Jump to content


EP performance differences in f5 and f6 scopes


Recommended Posts

A lot of postings on SGL mention, sometimes directly, sometimes in passing, that f6 is a tipping point in focal length to aperture ratio below which better quality EPs are needed.

Could someone please suggest what differences I should be able to see if I take one of my present EPs and swap it between my SW 200p (f6) and SW 250px (f4.7)? Note both scopes are 1200mm focal length - its just the primary mirror sizes which differ.

I have a number of SW and Celestron branded EPs. From my reading on SGL, I have been learning that, although labeled Plössl and Super Plössl, they are probably of 4 element symmetric design but not truly Plössl. By accident of second hand scope purchases, I also have a number of duplicate EPs (2 x 32mm etc).

Clearly, with any telescope the better quality implementations of a particular EP design concept will perform closer to the design potential than the lesser examples, whether resulting from manufacturing cost compromises or just being an odd one which has slipped through the QC testing stage. But with a particular example EP, what differences should I expect to see due to the change in aperture?

Thanks for your information and wisdom.

Link to comment
Share on other sites

Could you tell us please?!

I feel a review coming on.

You have both 'scopes, I'd honestly be interested to hear what you think the differences, if any, are.


I could give you a full run down on the physical differences already, but on the observing front it's very early days, and so far I've only had the opportunity for a one hour go/no go test on the 250px after an initial quick collimation. I was happy with it however because I did locate the Ring Nebula and M81, split the double double and was wowed at Albireo.

But when I get some further clear nights I will try out some side by side comparisons. That's why I asked the question about EPs and what I should be looking out for!

Ultimately I'll have to make the choice between the two Dob scopes and decide which is the keeper.

Link to comment
Share on other sites

as with most things in the world there's no actual cut off, just a scale from lots of coma at say f4 and hardly any at f11. also, eyepieces show more at f4 and less faults at f11. I have never worried about focal ratio and tend to choose my scope based on cost (of course!), aperture and focal length as the latter governs eyepiece height and magnification/possible field

  • Like 1
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.