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m00nless
super member
Reged: 08/06/08
Posts: 154
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What sort of performance should I expect from this mount?
Is it accurate enough for short unguided exposures of deep sky objects with an 80mm f/7.5 refractor?
Is the mount strong enough to hold both a C8 SCT OTA and an 80mm f/7.5 refractor OTA (either on a mounting plate or piggyback)?
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mclewis1
Thread Killer
Reged: 02/25/06
Posts: 3938
Loc: New Brunswick, Canada
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Yes, short unguided shots at around 600mm are very possible. I would personally also look into a focal reducer for both your 80mm and SCT. A faster f ratio helps with shorter exposures and takes some of the pressure off of the tracking accuracy.
The C8 plus 80mm plus mounting hardware will mean that you'll be around 25lbs, not too much for a CG-5 for visual work especially if you are careful with the balance. If you want to image with the C8 I would suggest looking at a lighter autoguiding setup (modified 50mm finder for example vs. your 80mm).
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Mark
C11, C6, APM/TMB115, and AT80ED - Tandem mount CGE and CG-5A, WO EZ-Touch and AT Voyager
25x100s and 8x56s, T-Mount Light, Mark 1 eyeballs - Modded 350D, DSI-P, SPC900, Mallincam
Just because you can doesn't necessarily mean that you should
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rmollise
Post Laureate
Reged: 07/06/07
Posts: 4561
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Quote:
What sort of performance should I expect from this mount?
Is it accurate enough for short unguided exposures of deep sky objects with an 80mm f/7.5 refractor?
Is the mount strong enough to hold both a C8 SCT OTA and an 80mm f/7.5 refractor OTA (either on a mounting plate or piggyback)?
I've had no problem with unguided 30 second exposures with the C8 at about f/4...800mm, give or take.
It does fine with an 80mm riding piggyback as long as you pay attention to proper balance.
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Uncle Rod
Rod's New Book:
Choosing and Using a New CAT
Available now!
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bebert
sage
Reged: 07/21/08
Posts: 246
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I second Rod's comment. Balance is key.
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Celestron C8
Orion 80 ED
Vixen 70s for guiding
CG5-ASGT
SSAG
PHD Guiding
Canon 300D(a)
Celestron 0.63 focal reducer
WO 0.8x FF/FR version II
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Takman
super member
Reged: 02/25/09
Posts: 190
Loc: Maple, ON - Canada
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This mount, for the money, is quite good and I held on to mine despite having other GOTO mounts. I think with good polar alignment (and balance), a 1 minute exposure is attainable.
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Takahashi TOA-130F
Celestron C-11/C-8 with FASTAR
William Optics Zenithstar 66 SD/ED Petzval
Astro-Physics Mach1 GTO/Celestron AS-GT
TeleVue Naglers 26mm, 13mm, 7mm and 3.5mm
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Patrick
Postmaster
Reged: 05/16/03
Posts: 7800
Loc: Franklin, Ohio
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Quote:
What sort of performance should I expect from this mount?
Is it accurate enough for short unguided exposures of deep sky objects with an 80mm f/7.5 refractor?
Is the mount strong enough to hold both a C8 SCT OTA and an 80mm f/7.5 refractor OTA (either on a mounting plate or piggyback)?
Typical CG5-GT PE is about +/- 20 arc seconds with a period of 600sec (IIRC). Plug your camera, exposure, mount and scope information into This Simulator. At the bottom of the box is the "pixel curve size". You want to be less than 1.0. Greater than 1.0 means a star will cross over more than 1 pixel which means you'll start to see star trailing.
That's just a rough approximation, but a starting point. I've found with my CG5-GT and 320mm focal length scope and a Canon 30D (6.4 micron pixels), that I could do 1 minute unguided exposures (90s exposures showed trailing). Guided, the best I can do is about 1524mm f/l for 60 seconds. The newer DSLRs have smaller pixels, which work against you with regard to star trailing.
I personally think the CG5-GT would be overmounted with a C8 and 80mm refractor for astrophotography. The total weight will be in closer to 30 lbs. My C6 SCT and AT66ED with brackets, camera's, etc weigh approx 24 lbs, and the C8 and 80mm refractor are significantly heavier and longer. For that kind of load, I think you would be better off with an Atlas class mount.
