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Equipment Discussions >> Mounts

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freestar8n
Post Laureate
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Reged: 10/12/07

Re: CGEM not suitable for serious astrophotography? new [Re: photodady]
      #5650699 - 01/29/13 04:31 PM

Quote:

This is completely untrue, you are conflating optical axis-RA non-orthogonality with non-orthogonality of the RA-Dec axis.




No - I am not conflating anything. It's unfortunate that apparently some people on CN also don't realize that orthogonality has nothing to do with dec. drift, but one person above agrees with my perspective. I am extremely familiar with pointing models and cone and so forth - and they do affect the pointing model - but once you have a telescope rigidly attached to a single rotating axis - orthogonality plays no role at all. The dec. axis shouldn't be moving in the first place - so you can bend and weld the entire system - except for the polar axis - and it will track perfectly.

I rather doubt that you had the telescope outside tracking a star and you did a proper drift alignment and determined that no matter what you did, the star would drift away.

You are certainly in a "write-only" mode when it comes to these matters, and I think CN could have been a good place for you to ask questions and learn how to get the best out of your mount. But I think that was a lost cause when last you dismissed the CN community, and I don't expect it to change during this unfortunate rehash.

Frank


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jrcrillyAdministrator
Refractor wienie no more
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Reged: 04/30/03

Loc: NE Ohio
Re: CGEM not suitable for serious astrophotography? new [Re: photodady]
      #5650702 - 01/29/13 04:32 PM

Quote:

Sorry I just assumed that those outspoken in an astronomy forum would have at least had a basic course in astronomy.




Ad hominem attacks won't improve your understanding of how EQ mounts work. A careful reading of comments by those who do know might.


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Stew57
Carpal Tunnel
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Reged: 05/03/09

Loc: Silsbee Texas
Re: CGEM not suitable for serious astrophotography? new [Re: jrcrilly]
      #5650730 - 01/29/13 04:51 PM

Are we discussing euler angle?

One thing besides the poining accuracy that a nonorthogonal dec/ra axis would affect is ASPA.

Edited by Stew57 (01/29/13 05:05 PM)


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gdd
Pooh-Bah


Reged: 11/23/05

Loc: N Seattle suburb, WA
Re: CGEM not suitable for serious astrophotography? new [Re: rmollise]
      #5650755 - 01/29/13 05:06 PM

Non-orthogonality will cause ra and dec drift only for GOTO slews and may prevent you from doing a GOTO to the area around the celestial pole. The mount's modeling features should compensate for this to some degree. However, once the imaging target is found tracking accuracy depends only on the correctness of the RA polar alignment, not on the cone error, orthogonality of the DEC axis to the RA axis or the OTA axis. Rotational error is caused by using a guide star that is not in the center of the image, again nothing to do with orthogality. What about RA single axis equatorial mounts? They have no concept of orthogonality yet they can track. This is strictly a GOTO issue for finding the imaging target, important for automated sessions but not so much of a problem for manual sessions.

Gale


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: CGEM not suitable for serious astrophotography? new [Re: gdd]
      #5651100 - 01/29/13 08:26 PM

perhaps dady is insisting on his lissajous pattern tracking because he is guiding in both RA and DEC.

but then he says he wants unguided.

i find it really hard to wrap my brain around his concept of tracking errors in both axes when the DEC is not moving!


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guyroch
Vendor (BackyardEOS)
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Reged: 01/22/08

Loc: Under the clouds!
Re: CGEM not suitable for serious astrophotography? new [Re: orlyandico]
      #5651322 - 01/29/13 10:49 PM

Ah, we should all write a blog entry on the blog the OP posted

The CGEM is very capable.

Guylain


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Patrick
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Reged: 05/16/03

Loc: Franklin, Ohio
Re: CGEM not suitable for serious astrophotography? new [Re: orlyandico]
      #5651336 - 01/29/13 11:00 PM

With modern camera's we DON'T NEED to track for hours on end. In my suburban area, a 2 minute sub with my DSLR pushes the skyfog mountain a third of the way from the left side of the histogram...and that's with a light pollution filter. Longer subs will not increase the SN significantly, so it's better to take a lot of shorter subs and stack them. With CCD camera's having 60 to 70% QE values it's possible to grasp a lot of light in a short amount of time and that only helps the situation. That takes a lot of pressure off the mount.

One other comment for Photodady...for most of us, this astronomical pursuit is a hobby. We are mainly "amateur" astronomers. Most of us have families and other things to spend our hard earned money on instead of very expensive astro imaging gear. We generally try to eek out as much as we can from what we can afford. It's pretty amazing what some of our more serious members here can do with 'cheap' gear.

I'd still like to see some of your astro images, sir. Perhaps you expected to be obtaining dazzling images of deep sky objects and are dismayed at what you're actually getting. You know you still have to go through the learning curve of actually using your gear, right? Astrophotography may be a subset of photography, but it has very different requirements from general photography. There is a steep learning curve and quite a few people give up because it's too tough. In my experience only those who are really serious continue in this part of the hobby.

Patrick

Edited by Patrick (01/30/13 09:45 AM)


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: CGEM not suitable for serious astrophotography? new [Re: Patrick]
      #5651349 - 01/29/13 11:10 PM

Patrick, I wish that were true. In my location light pollution is so bad. And I'm using an IDAS LPS V4 filter. Was trying for the Horse head last night. At 1880mm and f8. 20 minute subs weren't cutting it. I figure I need 40 minute subs. And - surprise? - the Mach1 can't do 40 minute guided subs with a C9.25 consistently...

