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Equipment Discussions >> Mounts

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orlyandico
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Re: CGE Pro: Anyone have a good one? new [Re: KDizzle]
      #5365660 - 08/12/12 11:37 PM

I was gonna bring that up... seems that a fair-to-middling mount with an AO unit should be able to outperform a high-end mount, at least in the roundness-of-stars category.

Of course reliability would be another ball of wax entirely. All the folks who rave about AP's and Tak's (and Paramounts?) must be at least partially singing the praises of reliability and predictability.


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Jared
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Re: CGE Pro: Anyone have a good one? new [Re: KDizzle]
      #5365794 - 08/13/12 01:40 AM

Here is the disadvantage to AO... You need a bright guide star in the field of view. If you really want to adjust things on the order of 2-5 Hz or even faster, that requires either an extremely fast scope or a very short focal length scope (in which case even a moderate quality mount can probably handle things without the help of AO).

Does it work? Sure. I had an AO7 that I used with great success with a 110mm refractor on a Losmandy GM8 for a while. It was wonderful with that combination. The Losmondy had some rough spots in its tracking that the AO unit was quick enough to take out where an ordinary autoguider couldn't. But as the focal length grows, the number of bright guide stars begins to shrink. When I switched to a 1,600mm focal length scope, the number of objects where the AO unit was an improvement on a traditional autoguider began to shrink since I couldn't find guide stars that would allow a fast sampling rate. Now I'm imaging at 2,000mm+ and I just don't see how to make AO work.


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orion69
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Reged: 05/09/10

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Re: CGE Pro: Anyone have a good one? new [Re: Jared]
      #5365833 - 08/13/12 02:36 AM

Quote:

Here is the disadvantage to AO... You need a bright guide star in the field of view.




Also, how much of improvement AO brings over mount that guides correctly <1000 mm? Is that even visible on pictures?
There is one more problem with using AO and that is backfocus. In my case backfocus for FF/FR I use is 55mm and there is no way I could squeeze AO with FW and camera.
Don't get me wrong, I would like to use AO, I just can't.

Knez


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freestar8n
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Re: CGE Pro: Anyone have a good one? new [Re: KDizzle]
      #5365872 - 08/13/12 03:53 AM

Quote:

Every time these PE discussions come up, AO never seems to enter the discussion. Doesn't it make sense to move a tiny refractive element super fast without a giant pile of weight on it? Or still everyone thinks it's best to move a huge mass ever-so-slightly while combating the manufacturing challenges of pi?




I consider AO to be proven technology that will make a mid-range or even lower grade mount perform, in terms of fwhm, like a high end mount - but it has many disadvantages.

Results from AO provided evidence for me many years ago that tighter guiding was possible with low latency and a good centroid. AO for amateurs was originally intended (I believe) to help correct seeing in the 10-20Hz range, but the benefits with mid-range equipment were obvious even when used at around 1 Hz, and much slower than seeing. I could tell from images I saw on the web whether an lx200 shot used AO or not - even when it was correcting at 1 Hz or slower.

If you go from a small guidescope to AO you are not only getting the lower mechanical latency of a small moving element - you are switching to OAG and low latency corrections without a shutter. But there is nothing that prevents a mount from responding in less than one second. So - as long as the autoguider feedback is prompt, a mount can achieve similar tightness of guiding without the need for added complexity of AO.

AO has several flavors nowadays - reflective or refracitive; guiding before the filters or after. But for correcting mid-range mounts (not seeing) I think you can get nearly the same benefit in the 1 Hz range with tight guiding and OAG.

Unfortunately there are "expert" web pages on autoguiding that say you should make as few corrections as possible because each correction introduces "noise." I would say instead that you should make corrections at the optimal rate - whatever it is - that is needed for your mount. If you steer a cruise ship across the Atlantic you probably don't want to jerk the steering back and forth rapidly - but if you are off-roading in a jeep you had better keep your hands on the wheel.

Also unfortunate - many people using OAG are not dialing in the brightest guide star available and instead they just expose longer - perhaps 5 seconds - until they can see a guidestar that happens to fall on the chip. This is fine for high end mounts because they are truly smooth over this time frame - but for mid-range mounts it will prevent prompt corrections to faster terms. I recommend dialing in the brightest guidestar available with OAG. This points out the inherent benefit of high end mounts, in that such details and added care may not be needed. But similar results can be obtained when added effort is made in directions that actually yield smaller fwhm.

Frank


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orlyandico
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Re: CGE Pro: Anyone have a good one? new [Re: freestar8n]
      #5365881 - 08/13/12 04:18 AM

Frank - using an OAG is tough. Finding guide stars is really hard because of the tiny pick-off mirror.

I have actually gone back to finder guider from OAG, in spite of the obviously superior results from OAG, because it is simply less hassle to use a finder guider.

