How dirty a mirror?
Scatter
I've just got a 2nd hand 8" dob and am thinking that I'm going to have
to clean the primary mirror (eek). Rather than just a fine layer of
dust, it seems to have a number of "splotches" on it.
Due to the weather, I haven't been able to view any stars through it
and am not sure if even this level of gunk would have an impact but it
can't be good.
Any thoughts? I know that I'm going to have to take extreme care
(orion provides quite good instructions on how to clean the mirrors)
and am wondering if others have had to perform this procedure.
Dennis Woos
"Scatter" <us...@eeepc-r.domain_not_set.invalid> wrote in message
news:slrnhpql2...@peewee.domain_not_set.invalid...
The best thing you can do is not clean. It is likely that you will think it
needs cleaning when it doesn't, as it takes a surprising large amount of
dirt to produce any noticeable degradation. Also, there is a significant
danger of scratching the mirror when removing it and then re-installing it,
so be extra careful. If you have to clean (for psychological reasons or
because it really is that dirty) then I recommend standing the mirror on
edge and spritzing it with distilled water, and using the tip of unscented
Kleenex to suck up any drops that won't run off the surface. I wouldn't use
canned air to assist with this, as I have seen the propellant shoot out and
mess up a mirror. This very light bath won't get it sparkling clean, but it
will do what needs to be done so long as you aren't dealing with animal
droppings, etc.
Dennis
palsing
wrote:
http://www.youtube.com/watch?v=9Y8xFnXFVGQ
http://www.visualastronomy.com/2008/10/how-to-clean-your-telescopes-mirror.html
http://www.dvaa.org/ATM/MirrorCleaning.html
\Paul A
Chris L Peterson
<us...@eeepc-r.domain_not_set.invalid> wrote:
>Any thoughts? I know that I'm going to have to take extreme care
>(orion provides quite good instructions on how to clean the mirrors)
>and am wondering if others have had to perform this procedure.
Personally, I'd clean it. I wouldn't want to start out with a new scope
and a dirty mirror. In practice, the dirt will only slightly affect
performance. But cleaning it will allow you to clearly see what you have
bought (and if it has any existing damage), as well as getting you more
comfortable with the process. While you shouldn't have to clean the
primary very often (perhaps once every year or two) it's something you
shouldn't be afraid to do. And despite all the warnings, it isn't
particularly difficult to clean a mirror without damaging it. Just
follow the instructions and use some common sense.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
palsing
wrote:
By the way, once you have successfully cleaned the mirror, don't be
discouraged if it has some of the coating missing (dark spots or
obvious flakes) or if there is some hazing on the surface, it is very
unlikely that this will affect the view in any significant way. I had
a 20" telescope (which is still being used regularly today), that has
not been re-coated since it was new in 1990, and that mirror has many,
many spots on it where the coating has simply gone away, but its
performance is still outstanding. I calculate that those spots are
only about 2% of the surface area...
I've observed with some very well-known amateurs at the Texas Star
Party, for example, and you might not believe the condition of their
mirrors after 4 or 5 days of those legendary Texas dust devils. With a
quick look at their mirrors you might doubt whether they could see
anything at all, the dust is so thick, but the views the last 2 nights
are essentially the same as the views the first night, simply
wonderful.
My own 25" mirror only gets cleaned about once a year, and so for most
of the year it would be classified as 'dirty' by most folks.
Worry less, observe more, and all will be well in your world...
\Paul A
Martin R. Howell
> I had
> a 20" telescope (which is still being used regularly today), that has
> not been re-coated since it was new in 1990, and that mirror has many,
> many spots on it where the coating has simply gone away, but its
> performance is still outstanding. I calculate that those spots are
> only about 2% of the surface area...
Which is not to say that this statistic indicates a 2% loss in light
reflectivity for the mirror as a whole. Even bare glass reflects light to
a degree. At 12 years old, I ground and polished a 4 1/4 inch primary from
Edmund's Scientific (this was more than 45 years ago) and couldn't afford
to have it aluminized. . .but it did serve up some fine views of brighter
objects like the moon, Jupiter, Saturn, and etc.
--
Martin R. Howell
Dennis Woos
"Martin R. Howell" <ilikestars...@universe.com> wrote in message
news:16kbiptneibxw$.1xy1pv4amu2fs.dlg@40tude.net...
A guy in our club claims that the nicest view of the moon you can get is
with an un-coated mirror.
Dennis
palsing
> A guy in our club claims that the nicest view of the moon you can get is
> with an un-coated mirror.
>
> Dennis
I wouldn't be a bit surprised...
\Paul A
Chris L Peterson
wrote:
>A guy in our club claims that the nicest view of the moon you can get is
>with an un-coated mirror.
I've tried that, and it's not true. You get lousy contrast due to all
the secondary reflections and scatter off the back of the mirror and the
mirror cell.
wsne...@hotmail.com
> On Tue, 16 Mar 2010 17:10:14 -0400, "Dennis Woos" <dpw...@gmavt.net>
> wrote:
>
> >A guy in our club claims that the nicest view of the moon you can get is
> >with an un-coated mirror.
>
> I've tried that, and it's not true. You get lousy contrast due to all
> the secondary reflections and scatter off the back of the mirror and the
> mirror cell.
Paint the back of the mirror flat black?
wsne...@hotmail.com
wrote:
While you have the mirror out check to see if a light is easily seen
through the coating. If so, it might be time to re-aluminize. Check
the diagonal mirror too.
Chris L Peterson
>Paint the back of the mirror flat black?
I'm sure that would be a big help. But there would still be some
secondary reflection off that rear surface.
Scatter
mirror and it's in pretty good condition with the coating only
slightly degraded (less than 1mm) at the edge in a few spots.
The secondary looks fine so I decided not to pull it out.
And then I panel beated the scope (an orion xt8 metal optical tube).
It'd obviously had it finderscope whacked as there is a dent around
it. The finderscope also didn't point anywhere near the correct axis.
I planed a block of wood to the shape of the optical tube, drilled
holes in it and another block, sandwiched them around the finderscope
mounting holes, put bolts through, and carefully tightened. Now the
finderscope actually points in the right direction.
What a great day, now if only I could fix my attrocious eyesight I'd
be fully set up for some quality skygazing.
Again, many thanks.
Dennis Woos
"Scatter" <us...@eeepc-r.domain_not_set.invalid> wrote in message
Don't neglect to collimate. Of course, after removing/installing the primary
and repairing the tube it is certain that the collimation is off. However,
even under the best circumstances regular collimation is required. It
doesn't need to take more than a couple of minutes, and makes a huge
difference.
Dennis