Anyone interested in cassegrain types

[Ajohn]

Wondering what to do with my flat field diffraction limited cassegrain project mirror I've downloaded a small application some one called Mike on Cloudynights wrote for people to rough out the design. Might be of interest to some adventurous people. It's here

http://www.cloudynights.com/index.php?app=core&module=attach&section=attach&attach_id=335273

It will do the sums for several types of cassegrains and is really intended to obtain numbers to put into an optical design package such as the free version of Oslo but I feel it's actually pretty accurate in it's own right.  It also calculates baffle tube dimensions. I need to ask about those results as they seem a bit big to me - could be that they assume the tubes are flared.

The ATM people on cloudynights are very helpful and generally always prepared to answer questions and even check designs. It's also a good place to ask about the various ways of testing mirrors but remember that any supplementary lenses needed have to be precision types really and all have some limitations one way or the other.

 I think that after spending rather a lot of time on my design that people who make dall kirkams with relatively low focal length magnifications aren't so mad after all but they still need a rather fast primary and have rather a lot of coma compared with the classical cassegrains. On the other hand they can be made with a lot less difficulty.

John

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[Peter Drew]

I've made several Schmidt, Maksutov, DK and classical Cassegrains up to 18" aperture so I qualify as "interested", however I'm not an optical worker and have relied on others for this so my main interest lies in the mechanics from a practical point of view. I have also made a 8" Slevogt and Schmidt cameras with perforated primaries that a small telescope could be fitted to examine star images at the film focal plane as a focusing aid. 

[Mr Spock]

As a planetary observer I have an observational interest in Cassegrains. A classical f20 with a small secondary would be perfect. I't a shame you can't get such a beast.

[Zakalwe]

As a planetary observer I have an observational interest in Cassegrains. A classical f20 with a small secondary would be perfect. I't a shame you can't get such a beast.

The Altair Astro Planeta truss Cassegrain is pretty close. 10", f18.

http://www.altairastro.com/altair-planeta-250mm-truss-tube-cassegrain-professional-grade.html

It has a place in my "dream observatory for when I win the Lottery (which will never happen as I don't play the Lottery! )". At nearly £4K I fear that that is where it will stay!

[Mr Spock]

It seems they can make those up to 22"... Lotto here we come

[Zakalwe]

It seems they can make those up to 22"... Lotto here we come

Why slum it with an Altair scope? APM do a nice 12" f20 at a mere €18,000

http://www.apm-telescopes.de/en/telescopes/reflecting-telescopes-ota/cassegrain/apm-professional-cassegrain-telescope-12.html

[Mr Spock]

On the 22" a 32mm eyepiece gives x300... woah!

[Ajohn]

For planetary and narrow field Dall Kirkam's  are very attractive. Several of the commercial cassegrains are reckonned to be DK's really. They have far more off axis coma than a cassegrain or even a newtonian of the same F ratio but planets don't exactly need wide field views. On axis they are just as good as a newtonian. The 2ndry is spherical so much easier to make and is also a lot less inclined to go out of alignment. The primary is a part formed parabola so less figuring is needed making it a little easier to make a fast mirror. The catch really is that the design depends on the rad of the 2ndry being correct for the design. Some people make a concave one 1st and then use that to check the actual mirror using fringes - never tried it but a green led should probably give decent ones, blue would be a bit more accurate.

The mirror main mirror is an ellipse so can nulled at it's 2 focal points (conjugates in Mike's program) I've seen comments that it's so difficult to set the test up that it's quicker to use Foucault. One design I just looked at would put the far point at around 350ins and that was for an F2.8 main mirror - needs a long room. Testing a mirror with a hole in it is reckoned to be tricky so generally people use plaster to stick the core back in after it has been cut out.

There is an update on the program but from what I can see it gives the same results

http://www.cloudynights.com/index.php?app=core&module=attach&section=attach&attach_id=440930

This will run under wine on Linux if winetricks is used to install the usual plus msfonts and the vbrun stuff. WineHQ show how to do that from the console.

 Sounds like cassegrain making is more profitable than newts. I'm interested in one around F8 but like I said off axis it wont match a newtonian but then these don't have very good off axis performance anyway. The design I looked at had around 14 arc seconds of coma at the edge of a 1/2 degree field.

John

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[Ajohn]

I just found a free ray tracing program. The actual download links are right at the bottom of the page.

http://www.busack-medial.de/download.htm

It comes with some example designs and doesn't look too complicated to use. It looks like it will also pre optimise designs but that will probably be more complicated to use. The doc's also contain a story of how optical designs are "stolen" by people at times.

I'm managing to run it on Linux via Wine with few oddities so far.   One thing it will allow people to see is how awful the off axis performance of Newtonians are especially as the F ratio goes down.

John

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