ASTR: Guide Scope

I just purchased an 8" LX200GPS. I have a DSIC and DSIC2 cameras and want to
use one for guiding. What is a good scope to use for guiding without
spending a fortune? I have spent enough already for the LX200. :-) I do have
an EXT90 sitting around. Would that work? Thanks.

Barry

Due to the problem of getting enough rigidity between two scopes I'd say
going with a good off axis guider would be both cheaper and yield better
results. Only if you plan to track comets or asteroids much of the time
would I go with a second scope. A scope with its own mount like the
ETX, unless you have only the OTA make lousy guiders as there's always
flex in that type of set up and the ETX is especially sloppy in this
regard then you still have mirror flop to deal with. 40 years ago I
tried to make the far more stable Questar work as a guide scope on its
mount. Total disaster. Even if the mount held rigid then the mirror
would flop. I'd think I tracked perfectly and have lousy shots due to
flex between the two systems. Only when I went to a rigidly mounted
2.4" refractor did I get good results and then only about half of the
time. There was no such thing as an off axis guider back in those days.
When the first one came out I was in heaven! No more shots that
appeared well guided but weren't. Finding a guide star was so much
easier. Rings are very frustrating to try and adjust and if you miss
tightening something enough things move and you don't know it. But
tightening often moved the star too much. With the off axis guider I
needed only one third the time to set up a shot and I knew by the end of
the shot if it was worth developing (unless I screwed up focus that is).

Rick

--
Correct domain name is arvig and it is net not com. Prefix is correct.
Third character is a zero rather than a capital "Oh".

BClay wrote:
I just purchased an 8" LX200GPS. I have a DSIC and DSIC2 cameras and want to
use one for guiding. What is a good scope to use for guiding without
spending a fortune? I have spent enough already for the LX200. :-) I do have
an EXT90 sitting around. Would that work? Thanks.

Barry

I do have an off axis guider by Meade but the problem I have with that is
one camera will be focused and the other is not.

"Rick Johnson" wrote in message
...
| Due to the problem of getting enough rigidity between two scopes I'd say
| going with a good off axis guider would be both cheaper and yield better
| results.

BClay wrote:

I do have an off axis guider by Meade but the problem I have with that is
one camera will be focused and the other is not.

"Rick Johnson" wrote in message
...
| Due to the problem of getting enough rigidity between two scopes I'd say
| going with a good off axis guider would be both cheaper and yield better
| results.

I'm not familiar with the Meade off axis guider. You may need to add an
extension tube to one or the other other camera (most likely the main
camera) to get them to focus the same. Note the off axis guider need
not be an exact focus like the main camera. Close is plenty good so you
should be able to get close enough just by using an extension tube as
needed and just slide the off axis camera back and forth in its draw
tube until it is reasonably focused when the main camera is at exact
focus. Just make sure both are rigidly set and there's no vignetting of
the main camera. Guiders that are designed for visual guiding would
focus differently when used with a CCD for guiding as the camera usually
needs more in focus than an eyepiece. I expect with the main camera in
focus the guider is too far out to reach focus. So you will likely need
the extension on the main camera to bring it out at as far as the
guider. I had to do that with my off axis guider.

Rick

--
Correct domain name is arvig and it is net not com. Prefix is correct.
Third character is a zero rather than a capital "Oh".

I did have the Celestron guider but the threads didn't quite match and was
very hard to screw onto my 6.3 Meade focal reducer. I got the Meade guider
because of the threading issue. I already fabricated a solid mount system
for the ETX90 to the LX200. We'll check that out and then if it that doesn't
work too well then I'll get some extensions for the main camera. Either one
of my cameras, without the nosepiece, will screw onto the back of the Meade
guider but it makes sense that they should be about the same distance. The
prism in the Meade guider is also twice the size of the Celestron guider. I
believe that would help as far as field of view for the guider? Thanks Rick.