The Innovations Foresight ONAG should help here as you can pick guide stars from the center of the field where the stars aren't shaped like bananas and are much brighter. It is rather pricey though. I am going to try building my own ONAG using a cold mirror... updates if ever I get that working.


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freestar8n
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Re: CGE Pro: Anyone have a good one? new [Re: orlyandico]
      #5365905 - 08/13/12 05:17 AM

There are good oag setups and bad ones, and the main problem I see is a small mirror far away from the guide chip - often with spacers inserted to push it even farther back. It's important to have the guide mirror be as large as possible and for the guide chip to be as close to it as possible - because the size of the mirror as seen by the guide chip places a limit on the effective f/ratio of the guider setup - regardless of the f/ratio of the OTA. I think many OAG setups are at f/30 or something due to geometry alone.

The second issue with OAG is to pre-plan the guidestar and select the brightest one available using a planetarium program and a field of view indicator. This, combined with angle readouts on the OAG, lets you dial in the guidestar ahead of time and find it easily. Examples of my setup can be seen here.

Even people with high end setups would tend to pre-plan the best guide star - partly because some people with dual chip setups are guiding through the filter, which may be narrow band, and they really need a bright guidestar. With OAG you are in front of the filter, but there is still benefit in using the best available guidestar.

I prefer the straight-through geometry of OAG rather than mounting the imaging camera off-axis and having an additional reflective element in the imaging path.

Although this is all specific to OAG technique - comparing one mount to another is less meaningful when one of them is not guided by techniques that realize its true capabilities.

Frank


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orlyandico
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Re: CGE Pro: Anyone have a good one? new [Re: freestar8n]
      #5365912 - 08/13/12 05:23 AM

because the size of the mirror as seen by the guide chip places a limit on the effective f/ratio of the guider setup

thanks for that information. no wonder i was having such problems with OAG.

i was always wondering why the exact same camera, with the exact same OTA, needed shorter exposures when "straight through" than with the OAG. It is the same objective right? so I thought the focal ratio was the same.


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orion69
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Reged: 05/09/10

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Re: CGE Pro: Anyone have a good one? new [Re: freestar8n]
      #5365943 - 08/13/12 06:45 AM

Quote:

There are good oag setups and bad ones, and the main problem I see is a small mirror far away from the guide chip - often with spacers inserted to push it even farther back. It's important to have the guide mirror be as large as possible and for the guide chip to be as close to it as possible - because the size of the mirror as seen by the guide chip places a limit on the effective f/ratio of the guider setup - regardless of the f/ratio of the OTA. I think many OAG setups are at f/30 or something due to geometry alone.





I'm not sure that is true because mirror is not lens. Also not sure that larger mirror would produce brighter stars on guiding camera chip, it would simply cover larger surface of the chip. If I'm wrong feel free to correct me.
More important is quality (sensitivity) of guiding camera, get Lodestar and all problems with finding guiding stars are gone (depending of FL, of course). That said I never had problems finding guiding stars with same configuration using Alccd5 (Qhy5).
I think main problem with new OAG users is that they give up too soon, guiding with OAG is actually NOT difficult.

Knez

P.S.

orlyandico, what scope (configuration) are you using?


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orlyandico
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Re: CGE Pro: Anyone have a good one? new [Re: orion69]
      #5365947 - 08/13/12 06:52 AM

I'm using a Meade DSI as my guiding camera. This is a pretty sensitive camera, but I am forced to go to 5-second (!) exposures to get decent guide stars. OTA is an Orion 100ED (f/9).

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orion69
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Re: CGE Pro: Anyone have a good one? new [Re: orlyandico]
      #5365958 - 08/13/12 07:01 AM

Quote:

I'm using a Meade DSI as my guiding camera. This is a pretty sensitive camera, but I am forced to go to 5-second (!) exposures to get decent guide stars. OTA is an Orion 100ED (f/9).




With Equinox F7.5, 900mm I was guiding 1-2 sec with Alccd5 (Qhy5), with FF/FR 0.85x and Lodestar I could go <0.5 sec, but always guiding at 1 sec with plenty of stars available (almost like with separate scope).
Don't you use FF/FR?

Knez


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freestar8n
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Re: CGE Pro: Anyone have a good one? new [Re: orion69]
      #5365972 - 08/13/12 07:16 AM

Quote:

I'm not sure that is true because mirror is not lens. Also not sure that larger mirror would produce brighter stars on guiding camera chip, it would simply cover larger surface of the chip. If I'm wrong feel free to correct me.




It is definitely true as long as the guide mirror is only accepting part of the light from the OTA. Often, as in my case with an SCT, the allowed light is restricted by a mixture of the guide mirror size and the secondary obstruction or the full aperture. Either way - the guide chip is only seeing part of the entrance pupil, and making the guide mirror larger would let it see more.

The geometry of f/ratio is simple - it is defined by the maximum cone of light arriving at focus from a star - which in this case is defined by the size of the mirror and its distance from the guide chip.

