f30 mirror

[NewMoon]

Is there anything wrong with making a telescope with a mirror say 100mm diameter and focal length 3000mm or more and use it for prime focus photography of the moon?
The idea is to have a higher magnification than my current 1000mm scope.

[rmcpb]

There would be nothing wrong with that scope except it would be a real bu**er to move around. With that focal length the central obstruction should be tiny so it would have great contrast - good for the moon and planets, especially when equatorially mounted and tracking so you can get some nice longer exposures.

In the old days (1800s) it was quite common for the scopes to have these really large focal lengths, much easier on the eyepieces of the day, but it would be a bit like looking down a piece of water pipe.

[ving]

yup, much as rob stated... but how are you going to mount a 3m long scope?

[NewMoon]

Thanks for your replies.

At this stage I'm only looking for any problems with the theory of it.

Would there be any problems in buying a mirror like that?

Apart from the increase in magnification over the 1000mm one I also want the increase in the critical focus distance (depth of field) to make focuing less critical.
The aim of trying to keep with prime focus method is to avoid the eyepiece + camera lens.

I reckon with the relatively short exposure exposure times for the moon (even at f30) the tracking would not have to be perfect.
I'll work on that side of it when (if?) it looks ok in theory!

[janoskiss]

A 3x barlow would be the way to go to get a 3000mm effective focal length with your scope. With a barlow you can do prime focus like you are doing now.

Yes it the f30 works "in theory", but if that theory is sound it would tell you that you would need to go to a lot of effort to make a 3m telescope stable and sturdy (just the scope) and more effort still to mount it properly. And in the end, a 4" refractor with a barlow will perform at least as well (probably better).

[AstroJunk]

At f30, the difference in saggita (depth of curve) between a parabola and a sphere is insignificant, so you can make a spherical mirror which is pretty easy.

Now, with a spherical mirror, you don't have to worry about on-axis alignment. Tilt the mirror and it will come to focus away from the main path, and you won't even need a secondary. If you can make a tube rigid enough to hold the componets, then it doesn't matter that it would be impractical to use visually. Many of the early big reflectors used this method.

I'll try to explain this better when I've more time! Must dash ...

[mickoking]

f10-f12 would be a more practical focal ratio, an Apo Reflector:P

[NewMoon]

Thanks All,

Yes Janoskiss, I was thinking of placing the camera offset at the 'open end' and then either accept any distortion or correct it later.

Thanks AstroJunk, a 3x barlow would be much better but the only barlow I have used gave a very poor image. I have assumed that they are a kind of gimmick.
Are there good quality barlows that would be as good as a medium quality eyepiece in terms of sharpness and chromatic aberation?

[janoskiss]

Good barlows are not a gimmick. They do what they are supposed to. I doubt you would be disappointed with a Televue 3x barlow.

[AstroJunk]

At f30 there would be no distortion due to the offset.

BTW, what you are suggesting is quite mad, but don't let that stop you. f20 is more reasonable (I have a half finished 125mm f20 mirror somewhere). Every piece of glass you place in the way of yoursellf and the object will by definition make things worse - but a powermate is much easier!

[janoskiss]

I have two f15 4.5" mirrors in my possession. I've been thinking binos since I got them few years ago. The coatings are looking pretty thin now, but I did used to gaze at the Moon with one of them, without any EPs except the dodgy ones in my eyes. You just look at the focal plane. Very nice. :)

[AstroJunk]

I feel a 3d camera coming ...

Go on, you know you want to.

[cometcatcher]

Only that no company makes them. You will either need to know someone that grinds mirrors or grind it yourself.

The slowest mirror I've ever made was f8.8. Beautiful planetary scope but even that was long and needed a ladder for an 8 inch mirror.