F/ Ratio questions

[spaceboy]

Hi

In last months sky at night cover cd there was a handy little bit of software SCOPE CAL 3.0 for working out TFOV, MAG etc depending on your app & F/ Length. While messing around I notice that if you barlowed an ep the f/ ratio changed ie: F/5 becomes F/10. I have also read that TV paracorr increases magnification by 1.5x in the same way as a barlow and this improves the edge sharpness. From what I gather this is the same in any high end EP, all have some kind of barlow arrangement with the lenses to acheive the same results. The point I'm trying to grasp is.... can you not just barlow or powermate cheaper ep's to resolve soft stars at edges as slower F/ ratios don't seem to have the issues that fast scopes do when it comes to edge sharpness. I know the argument then would be it defeats the object of having a short F/ratio as images are different. I only purchased a 250 dob as it gives me more app for the £££ not because it was a fast scope. I have seen in a previous SGL thread http://stargazerslounge.com/astro-lounge/77512-focal-ratio-vs-aperture-size.html that it is argued that F/ ratio doesn't make any difference anyway. I have always been impressed by barlowed EP's and always thought it to be the improved eye relief that made the difference but I have also noticed that the stars do also appear sharper, has anyone else noticed this?

Is it not cheaper to buy a good quality barlow of powermate and longer focal length EP's?? this way you don't need to splash the cash on expensive EP's (Baader make 68' fov for £99) you would benefit from good eye relief offered by longer focal EP's. Granted any EP barlowed can be like a scaffold bar hanging out the focuser but once balance there wouldn't be much weight differences between eyepiece changes.

Has anyone come across this question before? And has there been any reviewed tests carried out? or is it not an option worth considering?

[Matthew.Blake]

F ratio is more important for photography as faster scopes mean shorter exposures and thus less worry about poor tracking.

As for cheaper eyepieces, whilst slower scopes are more forgiving of eyepieces I think it's worth spending the extra. I just bought my first Nagler and I consider it an "eyepiece for life". Regardless of what scope I get in the future I know I will never need another 7mm eyepiece (at least until I can afford the Ethos range....)

[brianb]

The point I'm trying to grasp is.... can you not just barlow or powermate cheaper ep's to resolve soft stars at edges as slower F/ ratios don't seem to have the issues that fast scopes do when it comes to edge sharpness.

Yes, to a certain degree. But you do lose absolute field of view, and Barlows (especially cheap ones) tend to add their own aberrations. The longer effective focal ratio doesn't help with collimation issues, either, a short focus instrument needs collimating just as carefully when used with a barlow as without one.

Also - the TV Paracorr is 1.25x not 1.5x, and deliberately introduces aberrations to compensate for the coma inherent in fast Newtonians. It will definitely NOT improve the view in any other type of scope ... and the glass it contains does reduce the visual threshold by ~0.1 mags. It is however invaluable for visual low power work with large Dobsonians (over about 12"), which tend to work at about f/4 for reasons of keeping the tube length with sane limits.

[michael.h.f.wilkinson]

The main issue is you still have to deal with the light cone coming out of the fast objective. Whether it is the barlow or the EP, makes no difference. The "barlow part" of many EPs is also in place to increase eye relief. Besides, the aberrations of this "barlow part" are matched to those of the rear end of the EP, so that they cancel each other out as much as possible. This is not possible with a separate barlow, which therefore needs 3 or 4 lenses, rather than just two, adding to the amount of glass.

I use barlows mainly for imaging, not extending my effective EP-collection.

[spaceboy]

Thanks guys