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Peter in Reno
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Reged: 07/15/08

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Re: The end of OAG? new [Re: vpcirc]
      #6149904 - 10/21/13 01:39 PM

Quote:

6. The second part of the post with AP's new setup for refractors is a far better solution to OAG.




I will try to be as diplomatic as I can. I think once again you chose your words poorly. What do you mean by "better"? "Better" in terms of getting rounder stars than using OAG? IMHO, not necessarily. "Easier" would have been a better choice of word. I can see that placement of guide scope on OTA's tube or focuser rather than on dovetail/tube rings so the guide scope will follow the OTA's flexure and will be better in sync with the main camera. But it is not 100% foolproof because there could still be unwanted hidden flexure anywhere that can happen anytime that will result throwing out some subs but A-P guide scope setup is definitely better than traditional guide scope mounted on top of dovetail/tube rings. OAG may not be 100% foolproof but darn close to 100% since guide camera knows exactly what's happening with the main camera and guide scope does not.

Peter


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Peter in Reno
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Re: The end of OAG? new [Re: garret]
      #6149906 - 10/21/13 01:42 PM

Quote:

Quote:

QSI 683wsg with integrated filter wheel and OAG has back focus of 50mm to 53mm depending on T-thread or 2.156" adapter. Wouldn't this camera work?






I want to use 35x24mm ccd arrays...because it give me a large field.
My 5 dmk2 has a coolerbox (-22 Celsius inside) the imaging result are very good only limited by seeing, light-pollution, and poor guiding (I'm working on this: new guider telescope, PEC for mount etc.)

Garret van der Veen




Hi Garret,

Sounds reasonable. I wish you best of luck getting your guiding issues resolved. Let us know if you found a solution.

Peter


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Aquatone
sage


Reged: 03/23/06

Loc: Moraga, CA (Bay Area)
Re: The end of OAG? new [Re: Peter in Reno]
      #6150692 - 10/21/13 10:00 PM

Quote:

Quote:

6. The second part of the post with AP's new setup for refractors is a far better solution to OAG.




I will try to be as diplomatic as I can. I think once again you chose your words poorly. What do you mean by "better"? "Better" in terms of getting rounder stars than using OAG? IMHO, not necessarily. "Easier" would have been a better choice of word. I can see that placement of guide scope on OTA's tube or focuser rather than on dovetail/tube rings so the guide scope will follow the OTA's flexure and will be better in sync with the main camera. But it is not 100% foolproof because there could still be unwanted hidden flexure anywhere that can happen anytime that will result throwing out some subs but A-P guide scope setup is definitely better than traditional guide scope mounted on top of dovetail/tube rings. OAG may not be 100% foolproof but darn close to 100% since guide camera knows exactly what's happening with the main camera and guide scope does not.

Peter




Using a guidescope mounted on the actual telescope body (NOT rings) is certainly a lot easier than an OAG, especially in more barren areas of the sky. I have been testing the new Astro-Physics guidescope arrangement (using their 60mm finder scope) directly mounted on the backplate of an almost 4,000mm focal length Cassegrain and it works superbly at different pointing angles, thermal, and gravity loads. I have compared guiding results using an SBIG off-axis internal guide chip camera and see little if any difference. (Or at least any variation could just as easily be seeing related) Whereas I would not say that the results are "better" they are no worse. Obviously the mechanical/strutural integrity over the whole chain is critical, but as long as weak points have been identified and an intelligent approach is adopted, there appears no reason in principle why a guidescope solution cannot work at longer focal lengths.

I had a concern that the nylon tightening bolts in the AP Guidescope rings would not be as solid as metal, but at AIC George from AP told me that the nylon tipped metal bolts were harder to procure and I should see no difference. I find myself tightening those things as much as possible, but so far he is right and I have not noticed any difference. Good job AP!

Anyway, just one data point.

Chris


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vpcirc
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Reged: 12/09/09

Loc: Merced CA
Re: The end of OAG? new [Re: Aquatone]
      #6150815 - 10/21/13 11:08 PM Attachment (8 downloads)

That's very encouraging news. That may very well be the way I go for the planewave 14", the focal length is only around 2500. I told George he took my idea from pics I sent him last year and made it 10x better and correctly mounted!

