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Old 25-10-2020, 12:56 PM
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mental4astro (Alexander)

mental4astro is offline
Join Date: Jun 2008
Location: sydney, australia
Posts: 4,879

I am assuming your question is wholly about visual as your opening post is only referencing visual.

I have and had scopes ranging in focal length from 2000mm to 3100mm. This sort of long focal length in itself already means they are high magnification instruments and not expected to give expansive views of the whole of M42, etc. You won't be using very much under 66X to 100X with such scopes.

The only limit I find is eyepiece selection to match scope focal length with magnification range. From the eyepieces I used with my 8" SCTs to the 3100mm focal length Mak is I've added two eyepieces. I've added a 12mm that sits between the 14mm and 10mm I already had. Doesn't sound like much, but sometimes a mm or two can make the difference when seeing is just a little too touchy to go to a 10mm. The other is a 7mm as 5mm is used even less with this scope.

The 12mm I also found was the exit pupil sweet spot for this scope. During Sydney's lockdown I managed to do a planetary nebula marathon from home. You wouldn't think it, but with a 9" Mak I pinned 24 planetaries from my home in Sydney over two nights - doesn't sound like much but we are talking about inner Sydney and sketching each and every one.

Practical limits? No, I find no real difference between the them. As has been already mentioned seeing is the main limiting factor and it then does not matter the focal length. Yes you are reducing the maximum TFOV, but it is nothing that has concerned me. Really, just how many objects are larger than 1deg in size? Not very many. And if you have a goto mount, you are not really needing to pan to find an object at low magnification, are you?

When I went from the SCT's to the Maks, it was entirely about optical quality. I understood that with Maks it meant longer focal length, but it really was not anything that concerned me from the very start at all. It only meant adapting my eyepiece case a little and really, big deal.


Last edited by mental4astro; 25-10-2020 at 01:25 PM.
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