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GM8 rant



 
 
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  #1  
Old September 2nd 15, 04:20 PM posted to sci.astro.amateur
Sam Seagate
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Posts: 9
Default GM8 rant

I understand this mount is supposed to be one of the higher end mounts
in the astronomical community. After doing wide field astrophotography
for quite some time, I was given the opportunity to rent a GM8 along
with a small refractor for a month. My time is nearly up and I couldn't
be happier.

I couldn't spent every night working with this thing as I have to work
too, but the nights I did try to use it ended up in frustration. The
particular mount I had also came with Gemini 1 part 4. Even after
reading the 137 page manual over and over again, I still found the menu
system difficult to use to get to what I needed to. The buttons pushed
would sometimes switch and if I had used "Ra+" to access the DSO library
the first time, I might have to use "Ra-" the second. The there was
polar alignment.... forget it!!! Certainly not good enough to track
more than 30 seconds unguided. I laborously tried aligning the the
Losmandy polar scope to Polaris and the 2nd/ 3rd stars followed by the
"Polar Axis Correction" routine in the manual but tracking didn't
improve... spent at least 2 hours in the manual and menus each time
trying to get the thing aligned.

I was told that this was the mount to get if I was going to have to set
up on a regular basis, which is something I have to do. I was also told
that it was portable and easy to use. While I'll agree that it is
somewhat portable, as far as being easy to use, no way.

I had hoped to grab a few images of some prominent NGC and M objects
while the Summertime Milky Way was in place, but I spent the last three
weekends just trying to perfect the alignment on the darn GM8! I never
did reach the point where it would guide correctly and I DO find the
manual could have been better written.

If you are able to set this thing up in an hour with proper polar
alignment to take unguided imagery, I'd sure love to hear your story. I
certainly didn't and can't wait to return this thing tomorrow. Oh, one
person did suggest autoguiding, but if I'm already having trouble
aligning and getting unguided shots, how would shelling out $300+ on an
autoguiding set up make any difference? And no one I know of rents such
an accessory either.

You are welcome to the GM8, hope it works for you and I'm sure it does
in permanent set ups, but no good for those temporary users.

Sam
  #2  
Old September 2nd 15, 05:26 PM posted to sci.astro.amateur
Uncarollo2
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Posts: 803
Default GM8 rant

On Wednesday, September 2, 2015 at 10:20:16 AM UTC-5, Sam Seagate wrote:
I understand this mount is supposed to be one of the higher end mounts
in the astronomical community. After doing wide field astrophotography
for quite some time, I was given the opportunity to rent a GM8 along
with a small refractor for a month. My time is nearly up and I couldn't
be happier.

I couldn't spent every night working with this thing as I have to work
too, but the nights I did try to use it ended up in frustration. The
particular mount I had also came with Gemini 1 part 4. Even after
reading the 137 page manual over and over again, I still found the menu
system difficult to use to get to what I needed to. The buttons pushed
would sometimes switch and if I had used "Ra+" to access the DSO library
the first time, I might have to use "Ra-" the second. The there was
polar alignment.... forget it!!! Certainly not good enough to track
more than 30 seconds unguided. I laborously tried aligning the the
Losmandy polar scope to Polaris and the 2nd/ 3rd stars followed by the
"Polar Axis Correction" routine in the manual but tracking didn't
improve... spent at least 2 hours in the manual and menus each time
trying to get the thing aligned.

I was told that this was the mount to get if I was going to have to set
up on a regular basis, which is something I have to do. I was also told
that it was portable and easy to use. While I'll agree that it is
somewhat portable, as far as being easy to use, no way.

I had hoped to grab a few images of some prominent NGC and M objects
while the Summertime Milky Way was in place, but I spent the last three
weekends just trying to perfect the alignment on the darn GM8! I never
did reach the point where it would guide correctly and I DO find the
manual could have been better written.

If you are able to set this thing up in an hour with proper polar
alignment to take unguided imagery, I'd sure love to hear your story. I
certainly didn't and can't wait to return this thing tomorrow. Oh, one
person did suggest autoguiding, but if I'm already having trouble
aligning and getting unguided shots, how would shelling out $300+ on an
autoguiding set up make any difference? And no one I know of rents such
an accessory either.

You are welcome to the GM8, hope it works for you and I'm sure it does
in permanent set ups, but no good for those temporary users.

Sam


The GM8 is a very sturdy affordable mount. It have a precise worm drive and a somewhat complicated computer hand control, but having said that, it can be operated successfully by the novice as well as the seasoned astrophotographer.

Astrophotography is a difficult art, in that even with a perfectly polar aligned mount, you will not get the stars to sit still for long periods of time. The main reason is that only at the zenith do the stars actually move at the sidereal rate. Everywhere else they move slower due to atmospheric refraction. And even that is a variable dependent on air temperature and pressure.

Nevertheless, one should be able to properly polar align a mount without any fancy computer aids, assuming one knows how the stars move around the pole. One sure fire method is called the Drift method, which can be done in 10 to 15 minutes before the sky is really dark if you can see a few 1st mag stars. Google Drift method to find out how it is done. This method is the gold standard and is used by most advanced astrophotographers.

The method that I use with my Astro-Physics mounts is called the Quick Drift method, and only takes a few moments. It requires a GoTo mount such as the GM8 with Gemini that you used. Here is the method:

A) Altitude adjustment (3 to 5 minute routine)

1) find a bright star near the zenith and send the mount there. Using a simple right angle finder scope with crosshairs, center the star on the crosshair and recal or sync on that position.

2) Now flip the mount to the other side and acquire the same star with your GoTo. The star most likely will not be on the crosshair in the North-South direction (ignore any East-West error). Move the star half way to the crosshair using the polar axis altitude adjust and then the rest of the way with the hand control centering buttons. Now recal or sync that position of the star.

3) At this point theoretically your altitude axis is aligned correctly with the pole height, but to check that, just flip the mount back across the meridian to pick up the star on the opposite side again. If the star is on or very nearly on the crosshair in the N-S direction, you are done with the altitude adjustment. If not, simply repeat the centering of the star as in #2. Typical time spent is about 3 - 5 minutes. Now lock the altitude axis and do not adjust it further.

B) Azimuth adjustment (3 to 5 minute routine)

1) starting out with the star near the zenith on the finder crosshairs and scope on the East side, send the mount to a star somewhere in the west near the celestial equator, perhaps 40 to 45 degrees away from the zenith. Now bring the star to the N-S finder crosshair using the azimuth adjustment (ignore any E-W error).

2) normally that is all you will need to do, however you can check by going back to the zenith star, recenter it using the hand control centering buttons and recal or sync. Then go back to the star in the West and tweak your azimuth setting once more.

After 2 iterations you will be as polar aligned as needed for almost any kind of imaging. You should be able to get 10 minute exposures with round stars at low to medium focal lengths (500mm to 1000mm) and a typical CCD camera. You will need to turn on the programmed PE correction in the mount, otherwise you may get a small E-W error due to periodic error of the worm drive.. This is normal even in the most expensive high end mounts available today..

Good luck and don't give up so easily.

Uncaimaging

  #3  
Old September 4th 15, 04:30 AM posted to sci.astro.amateur
RichA[_6_]
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Posts: 1,076
Default GM8 rant

I wonder about Losmandy. I had a GM8. You could slice your fingers to ribbons if you accidentally got one caught in the lock holes where the head attaches to the tripod. They didn't even chamfer the frigging edges!!!
 




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