So its a matter of expectation. Certainly the Mach 1 is a great mount - but it also has its limits. And I'm realistic about it.

Dady's blog post and responses seem to be a combination of inadequate practice, and unrealistically high expectations. Dec drift induced from the RA movement points to polar misalignment..


Guess I should go back to my f6 refractor. That one pulls in decent detail with 20 minute subs... But the horse is small...


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Patrick
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Reged: 05/16/03

Loc: Franklin, Ohio
Re: CGEM not suitable for serious astrophotography? new [Re: orlyandico]
      #5651383 - 01/29/13 11:25 PM

Singapore? Yes, I get that. You would be doing good to image the moon.

What camera are you using?

Patrick


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Bowmoreman
Clear enough skies
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Reged: 09/11/06

Loc: Bolton, MA
Re: CGEM not suitable for serious astrophotography? new [Re: orlyandico]
      #5651693 - 01/30/13 07:07 AM

Quote:

Patrick, I wish that were true. In my location light pollution is so bad. And I'm using an IDAS LPS V4 filter. Was trying for the Horse head last night. At 1880mm and f8. 20 minute subs weren't cutting it. I figure I need 40 minute subs. And - surprise? - the Mach1 can't do 40 minute guided subs with a C9.25 consistently...

So its a matter of expectation. Certainly the Mach 1 is a great mount - but it also has its limits. And I'm realistic about it.

Dady's blog post and responses seem to be a combination of inadequate practice, and unrealistically high expectations. Dec drift induced from the RA movement points to polar misalignment..


Guess I should go back to my f6 refractor. That one pulls in decent detail with 20 minute subs... But the horse is small...




Horse head is tough at best, try an Ha filter...

If you have bad LP, longer exposures will just drown it in the sky fog otherwise... It's all about signal to noise, if the signal, in this cas HH, is dimmer than your LP, then longer does nothing...


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Patrick
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Reged: 05/16/03

Loc: Franklin, Ohio
Re: CGEM not suitable for serious astrophotography? new [Re: Bowmoreman]
      #5651870 - 01/30/13 09:24 AM

Samir's Discussion on Optimal Subframe Exposures is a pretty good treatise on short vs long exposures using a DSLR in light polluted environs with less than ideal mounts. It's a good read, btw.

Here's the math for determining the most efficient Signal to Noise Ratio and Subexposure Duration.

Here's one on Measuring Skyfog and one on using LP Filters...very good stuff!

The bottom line here for me is that technique is a big part of the imaging process. Knowing how to use one's equipment with all it's inherent flaws is part of the process.

Patrick


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dickbill
scholastic sledgehammer


Reged: 09/30/08

Re: CGEM not suitable for serious astrophotography? new [Re: Patrick]
      #5653943 - 01/31/13 09:48 AM

Well, i realigned last night and it turn out that it was still very close from the previous polar alignment, so the mount didn't loose it as i thought.
The gotos were excellent, near the center of the field at 240X, so i proceeded anyway to gain maybe a few arc minutes in the polar alignment.
The problem is that i still got declination drift on the ccd during imaging. I believe it is possibly because of the inability of PHD to correct in south declination during calibration. During guiding, the guide star was actually excentered in the guide box.

I had another incident unfortunatly which is worth opening a new thread, for the other cgem users.


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dickbill
scholastic sledgehammer


Reged: 09/30/08

Re: CGEM not suitable for serious astrophotography? new [Re: dickbill]
      #5660685 - 02/03/13 08:41 PM

I come back to this thread because i think the title was a bit harsh.
Well, with some patience I managed to autoguide my cgem for 4 min at...4700 mm, C9.25 plus barlow. It was not easy and still is not perfect (still some drift, soft focus, no darks, piggyback scope not parallel, i was in a rush etc) but it's a testimony to the mount. Needless to mention it would not have been possible with a cg5.

4 minutes in H-alpha on NGC2392, the Eskimo.


Edited by dickbill (02/03/13 08:42 PM)


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: CGEM not suitable for serious astrophotography? new [Re: dickbill]
      #5660765 - 02/03/13 09:24 PM

Dick, that is pretty darn good. Didn't think it was possible with a CGEM.

Question, what is the % keeper rate on those 4 minute subs...


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dickbill
scholastic sledgehammer


Reged: 09/30/08

Re: CGEM not suitable for serious astrophotography? new [Re: orlyandico]
      #5660803 - 02/03/13 10:00 PM

Question, what is the % keeper rate on those 4 minute subs...

answer: 100% because it is only 4 subs. i didn't have time to do more before i hit the dreaded meridian.
i posted the full composite in the ccd section


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tboconnor
member


Reged: 01/14/10

Re: CGEM not suitable for serious astrophotography? new [Re: dickbill]
      #5678747 - 02/14/13 03:19 AM

just a mini followup - Im having a ball with the CGEM Unguided So far only 30 sec - 1 minute with my refractor. (a megrez 90) I havent tried too much longer, because LP kills it anyway. No signs of trails at these exposures, so Im sure I could go longer.

Im only using the All-Star alignment routine for polar alignment, and its good enough for the above.

Im sure under dark skies and with an autoguider I could do some 'serious' astrophotography


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: CGEM not suitable for serious astrophotography? new [Re: tboconnor]
      #5678820 - 02/14/13 06:20 AM

i can do anywhere from 4-6 minutes unguided with my CGEM using the ASPA. but only at 336mm focal length. it is kinda fun.

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