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Upper End Mounts worth it Anymore

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#1 starbob1

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Posted 13 April 2009 - 03:20 PM

I have been thinking'Dangerous I know;Being that the new Cgem 'CGE' EQ6-ect can be found at bery good prices.And with the advent of PHD guiding.Is it really worth paying big bucks for a TAK AP ect mount.If one can guide these cheap mounts and get excellent tracking.Why pay more.Am I missing something. :question:

#2 skybsd

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Posted 13 April 2009 - 03:38 PM

Hi,

I have been thinking'Dangerous I know;Being that the new Cgem 'CGE' EQ6-ect can be found at bery good prices.And with the advent of PHD guiding.Is it really worth paying big bucks for a TAK AP ect mount.If one can guide these cheap mounts and get excellent tracking.Why pay more.Am I missing something. :question:


Whilst I applaud the notion of your asking (in my relatively short time in this hobby) probably the most agonizing of questions for most of us that share in this pastime, you've (knowingly :question:) loaded your question with the key points with which we all wrestle: -

1] "at very good prices"
2] "worth paying big bucks"
3] "excellent tracking"

All of the above mean VERY_DIFFERENT_THINGS to quite a lot of folks, thus leading to the reasons why for some, "paying more" does actually make sense :)

Again, great question!

Indeed, in these times we now live in, its a question I'm sure we ask ourselves even more so..,

Best wishes..,

Regards,

skybsd

#3 David Pavlich

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Posted 13 April 2009 - 04:02 PM

If it's in the budget, then I'd say yes. Mounts are not different than scopes, than eyepieces, than cameras, than, well, you get the picture.

Is the performance of an AP 900 2.5 times a CGE or G11 once you factor in a guider? No, but it you want that satisfaction of watching the guide inputs, or lack thereof that you get with a mount of the 900's calibre, then it's worth it.

After I got a good polar alignment on my MI250 and started doing an imaging run, at first I thought that there was something wrong with the guider or PHD. There were guide inputs, but there seemed to be an awful long duration between inputs. But...that's what you get with a great mount and a good PA.

David

#4 groz

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Posted 13 April 2009 - 05:01 PM

First, you have to define 'good tracking', and what that means to _you_. Allow me to illustrate with our own case(s) here.

Case 1, Williams 80 mounted on an HEQ5, using a 9x50 with starshoot autoguider to guide it. Camera is a Nikon D-50, which results in an imaging scale of 2.95 arcseconds per pixel. The guider has a resolution of 6.1 as/p. Tracking of +- 1 arcsecond is more than adaquate to produce very good images out of this system, and, the guider is quite capable of keeping our HEQ5 running at that kind of accuracy. So, I can see absolutely no reason to upgrade the mount underneath it, none at all.

Case 2, 8 inch SCT mounted on an EQ6-Pro, using an ST80 on the guide side, another starshoot autoguider in that one, and a canon 350 on the SCT, which runs mostly without any reducers. That imager runs about 0.66 arcseconds per pixel. To get good images out of this one, I need tracking far superior to the above kit, on the order of +-0.25 arcseconds. Again, the guider on this one is capable of producing that kind of accuracy, so, as above, no real point in upgrading the kit underneath it. But, I will also mention, the guider really does work on this kit. Again, that's why I bought it, to make corrections to the mount. It would be a waste of money buying guide kit if the mount doesn't need any corrections.

BUT, I'm happy with a system where the guider is providing inputs at a relatively high frequency. That's exactly why I bought it, to do just that. In all honesty, I dont look at guider output, I look at the final images on the imaging side. But, some folks are not going to be happy with a system that sends guider updates every 5 seconds (second harmonic of the first gear mesh). For those folks, then neither of our mounts will be considered 'good enough', and they'll have to upspend a bunch to get a mount that wont need such aggressive correction at the resolutions we use. That fine with me, its their perogative (and money).

