors work, it is my belief that I should also overcorrect my mirror
> > > to be somewhat hyperbolic in order for the MPCC to have the most positive

> > > effect.
> > >
> > > My question is this. By how much should I overcorrect my mirror? An answer
> > > in units of either Schwarzchild constant, eccentricity, or wavelengths of

> > > 550nm light would be most appreciated.
> > >
> > > I thank you in advance for your help.
> > >
> > > Sincerely,

> > > Jimbo S. Harris
> > > Newark, CA, USA
> > > 37° 31′ 44” N

> > > 122° 00′ 42” W
> > > http://www.jimbo.net/astro/
> > >