Regards,
Patrick
EDIT: I should add that for visual use, the C8/80mm refractor combination can be carried by the CG5-GT with little sweat. If you're going to image unguided you won't need to mount the C8, obviously, so that's a help. Nevertheless, the main problem will just be the focal length and the mount's PE.
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10" f/6 Truss Tube Newtonian
Celestron C6S-GT SCT
AT66ED Refractor
Oberwerk Ultra 15x70 Binocular
475B Geared Tripod & 501HDV Head
Celestron Regal 8x42 Binocular
Canon XSi; Meade DSI;SPC900-NC
Vixen GP2 Photo Guider Mount
My Astronomy Pages
Edited by Patrick (10/29/09 07:50 PM)
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Takman
super member
Reged: 02/25/09
Posts: 190
Loc: Maple, ON - Canada
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Quote:
I personally think the CG5-GT would be overmounted with a C8 and 80mm refractor for astrophotography. The total weight will be in closer to 30 lbs.
I've done exposures with roughly this load and was able to attain marginally usable photographs. As you've suggested, however, a more robust mount would be the preferred solution unless your expectations are low.
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Takahashi TOA-130F
Celestron C-11/C-8 with FASTAR
William Optics Zenithstar 66 SD/ED Petzval
Astro-Physics Mach1 GTO/Celestron AS-GT
TeleVue Naglers 26mm, 13mm, 7mm and 3.5mm
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rmollise
Post Laureate
Reged: 07/06/07
Posts: 4561
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Quote:
I personally think the CG5-GT would be overmounted with a C8 and 80mm refractor for astrophotography. The total weight will be in closer to 30 lbs. My C6 SCT and AT66ED with brackets, camera's, etc weigh approx 24 lbs, and the C8 and 80mm refractor are significantly heavier and longer. For that kind of load, I think you would be better off with an Atlas class mount.
Regards,
Patrick
I believe that it's doable with an 80, and do use one on my C8 at times, but, like you, I think the 66 is a better solution and that's what I use as a guidescope/widefield imager most of the time when the C8 is in the mix.
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Uncle Rod
Rod's New Book:
Choosing and Using a New CAT
Available now!
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HaleBopper
professor emeritus
Reged: 01/14/08
Posts: 500
Loc: Great White North
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With a C8, on my CG5, and an 80 mm Orion ShortTube piggybacked, I was able to get 5 min guided exposures like this one of M106. It is possible to get half decent images, when the conditions are just right. I eventually decided to upgrade to a beefier mount.
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8" SCT Losmandy G11, CG5 mounts
Canon Digital Rebel 400
Kodak Easyshare 2.0 Megapixels
5, 8, 13, 17, 24mm Orion Stratus
Hyperion 31 mm Aspheric
Pentax 10 mm XW
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Leo.S
member
Reged: 09/21/09
Posts: 67
Loc: Toronto, Ontario
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the pic is just amazing..... I wish I could do it with my 8se one day.
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Telescopes: Celestron Nexstar 8SE, WO Megrez 72ED APO, Orion 9x50 RACI Finder
Eyepieces: 8mm 13mm TV Ethos, 26mm 31mm TV Naglar
Filters: Lumicon 2" Variable Polarizer/2" UHC
Mount: Celestron Nexstar GoTo, CG-5 ASGT
Accessories: Moolite Focuser,2"Dura Bright/2"Quartz WO Diagonals,WO P-Flattner 3,CN16 GPS,NexImage,Starry Night Pro Plus 6.3
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HaleBopper
professor emeritus
Reged: 01/14/08
Posts: 500
Loc: Great White North
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Thank you. Things just went right that night. No wind, dark skies. I have lots of stinkers too though 
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8" SCT Losmandy G11, CG5 mounts
Canon Digital Rebel 400
Kodak Easyshare 2.0 Megapixels
5, 8, 13, 17, 24mm Orion Stratus
Hyperion 31 mm Aspheric
Pentax 10 mm XW
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Adrian Lopez
sage
Reged: 10/22/08
Posts: 267
Loc: Puerto Rico
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Quote:
Guided, the best I can do is about 1524mm f/l for 60 seconds.
Does this mean there's a limit on how long an exposure you can have while guiding? I thought the idea behind guiding was to correct any drift before it has a chance to register on camera, and that the process could last a lot longer than 60 seconds. Is there some factor that prevents such corrections after an interval as short as 60 seconds?