Another factor determining guide star brightness and quality is the presence of aberration in the outer part of the field. This is where field correction in the OTA has a second benefit by improving the guide star quality - and therefore the guiding. I was able to guide with a normal c11 at f/10 but the guidestars are better with the 6.3 reducer. I expect they would be even better with EdgeHD.

Many people say the shape of the guidestar doesn't matter and you can even guide on a star shaped like a line - but I would say that centroid accuracy is best when the star is small and round and the guide pixel scale is < 1". This also lets you use shorter exposures and react to errors promptly.

Frank


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orlyandico
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Re: CGE Pro: Anyone have a good one? new [Re: freestar8n]
      #5366003 - 08/13/12 07:52 AM

I am using an FR/FF, but its spacing requires that it sits after the OAG (I am using a Vixen OAG, not one of those low-profile TS ones).

So I am seeing the edge of the f/9 light cone. and yes the stars look like lines!!! (an f/9 refractor should have a decently flat field, so I think the star shape is due to the shape of the pick-off mirror).


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orion69
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Reged: 05/09/10

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Re: CGE Pro: Anyone have a good one? new [Re: orlyandico]
      #5366037 - 08/13/12 08:45 AM

Quote:

I am using an FR/FF, but its spacing requires that it sits after the OAG (I am using a Vixen OAG, not one of those low-profile TS ones).





This is not good, OAG should be placed between FF/FR and filter wheel if you have one. If you can't place OAG after FF/FR you'll have to buy new OAG.

Knez


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orlyandico
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Re: CGE Pro: Anyone have a good one? new [Re: orion69]
      #5366052 - 08/13/12 09:06 AM

D'oh. I suspected as much, because my DSI Pro would not reach focus unless I added a 1.25" diagonal to the guide port! Guess I'll try putting the reducer in front and see how bad things get.

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Alph
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Re: CGE Pro: Anyone have a good one? new [Re: orion69]
      #5366287 - 08/13/12 11:42 AM

Quote:

Also not sure that larger mirror would produce brighter stars on guiding camera chip, it would simply cover larger surface of the chip. If I'm wrong feel free to correct me.



You are correct. A larger pick-off prism will not produce a brighter guide star. Otherwise, a larger CCD would produce brighter stars too.


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freestar8n
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Re: CGE Pro: Anyone have a good one? new [Re: Alph]
      #5366396 - 08/13/12 12:44 PM

Quote:

You are correct. A larger pick-off prism will not produce a brighter guide star. Otherwise, a larger CCD would produce brighter stars too.




Things sure are different up there on Melmac. Go ahead then - make the prism 1mm square and see how much light you get when it is 100mm from the guide chip. At best it would be operating at f/100 even if the OTA is f/2. At the same focal length, of course.

The minimum possible f/ratio seen by the guide chip is the distance to the prism divided by the prism width. If that number is greater than the f/ratio of the OTA, then increasing the prism size will increase the brightness of guide stars, by decreasing the f/ratio. Once the prism is large enough that it is not clipping the available cone of light - then a larger prism will not help anything and the f/ratio will equal that of the OTA.

Most people are operating in a regime where the prism is greatly limiting the cone of light - and therefore the effective f/ratio of the guide system. You can decrease the effective f/ratio by increasing the prism size and/or moving the guide chip closer. The difference can be dramatic.

Frank


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orlyandico
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Re: CGE Pro: Anyone have a good one? new [Re: freestar8n]
      #5366424 - 08/13/12 01:00 PM

Frank - bingo.

I think this is why my OAG experiences have been really bad. I put the flattener behind the OAG, which requires a lot of in-focus, so I actually needed a 1.25" diagonal at the guide port so the guide chip would come to focus.

What this means is... the guide chip is very, very, very far from the prism. Which accounts for my needing 5-second exposures.


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David PavlichAdministrator
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Re: CGE Pro: Anyone have a good one? new [Re: orlyandico]
      #5366446 - 08/13/12 01:11 PM

A good topic, OAG, but if we could, get this back on the CGE Pro topic? Thanks!

David


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James Cunningham
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Re: CGE Pro: Anyone have a good one? new [Re: David Pavlich]
      #5366562 - 08/13/12 02:15 PM

A friend just purchased a 14 inch SCT on the CGE Pro mount. The instructions say nothing about how to get a Polar alignment. It just says pick the all star and let the scope slew to the first star. The manual also says nothing about where to orient the scope. It does not even mention the Polaris star. How do you srart to get an alignment with the CGE Pro mount? Thanks.

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Alph
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Reged: 11/23/06

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Re: CGE Pro: Anyone have a good one? new [Re: orlyandico]
      #5366584 - 08/13/12 02:28 PM

Quote:

Frank - bingo.



Unfortunately he is wrong about the effect of the pick-off prism size on the guide star brightness. QSI has just confirmed it.


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