Just picked up a nice older refractorPosted By John Biretta |
Great stuff! The Henry Paul story is interesting.
Posts Made By: John Biretta
May 20, 2010 08:57 AM
Forum:
Refractors
May 23, 2010 04:02 AM
Forum:
CCD Imaging and Processing/Solar System
Moon through a Questar 3.5, 5/19/10Posted By John Biretta |
Beautiful shot!! Excellent detail and scale.
May 28, 2010 07:30 PM
Forum:
Equipment Talk
What's it called?Posted By John Biretta |
I expect Mal Speer at Malco Precision Products
would know.
http://www.malcoprecision.com/
would know.
http://www.malcoprecision.com/
June 5, 2010 08:31 PM
Forum:
Equipment Talk
Power Cord Classic C8 SCTPosted By John Biretta |
Don Rothman is worth a try...
http://astronomy-mall.com/astro.parts.outlet/
http://astronomy-mall.com/astro.parts.outlet/
June 6, 2010 08:53 PM
Forum:
Equipment Talk
How to determine astigmatism source?Posted By John Biretta |
Firstly, I assume you are allowing maybe
1 hour for the scope to equalize temperature
with the outdoors. Also, it is best to test
looking nearly straight up.
Next... test the primary...
Carefully note the direction of the star pattern
elongation inside of best focus. Then rotate the
primary mirror by 90 degrees in its cell
(un-install mirror / rotate / re-install / re-collimate
as needed). The check the direction of the elongation
again. If it did not rotate at all, then its
pretty clearly the secondary mirror.
Next step would be to remove the secondary and
see if anything is causing undue stress on it.
Sometimes there is pillow batting behind the
secondary to press it in place, and removing
some of it will reduce the pressure on the mirror.
After that, maybe try replacing the secondary.
If both the secondary and primary are astigmatic,
you can usually find a rotation of the primary that
will minimize the total astigmatism.
If the astigmatism is small and in the proper
direction, you can sometimes make it go away
by heating the rear of the secondary mirror
with a small resistor. This will cause the
secondary to go slightly concave, hence
adding (or subtracting) a little astig.
1 hour for the scope to equalize temperature
with the outdoors. Also, it is best to test
looking nearly straight up.
Next... test the primary...
Carefully note the direction of the star pattern
elongation inside of best focus. Then rotate the
primary mirror by 90 degrees in its cell
(un-install mirror / rotate / re-install / re-collimate
as needed). The check the direction of the elongation
again. If it did not rotate at all, then its
pretty clearly the secondary mirror.
Next step would be to remove the secondary and
see if anything is causing undue stress on it.
Sometimes there is pillow batting behind the
secondary to press it in place, and removing
some of it will reduce the pressure on the mirror.
After that, maybe try replacing the secondary.
If both the secondary and primary are astigmatic,
you can usually find a rotation of the primary that
will minimize the total astigmatism.
If the astigmatism is small and in the proper
direction, you can sometimes make it go away
by heating the rear of the secondary mirror
with a small resistor. This will cause the
secondary to go slightly concave, hence
adding (or subtracting) a little astig.
June 12, 2010 02:13 PM
Forum:
CCD Imaging and Processing/Deep Sky
Antares/Rio Oph regionPosted By John Biretta |
I tried tweaking the contrast with some curves in
Photoshop. Lotta nice stuff in your image! It could
use some flat-fielding to bring the corners up.
Which version of the Canon 200mm is this lens?
Photoshop. Lotta nice stuff in your image! It could
use some flat-fielding to bring the corners up.
Which version of the Canon 200mm is this lens?
June 17, 2010 10:54 AM
Forum:
Eyepieces
field stop measurement for eyepiece fov-works?Posted By John Biretta |
These equations relating apparent FOV and the field
stop diameter are only an approximation, and do
not include effects of geometric distortion. Typically
one assumes that apparent FOV and the field stop
diameter are proportional (i.e. linear relationship),
but there can also be terms in the equation proportional
to the square, cubed, etc. of the field stop diameter.
For example, the Panoptics are noted for having
significant geometric distortion, so these higher order terms
become important.
stop diameter are only an approximation, and do
not include effects of geometric distortion. Typically
one assumes that apparent FOV and the field stop
diameter are proportional (i.e. linear relationship),
but there can also be terms in the equation proportional
to the square, cubed, etc. of the field stop diameter.
For example, the Panoptics are noted for having
significant geometric distortion, so these higher order terms
become important.
June 22, 2010 09:34 PM
Forum:
Mounts
crazy ideaPosted By John Biretta |
I've also wondered about using batteries for counterweights. One less heavy item to cart around. You could probably make one from a pair of 12V 17AH gel cells -- make yourself a battery carrier to hold both with the Dec shaft up the middle -- with an appropriate sized set of bushings and lock screw. No reason it needs to be round.
I suppose when the battery becomes exhausted you are no worse off than if you had a conventional counter weight -- just connect a standard battery as backup.
If you managed to drop the thing, you'd have a bit more of a mess -- battery acid -- but probably not too bad with a gel cell. Dropping a conventional counterweight is destructive, too.
I suppose when the battery becomes exhausted you are no worse off than if you had a conventional counter weight -- just connect a standard battery as backup.
If you managed to drop the thing, you'd have a bit more of a mess -- battery acid -- but probably not too bad with a gel cell. Dropping a conventional counterweight is destructive, too.
June 24, 2010 04:15 PM
Forum:
Telescope Making
Tube rings for a 4" DynascopePosted By John Biretta |
I'd be surprised if you couldn't find some cheap rings
meant for some 90mm to 100mm Chinese scope here on AM.
That and some felt of proper thickness (hardware store)
would probably do the job with least effort.
meant for some 90mm to 100mm Chinese scope here on AM.
That and some felt of proper thickness (hardware store)
would probably do the job with least effort.
June 25, 2010 08:23 AM
Forum:
CCD Imaging and Processing/Solar System
White screen when webcam hooked up :(Posted By John Biretta |
If it worked OK indoors without the scope, it should work fine
with the scope. Are you sure it is pointing where you think it
is -- were you possibly pointed at blank sky above the trees?
These typically have a very narrow field of view due to small
chip. Try placing a 6mm eyepiece in the scope and see where it
is pointed.
Try turning the gain / brightness / exposure times lower so
you see some grey levels -- not just pure white.
It is possible it is not reaching focus with the scope.
This is pretty common problem with cameras.
Try adding or removing any star diagonal in the light path.
Put a piece of paper behind the focuser and see where the
focal plane is located; you will need the CCD chip at this
same location.
with the scope. Are you sure it is pointing where you think it
is -- were you possibly pointed at blank sky above the trees?
These typically have a very narrow field of view due to small
chip. Try placing a 6mm eyepiece in the scope and see where it
is pointed.
Try turning the gain / brightness / exposure times lower so
you see some grey levels -- not just pure white.
It is possible it is not reaching focus with the scope.
This is pretty common problem with cameras.
Try adding or removing any star diagonal in the light path.
Put a piece of paper behind the focuser and see where the
focal plane is located; you will need the CCD chip at this
same location.
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