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Celestron Omni XLT 120 (f/8.3) Refractor
William Optics 1.25" Dielectric Diagonal
Celestron 4, 6, 9, 15, 25, and 32mm EPs
Nikon Action Extreme 8x40 Binoculars
Canon Digital Rebel XT
RA Sweep Calculator (find objects without GoTo)
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Patrick
Postmaster
Reged: 05/16/03
Posts: 7800
Loc: Franklin, Ohio
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Quote:
Does this mean there's a limit on how long an exposure you can have while guiding? I thought the idea behind guiding was to correct any drift before it has a chance to register on camera, and that the process could last a lot longer than 60 seconds. Is there some factor that prevents such corrections after an interval as short as 60 seconds?
To clarify...without throwing out at least 40% of the frames. You are correct that the goal of guiding is to correct the periodic error of the mount, but there is a limit to what guiding can correct depending on how bad the error is and the focal length of the instrument. Also, it depends on where you're imaging in the sky, polar alignment, etc.
The time factor has to do with how much of the PE curve is being used. If you've ever looked at the PE curve you will know it's cyclical. It rises and falls over it's period. I believe the CG5 has a 600 second period. If you start out at zero, at 150 seconds (600/4)it will reach it's first peak. At 300 seconds, it will be back at zero. Then at 450 seconds it will be at it's low point. At some point the focal length of the scope, plus the camera's pixel size, plus the mounts PE...even when guided...will swamp the setup and star trails will result. At certain points in the cycle, the mount tracking is a little faster than at others, plus there are spikes.
If the mount's native PE is +/-20 arc seconds with average spikes, will the guiding be able to keep up with it? Maybe you'll get +/- 5 arc seconds instead? At long focal lengths, even that may not be good enough for extended exposures.
The other factor just has to do with weight, wind, and vibrations. A heavy load on a lighter weight mount tends to vibrate more by little things.
If you get a chance download the Simulator I mentioned above. Play with it a little to figure out how it works and see how adjusting the various variable affect things.
Patrick
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10" f/6 Truss Tube Newtonian
Celestron C6S-GT SCT
AT66ED Refractor
Oberwerk Ultra 15x70 Binocular
475B Geared Tripod & 501HDV Head
Celestron Regal 8x42 Binocular
Canon XSi; Meade DSI;SPC900-NC
Vixen GP2 Photo Guider Mount
My Astronomy Pages
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Adrian Lopez
sage
Reged: 10/22/08
Posts: 267
Loc: Puerto Rico
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Ah... so it's basically a matter of having to deal with noise-like errors too big to correct via guiding.
Perhaps these kinds of errors can be minimized by stacking short exposures together?
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Celestron Omni XLT 120 (f/8.3) Refractor
William Optics 1.25" Dielectric Diagonal
Celestron 4, 6, 9, 15, 25, and 32mm EPs
Nikon Action Extreme 8x40 Binoculars
Canon Digital Rebel XT
RA Sweep Calculator (find objects without GoTo)
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Patrick
Postmaster
Reged: 05/16/03
Posts: 7800
Loc: Franklin, Ohio
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Quote:
Perhaps these kinds of errors can be minimized by stacking short exposures together?
Yep. That's part of the digital revolution in astrophotography. With film, it was quite common to have to take single exposures 15min to 30min long. With CCDs and DSLR's the exposure times can be cut to a couple of minutes and images stacked. That has reduced the demand on the mount, but there is still a limit to what can be done.
Patrick
--------------------
10" f/6 Truss Tube Newtonian
Celestron C6S-GT SCT
AT66ED Refractor
Oberwerk Ultra 15x70 Binocular
475B Geared Tripod & 501HDV Head
Celestron Regal 8x42 Binocular
Canon XSi; Meade DSI;SPC900-NC
Vixen GP2 Photo Guider Mount
My Astronomy Pages
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rmollise
Post Laureate
Reged: 07/06/07
Posts: 4561
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Quote:
Quote:
Guided, the best I can do is about 1524mm f/l for 60 seconds.
Does this mean there's a limit on how long an exposure you can have while guiding? I thought the idea behind guiding was to correct any drift before it has a chance to register on camera, and that the process could last a lot longer than 60 seconds. Is there some factor that prevents such corrections after an interval as short as 60 seconds?
