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Typical Guided Performance of GM8G, GM811G, and G11

 

I am looking to setup an EAA rig for live stacking at outreach events (I am new to astrophotography). I am leaning toward using an autoguider to maximize the number of usable frames and allow for longer exposures (probably won¡¯t want to go beyond 4 minutes at outreach events). I am considering getting a Losmandy mount for this setup. Based on the datasheets it appears that the GM811 and G11 have better unguided tracking performance than the GM8. Does this translate to significantly better tracking when guided? What would be a typical level of performance for each of these mounts when guided? Is it reasonable to expect sub-arcsecond performance from a guided GM8G?
 

Liam,

A partial answer:

The GM8 RA drive uses a ring gear with 180 teeth around it. The GM811 and G11 use a 360 teeth gear that is about 2x the diameter of the GM8 design.? The larger ring gear with double the teeth gives about twice the sensitivity given identical worms.? So for better accuracy and stronger payload the G11 and GM811 can outperform the GM8.??

Getting subarcsec with autoguiding is achievable.? It requires excellent polar alignment and other factors to be set up right.? That's the effort required but many people do that all the time.? The choice of scope matters too...reflectors must keep the mirrors unconstrained, and you gave to be wary of "mirror flop".? Refractors don't have that issue... triplets are now super good quality and optical coatings are generally fabulous.??

Have fun...hope this helps.

Michael

On Thu, Feb 25, 2021, 9:36 AM <Liam.J.Cheney@...> wrote:
I am looking to setup an EAA rig for live stacking at outreach events (I am new to astrophotography). I am leaning toward using an autoguider to maximize the number of usable frames and allow for longer exposures (probably won¡¯t want to go beyond 4 minutes at outreach events). I am considering getting a Losmandy mount for this setup. Based on the datasheets it appears that the GM811 and G11 have better unguided tracking performance than the GM8. Does this translate to significantly better tracking when guided? What would be a typical level of performance for each of these mounts when guided? Is it reasonable to expect sub-arcsecond performance from a guided GM8G?

Sonny Edmonds
 

Hi Liam!
I got into this Astromaddness specifically for imaging. And I decided I'd need to be guiding before I began ordering anything.
I did try two Off-Axis Guiders before I finally gave up on that idea.
When I got a guide scope, and put my original guide camera on it, things started working well.

But if you have much hope of imaging, in my opinion you will need guiding.
many years ago of my AVX mount and an unguided run with it.
It's a funny 7 seconds!

--
SonnyE


(I suggest viewed in full screen)
 

Thanks all, I really appreciate the thoughtful responses. It sounds like the GM8G with a guide scope should be a pretty decent setup to start with. Since my focus is on live stacking at outreach events, I will probably mostly use it for the larger and brighter objects. I currently have a 750mm FL 6-inch Newtonian on a stock mount and a 200mm FL DSLR lens for wide field. Eventually I may look into getting an APO refractor somewhere in between those focal lengths.
 

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I just finally got a decent DSO image, so count me in the ¡°if I can do it, anyone can do it¡± category..

?

Tonight was below the normal.. 3 hours and I was at 0.92 arcseconds total RMS on both axis.. Normal is ~0.75, but my seeing was bad.. Polar Alignment is off by 5 arcmins..

?

Old style G11 with Gemini 2 bought used 3 years ago.. 130mm Triplet guided by an 80mm doublet, both at f/7..

?

Derek

From: [email protected] [mailto:[email protected]] On Behalf Of Liam C
Sent: Thursday, February 25, 2021 9:50 PM
To: [email protected]
Subject: Re: [Losmandy_users_io] Typical Guided Performance of GM8G, GM811G, and G11

?

Thanks all, I really appreciate the thoughtful responses. It sounds like the GM8G with a guide scope should be a pretty decent setup to start with. Since my focus is on live stacking at outreach events, I will probably mostly use it for the larger and brighter objects. I currently have a 750mm FL 6-inch Newtonian on a stock mount and a 200mm FL DSLR lens for wide field. Eventually I may look into getting an APO refractor somewhere in between those focal lengths.

?

Virus-free.

 

Liam,

The G11 is, in my opinion, the best bang for the buck. It has a greater weight capacity and doesn't cost that much more. Not hard to set up either unless you have serious back problems. This 70+ year old handles it easily.

Unless you go bigger than, say, a 10 inch Newtonian, the G11 will serve you well for years.

Of course, just one persons opinion.

And, of course, for guiding is necessary. Just don't get too greedy on the focal length to start with - guiding is more critical and exposures become long with high f/-ratios.

Mark Christensen
Sonny Edmonds
 

Hi Liam,
I just want to point out that for AP there are telescopes that work, and telescopes intended for visual observing only, they don't work so great with cameras.
The best explanation I know of comes from Forrest Tanaka. Watch how he describes the differences. Particularly for Newtonian telescopes, and the placement of the reflector mirror. He modified his own Newt.
https://youtu.be/9d0292TBMHo?t=95
--
SonnyE


(I suggest viewed in full screen)
 

Thanks again for all the helpful information! I see the benefit of going straight to the G11 instead of the G8, especially because I would probably want to upgrade to a G11 eventually. I¡¯ll just have to patiently save a little longer for it and practice with my current setup in the meantime.

I ran into the the DSLR back focus problem on my Newtonian with my DSLR. I ended up modifying the Focuser to achieve focus. Of course, I think my new dedicated astronomy camera will be able to achieve prime focus without the adapter (and without torquing the tube as much as the heavy DSLR).
 

I¡¯ve been imaging M42 and NGC2264 and find the GM811G isn¡¯t very good past 30 seconds, its not about tracking its like the image is on a bumpy road with stars moving left/right/up/down like too much coffee. ?Since I am just hunting for new objects in the sky I am NOT using guide software (PHD2) but everything is level and balanced. ?I thought the Nikon DSLR was bad but its really this GM811. ?Serious imaging will require Guide software which is easy to setup and gives good results. ? ? ? ?AT115 reflector and Nikon DSLR bare bones tonight.
?
 

Liam,

I have both G11 and GM8. I use G11 with 10" RC @2000mm FL in my home observatory and it is great.
On the other hand, I bought GM8 specifically for travel imaging due to its lower weight.?
I tried using it with 8" RC but was not very happy with it, so I switched to 750mm refractor and it works very well for this payload with guiding.
Here are some results:
https://www.astrobin.com/nn5su1/?nc=user


Alex

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