Tracking down astigmatism (was: Sanding down secondary mount...)
- --- Joe Huber wrote:
> How do you tell if the astigmatism in from the secondary?Joe-
Astigmatism seems to be an error born more of the way an optic is
held in place, rather than the figure of the optic itself. Not
always, but usually (an example of the exception: I believe Suiter
talks about secondary mirrors being made using non-flat surfaces, and
that astigmatism can then be manufacturered right into the mirror
itself, but I believe that's rare). So, the good news is that it's
So, there are basically four potential sources of astigmatism:
Your eyes - presumably you would know if your vision is astigmatic,
so most of us can rule this one out right away.
The eyepiece - I've heard that eyepieces can be the source, but I
don't think I've ever heard anyway say "I found such and such an
eyepiece to demonstrate astigmatism". It could be that we're usually
dealing with higher-quality eyepieces these days, so this is largely
an artifact of older eyepieces? In any event, you can rotate an
eyepiece in the barrel; if the astigmatism doesn't rotate in a
corresponding manner, then you can rule out the eyepiece.
Primary mirror - I can't remember if astigmatism is the same (or has
similar manifestations to) pinched optics. In any event, if the
clips on the primary are too tight, it will pinch the mirror and
deform the image. So, the clips on the primary should be one of the
first things checked (probably *the* first thing) if you notice some
evidence of astigmatism/pinched optics.
Secondary mirror - Most of us go through the prior list, and still
have some remaining astigmatism. This leaves the only remaining
source - the secondary mirror/mount. Assuming it's not the mirror
itself (which may or may not be a valid assumption, but it's probably
the best starting point), you should examine the secondary mount to
make sure that there is no pressure on the optic. Phil or someone
else can talk about the Synta holders, but the Guan Sheng holders are
notorious for imparting some astigmatism. That's one of the reasons
that I'm upgrading my holder, as I've reached the point where I can't
rid the system of astigmatism (I haven't tried Phil's latest
suggestion, but probably won't, since there are other reasons that I
want to replace the secondary mount as well).
Good luck - it sounds more daunting in writing than it really is.
Just proceed step by step, and I'm sure you'll be able to isolate it,
then figure out the best way to rememdy it (if need be).
- --- Phil Lefever wrote:
> Your holder is the same as mine (a bit smaller though) and RTV wouldAh, I forgot about that lip, and didn't actually realize that it came
> work just fine! The mirror rests against a flat lip all around the
> rear edge. There is a large opening into the holder body but ignore
> that. All you need is 3 pea sized dabs of RTV on the lip around the
> edge. Clean everything with alcohol first and you will be swearing
> when you need to remove the mirror!
into play (or *could* come into play, given the presence of the
foam). In any event, sanding down is one thing, but that level of
modification, given that I'm very likely replacing the secondary
anyway, is probably overkill for me. I really wanted to do something
to ensure that the secondary mirror was ok, but I think I'll do that
by simply ordering the mount first, using the older mirror, and
checking the image.
Incidentally, I don't think I have a way to measure the minor axis on
my secondary. Do you (or anyone else) know whether the secondary on
the older (Guan Sheng) XT8 is 1.83" or 1.85" - someone a few posts
back alluded to the fact that it might be 1.85". I can probably call
Bryan at Protostar too, to see if he has any knowledge about this,
since I'm sure he has a reasonable amount of experience with
secondary upgrades of these scopes.
> Also if you ever thought of a dew heater on the secondary installThanks, although I'm doing my very best to stay caveman (non-
> it before you glue the mirror in place. A DewGuard from
> Astrosystems is a excellent and simple choice!
electric) on my dob!
- --- In firstname.lastname@example.org, "Chris Nicholl"
> --- Joe Huber wrote:astigmatic, so most of us can rule this one out right away.
> So, there are basically four potential sources of astigmatism:
> Your eyes - presumably you would know if your vision is
And you CAN test for it, to find out if the amount you have is the
cause of the troubel, or you still have to work with the optics of
the scope: simply rotate your head while looking through the
eyepiece, and see if the astigmatic image rotates too.
> Primary mirror - I can't remember if astigmatism is the same (orhas similar manifestations to) pinched optics.
Physically, it is the same phenomenon, but because the mirror is
usually held down by three clips, the out of focus image is
triangular in symmetry and the triangle flips as you pass through
focus, whereas in astigmatism, it is the long axis of the ellipse
that flips as you pass through focus. If the hold down clips bend
the axis of distortion along one direction, you experience it as
astigmatism. If they set up three axes of distortion, you
experinece it as "pinched optics".
>Ah, I forgot about that lip, and didn't actually realize that it cameChris,
>into play (or *could* come into play, given the presence of the
>foam). In any event, sanding down is one thing, but that level of
>modification, given that I'm very likely replacing the secondary
>anyway, is probably overkill for me. I really wanted to do something
>to ensure that the secondary mirror was ok, but I think I'll do that
>by simply ordering the mount first, using the older mirror, and
>checking the image.
I think your best upgrade route it to cut off the lip of your holder
and RTV the mirror in place. It might completely solve your problem.
The question will lie in how you would attach your oversized GS
mirror to the new holder, you will likely have to glue it to the new
one as well as the shroud may not slip over it.
If your specific goal is to eliminate astigmatism RTVing the mirror
to your holder would give you a good enough test to tell if the error
was in the glass. This would allow you to know if you need a new mirror.
Heck it might even keep you from spending $100+....
I do understand about being squeamish about it though, but you survived
the 2" focuser swap so it should be easy!
Phil Lefever - Burnsville, MN
Amateur Radio Callsign - KB0NES EN34jt
C8-SP XT-10 C102 80WV
MNAA - Minnesota Amateur Astronomers