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Equipment Discussions >> Mounts

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Astropin
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Reged: 08/30/07

Loc: Michigan
Longest Exposures Auto Guiding a CG-5 GT?
      #5281275 - 06/20/12 08:19 PM

I've read over and over that with proper alignment a CG-5 can do 1-2 minute exposures....unguided.

Well if I will be auto guiding how much longer is possible?

My total load weight will be right around 15 lbs.


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cam1936
sage


Reged: 08/01/08

Loc: Alberta, Canada
Re: Longest Exposures Auto Guiding a CG-5 GT? new [Re: Astropin]
      #5281290 - 06/20/12 08:31 PM

What focal length?

At 50mm (camera lens) I can do 5 min + unguided. At 2000mm not so much.

Lots of variables. Consensus is about 5min in most cases.


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Falcon-
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Reged: 09/11/09

Loc: Gambier Island, BC, Canada
Re: Longest Exposures Auto Guiding a CG-5 GT? new [Re: cam1936]
      #5281384 - 06/20/12 09:32 PM

I do 10-minute guided exposures on my CG-5. At 300mm it is no problem, at 912mm I have some rejected frames, at 1370mm it is a frustrating thing to try and do such long exposures (but can be done with large numbers of rejected frames).

I think if I adjusted the mount I *might* make it more useable for the 912mm and under focal length, but for over 1000mm and higher focal lengths with such long exposures.... well that is why I purchased a CI-700 recently!


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Patrick
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Reged: 05/16/03

Loc: Franklin, Ohio
Re: Longest Exposures Auto Guiding a CG-5 GT? new [Re: Astropin]
      #5281412 - 06/20/12 09:56 PM

Quote:

I've read over and over that with proper alignment a CG-5 can do 1-2 minute exposures....unguided.

Well if I will be auto guiding how much longer is possible?

My total load weight will be right around 15 lbs.




As others have noted, it depends a great deal on your focal length. Here is my Andromeda shot unguided on a CG5 with a 320mm focal length scope with 60 x 1 minute exposures. I found that if I increased the exposure lengths to 90 seconds I started to get a lot of rejects, so I kept it to 60 seconds.

With autoguiding, you should be able to guide as long as you need. But the question you have to ask is how long do your sub frames really need to be? If you're not at a really dark sky site you may find that the sky glow will take over quite quickly. In that case it's better to take more short exposures than one really long one. But each case is different. If you're using narrowband filters for instance, you're automatically going to need to take longer exposures. I think in my case, I had to increase my exposures by about 40% to compensate for my light pollution filter, going from 60 seconds to 90 seconds. (I now use autoguiding and am imaging at 720mm focal length on a CGEM). The way to tell how long you need to image is by looking at the histogram. You want the big hump on the histogram to be about 10-15% from the left edge.

See Samir's Minimal Exposures article for more information.

Patrick

Edited by Patrick (06/20/12 10:06 PM)


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oldstargazer
Carpal Tunnel
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Reged: 08/03/11

Loc: Western Oklahoma
Re: Longest Exposures Auto Guiding a CG-5 GT? new [Re: Patrick]
      #5281533 - 06/20/12 11:18 PM Attachment (157 downloads)

I can get some pretty long exposures with my CG5-GTa by doing rough polar align and then polar align by hand controller. Here is a 3 minute exposure with my T3i attached to my C8 with focal reducer makeing it a 1280mm, this is single shot just resized for posting.

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Astropin
super member


Reged: 08/30/07

Loc: Michigan
Re: Longest Exposures Auto Guiding a CG-5 GT? new [Re: oldstargazer]
      #5281658 - 06/21/12 01:03 AM

My standard focal length is 655mm but will probably take many (most) using a .8x reducer/flattener for a focal length of 524mm at f4.76.

I'd like to be able to hit 8 minute subs (guided) for DSO targets. My skies are fairly dark....I'm in a small town in northern Michigan.....some light pollution but nothing like populated areas. My house is located in a "green" zone but I'm only minutes from blue zones and can even hit a black zone about 45 minutes from my house.

Edited by Astropin (06/21/12 01:13 AM)


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Falcon-
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Reged: 09/11/09

Loc: Gambier Island, BC, Canada
Re: Longest Exposures Auto Guiding a CG-5 GT? new [Re: Astropin]
      #5281680 - 06/21/12 01:19 AM

Astropin: sounds a lot like my location in some ways... Green zone with Black an hour and a half in one direction and White an hour in the other direction!

With the focal length and weight you are talking about I think you could do fine with a CG-5. My own 912mm scope is a massive MCT that is really too heavy and long for a CG-5, yet I squeezed a fair amount out of it.

Something like the Orion Mini Guider or KWIQ Guider (or a home-built 50mm finder-guider) helps keep the weight down and works very well. With my own CG-5 I had some issues with sticktion in the DEC access that a cleaning re-grease helped a lot with. A slight east-heavy imbalance in RA and not quite perfect polar alignment so that the DEC only has to drive in one direction also helps. (Not that I ever tried to get an imperfect polar alignment.... I just never quite managed to get a perfect one is all )

All that said... If you can afford something like a iEQ30, Atlas/EQ6, CGEM or iEQ45 you might have a slightly more troubleshooting-free time getting to 8 mins and end up with a mount easier able to extend to longer focal lengths if you ever wish to do so.


