Camera lens as guidescope

[Dave_D]

So, on my never ending quest for better this, that and the other, I'm turning my attention once more to my guide scope.

Has anyone used something like a 500mm mirror lens in conjunction with a guide camera?

Now, I've read up a bit on main f/l to guider f/l and a lot of people are saying a 50mm finder is fine, but to be honest, I'd be happier if the ratio was somewhere between 25% and 50% of my 1200mm f/l newt.

I moved my guide scope onto the tube rings for better stability a while ago, so fixing a lens with a 3/8" (i think) tripod fixing is no issue whatsoever.

Any thoughts?

[VigdisVZ]

I think people with "50mm guiders" are talking about aperture, not focal length.

A 500mm focal length lens should work fine as a guidescope. As long as you can work out mountings and focus with it.

[Laurie61]

The only problem I can see is the speed of the 500 mm lens, they are quite often slow-ish f8 which would reduce the number of stars, bright enough, around the target you would like to image. 

[Stargazer33]

The only problem I can see is the speed of the 500 mm lens, they are quite often slow-ish f8 which would reduce the number of stars, bright enough, around the target you would like to image. 

You are also reducing your field of view; so again, reducing the amount of guide stars available to you.

[Psychobilly]

I have used a number of cheap telephoto lenses as guide scopes  something like a 400/5.6 or 500/f8 will work well if you have a sensitive enough guide cam...  With the 500/8 I never failed to find a guide star using Meade DSI IIC's as guidecams...

I used to have the guide scope co-aligned pretty accurately with the two imaging scopes and also used to use it as an electronic finder...  for alignment etc...  have never had to move a guide scope in the rings to get a  guide star either...

Peter...

[lensman57]

So, on my never ending quest for better this, that and the other, I'm turning my attention once more to my guide scope.

Has anyone used something like a 500mm mirror lens in conjunction with a guide camera?

Now, I've read up a bit on main f/l to guider f/l and a lot of people are saying a 50mm finder is fine, but to be honest, I'd be happier if the ratio was somewhere between 25% and 50% of my 1200mm f/l newt.

I moved my guide scope onto the tube rings for better stability a while ago, so fixing a lens with a 3/8" (i think) tripod fixing is no issue whatsoever.

Any thoughts?

Hi Dave,

A 500mm mirror lens F8, far too slow and narrow for guiding. A 50mm guider has an F ratio of 3.2 so you can see how much brighter and wider it is compared to an F8. It is also more than good enough for up to and slightly beyond 1 meter of FL.

A.G

[ollypenrice]

Remember that you are not trying to resolve guide stars, you are trying to allow their centroid to be calculated. SInce this is done to a fraction of a pixel I doubt that the FL matters much. Craig Stark recommends being in soft focus as well. A far bigger problem for you will be slight mirror shift in the Newt and only an OAG can counteract its effects. You may be tryng to solve a problem which doesn't exist.

Friends using refracting camera lenses for guiding have had good results. CHeap and fast.

Olly

[StuartJPP]

I have used a Canon 70-200mm f/4 L IS as a "guide scope" coupled to a QHY5L-II mono camera mounting it via a hand-crafted EOS to 1.25" eyepiece adapter. Works rather well. Even at 70mm there are a lot of stars that show but I think that at 200mm it is better scale for my imaging at 500mm.

I am determined to never buy a scope of any description and only use lenses that I have already got...though I never say never...

[Merlin66]

I'm using an ol' Zuiko 135mm telelens with an ATiK16ic as an electronic finder....works 100%