| I'm not familiar with the Meade off axis guider. You may need to add an
| extension tube to one or the other other camera (most likely the main
| camera) to get them to focus the same. Note the off axis guider need
| not be an exact focus like the main camera. Close is plenty good so you
| should be able to get close enough just by using an extension tube as
| needed and just slide the off axis camera back and forth in its draw
| tube until it is reasonably focused when the main camera is at exact
| focus. Just make sure both are rigidly set and there's no vignetting of
| the main camera. Guiders that are designed for visual guiding would
| focus differently when used with a CCD for guiding as the camera usually
| needs more in focus than an eyepiece. I expect with the main camera in
| focus the guider is too far out to reach focus. So you will likely need
| the extension on the main camera to bring it out at as far as the
| guider. I had to do that with my off axis guider.
|
| Rick
|
| --
| Correct domain name is arvig and it is net not com. Prefix is correct.
| Third character is a zero rather than a capital "Oh".
|

Mirror flop in the ETX will be a real problem. You can lock the mirror
in a LX200GPS but not the ETX. If you have an old LX200 without mirror
lock then the problem will be even worse but think you have a new one.
Just be sure to use the mirror lock! That will mean an external
focuser. My mirror lock changes focus very slightly when engaged so I
can't focus using the mirror focus then engage the mirror lock. But I'm
working at 14" not 8" so that may make a difference, though I doubt it.
Also once locked remember not to use the mirror focus or you can
damage the mirror lock and possibly the focuser. A friend did just that
to his 10" GPS. It's now at the factory being fixed. I used Meade's
external focuser that comes with the GPS with good results except it
couldn't support my STL camera without flexing. It could handle the
ST-7 just fine. You likely got one with your GPS and it should carry
your camera without a problem. I also found some vignetting with it but
I'm using a 35mm size chip. It would handle your set up just fine.

The mirror in my ancient Questar is far more stable than any ETX I have
used and it flopped too much with my Newtonian 10" F/5. Short exposures
in a small area of the sky will reduce the problem. But be ready to
throw out those frames the flop got you on.

When using a separate scope to guide there's a strong tendency to move
it too far away from the FOV of the camera to find a nice bright guide
star. This will cause rotation of the field of view of the camera
unless your polar alignment is dead on. With the guider just outside
the FOV of the camera then you can survive a so-so polar alignment that
would be fatal if you go very far from the FOV for one. That was
another trap I fell into as a novice. When you combine that with flex
and with mirror flop you have three variables and determining which is
ruining your shot is a major headache, especially when you have two or
even all three of the problem.

For polar alignment use the drift method. As long as it stays on the
crosshair (north south one that is) for a couple minutes you should be
good enough for guiding IF you guide in or right next to the FOV of the
camera. Go a degree or so away and you could have problems, assuming
you survive the flex and mirror flop issues.

In my astronomy club we've had many beginners in astrophotography. Not
one could make a separate guide scope work very well except for piggy
back photos with a camera and telephoto lens of 300mm or less focal
length. Once experienced, yes but not as a novice.

In short I'd use track and accumulate with all its deficiencies before
I'd try and use an ETX atop a SCT for guiding. I've been taking deep
sky shots for nearly 50 years now and I doubt I could get it to work.
If others here have please let us know how you did it!

Rick


BClay wrote:
I did have the Celestron guider but the threads didn't quite match and was
very hard to screw onto my 6.3 Meade focal reducer. I got the Meade guider
because of the threading issue. I already fabricated a solid mount system
for the ETX90 to the LX200. We'll check that out and then if it that doesn't
work too well then I'll get some extensions for the main camera. Either one
of my cameras, without the nosepiece, will screw onto the back of the Meade
guider but it makes sense that they should be about the same distance. The
prism in the Meade guider is also twice the size of the Celestron guider. I
believe that would help as far as field of view for the guider? Thanks Rick.


| I'm not familiar with the Meade off axis guider. You may need to add an
| extension tube to one or the other other camera (most likely the main
| camera) to get them to focus the same. Note the off axis guider need
| not be an exact focus like the main camera. Close is plenty good so you
| should be able to get close enough just by using an extension tube as
| needed and just slide the off axis camera back and forth in its draw
| tube until it is reasonably focused when the main camera is at exact
| focus. Just make sure both are rigidly set and there's no vignetting of
| the main camera. Guiders that are designed for visual guiding would
| focus differently when used with a CCD for guiding as the camera usually
| needs more in focus than an eyepiece. I expect with the main camera in
| focus the guider is too far out to reach focus. So you will likely need
| the extension on the main camera to bring it out at as far as the
| guider. I had to do that with my off axis guider.
|
| Rick
|
| --
| Correct domain name is arvig and it is net not com. Prefix is correct.
| Third character is a zero rather than a capital "Oh".
|