Re: Slightly OT: Comet Lulin and Kalman Filtering and signal averaging

On Feb 26, 12:45 pm, Jerry Avins <j...@xxxxxxxx> wrote:
c...@xxxxxxxxxxxxxxx wrote:
On Feb 26, 11:12 am, Jerry Avins <j...@xxxxxxxx> wrote:
c...@xxxxxxxxxxxxxxx wrote:
On Feb 25, 5:44 pm, Jerry Avins <j...@xxxxxxxx> wrote:
c...@xxxxxxxxxxxxxxx wrote:
   ...
I'm using a CCD camera on the guide scope and a CMOS based DSLR for my
main imaging. There are a lot of ways to go with this. The big problem
with using a DSLR is it blocks the hydrogen alpha wavelength (the red
associated with many emission nebulae). Some modify their DSLRs by
removing the deep red blocking filter.
With its sensor at the prime focus if a mirror, doesn't the camera body
block a lot of light?
Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Hello Jerry,
Prime focus doesn't mean the camera has to be inside of the
telescope's main tube. The camera (without a lens) mounts in place of
the eyepiece. So I use a Nikon bayonet mount to 2 inch eyepiece
adapter. More often one uses a T mount (which is a particular 37 mm
thread mount) consisting of two pieces. One is the camera bayonet to
female T thread and the other piece is a eyepiece barrel (either 1.25
or 2 inch) to male T thread. This way you can find the right adapter
for almost any SLR.
Two other ways of coupling the camera to the telescope involve using
an eyepiece. In one method the eyepiece projects into the camera body..
The other way the camera with its lens focused at infinity is place
right up against the eyepiece.
These last two methods allow for high magnification will all of the
attendant difficulties. One can magnify the prime focus method with a
barlow lens or its improved version called a powermate made by
televue.
Clay,

I'm still confused. In my Schmidt (in every SCT I know) the eyepiece is
behind a secondary. Unless the secondary mirror is flat and the
telescope oddly proportioned, the eyepiece is at the secondary focus.

I use T-mounts both on my 5" Schmidt and on the 80 mm refractor pictured
at the end ofhttp://users.rcn.com/jyavins/photoadapter.htm. The lens
isn't removable on my little pocket camera. The article is about using
it anyway.

Jerry
--
Engineering is the art of making what you want from things you can get..
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯- Hide quoted text -

- Show quoted text -

I think I see the problem - I'm using a Schmidt Newtonian and not a
Schmidt. So like the Newtonian it has a primary mirror followed with a
45 degree diagonal to pass the light out of the side of the tube just
like a Newtonian. But unlike the Newtonian, the Schmidt-Newtonian uses
a spherical primary mirror and has a Schmidt corrector plate at the
front ent of the scope. This design has a lower cost (than a Newt's)
primary mirror and the cost due to the corrector is more than made up
in the savings on the primary. This design has reduced coma when
compared to Newtonians if you are working with low F ratios. Typically
Schmidt Newts are F/4 or F5.6. If a Newtonian is made that fast you
will have strong coma at the edge of the field. However, Televue makes
Paracor which goes into your focuser and then the eyepiece fits into
the Paracor. The Paracor is an adjustable coma corrector specifically
for Newtonians and the reports I've heard about it are quite positive.

My scope despite its having a 10 inch aperature was quite inexpensive.
Its focuser and spotting scope were both quite crappy - I picked it
for its main optics. I replaced the spotting scope with a Telrad (I
highly recommend one) and replaced the focuser with a high quality
Crayford focuser made by JMI. I rarely use the computer's goto feature
- I can use star charts more quickly to find my targets. Back in the
80s, Celestron made a 5 inch Schmidt Newtonian called a Comet Catcher
(They were bootstrapping off of the interest in comet Halley's return
in 1986). Now Meade is the only mainstream manufacturer of Schmidt
Newts.

There is a company now offering Ritchy Cretians OTAs (optical tube
assemblies) for pretty chreap (compared to other suppliers of RC's).
These work very well as astrographs and they are completely coma free.

Thanks. I'm no longer confused. :-)

Did you look at the digiscope adapter inhttp://users.rcn.com/jyavins/photoadapter.htm? It lets me photograph
through any scope that uses 1.25" eyepieces, but since the camera's
optical viewfinder is then useless, focusing in daylight can require a
dark cloth, like an old-fashioned view camera. Here are pictures taken
with it:http://users.rcn.com/jyavins/downy_lady.jpghttp://users.rcn.com/jyavins/woodpecker1.jpg

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯- Hide quoted text -

- Show quoted text -

I like the woodpecker shot. Digiscoping is becoming common among
birders. I know what you mean about the LCD being hard to see in the
daylight. Looks like you made a slip collar for your lens - cool! If
the lens has filter threads then you may be able to find a step up
adaptor or combination to get to a T-thread. If your camera is held
well enough to not tilt the image plane, you may be fine with what you
have. The images look good!

I can actually see some nebulae, galaxies etcs through my DSLR when
connected on the telescope. This is more useful for centering the
image rather than checking actual focus. Although I can get close to
correct focus by looking at the stars in the viewfinder. For
finetuning the focusing I can shoot a series of digital pics until I
find the one that is sharpest. And to help with this I use the numbers
on the focuser's adjustment knobs. Going back to the best number from
a previous viewing session starts me close. I have yet to try a
focusing aid where you cut a circle of cardboard to fit over the front
end of the telescope and the cardboard circle has three equilateral
triangular cutouts spaced every 120 degrees. When focusing on stars,
out of focus is shown as a tripling of each star. I'll may make one
this weekend since we are supposed to be cloudy and rainy. I wonder
how well such an aid works. People advertise these aids for quite a
bit on the web and I have plenty of cardboard and a razor knife!

Clay

.

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