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Equipment Discussions >> Mounts

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Elliottastronomy
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Reged: 09/20/12

Tracking...
      #6054834 - 08/30/13 05:47 PM

I have a Cg4 Omni XLT dual axis motor drive mount....and I have the polar scope attached to it...my question I have is what is the best tracking I can go with it and I've been having some major problems polar aligning it as someone touched the alzmith knobs and has thrown the polar alignment way off because of it...I can get between 70-90 seconds as my best but there is a minor problem.....the 1st 90 second frame may look good but the 2nd looks off and then it reverts back to pinpoint on the 3rd....and this repeats its self throughout the night....why would it do this...and what would you suggest....I would ultimately like to go for 200-300 second frames and how could I get to that point is it even possible with this mount?

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Elliottastronomy
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Reged: 09/20/12

Re: Tracking... new [Re: Elliottastronomy]
      #6054838 - 08/30/13 05:49 PM

Or in other words what's the maximum exposure time I can go before the star drifts....I'm willing to hear everything....

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gdd
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Reged: 11/23/05

Loc: N Seattle suburb, WA
Re: Tracking... new [Re: Elliottastronomy]
      #6055424 - 08/31/13 01:30 AM

Hi Elliott,

This sounds like expected behavior. I don't believe the CG4 autoguides, so you must be doing unguided tracking. Your longest exposure will depend not only on the CG4, but on the accuracy of your polar alignment, the focal length of your lens or telescope, and the size of the pixels in your camera.

To get pinpoint stars for all of your images use a wide angle lens. If you use a somewhat longer focal length lens you will get the behavior you described due the the periodic error of of the mount. Instead of going at a constant speed it will alternately slow down and speed up, but briefly the speed will be correct.

Do some experiments to discover the longest focal length lens you can use to achieve 200-300 second exposures. For the longer focal lengths try to find an exposure length where half of your images are good and discard the rest.

Good luck,

Gale


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Elliottastronomy
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Reged: 09/20/12

Re: Tracking... new [Re: gdd]
      #6055799 - 08/31/13 10:36 AM

I have a 6" Celestron reflector and my camera is a canon t3i so I dot know if that helps

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Cliff Hipsher
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Reged: 12/31/08

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Re: Tracking... new [Re: Elliottastronomy]
      #6055840 - 08/31/13 11:00 AM

OK, I'm not gonna sugar coat this:

Unless you install a Radial Guider and use a dual cross hair reticule eyepiece and manually guide the mount, the best tracking you can achieve with this mount will never give you the exposure lengths you want. And quite honestly, I seriously doubt you could manually guide for 5 full minutes and get perfectly round stars anyway...

The bottom line is, you need a better mount.

And, if you are using a Newtonian reflector, you are going to get field rotation.


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gdd
scholastic sledgehammer


Reged: 11/23/05

Loc: N Seattle suburb, WA
Re: Tracking... new [Re: Cliff Hipsher]
      #6055868 - 08/31/13 11:16 AM

You will not be able to do unguided 200-300 second frames with a CG4 for a 6" reflector. Is it an F/5 (750mm focal length)? You will need to autoguide, which requires a modification to the hand controller for this mount.

See http://www.store.shoestringastronomy.com/products_gp.htm

Even so may be inadequate for the lenghy exposures you are seeking. You are best off using camera lenses for long exposures with a CG4. Since you are having some luck with 90 second exposures I would stick with 30-60 second exposures with the reflector. Use DSS (Deep Sky Stacker) software to to stack multiple short exposures to allow longer integration times.

For more pleasing results you will need a larger and more precise mount.

Edit:
For an idea of what is possible with the CG4, go to www.astrobin.com and do an Advanced Search for CG4. There you will find some examples of unguided images of galaxies and nebula using 6" reflectors and small refactors. All of the images were taken by stacking 30-90 second exposures. I was surprised by how good some of them were. You will see some star enlongation at full resolution on many of them, but the look good when reduced to 25% to 50% size (which you need to do anyway to avoid bloated stars).




Gale

Edited by gdd (08/31/13 12:06 PM)


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shawnhar
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Re: Tracking... new [Re: Elliottastronomy]
      #6055934 - 08/31/13 12:04 PM

Quote:

Or in other words what's the maximum exposure time I can go before the star drifts....I'm willing to hear everything....



The reason you get a good frame then a bad, then a good, i think, is the periodic error in the worm gear. So if you can get a good 90 sec frame out of 3 then I would say that's your best. The only way to get longer without tremendous frustration is to not use the scope. Just mount the camera on there and use a 50mm lens, you should be able to get 300 sec subs with good round stars.


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Elliottastronomy
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Re: Tracking... new [Re: shawnhar]
      #6057770 - 09/01/13 02:26 PM

Thanks for all the comments I am new and trying to get better at all of this and I see everyone doing 300sec exposures and I got jealous....I don't have the money for a new mount unfortunately...other wise I'd go out and get a bigger and better mount.....so I'm stuck with what I got....I'm somewhat satisfied with 90sec subs but since I got a h alpha filter to attach to my dslr I thought someone said you needed longer subs to make up for the filter....my camera is already modified to pick up h alpha but I figured go all out and get as much h alpha as possible...is it a dumb idea getting the astronomik h alpha 12nm and sii filters....Because I haven't tested out yet and I would like opinions....

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orlyandico
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Re: Tracking... new [Re: Elliottastronomy]
      #6057779 - 09/01/13 02:31 PM

That H-a filter is going to be...

For reference, without an H-a filter I would saturate my camera is 2-3 minute exposures. With an H-a filter I can go 20 minutes - that's 1200 seconds - and the histogram is only 1/3 of the way from the left.

I never thought anyone would try to do narrowband imaging with a mount in this class... also a DSLR will have tremendous noise with the long exposures you'd need for narrowband.

