Off Axis Mask question...

I am relatively new at this (astronomy and newsgroups) and have been
exploring what I can with an 8" SkyViewPro. I have been learning a
good bit just "lurking" in here and didn't realize lurking was
considered impolite.

I am wondering if an off axis mask (perhaps made of cardboard at
first) would improve contrast on planets. I noticed improvements in
Mars with a polarizing filter and I am curious if eliminating the
secondary & spider from the light path (1st pass) would help even
more? If so, how critical is it to have the aperture perfectly
circular?

I thank you for sharing your experience.

David

On 24 Sep 2003 21:03:23 -0700, (David Showers) ...reflected:

I am relatively new at this (astronomy and newsgroups) and have been
exploring what I can with an 8" SkyViewPro. I have been learning a
good bit just "lurking" in here and didn't realize lurking was
considered impolite.

Lurking is by no means regarded as impolite, for in that you claim
to be learning by so doing, rest assured that your "tutors" consider it
anything but.

snip

Alan

Hi David !

It can help in ways that might not seem obvious at first. Bad seeing
effects can be reduced with a mask, but not always. Contract and cutting
down on the brightness of the planet to help see details is also helped with
a mask. One effect of a mask is to make the focal ratio of the scope higher
by effectively reducing the aperture, and this also helps with viewing the
planets.

Try it and see !
--
----------------------------------------------------------------------
Pinprick holes in a colorless sky
Let inspired figures of light pass by
The Mightly Light of ten thousand suns
Challenges infinity, and is soon gone




"David Showers" wrote in message
om...
I am relatively new at this (astronomy and newsgroups) and have been
exploring what I can with an 8" SkyViewPro. I have been learning a
good bit just "lurking" in here and didn't realize lurking was
considered impolite.

I am wondering if an off axis mask (perhaps made of cardboard at
first) would improve contrast on planets. I noticed improvements in
Mars with a polarizing filter and I am curious if eliminating the
secondary & spider from the light path (1st pass) would help even
more? If so, how critical is it to have the aperture perfectly
circular?

I thank you for sharing your experience.

David

Hi David !

It can help in ways that might not seem obvious at first. Bad seeing
effects can be reduced with a mask, but not always. Contract and cutting
down on the brightness of the planet to help see details is also helped with
a mask. One effect of a mask is to make the focal ratio of the scopehigher
by effectively reducing the aperture, and this also helps with viewing the
planets.

On the other hand, an aperture mask reduces the aperture by a factor of more
than 2, in this case reducing an 8 inch scope to 3 inches or less.

Since aperture is the number one factor in determining resolution, one is
paying a big price here. A filter is a more effective way of reducing the
brightness if that is the issue.

The 8 inch Sky View Pro is an F5 Newtonian. If the images are not sharp then I
would look first to collimation. Good collimation is critical for planetary
detail. Second of course is to make sure the telescope has had adequate time
to cool down and lastly is the seeing, the atmospheric stability.

A mask is relatively easy to make but giving up that added aperture in my view
is unlikely to provide more detailed images. In my experience it takes a
pretty bad night for a 3 inch scope to compete with a decent 8 incher.

jon

I am wondering if an off axis mask (perhaps made of cardboard at
first) would improve contrast on planets.

Hi:

An off axis mask can have its uses. If the seeing is bad, stopping down the
aperture makes planets _look_ better (you'll still be able to make out more
detail at full aperture by the end of the evening). They can also help scopes
with problem optics--sometimes. But usually the result is far worse than what
you'll have at full aperture.



Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html

Contract and cutting
down on the brightness of the planet to help see details is also helped
with
a mask.

A much better solution is to buy a neutral density filter. A 95% moon
filter can be used on mars to cut down the brightness without decreasing
resolution.

--
http://www.zansstuff.com/contact.html

I am curious if eliminating the
secondary & spider from the light path (1st pass) would help even
more?

