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f-ratio for imaging/ guiding

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Imaging; fast is good but very fast is also slightly tricky unless you pay a lot of money for the scope. It is hard to make fast scopes both from an optical and mechanical point of view. The cheap ones, like the budget imaging Newts, need careful input from the user before they will perform to the book. Beware the free lunch! I would avoid any budget instrument, myself, that went below about F6 because I want to go outside and get started. I don't want to be adjusting optics - though lots of people don't mind and I admire them for it.

So... if spending a lot I would consider any fast F ratio, right down into the F3s or high 2s like the Tak Epsilon. If spending on a budget I'd settle for slower, like an ED80 with reducer at F6.37. I would not consider anything below F8 and even at that there would need to be a reason for accepting something so slow.

(For example, only having a smallish mount and wanting to have the longest possible focal length it could carry. Or having demon large refractor optics where F7 is about as good as it gets.)

Guiding? The F5 ST80 is ideal because with a fast guidscope you can fit it with a rigid mounting confident that it will pull out guidable stars. Not having guide rings makes for a stiffer system and saves a bomb, but a fast guidescope is needed.


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Olly, interesting question this, I wonder what your take is on guiding in terms of matching the guide scope to the imaging scope. Some say half focal length or even the same is best. But that is quite a sweeping statement isn't it?

I use the 80ED to guide (so far in rings to find a bright enough star using only a webcam) but I am waiting for the lodestar and I'm playing with the idea to then reduce the 80ED with just a cheapo 0.5 reducer to get wider field and pin point guidestars. God or bad idea?

The RC I have reduced and flattened to F5.36 hoping to be able to keep it in check with only my sig mount...

Or in one question: What do you normally guide with?



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John, reassuring to hear! I put my kit on the bathroom scales and it's around 17 kg with every nut and bolt - all balanced by 15 kgs at the other end so I guess I am entering territory not ideal, but for my normal use seeing seems to limit anything I do so I can't really determine if guiding could or needs to be improved...


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I guess you can image at most f-ratios, but personally I wouldn't go past f10 as it would just take so long.

I would say the focal length of the guider is more important than the f-ratio. The f-ratio will give you more star selection in your guidescope so, fast is better. In my experience the focal length is more critical. If I use my finderguider which as 125mmFL with my 1200mm reflector then star trailing is obvious in 5min exposures. If I use it on my 675mm reflector then I can get 10mins.

With an OAG and sensitive guide camera (SX Lodestar), then the question of FL becomes irrelevant :-)

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