Posts Made By: Renato Alessio

I've used my 14mm UWA with a standard 2" Barlow that I got from University Optics without any problem - no vignetting. And the eye relief stays the same, it can't get "thrown" out.

I would have thought that the only times you'd strike noticeable vignetting problems was with low power 20 to 40mm eyepieces. And then, mainly with shorty type barlows.
Renato

I hope you like Barlows - I'm not ultra fond of them (though, I've got four of them). As far as I'm concerned the view is never quite good as in the straight eyepiece, though I'm told Powermates are much better

The best exit pupil for detecting and viewing galaxies and most other faint DSOs is 2mm. Your 10mm delivers that. If that's the focal length you use the most, then upgrading to a Nagler makes sense, the only question is whether to get a 9 or 11mm - I'd go for the 9mm.

Anyhow, if I were you, before starting the spending spree, I'd just get one 2X Barlow and start getting a feel for what powers you use the most during actual observing. Also, if you get an extension tube (e.g. like a long filter adaptor) it will increase the magnification of your Barlow.
Renato

Yes, I've found it handy visually, for example,
- To get double duty out of an Ultrawide eyepiece.
- To get a big exit pupil for use with the H-Beta filter.

That said, whenever I did the exercise of tracking down galaxies with and without the reducer (using eyepieces that gave roughly equal magnifications), I've always found it more difficult finding faint galaxies with the reducer attached.
Renato

I didn't like my 14mmUWA much at first - I had difficulty seeing the entire field.
Finally I realised that unlike my 9mm Nagler, it really needed to have half it's rubber eyecup raised up (obviously, my dark site isn't as dark as it should be).

I'm not sure why the 4.7mmUWA is so unloved, I think it's brilliant - with eye relief no worse than comparable orthoscopics, which I've also happily used.

My 6.7mmUWA I don't like much, with star images being less sharp at the edges. I did buy it second hand though, so I'm unsure as to whether it's performance is typical of a new one.
Renato

It'll work, but you'll find it fairly annoying as I did. The best aperture mask is one having the single biggest hole that just fits in between the secondary and primary mirror and misses the vanes.

That said, the four hole aperture mask is a pretty good way of checking collimation in a big fast dob. Defocusing the image should give four equal bullseyes.
I once used the four hole aperture mask to prove to a colleague that his dob was hopelessly out of collimation, despite his laser collimator telling him that it was perfect.
Renato

On my reflector's instruction manual, it explicitly stated that the little aperture was for eyepiece projection of the sun (otherwise the eyepieces would get too hot) and for viewing a bright moon.
Renato

Any focal length is useable, it just may not be optimal or pleasing.

In your f/4 scope, you'll get a 7mm exit pupil from a 28mm eyepiece. That's as bright an image as you can see if your pupil expands to the full 7mm.

The older you get, the less your pupil can expand, till a 20 or 24mm eyepiece may give the brightest image you can see.

Suppose your age and/or stray light limit your pupil size to 5mm. Then the image in your 20mm eyepiece (exit pupil=5mm) is as exactly as bright as in your 40mm eyepiece (exit pupil=10mm), it's just that the magnification is bigger in your 20mm eyepiece, so that you're optimising the view.

In your f/14 telescope, the 40mm eyepiece will only give you a 3.5mm exit pupil. If you got a 70mm eyepiece, you'd get a nice bright view with a 5mm exit pupil. But you'd also have a tiny field and the feeling that you're looking down a long well - which is why they don't bother making them.

That said I actually like unoptimised viewing. I suffer from astigmatism, such that at low powers I get flares out from the sides of all stars. With a 9mm or larger exit pupil, I can twist my eye off in one direction, and eliminate the flaring. I can't do the same with a 6 or 7mm exit pupil eyepiece, because moving my eye reduces the field I'm looking at, as I can't see one edge in the eyepiece.
Renato

I did a similar comparison, using a faint open cluster in Orion. It was invisible in my C8 in suburbia, but easily visible with my 80mm scope at a moderately dark site.
Renato

I tried a 4 hole aperture mask on my C8 some three or four years ago, mainly as a collimation tool. The holes were 66mm in diameter.

The images were okay.

But the images were even better when I just used one hole - the quality easily exceeding that of my 80mm ED glass refractor.
Renato

It doesn't matter where you place the mask, so long as the light going through it doesn't intersect any vanes. At my local club, a few people have built telescopes with a spot above their mirror box to slide the mask into.
A circle will do.
Renato