CGEM testing

[jambroseus]

I went out Wed to test my CGEM mount after having Astrotroniks tune it. As I stood in a small park close to home at 9 o’clock with the sweat pouring off me just standing there doing nothing and the mosquitoes buzzing around my head, I thought, THIS WAS NOT A GOOD IDEA. I almost threw in the towel twice but finally decided to stay, I was already soaked and had to shower anyway so what the heck.

I didn’t do any imaging but just used my cross-hair eyepiece with barlow to see how well she tracked. After some iterations with Polar Alignment and two-star alignment I finally got the GOTO dead on and was able to sit on Deneb for several minutes. I defocused the star and placed the cross-hairs in center and watched.  At 150 power I saw steady tracking, no bumps or skips – smooth as silk.    Excellent job Jason!  After several minutes there was a slight drift but this was due to my alignment. If I had plugged in my camera I feel I could have imaged a couple minutes or more with great results.

A couple tips for CGEM users: I found that after you do a Precision GOTO you can Sync on the object to improve tacking. A Precision GOTO just gets you there and centered but doesn’t sync the tracking to that location in the sky.

Next, using the Polar Alignment command you really have to make sure your GOTO is dead on. After following the procedure you should swap out one of the alignment stars with the star you were using, check the alignment error again and maybe repeat the procedure. Iterating on this may be the only way to get a really good PA.

Hope you find this helpful and thanks Astrotroniks!

John

[David (djrLX90)]

Awesome John, sounds like you are almost there and should be taking exposures of a few minutes in no time.

I just wanted to clarify one thing related to GEM operation.  You mentioned:

A couple tips for CGEM users: I found that after you do a Precision GOTO you can Sync on the object to improve tacking. A Precision GOTO just gets you there and centered but doesn’t sync the tracking to that location in the sky.

Syncing has no effect on tracking what so ever.  It only helps improve the pointing model in that specific portion of the sky making subsequent  GOTOs more accurate.  The tracking rate is fixed as a function of the mount's computer.

[jambroseus]

Thanks David. Agree syncing improves the pointing model but by doing so it tells the computer the true location of the object and thus tracking is improved. I've used this many times in the past with my LX200 with good results. But I think we are probably saying the same thing.

John

[David (djrLX90)]

Almost the same thing...the telescope will track at the exact same rate regardless of its position in the sky or what the pointing model looks like.  It is a fixed rate, designed in the hardware, that is as close to true sidereal rate as possible.

I think there is a semantics muddling happening :-)

[Craig & Tammy]

There is no real "computerized logic" in tracking for a GEM.  It is a simple motor turning a series of gears at a certain rate, dependent on NOTHING ELSE.  Tracking accuracy is based on polar alignment, gear precision and motor accuracy.  No syncing on stars or other GoTo alignments have any affect on this at all.

[David (djrLX90)]

ding ding ding ding .......

[El Paso Eric]

Sounds like you have your mount running well, John.  Thanks for sharing the results.   

Interestingly, when I was building my little mini-tracker, I used a microcontroller to control the stepper motor.  I used one, not because I had to change the rate, but rather to insure a constant rate.  It's a motor drive, but the microcontroller is used to control the phases and timing for the stepper motor.  "Servo motors" used on most mounts work similarly, requiring some sort of timing & feedback to drive them at the proper constant (sidereal) rate. 

But, it turns out that the Sidereal rate is not 1 revolution in 24 hours, as most would think.  It's actually one revolution in 23.9344696 hours (somewhat of a surprise to me).  So when doing the calculations for my gear train and stepper motor, the numbers were not nice even decimals.  The microcontroller I used was 8 bit and not a real math machine.  So I could only calculate Sidereal rate to 3 decimal places. 

However, when I did some error calculations, it turned out that the error between Sidereal and 24 hours was about 0.014% per hour (about 6 arc sec/hour).  That's significant for a telescope/camera set-up, but just using a camera lens of 70mm or so on a DSLR, this is many fractions of a pixel of error. 

It's also interesting to note that near the horizons, the rate is very slightly different (slower), since there is atmospheric refraction that takes place.  Losmandy takes care of this using what's called "King" rate.  It's still a constant rate, but just slightly slower.  This is why guiding is so important to good astro photos.  Regardless of where we're pointed, we can never control the atmosphere and how it affects the location of the stars.  Guiding will help smooth-out these atmospheric effects, as an average.  Some of the new AO devices will take it a step further, and try to correct some of the atmospheric effects in a much smaller time scale.

Sorry for the diversion, I was just thinking about my old star tracker.