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CGEM not suitable for serious astrophotography?

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#51 EFT

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Posted 29 January 2013 - 01:13 PM

OK, you have a point. The CGEM drifts about one arc-second every twenty seconds, just in the declination axis, due to non orthogonality.

This is 1/60th of the performance of the Astro-Physics mount, so then I should be satisfied if my Accord has a top speed of only 4 MHP?

Five minutes of accurate unguided performance is not that unreasonable.


Unguided AP for anything but the brightest objects, fastest focal lengths and shortest exposures, particularly at this price point, is unreasonable. Depending on the scope and imaging setup, it can be unreasonable for even a much more expensive mount. Any amount of research on the CGEM would show that. I agree that about the only thing the CGEM and an AP mount have in common is that they are both telescope mounts.

#52 freestar8n

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Posted 29 January 2013 - 01:20 PM

The CGEM drifts about one arc-second every twenty seconds, just in the declination axis, due to non orthogonality.


Non-orthogonality of the axes has no connection with dec. drift at all. The axes could be off by 20 degrees and it would certainly affect GoTo accuracy - but once you had the scope somehow aimed at an object, and if the polar axis is aligned, and if you are smoothly rotating around that polar axis, then the tracking will be no different from that of a perfectly orthogonal mount.

Polar tracking just involves rotation around a single axis. As long as the rest of the geometry is fixed relative to that axis - there is no way to induce additional rotation in a different direction.

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#53 jrcrilly

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Posted 29 January 2013 - 01:48 PM

OK, you have a point. The CGEM drifts about one arc-second every twenty seconds, just in the declination axis, due to non orthogonality.


That would amaze me; I've never heard of orthogonality issues causing drift of that magnitude. Sounds more like a severe polar alignment error.

#54 rmollise

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Posted 29 January 2013 - 01:49 PM

OK, you have a point. The CGEM drifts about one arc-second every twenty seconds, just in the declination axis, due to non orthogonality.

This is 1/60th of the performance of the Astro-Physics mount, so then I should be satisfied if my Accord has a top speed of only 4 MHP?

Five minutes of accurate unguided performance is not that unreasonable.


Your problems, whatever they are, are not caused by "non-orthogonality." Look to your polar alignment. How did you perform it?

#55 wolfman_4_ever

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Posted 29 January 2013 - 02:46 PM

That's exactly what I was going to say Rod.

This guys blog post screams user error..

The mount is only going to be as good, or as smart, as the user is.. I don't care what mount you are using.. As they say.. "stupid is as stupid does"

#56 gdd

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Posted 29 January 2013 - 03:15 PM

Your problems, whatever they are, are not caused by "non-orthogonality." Look to your polar alignment. How did you perform it?




I don't understand what non-orthoganality has to do with tracking accuracy though I can see it messing up goto accuracy. You don't even need to have a dec axis to track (think of AstroTrac).

Gale

#57 jrcrilly

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Posted 29 January 2013 - 03:21 PM

I don't understand what non-orthoganality has to do with tracking accuracy though


If it were off by tens of degrees you'd see drift. If it were critical in small amounts, folks would have to worry about alignment of the optical axis of the telescope to the mount. Some do worry about that, but most do not.

#58 photodady

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Posted 29 January 2013 - 03:47 PM

"Non-orthogonality of the axes has no connection with dec. drift at all."

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

For the former misalignment your statement is true and is called Cone Error, but for RA-DEC nonorthogonality the error is not constant, rather the scope will travel both below and above the intended path. The two extrema will be 90 degrees from the zero crossings. When the path traveled is viewed from the side it will form a lissajous pattern, or a figure eight.

The periodic error for the corrections using a guiding system in the declination axis will have a period of 24 hours. For my particular scope the peek-to-peek error is about 1/2 degree, or about the angular width of the Sun or Moon.

A similar effect occurs when viewing the Sun at the same time every day and plotting its position, I think this is called the Sun's analemma. This is also due to the nonorthogonality of the rotation of the earth's axis in relation to its orbital plane.

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

#59 dickbill

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Posted 29 January 2013 - 04:09 PM

Misalignement also translates into field rotation, that's easy to see.
The simplest think to explain a dec drift, as Rod said, is inaccurate polar alignment. Do not assume it is good just because 'before' it was good. My experience is that if you are going to image, check it, even if two days before it was indeed very good and you havn't move the mount.

First using the polar routine after top notch 2stars align + 2, the mount put the Esquimo dead center of the C925 at f10, 120X. So then i did the routine for polar alignement and had little drift in RA and dec thereafter.
Well, 2 days later i thought i should not realign since it was good before and i didn't move the mount. And this is when my declination drift showed up. In between I had lots of winds. I think the altitude can not change so easily under transversal winds so it's perhaps just the azimuth that has shifted, or perhaps something about the central spreader and screws that get loose, I'll know next time i realign, but anyways, as long your mount is not on a solid permanent pier shielded from the winds, polar alignement should be checked regularly IMO.

#60 rmollise

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Posted 29 January 2013 - 04:24 PM

"Non-orthogonality of the axes has no connection with dec. drift at all."

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

For the former misalignment your statement is true and is called Cone Error, but for RA-DEC nonorthogonality the error is not constant, rather the scope will travel both below and above the intended path. The two extrema will be 90 degrees from the zero crossings. When the path traveled is viewed from the side it will form a lissajous pattern, or a figure eight.

The periodic error for the corrections using a guiding system in the declination axis will have a period of 24 hours. For my particular scope the peek-to-peek error is about 1/2 degree, or about the angular width of the Sun or Moon.

A similar effect occurs when viewing the Sun at the same time every day and plotting its position, I think this is called the Sun's analemma. This is also due to the nonorthogonality of the rotation of the earth's axis in relation to its orbital plane.

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


The telescope is only being driven in RA. If it is properly polar aligned, you won't get drift. Forget this as the source of your problem. ;)

#61 freestar8n

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Posted 29 January 2013 - 04:31 PM

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

#62 jrcrilly

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Posted 29 January 2013 - 04:32 PM

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.

#63 Stew57

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Posted 29 January 2013 - 04:51 PM

Are we discussing euler angle?

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

#64 gdd

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Posted 29 January 2013 - 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

#65 orlyandico

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Posted 29 January 2013 - 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!

#66 guyroch

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Posted 29 January 2013 - 10:49 PM

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

The CGEM is very capable.

Guylain

#67 Patrick

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Posted 29 January 2013 - 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

#68 orlyandico

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Posted 29 January 2013 - 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...

#69 Patrick

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Posted 29 January 2013 - 11:25 PM

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

What camera are you using?

Patrick

#70 Bowmoreman

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Posted 30 January 2013 - 07:07 AM

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...

#71 Patrick

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Posted 30 January 2013 - 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! :smirk:

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

#72 dickbill

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Posted 31 January 2013 - 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.

#73 dickbill

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Posted 03 February 2013 - 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.
Posted Image

#74 orlyandico

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Posted 03 February 2013 - 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...

#75 dickbill

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Posted 03 February 2013 - 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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