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elbee
professor emeritus


Reged: 05/02/09

Loc: Arizona
Re: The end of OAG? new [Re: vpcirc]
      #6151018 - 10/22/13 02:35 AM

Quote:

I told George he took my idea from pics I sent him last year and made it 10x better and correctly mounted!




I suspect that was said somewhat in jest :-)

fact is, roland has been advocating direct mounting of a guidescope onto the optical tube for years in the ap yahoo groups. he has repeatedly advised those that were mounting a guidescope on a top plate it was a recipe for trouble.

I started using this setup last year. it is the sbig 100mm focal length e-finder with an STL remote guide head installed on a two generations old A-P finder bracket. I've used it so far with a refractor at 1120mm focal length and it guides extremely well up to 30min exposures.



btw, I did replace the machine screws for metal with nylon tips. not because I was worried about them being strong enough, but because I needed slightly longer ones than what comes with the finder bracket.


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freestar8n
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Re: The end of OAG? new [Re: Aquatone]
      #6151092 - 10/22/13 04:25 AM

Quote:

I have compared guiding results using an SBIG off-axis internal guide chip camera and see little if any difference.




A key source of flexure in a mirror scope is within the OTA itself - and more rigid mounting of the guidescope won't help with that. In order to assess how small the flexure is, it needs to be expressed in arc-seconds per minute - i.e. as a rate. If you aren't guiding with a common path system like OAG, then there will always be some non-zero flexure rate. That's why this sbig thing has been in the works for so many years. Whether it finally works now I don't know because I have never seen results from that system - but I have sure heard about it for a long time.

So if you find that a small guidescope works well for you that's great - but as a data point it would be good to estimate the actual flexure rate you see - because it will not be zero, and it will prevent getting consistently small and round stars in long exposures and long focal length. 0.1"/minute is a very tiny amount - but in a 10m exposure with 2" stars it would be a problem. With 4" stars it might be tolerable.

Refractors are much less affected by flexure so I can imagine the guidescope mounting plays a bigger role. They also don't create stars that are 2" and below fwhm - so they are much more forgiving in many ways than a mirror scope.

Professional work with large cassegrains is either self-guided or common-path (OAG) guided. They may internally use infra-red light sources for slow active optics alignment of the telescope during imaging - but the guiding itself would be with a real star using the actual optical path at the full focal length.

Frank


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vpcirc
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Reged: 12/09/09

Loc: Merced CA
Re: The end of OAG? new [Re: Aquatone]
      #6151234 - 10/22/13 08:32 AM

Of course my comment was in jest, but I do have a serious question. Before anyone gets their feathers ruffled, I'm not pointing or stating, I'm asking!!! I feel like the term flexure is thrown around loosely at times, so here's the question. How do you know how much is flexure, and how much is the construction quality of the OTA, the PE of the mount, and how much is caused by setup? Would you have to somehow measure each? When I asked Tom Bash about this last year he told me that guiding has changed dramatically in the past 5 years. The old thought was the f-ratio between the guide scope and the OTA couldn't be different than xxx but that doesn't apply anymore. If you're getting great guiding with AP's setup at a 4000 FL, is it because of the stability of the overall system, mirrors that are temp regulated, a solid mount? I am really confused and making the right choice for my new OTA could be a difference of 4-5k in the total cost.

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Alex McConahay
Carpal Tunnel


Reged: 08/11/08

Loc: Moreno Valley, CA
Re: The end of OAG? new [Re: vpcirc]
      #6151274 - 10/22/13 09:08 AM

>>>>>How do you know how much is flexure, and how much is the construction quality of the OTA, the PE of the mount, and how much is caused by setup? Would you have to somehow measure each?

I am not sure you could ever know the source of all deviations of the locations of the stars. I suspect somebody with lots of equipment may be able to isolate some of the causes, but never all of them.

I have not seen the whole setup that SBIG was touting, although I did see the presentation in Santa Clara. But it seems to me they are adding parts to a system to try to get the flexure out. The more parts, the more chance for a new place to flex, in my opinion. I cannot tell how the LED source for the artificial star is affixed to the camera or the tube, or the focuser, but that seems to be a place for a problem, as well as the location of where and how the retroreflector is.