It is a pretty inexpensive experiment. Give it a try. Observe with and without
mask.

If so, how critical is it to have the aperture perfectly
circular?

It is critical. And as smooth edged as is possible.

Also critical in my experience, is that your scope must be cooled down fully,
with a mask on a solid tube newt. Otherwise the tube currents will be funneled
out the mask opening. A counter measure of this is that you mount the mask with
a gap between it and the end of the tube. This helps!

Alternate counter measure B, is to install a fan beneath the mirror to pull air
down the tube and out the bottom, eliminating currents percolating up the tube
and out though your mask opening. This can also help clean the boundary layer
air off the mirror surface. Ah, complexity.

In truth, as others have said, if you are critically collimated, and properly
cooled down, then you may not be using an off axis mask much. I sure don't.

I have used it for sharpening doubles. But difficult ones require more
aperture. So it only works there up to a point. I used to use it on planets and
Luna, but the resolution of detail is clearly not there, and the dimmer image
reveals too many floaters nowadays!









john

I tend to agree with the general sentiment that collimation and adequate
cooling are much more important than seeing. What are the effects of using a
mask:
smaller aperture
no secondary obstruction
longer focal length
That will turn your scope into the equivalent of a 3" f/15 super achromatic
refractor..now if that is better than an 8" f/5 newt. I leave up to discussion.
One thing which hasn't been mentioned is the depth of focus..due to seeing th
efocal plane tends to move a bit. IN fast scopes, this is not a problem because
the depth of focus is big enough so that the motion stays within it.
Fast scope in bad seeings however are another story: there can be cases where
the focal plane motion due to seeing is larger by far tha the focus depth. IN
which case you'll never be at focus and the image shows poor contrast...in that
case stopping down the aperture will increase th edepth of focus which will
result in a focuseable image...but of course yu'll have less resolution which
in practice means that you have to have pretty bad seeing for aperture
reduction to do the trick
Alex

"MEIE6" wrote in message
...

One thing which hasn't been mentioned is the depth of focus..due to seeing
th
efocal plane tends to move a bit. IN fast scopes, this is not a problem
because
the depth of focus is big enough so that the motion stays within it.

I think I get the jist of what you're saying, provided the previous comment
intended to state that _slow_ scopes have a greater depth of focus, or,
stated otherwise, the range of "good focus" is wider than for a fast scope.
I've seen this documented in pictures that show how a fast scope has a
narrow range over which to achieve best focus, where a slower scope has a
wider range.

It makes total sense to me now that the wider the range of "good" focus, the
more adept the scope is at dealing with the change of focus brought on by a
"wiggly" atmosphere (scintillation (?)). IOW, a fast scope on planets is
going to perform more worser than a slow scope, simply because the slow
scope has a larger range over which focus is good (a sort of built-in AO).

This would also explain a lot wrt the OA Newt. I made the claim back in one
of those threads that the OA6.5 handled the seeing better than any other
scope we have at our disposal. At a direct F10 (as opposed to the F2 with 5x
secondary of the SCT), it is the slowest scope that we use in our group.

I'm not certain about the result between the SCT and the F5 Newtonian. I
think we've had this discussion here, but as usual I suffer from CRS (Can't
Remember Sh_t). The question is whether the F2 primary of the SCT actually
makes it a faster scope (more narrow focus range) than the F5 of the Newt.

-Stephen Paul

"Zan Hecht" wrote in message
...
Contract and cutting
down on the brightness of the planet to help see details is also helped
with
a mask.

A much better solution is to buy a neutral density filter. A 95% moon
filter can be used on mars to cut down the brightness without decreasing
resolution.

Zan,


I never liked neutral density filters. On the Moon, they make the whites a
bit muddy and the blacks a bit gray. I'd recommend avoiding neutral density
filters. If you do a lot of observing, your eye/brain will get accustomed
to the brightness level and the image will no longer look overly bright.

Clear skies, Alan