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Astropin
super member


Reged: 08/30/07

Loc: Michigan
Re: Longest Exposures Auto Guiding a CG-5 GT? new [Re: Falcon-]
      #5281691 - 06/21/12 01:28 AM

Yeah....I was eyeballing the iEQ30 but things are a little tight at the moment and I ended up finding a CG-5 GT for $350 so I really couldn't pass that up.

If I really enjoy this I think I could get into an iEQ within a couple years.

I did pick up the Orion mini guider package (cost the same as my mount!).

Can I run the guiding program and something like Backyard EOS from a little netbook computer running Windows 7 light?


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rmollise
Postmaster
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Reged: 07/06/07

Re: Longest Exposures Auto Guiding a CG-5 GT? new [Re: Astropin]
      #5281874 - 06/21/12 07:55 AM

Balance is the key. With the mount properly balanced, just somewhat east-heavy, I was able to auto-guide without a hiccup for 10-minutes plus (as long as I cared to go) with a C8 at f/6.3. It is not enough to balance once for whichever side of the Meridian you are on. You must rebalance if you move significantly. Which is a pain, but not a big deal, since most times you will only be imaging one or two objects a night anyway.

Edited by rmollise (06/21/12 07:56 AM)


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Falcon-
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Reged: 09/11/09

Loc: Gambier Island, BC, Canada
Re: Longest Exposures Auto Guiding a CG-5 GT? new [Re: rmollise]
      #5282132 - 06/21/12 11:28 AM

Quote:

Can I run the guiding program and something like Backyard EOS from a little netbook computer running Windows 7 light?



Well I run PHD (guiding), Backyard EOS, Nebulosity and sometimes also Stelarium all from my old 1st gen Atom netbook at the same time.... so yes, I suspect your newer win7 netbook will do fine.


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Astropin
super member


Reged: 08/30/07

Loc: Michigan
Re: Longest Exposures Auto Guiding a CG-5 GT? new [Re: Falcon-]
      #5282280 - 06/21/12 01:34 PM

Quote:

Quote:

Can I run the guiding program and something like Backyard EOS from a little netbook computer running Windows 7 light?



Well I run PHD (guiding), Backyard EOS, Nebulosity and sometimes also Stelarium all from my old 1st gen Atom netbook at the same time.... so yes, I suspect your newer win7 netbook will do fine.




Do you need all of those running? Can I guide with PHD and control my camera with Backyard and just let those two do their thing?


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Falcon-
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Reged: 09/11/09

Loc: Gambier Island, BC, Canada
Re: Longest Exposures Auto Guiding a CG-5 GT? new [Re: Astropin]
      #5282288 - 06/21/12 01:37 PM

Yes you can run just those two apps - however I am generally running two cameras (a 350D/Xt and a 1000D/XS) and often find it easier to control one via BackyardEOS and one via Nebulosity, so I end up running both. Stelarium is just for target-hunting if I did not plan ahead.

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Jared
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: Longest Exposures Auto Guiding a CG-5 GT? new [Re: Astropin]
      #5282427 - 06/21/12 03:11 PM

Quote:

I've read over and over that with proper alignment a CG-5 can do 1-2 minute exposures....unguided.

Well if I will be auto guiding how much longer is possible?

My total load weight will be right around 15 lbs.




If the focal length is reasonable, say, under 800mm or so, then there should be no limit on how long a guided exposure you can take. Basically, if guiding is working properly it should continue to work properly for any length of exposure. Obviously, the longer you go the more risk you are taking that an airplane will intrude or that you'll accidentally bump the mount or that a gust of wind will ruin an exposure, etc., etc., so the general recommendation is to shoot long enough exposures that read noise is negligible compared to shot noise, and then concentrate on integration time rather than subexposure duration. If you are taking longer sub exposures than you need, then you are losing dynamic range (as more and more stars saturate) and you aren't really gaining anything in terms of better SNR.

If 8 minutes is your target and autoguiding is effective, there should be no problem achieving your goal.


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Astropin
super member


Reged: 08/30/07

Loc: Michigan
Re: Longest Exposures Auto Guiding a CG-5 GT? new [Re: Jared]
      #5282452 - 06/21/12 03:35 PM

I've noticed that many people take exposures of varying lengths. I get that this is to avoid overexposure of one area (like the core of M31) while still being able to properly expose the outer edges and dust lanes. One of the best shots I found of M31 (using the same DSLR that I have...1000D) used exposures of 30 seconds, 2 minutes & 8 minutes (for a total of 2.4 hours). Of course the focal length and "speed" of the optics are all going to affect this as well.

Trial and Error I guess. Just trying to get off to good head start.