More folks will be shooting at yet higher resolutions, swap out my 8 inch at 2000mm focal length, swap in something closer to 4000mm, and I dont know if my EQ6+guider will be quite as good underneath it anymore. With pixel resolutions on the order of 0.33 arcseconds, we'll need tracking accuracy at 0.1 arcsecond or better, and that's going to be a tall order for my kit. At that point, I'd have to do some tests with the PEC on board, and see if that can bring it into line with what I want/need, but, up to now I haven't had to experiment with that, altho it's in the plan for the not to distant future. In theory, properly trained for PEC, even my mounts _should_ be up to a job far more demanding than I'm putting on them today.

#5 Mike Clemens

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Posted 13 April 2009 - 05:13 PM

You can wage a war against any mount to improve tracking and periodic error, gear noise, etc. Or, you can buy a mount with killer characteristics right out of the box. That alone is worth it to some people.

When you need to point an 80 pound telescope with a 4000 mm focal length, thats worth it.

Really groz says it all.

My AP1200 had good tracking out of the box (red line), and then PemPro made it ridiculous (blue line). http://www.pbase.com.../image/94812800 So I never fought the mount battles. Was that alone worth the cost premium? Not sure. Even if it wasn't, I had an 1800mm focal length scope that weighs 70 pounds configured to point, what can you do. The expensive big insane precision mounts will always have a place/use.

#6 fetoma

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Posted 13 April 2009 - 05:52 PM

I would love to have an AP1200GT, they are beautiful. Exactly for the reason of Mike's mount performance above, but at a cost of $9K, it is way out of my budget. I'll have a high end mount someday, but until then, I'll be guiding out the error on my CGE. :undecided:

#7 David Pavlich

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Posted 13 April 2009 - 06:54 PM

I would love to have an AP1200GT, they are beautiful. Exactly for the reason of Mike's mount performance above, but at a cost of $9K, it is way out of my budget. I'll have a high end mount someday, but until then, I'll be guiding out the error on my CGE. :undecided:


Don't worry, Frank. Your CGE will do just fine. If it weren't for my desire for more capacity, I'd still be using a CGE. If does VERY well.

David

#8 nik hodges

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Posted 14 April 2009 - 05:36 AM

You can wage a war against any mount to improve tracking and periodic error, gear noise, etc. Or, you can buy a mount with killer characteristics right out of the box. That alone is worth it to some people.

When you need to point an 80 pound telescope with a 4000 mm focal length, thats worth it.

Really groz says it all.

My AP1200 had good tracking out of the box (red line), and then PemPro made it ridiculous (blue line). http://www.pbase.com.../image/94812800 So I never fought the mount battles. Was that alone worth the cost premium? Not sure. Even if it wasn't, I had an 1800mm focal length scope that weighs 70 pounds configured to point, what can you do. The expensive big insane precision mounts will always have a place/use.


Mike thats a great looking pec curve, hoping my AP900 will get close to this when i get it all set up

Nik

#9 raf1

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Posted 14 April 2009 - 08:17 AM

Hi Mike - obviously very impressive performance with the 1200. Since A-P programs PEC before shipping, did you use considerably more worm cycles to get to zero? Also, did you train with an eyepiece or autoguider?

Thank for the info.

#10 freestar8n

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Posted 14 April 2009 - 09:33 AM

In theory, properly trained for PEC, even my mounts _should_ be up to a job far more demanding than I'm putting on them today.



PEC can only help remove slowish variations that are commensurate with the worm period. Faster terms, particularly those related to the gearbox that are not harmonics of the worm period, cannot be removed by PEC. These terms up being what must be chased in a mid-range mount, and they are blissfully absent in a much more expensive mount.

I guide a cge with MetaGuide using 1 second, low latency corrections and I get sub 2" fwhm at 0.5" per pixel with OAG. So I think cheaper mounts can do well nowadays - but PEC only helps slightly and the main win is rapid, low latency corrections with an accurate centroid.

Frank

#11 EddWen

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Posted 14 April 2009 - 03:12 PM

"I guide a cge with MetaGuide using 1 second, low latency corrections and I get sub 2" fwhm at 0.5" per pixel with OAG."

