Help Picking an Off-Axis Guider, Please!

[Sox-n-Eagles Fan]

I'm thinking of ordering an off-axis guider (such as the Lumicon Cassegrain
Easy Guider or the Celestron Radial Guider) for astrophotography (I'm just
starting out). I have a Celestron 9.25 SCT Advanced GT. I've not tried any
astrophotography yet and wanted to know if this is a good option. I have a
Meade LPI that I figured I could use as the guide camera and then mount my
Nikon D70 for the actual photography.

Or is a better option to purchase a separate telescope to piggy back on the
9.25 to be used as a guide scope? I just thought the off-axis guider might
be the best option. Any suggestions would be welcome, including scope
types, mounts, piggy back accessories, etc. I'm very new to this exciting
hobby and don't want to just throw money away. Thanks!!!

Tom

[Chris L Peterson]

On Sat, 16 Apr 2005 16:45:42 +0000 (UTC), "Sox-n-Eagles Fan"
<tom_wade1996@nospam.yahoo.com> wrote:


I'd recommend a piggyback guider- a fast refractor between 200mm and
800mm focal length (it does not need to be very good). This will give
you a much wider field of view than you would get with an OAG, making it
a lot easier to get a guide star in the field. It also helps keep your
instruments close to the rear of the scope, which reduces problems with
flexure, puts you closer to the optimum optical position, and often
results in better guiding.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

[Armando]

Chris L Peterson wrote:

I don't agree with You: even if magnification is determinated by (focal
lenght)/(eyepiece focal lenght), only a long focal lenght (also by using a
Barlow) is adequate for guiding.
Remember that the Celestron 9.25'' f/10 focal lenght is 2350mm...
Moreover an eyepiece with a very little focal lenght is also not comfortable
and so it will affect guiding.
The guider resolving power has to be adequate to resolve the minimum allowed
stellar shift (according to the film used or the CCD pixel linear
dimension).

Clear skies, Armando

[Sox-n-Eagles Fan]

Both good points. So, do you also feel that a piggy backed guider scope is
preferred to a OAG?

Tom


"Armando" <a.b@c.d> wrote in message
news:hYb8e.68178$zZ1.1914434@twister1.libero.it...

[Chris L Peterson]

On Sat, 16 Apr 2005 17:29:49 GMT, "Armando" <a.b@c.d> wrote:


I don't follow. The OP wants to use an autoguider, so there is no issue
of eyepieces or magnification.



Precisely. And a good rule of thumb is that the guider-to-imager pixel
scale can be about 10:1 (that is, a guider pixel covers about 10 times
the area of sky an imager pixel covers).

I guide with a piggyback ST237 on a 200mm scope, and image with an ST8i
on a 2300mm scope. That is a pixel scale ratio of 9.2:1.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

[Armando]

Sox-n-Eagles Fan wrote:

If You find a star useful to guide by off-axis guider You could prefer
off-axis guider.
A Celestron C9.25 only to guide is excessive :-)
Remember also that if You use a piggy back guider scope it has to be fixed
"perfectly" to the OTA used for astrophoto, without any possible relative
movement (possible because of the rotation and "thermal" changes that can
cause expansions or "deformations" of the pieces used to couple the two
scopes).
Since many SCT are often affected by primary mirror shift, because of
primary mirror weight and focusing method, if You use a SCT to guide the
errors You detect and the corrections You make can be wrong: if You can
guide by off-axis guider prefer it! :-)

However sometimes You won't be able to guide by OAG :-(

Clear skies

Armando

[Richard Crisp]

the biggest problem with a piggybacked guide scope/camera is the liklihood
of differential movement between the mirror and guider.

I had a lot of problems with my C14 until I locked the primary mirror where
it couldn't move so easily.

http://www.narrowbandimaging.com/mir...e_c14_page.htm


The best solution wound up to be using a beam splitter and AO7 but
admittedly that is beyond what is absolutely necessary. An off axis guider
with a pickoff for the guider ahead of the filters will get you most of the
way there and still avoid the differential movement problem.

http://www.narrowbandimaging.com/dic...itter_page.htm


Chris is right that it can make the guidestars harder to find but you are at
least guiding without having your guider behind the filter which is the
worst aspect of the SBIG self-guided cameras. It is just better to get the
guider out from behind the filter.

http://www.narrowbandimaging.com/exposure_time_page.htm



"Chris L Peterson" <clp@alumni.caltech.edu> wrote in message
news:2tk261ll3dc0mgrpc95ka3qec88o6vhm4i@4ax.com...

[Stephen Paul]

I can't argue which is "better", but I'm guessing the OAG will add less
stress to the mount than a piggy backed solution. Considering that you're
using a 9.25 on the CG-5GT, it's something to think about.

"Sox-n-Eagles Fan" <tom_wade1996@nospam.yahoo.com> wrote in message
news:d3rffl$ov9$1@titan.btinternet.com...

[Chris L Peterson]

On Sat, 16 Apr 2005 18:22:46 GMT, "Richard Crisp"
<rdcrisp@sbcglobal.net> wrote:


True, it takes a lot of care to make things rigid. However, in this case
the OP is planning on using a D70 for imaging, so the exposures will
only be a few minutes at most- differential movement shouldn't be a
problem (with an unlocked mirror on my 12" LX200, I can go 30 minutes
before mirror shift causes differential movement).

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

[Sox-n-Eagles Fan]

Then my next question is: If I use a OAG for guiding, is my Meade LPI good
enough of a guide camera or do I need something more precise?

And thanks to all who are contributing to my education!

Tom



"Armando" <a.b@c.d> wrote in message
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