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Approx length of subs on new CGEM?

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#1 amdizack

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Posted 21 November 2012 - 12:03 PM

I am about to pull the trigger on a new CGEM mount. I know this question depends on a lot of other elements, but can anyone tell me how long I can expect to be able to track unguided? Also, what length can I expect with the Celestron guidescope and Celestron auto guider?

Thanks!

#2 orlyandico

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Posted 21 November 2012 - 12:25 PM

well if you get a good CGEM it will have maybe 20" of periodic error (peak-to-peak).

if you're imaging at a scale of 2" per pixel (around 700 to 800mm focal length) the typical FWHM would be about 3.5" to 4" or about 2 pixels. So I would say probably 2 minutes would yield round or round-ish stars.

If on the other hand you get a "typical" CGEM which has 40" of periodic error, you'd be stuck with 1-minute subs. My CGEM has 40" and i consider it unusable for unguided unless you're using a camera lens or really short refractor like a 70mm.

A good PEC training can halve the periodic error (the PEC cannot get rid of the 8/3 harmonic although there are persistent rumors that Celestron is coming up with a fix for that). So perhaps you can go 4 minutes with a good CGEM and good PEC training, but only 2 minutes with a typical CGEM and good PEC training.

If the 8/3 fix ever materializes, it should reduce the PE to 5" level or so (my best guess) which means probably 5-8 minutes with a good CGEM or 4 minutes with a typical one.

with any sort of guider, I'd say 5 minutes is easy. Unless you get the dreaded DEC cogging. I believe mine can do 10 minutes most of the time but with some throwaways. Others have had better luck. Personally I think 20 minutes is really pushing it, but some people report success.

#3 Raginar

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Posted 21 November 2012 - 01:40 PM

Guided photos will be dependent on the weight you place on the mount and the focal length of the imaging scope. With a 6-800mm scope? I know I could take 10 minute subs without any issues assuming I kept the weight manageable. With a 2000mm scope and 35# of gear on it? It was considerably harder.

Unguided photos on a CGEM can be done on the order of 1-2 minutes; I think it's an exercise in frustration though.

#4 Mike Wiles

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Posted 21 November 2012 - 01:51 PM

Unless you're shooting with really fast optics, it'll probably be difficult to get anything useful in an unguided setup. When I had my CGEM I was able to shoot 90 seconds up to about 2 minutes and get acceptable results - but I was imaging at 560mm focal length and f/2. Brighter star clusters and things that don't require long exposure time will be good targets for unguided imaging at focal lengths and ratios any larger than that.

You'll definitely want to add an autoguider. The finder scope guiders that are all the rage do work quite well and won't break the bank or add a ton of weight. The CGEM's biggest problem is going to be guiding accurately if you find it difficult to keep the weight down. I was loading right at 30 pounds of gear onto mine back then and it was at its absolute limit.

#5 orlyandico

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Posted 21 November 2012 - 02:27 PM

here is my CGEM's guiding performance with 15lb on it (quite modest) - orion 100ED plus accessories

https://dl.dropbox.c...22-cgem-m57.PNG

#6 amdizack

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Posted 22 November 2012 - 02:55 AM

You'll definitely want to add an autoguider. The finder scope guiders that are all the rage do work quite well and won't break the bank or add a ton of weight. The CGEM's biggest problem is going to be guiding accurately if you find it difficult to keep the weight down.


Mike,
Thinking about the Celestron guidescope and autoguider on an 8" dovetail. Looking at the total weight, I would be way under limits. I have failed to find any real downsides to this combination for guiding. Your thoughts?

#7 EFT

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Posted 22 November 2012 - 01:10 PM

I think the advice on unguided and guiding imaging is right on. In regards to the Celestron guiding system, you should definitely look at some of the comments and threads regarding this system. There have not been many, but I don't recall seeing much in the way of positive comments on the Celestron guide camera at this point.

#8 amdizack

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Posted 29 November 2012 - 08:36 PM

here is my CGEM's guiding performance with 15lb on it (quite modest) - orion 100ED plus accessories

https://dl.dropbox.c...22-cgem-m57.PNG


Can you give me a little info as to what I'm looking at? Are the red and blue horizontal lines showing variance to perfect guiding? Sorry but this is all new to me ;)

#9 orlyandico

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Posted 29 November 2012 - 08:40 PM

the blue line is RA, and the red line is DEC.

and yes an "ideal" mount would have both lines flat.

the important number is the RMS which is 0.18 in the above image. A premium mount ($6000+) would give maybe 0.03 to 0.05

#10 JS999R

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Posted 29 November 2012 - 11:38 PM

Will the performance improve with the upgraded CGEM-DX even though I understand the mount is unchanged? Its clear the tripod has been beefed up with thicker legs and the support spines,which to me will increase stability. Does unguided tracking have more to do with the mount mechanics or the strength and stability of the mount? I don't image, but I would think both must play a role. It would be interesting to pose the same question to those CGEM-DX owners and then compare the results. Notwithstanding transportability and OTA weight issues, those results could very well persuade people who image to consider making a mount change or not.

#11 orlyandico

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Posted 29 November 2012 - 11:43 PM

Well a beefier tripod will always help reduce wind induced vibrations. But it will not reduce the periodic error, which is a function of the gearbox and main worm gear.

#12 EFT

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Posted 29 November 2012 - 11:59 PM

Well a beefier tripod will always help reduce wind induced vibrations. But it will not reduce the periodic error, which is a function of the gearbox and main worm gear.


Orly is correct. Mechanically, the mount will perform no better. That is, it will have the same range of periodic and random error as a standard CGEM. It may be more stable in regards to the tripod. However, I can say that the CGEM DX motor board does seem to run the motors smoother and there seems to be very little discussion of problems with the CGEM DX mount overall. If they start to sell the CGEM DX motor boards, I will seriously consider changing the board in my standard CGEM (I do have both mounts). Thus, in some respects, you may find the performance improved, while in others, it will be the same.

#13 Mike X.

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Posted 30 November 2012 - 02:01 AM

I have the feeling that the new CGEMs might have included the upgraded DX motor board

I copy and paste from the Celestron support (FAQ) page:
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" What are the differences between the CGEM and CGEM DX mounts?


The CGEM DX mount differs from the CGEM:

1) The CGEM DX has a 1-1/4” counterweight shaft versus ¾” and correspondingly bigger counterweights.

2) The CGEM DX has a heavier, bigger tripod. It uses the same tripod as the CGE Pro.

3) The CGEM DX has an adapter bell to hold heavier payloads on the tripod.

The two mounts use the same motors, motor boards and the same gear box."
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So could it be that they use the same board at this moment on the new CGEMs out?
If it is so..the CGEM could be a "better" choice considering that if needed a better tripod and/or CW shaft can be added (3rd party) in a later moment.

#14 EFT

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Posted 30 November 2012 - 10:35 AM

The two mounts use the same motors, motor boards and the same gear box.


To my knowledge, this statement is incorrect. I would be happy were it true and will certainly let you know if I see evidence of it in the mounts.






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