Those are good numbers, but of course, the limiting factor then becomes the need for a bright guide star in the appropriate location. Better quality mounts can be relied on to track longer between corrections, thereby being able to use longer guiding exposures.

#12 freestar8n

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Posted 14 April 2009 - 03:46 PM

Those are good numbers, but of course, the limiting factor then becomes the need for a bright guide star in the appropriate location. Better quality mounts can be relied on to track longer between corrections, thereby being able to use longer guiding exposures.



With video guiding at 8 fps, using 1 second of frames and effective latency of 0.5s, and OAG with a c11, my limiting guidestar magnitude is about 10. This accomodates most objects easily with a pre-selected guidestar. For other objects I have to use different centroiding with greater latency and less tight guiding - but that is rare.

So - I would say the range of guiding software options today makes autoguiding easier, but to do it well with a mid-range mount requires rapid corrections, an accurate centroid, and low latency. With the right technique and software this can be achieved for most objects - but if you want to be more casual about the corrections and still achieve small fwhm, then you will need a high end mount and its better gears/bearings.

Frank

#13 fetoma

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Posted 14 April 2009 - 04:24 PM

Frank,

What camera is used for video guiding?

#14 Miguel Lopes

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Posted 14 April 2009 - 04:31 PM

Frank, I think that 1 sec exposure should give you mag 12 or 13... Why 10?

#15 freestar8n

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Posted 14 April 2009 - 05:55 PM

Remember - I am using video guiding and OAG at 8-30 fps. Mostly I use a lumenera SKYnyx 2-0, which I also use for planetary imaging and collimation.

The combination of video and OAG with a pickoff mirror reduces the limiting magnitude I can guide on, but it allows very low effective latency - plus a variety of centroiding games that are only possible with video. I call it a form of Lucky Guiding. The basic idea is that 1s guidestar images are far from Gaussian, and simple center of gravity algorithms don't give the best possible guide error.

So - this all fits under whether modern guide software makes a mid-range mount work as well as a high end mount. I would say that you have to chase the mount errors aggressively, and that requires an accurate centroid and low latency. I do this with video, OAG, novel centroiding, and 1s effective exposures with low latency. This limits me to mag 10, but the results are consistently good and achieve much of the win of active optics without the overhead - and helps me get sub 2" fwhm with a cge in the northeast US, where seeing is average.

Frank

#16 fetoma

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Posted 14 April 2009 - 06:16 PM

Frank,

This is interesting. Can video guiding be done with the Orion autoguider or some other "low end" camera? What software are you using?

#17 freestar8n

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Posted 14 April 2009 - 06:42 PM

Can video guiding be done with the Orion autoguider or some other "low end" camera? What software are you using?



Hi-

I am using, and I wrote, MetaGuide which is free software that makes novel use of video for collimation and guiding. There are many examples and documentation on the web site.

Because it is focused on the advantages of video, it only supports web/video cameras, either usb, firewire, or even analog security cameras via a video2usb converter. If you don't use video you lose a lot of benefits of MetaGuide that are particularly helpful for a mid-range mount. Nonetheless it is easy to use, supports ascom, etc.

It also has a novel 'pro-active' guiding feature that is like PEC, but locks onto a user-specified frequency in realtime and corrects for it. This is intended for the removal of gear noise that PEC can't help.

All this stuff is aimed at getting the most out of a mid-range mount so it performs like a high-end mount.

Frank

#18 fetoma

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Posted 14 April 2009 - 08:27 PM

Frank,

Thanks. Maybe I'll give it a try with my SPC900NC webcam or Atik 2C.

#19 jaynes

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Posted 15 April 2009 - 10:10 AM

The guider has a resolution of 6.1 as/p. Tracking of +- 1 arcsecond


Are you really getting +- 1 arcsecond guiding with a resolution of 6.1 arc/pixel? I know 1/2 or 1/3rd subpixel guiding is very possible, but 1/6th subpixel guiding? Really?

#20 Kolenka

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Posted 15 April 2009 - 11:35 AM

Really just depends on the seeing. 0.15 pixel motion is possible to capture, but unless you have very good seeing, it is very hard to keep guiding at that level.






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