Then again. Here's an H-alpha on a modest mount. But it was done with a 50mm lens. And a cooled camera was used to get the long exposures.

http://www.flickr.com/photos/91784720@N00/4296782609/


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orlyandico
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Re: Tracking... new [Re: orlyandico]
      #6057793 - 09/01/13 02:37 PM

These are 5-minute (300-second) subs.

http://stargazerslounge.com/topic/150870-cygnus-ha-50mm-widefield/

And this guy used a CG-4 with a Canon DSLR.

http://www.astrobin.com/32336/B/

60 x 60 seconds with a 50mm lens. At which focal length guiding is not needed.


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jrcrillyAdministrator
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Re: Tracking... new [Re: orlyandico]
      #6057800 - 09/01/13 02:41 PM

Quote:

90- to 300-second exposures in my opinion won't get you anything with an H-a filter. 600 seconds minimum..




On an Ha-rich target I sometimes get away with 5 minutes plus a lot of extra processing - but certainly no less than that. And forget short-exposure SII - it requires much longer exposures than Ha. I usually see no SII nebulosity at all in ten minutes.


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Elliottastronomy
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Re: Tracking... new [Re: jrcrilly]
      #6057882 - 09/01/13 03:22 PM

Wow....so it is possible with the h alpha but I would need ultimate polar aligning or very dark skies....and the Sii is bad?

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Elliottastronomy
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Re: Tracking... new [Re: Elliottastronomy]
      #6057894 - 09/01/13 03:27 PM

I just saw images of hydrogen emission and thought if I get an astronomik filter and put it in front of my camera I would get the same....not knowing that this takes thought and I had poor planning....

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Elliottastronomy
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Re: Tracking... new [Re: Elliottastronomy]
      #6057906 - 09/01/13 03:32 PM

And my other question here....I keep hearing "unguided"....is it possible if you get this mount to be guided....and what would it take to get it there.....I know my options....CCD camera can't afford...a CGEM mount again to expensive....I've hear of an auto guider but I don't think you can do this with a CG4 but I'm open to anything

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Jon Isaacs
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Reged: 06/16/04

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Re: Tracking... new [Re: Elliottastronomy]
      #6057927 - 09/01/13 03:53 PM

I have done some A-P with an Orion Astroview mount, it is the same mount as the older CG-4 ... I was doing film and manually guiding with a guide scope and a dual reticules guiding eyepiece.

The PE was quite good, manageable, most of the time. However there was one bad spot, I can still remember those terrible moments as I fruitlessly tried to guide away the error but I just couldn't keep up..

Jon


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Elliottastronomy
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Re: Tracking... new [Re: Jon Isaacs]
      #6057985 - 09/01/13 04:35 PM

So the verdict that has been laid down is that the most I can do and still get round stars is 90-100sec and that in order to go to 200-300sec I would need to get a better mount and with h alpha doing with the scope will cause noise and won't benefit me?? Because if so then the h alpha I have is not worth it then (it's the 1.25") and I made an adapter for it so it looks like the CLS clip in filter for my camera

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orlyandico
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Re: Tracking... new [Re: Elliottastronomy]
      #6058231 - 09/01/13 07:59 PM

Well that's the thing... when you said H-a I thought clip-in or 2" and that costs 1/2 of a used CG-5. My first thought was, wow, for the cost of an H-a and S-II you could get a used CG-5 which, while still non-ideal, would be a lot better than that CG-4...

I still got to say - I never tried narrowband with my CGEM that I used to have. I'm that paranoid.


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Elliottastronomy
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Re: Tracking... new [Re: orlyandico]
      #6058293 - 09/01/13 08:44 PM

Yeah I got the 1.25" filters for both h alpha and Sii....thought they were going to work...still haven't tried them out....I'm a little afraid now because now I know i can't do long exposures so I don't know if those two are worth it and if I should sell them because it will do more harm then good....

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orlyandico
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Re: Tracking... new [Re: Elliottastronomy]
      #6058306 - 09/01/13 09:04 PM

Like stated above... use a short fast lens like a 50/1.8 or 100/2 (the fast focal ratio will mean you can shoot shorter exposures, and the short focal length will mask tracking errors).

I was able to do 2-minute unguided exposures with a 50mm lens, and results weren't too bad. Now these are not H-a (just plain unfiltered) but with a good polar alignment I don't see how you can't do 4-5 minutes.



I used a Pentax K20D (a fairly noisy DSLR by today's standards) and a Vixen Polaris, which is the original mount from which the CG-4 was copied. Only an eyeball polar alignment, no guiding or anything. You can see the Horsehead and Flame Nebulas (faintly) in the above photo. 50mm f/2 manual focus lens at f/4 I believe. You could get a bigger image if you used a 100mm f/2 Canon lens.


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gdd
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Reged: 11/23/05

Loc: N Seattle suburb, WA
Re: Tracking... new [Re: orlyandico]
      #6058381 - 09/01/13 10:06 PM

Quote:

Like stated above... use a short fast lens like a 50/1.8 or 100/2 (the fast focal ratio will mean you can shoot shorter exposures, and the short focal length will mask tracking errors).

I was able to do 2-minute unguided exposures with a 50mm lens, and results weren't too bad. Now these are not H-a (just plain unfiltered) but with a good polar alignment I don't see how you can't do 4-5 minutes.







Or, if your mount is not up to 4 minute exposures for the focal length you want to image at, just take twice as many 2 minute exposures. That normally works for photos like Orlyandico took without the Ha filter. With the Ha filter you will be filtering out more LP and also a lot of the stars. I suspect the 20 minute exposures are to make the stars bright enough that the stacking software can align the images (Hey guys, am I right on that?).

Gale


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