Sure, theoretically you should be able to autoguide as long as you want. And at times you can. I've done 15 - 20 minutes on the CG5 with my ST2000. But then comes a gust of wind, or your buddy comes by and taps the tube (WATCHA DOIN?) and Katie bar the door. For that reason it's usually better to take shorter subexposures. I usually do two or three minutes each.
--------------------
Uncle Rod
Rod's New Book:
Choosing and Using a New CAT
Available now!
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Leo.S
member
Reged: 09/21/09
Posts: 67
Loc: Toronto, Ontario
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I'm new to this hobby, just have a question related to the topic, when you guys talking about the payload, is it include everything on top of the mount (OTA, diagonals, eyepieces, focusers, finder/guide scope).
I recently purchased a CG-5 ASGT as I roughly calculated everything together(8SE + WO 72ED + diagonals,focuser, eyepieces) the total weight of my equipment would be over 35 lb and probably will end up around 40 lb, when I purchased the mount I also ordered an extra 11 lb counter weight is that going to help? I was planning to do some short exposure APs with my DSLR for fun, but I'm just a little bit worried after reading comments about CG-5 payload. I don't know what should I do right now, am I going to be ok? any advice would be helpful.
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Telescopes: Celestron Nexstar 8SE, WO Megrez 72ED APO, Orion 9x50 RACI Finder
Eyepieces: 8mm 13mm TV Ethos, 26mm 31mm TV Naglar
Filters: Lumicon 2" Variable Polarizer/2" UHC
Mount: Celestron Nexstar GoTo, CG-5 ASGT
Accessories: Moolite Focuser,2"Dura Bright/2"Quartz WO Diagonals,WO P-Flattner 3,CN16 GPS,NexImage,Starry Night Pro Plus 6.3
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HaleBopper
professor emeritus
Reged: 01/14/08
Posts: 500
Loc: Great White North
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Well for visual use you can get away with a heavier load although I would not exceed the mount capacity. Make sure it's well balanced either way.
For AP, there is a "50%" rule I was told about. Simply put, for AP your total load should not exceed 50% of the mount's payload. More experienced astrophotographers on CN can give you a better idea.
On my scope, my C8 at about 13 pounds plus the Orion ShortTube at ~5 pounds with Scopestuff piggyback rings, plus my finder, DSLR, T-adapter, and focal reducer would put me just over 20 pounds I think. I never actually determined it. I also have an extra 11 pound CW. So I have already exceeded 50% capacity.
You can do AP with such a setup, but if AP is what you would really like to get into. Go for a sturdier mount.
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8" SCT Losmandy G11, CG5 mounts
Canon Digital Rebel 400
Kodak Easyshare 2.0 Megapixels
5, 8, 13, 17, 24mm Orion Stratus
Hyperion 31 mm Aspheric
Pentax 10 mm XW
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Leo.S
member
Reged: 09/21/09
Posts: 67
Loc: Toronto, Ontario
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Thanks Hale, now I got confused by something else, the spec for 8SE written on Celestron website says the tube weight for 8SE is 24 lb, but I tried to weight it by using 2 different home use weight calculator(one regular one electronic) all says the 8SE tube weight is 13 lb...
If it's in that way my total weight will again change to around 30 lb.
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Telescopes: Celestron Nexstar 8SE, WO Megrez 72ED APO, Orion 9x50 RACI Finder
Eyepieces: 8mm 13mm TV Ethos, 26mm 31mm TV Naglar
Filters: Lumicon 2" Variable Polarizer/2" UHC
Mount: Celestron Nexstar GoTo, CG-5 ASGT
Accessories: Moolite Focuser,2"Dura Bright/2"Quartz WO Diagonals,WO P-Flattner 3,CN16 GPS,NexImage,Starry Night Pro Plus 6.3
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mclewis1
Thread Killer
Reged: 02/25/06
Posts: 3938
Loc: New Brunswick, Canada
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The 8SE "tube" weight on the website is for the ota and the fork arm and base, basically everything but the tripod. Take the C8 ota off of the fork arm and as you've said it will weigh 13lbs.
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Mark
C11, C6, APM/TMB115, and AT80ED - Tandem mount CGE and CG-5A, WO EZ-Touch and AT Voyager
25x100s and 8x56s, T-Mount Light, Mark 1 eyeballs - Modded 350D, DSI-P, SPC900, Mallincam
Just because you can doesn't necessarily mean that you should
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