Flare

The other night I observed Saturn at low power with my 4" APO. I was using a 40mm Pentax XL and the 2" dielectric coated diagonal that came with the scope. The image of Saturn at low power had a bright orange flare on one side that totally obliterated image details on that side. I could not get rid of the flare with focus. It looked like an orange flame shooting out of the side of the planet. I never normally look at the planets at such low power.

I've always had focus issues with this scope, mostly on bright images. At first I thought they were due to astigmatism. The manufacturer checked it out and pronounced scope and diagonal to be fine.

Next I put a Starlight microfocuser on it (pinion assy only) as I thought I was overshooting focus. By doing that I found that the scope actually had a fairly wide focus range, but that the bright images really didn't clean up with focus. Apparently, the flare is there to some extent on all bright images.

Next good night I plan to set it up side by side with my other APO (a 3" Tak which always renders clean images) and try to get to the bottom of this. I plan to swap diagonals and eyepieces back and forth and also try straight-thru viewing to see if the flare follows any of the lesser components.

I'm wondering if anyone here has additional ideas that I might try. I'd like to get this fixed.
Bob

I'm wondering if anyone here has additional ideas that I might try. I'd like to get this fixed.
Bob

Bob:

You didn't say much about the scope in question, primarily the focal ratio. My first guess is that the problem is not with the scope, the diagonal or the eyepiece but rather with your eye.

A 40mm eyepiece in a F/9-F7 scope provides an exit pupil 4.5mm-5.7mm. These are large enough that a small amount of astigmatism in your eye will cause flaring as you describe as well as spikey stars. The test is to rotate your head and see if the flares move with it. I don't even bother with the low power views of something like Saturn or Jupiter because they will flare. The moon is a bit different as it is bright and large enough that you eye will contract to day light levels you are actually operating at a small exit pupil.

In general one loses detail when using such large exit pupils. There's a lot of light and it is concentrated in one place on the eye. If there were a problem with the optics of the equipment or if it was not thermally equilibrated, I would expect it to be worse at higher magnifications.

Jon

>>The other night I observed Saturn at low power with my 4" APO. I was using a 40mm Pentax XL and the 2" dielectric coated diagonal that came with the scope.>>

Rule of thumb: At low powers the eye defects are totally dominant, overshadowing everything else in the optical train. At high powers the optical system is totally dominant, overshadowing every defect in the eye's lens (except for floaters which are not in the eye lens).

There is absolutely no reason to look at planets with low powers. It is also quite common for a beginner to attribute optical defects seen at low powers to the telescope optics. Low power defects point to defects at the eye end (and this will include the eyepiece). High power defects point to defects in the front end (lens, mirror etc).

Rolando

If you see some aberration or error on one side of the image,
a good test procedure is to start rotating stuff. I agree
with others that it could be an eye aberration. Try rotating
your head relative to the eyepiece. Maybe stand on
one side of the scope, and then on the other side -- see
what happens.

Can also try rotating the eyepiece,
diagonal, scope in the rings, maybe put scope on other side
of the mount. Eventually you may find something which
you rotate where the error maintains its direction relative
to the rotation.

Could also be warm air pooling in the tube,
which will tend not to rotate with anything, but will be
minimized by pointing vertically.

Bob Young said:
....The image of Saturn at low power had a bright orange flare on one side that totally obliterated image details on that side.