Edited by Astropin (06/21/12 08:12 PM)


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Mike X.
scholastic sledgehammer


Reged: 06/28/10

Loc: Greece-Athens and Rome-Italy
Re: Longest Exposures Auto Guiding a CG-5 GT? new [Re: Astropin]
      #5282495 - 06/21/12 04:00 PM

Unguided i managed to reach lights of 50 seconds at 1500mm (C6)


Lights of 60 seconds at 945mm (C6 reduced at f/6.3)


Guided...i regularly do exposures of 10 minutes at 500mm
(i don't have a flattenere so don'nt mind the stars at the edges.)


another example:


It's all about polar alignment, patience, and good balance

The 10 minutes is not the limit at all.Once the polar alignment is good enough there is no limit virtually...it all depends from the sky

I believe that the CG5 is a nice mount to guide with up to 1 meter max without serious concerns.

Hope i've been of help

Edited by Mike X. (06/21/12 04:33 PM)


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nomosnow
sage


Reged: 03/21/11

Loc: Fort Saskatchewan,Ab ,Canada
Re: Longest Exposures Auto Guiding a CG-5 GT? new [Re: Mike X.]
      #5282803 - 06/21/12 07:36 PM

My CG-5 has annoying declination backlash which turns auto guiding into operator intervention guiding . The dec shaft is not true so I have varying backlash depending on the declination of the object.
The mount gave unacceptable results at 750 mm focal length .
I gave up and bought a different mount for guided astrophotos (one that has a spring loaded dec worm). Note very happy with his one!! Granted that maybe if I took the time with precise balance or hypered tuned it or ?? I could have got it to work but I moved on.
I still have the CG-5 for visual though.


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Astropin
super member


Reged: 08/30/07

Loc: Michigan
Re: Longest Exposures Auto Guiding a CG-5 GT? new [Re: nomosnow]
      #5282847 - 06/21/12 08:17 PM

Very nice Mike X!

Excellent shot of M31. That's what I'm shooting for.


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Mike X.
scholastic sledgehammer


Reged: 06/28/10

Loc: Greece-Athens and Rome-Italy
Re: Longest Exposures Auto Guiding a CG-5 GT? new [Re: Astropin]
      #5283292 - 06/22/12 04:05 AM

Thank you Astropin!
nomosnow, i can understand what you say.I had to do some tinkering too with my cg5.It does have the same varying backlash problems yours has.Actually i do believe the problem is that the axis is not a perfect cycle at all.
What i found to " resolve" practically the problem was a carefull balance slightly to 1 side to eliminate backlash and a carefull regulation of the backlash periodically during the year.The changhe from cold to hot seasons affects backlash actually.What deffinatelly helped me though was a very very carefull polar alignment so that i had less need ot corrections in DEC during guiding.That helped a lot.

I am looking forward to the IEQ45 myself both for an upgrade in load and the spring loaded dec worm.How is it in confront with teh CG5 in terms of dimensions?Is it closer to the CGEM or to the CG5?


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Jared
Post Laureate
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: Longest Exposures Auto Guiding a CG-5 GT? new [Re: Astropin]
      #5284161 - 06/22/12 04:55 PM

Quote:

I've noticed that many people take exposures of varying lengths. I get that this is to avoid overexposure of one area (like the core of M31) while still being able to properly expose the outer edges and dust lanes. One of the best shots I found of M31 (using the same DSLR that I have...1000D) used exposures of 30 seconds, 2 minutes & 8 minutes (for a total of 2.4 hours). Of course the focal length and "speed" of the optics are all going to affect this as well.

Trial and Error I guess. Just trying to get off to good head start.




Not many objects have such a wide dynamic range that this is required, though it does depend a bit on your camera. Orion is one object that needs this technique. M45 can sometimes benefit from a range of exposure durations (in order to minimize star bloat more than anything else). M31 usually does not require variable exposure durations, but some people have had success handling it this way. It can make processing a little easier since you won't need quite as much subtlety in your stretches. However, this is not a technique that is required or used by most advanced amateurs on most deep sky objects.


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Jared
Post Laureate
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: Longest Exposures Auto Guiding a CG-5 GT? new [Re: nomosnow]
      #5284167 - 06/22/12 04:58 PM

Quote:

My CG-5 has annoying declination backlash which turns auto guiding into operator intervention guiding . The dec shaft is not true so I have varying backlash depending on the declination of the object.
The mount gave unacceptable results at 750 mm focal length .
I gave up and bought a different mount for guided astrophotos (one that has a spring loaded dec worm). Note very happy with his one!! Granted that maybe if I took the time with precise balance or hypered tuned it or ?? I could have got it to work but I moved on.
I still have the CG-5 for visual though.




If you have excessive or variable backlash in declination, the usual solution is to allow the mount to guide in only one direction in declination. Imperfections in polar alignment will generally cause the mount to drift in only one direction (for a given side of the meridian), so you can just turn off corrections in the other direction. Obviously, periodic error is not an issue in declination since it is not a driven axis. A better mount, of course, will bring its own advantages. Just offering this as advice to others who night encounter the same issue.


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