I think this uncertainty in knowing where the flexure or other error comes from is the strength of OAG. You have the guide sensor and the imaging sensor in exactly the same relationship (although even here, there is room for flexure--but so little).

As for whether OAG is a pain or not--I have to say it can be. It can be hard to find a star sometimes. But, really, with a sensitive enough guider, it is almost always possible to find a guide star--and that is about the only trouble I know about.

Alex


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dawziecat
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Reged: 10/20/10

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Re: The end of OAG? new [Re: Alex McConahay]
      #6151544 - 10/22/13 11:50 AM

I have used OAG on only two nights so far. But it worked so well, I am determined to use it always now, on all OTAs, even those of ridiculously short FLs.

My experience with external guide 'scopes, both "mini" and "maxi" has been a battle. I will never be sure just what was flexing, but elongated stars on 15 minute 530mm exposures became round as soon as I used the OAG-8300.

I was surprised at how plentiful guide stars were but appreciate such limited experience, and with such a short FL, is no test at all.

I did very quickly run out of back focus however. With the FSQ106EDXIII, the QE reducer, a MMOAG, the shortest adapter Precise Parts can make and an STL with internal FW left me too long in focus by about 1mm. That is AFTER allowing 2.3mm for the filter, CCD window and coverslip. Had I the external, 8 position filter wheel for the STL, it seems I could not use the MMOAG with the QE reducer at the recommended focus at all.

The REAL problem with OAG, as I see it, is not the PITA factor, but the backfocus issue with some gear combinations.

Edited by dawziecat (10/22/13 11:51 AM)


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orion69
scholastic sledgehammer


Reged: 05/09/10

Loc: Croatia
Re: The end of OAG? new [Re: vpcirc]
      #6151587 - 10/22/13 12:16 PM

Quote:

How do you know how much is flexure, and how much is the construction quality of the OTA, the PE of the mount, and how much is caused by setup?




Easy, install OAG and difference in star quality is flexure.


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orion69
scholastic sledgehammer


Reged: 05/09/10

Loc: Croatia
Re: The end of OAG? new [Re: dawziecat]
      #6151590 - 10/22/13 12:19 PM

Quote:

The REAL problem with OAG, as I see it, is not the PITA factor, but the backfocus issue with some gear combinations.




Correct.


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vpcirc
Post Laureate


Reged: 12/09/09

Loc: Merced CA
Re: The end of OAG? new [Re: orion69]
      #6151626 - 10/22/13 12:40 PM

Now we're getting back to opinions again, for many people OAG is a PITA, but I'm sure you guys have mastered it so it's not for you. Here's my issue. I'm buying a 14" Planewave. To add the combination rotator focuser is an extra $3500. Now if I add a MMOAG it's another $900. Why on God's earth would I do that if I can achieve what Aquatone is achieving great success with AP's new design?

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Peter in Reno
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Reged: 07/15/08

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Re: The end of OAG? new [Re: orion69]
      #6151631 - 10/22/13 12:42 PM

That's true. I was somewhat forced to replace my external OAG/FW/CCD camera setup with QSI with integrated FW and OAG to reduce back focus. My new TEC 140 APO with flattener requires 85mm back focus. Previous scope was C-8 EdgeHD which had a whopping 133mm back focus. QSI wsg cameras' back focus is either 50mm (T-Thread) or 53mm (2.156") depending on telescope side adapter.

Peter


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Peter in Reno
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Re: The end of OAG? new [Re: Aquatone]
      #6151637 - 10/22/13 12:45 PM

Quote:

Using a guidescope mounted on the actual telescope body (NOT rings) is certainly a lot easier than an OAG, especially in more barren areas of the sky. I have been testing the new Astro-Physics guidescope arrangement (using their 60mm finder scope) directly mounted on the backplate of an almost 4,000mm focal length Cassegrain and it works superbly at different pointing angles, thermal, and gravity loads. I have compared guiding results using an SBIG off-axis internal guide chip camera and see little if any difference. (Or at least any variation could just as easily be seeing related) Whereas I would not say that the results are "better" they are no worse. Obviously the mechanical/strutural integrity over the whole chain is critical, but as long as weak points have been identified and an intelligent approach is adopted, there appears no reason in principle why a guidescope solution cannot work at longer focal lengths.

I had a concern that the nylon tightening bolts in the AP Guidescope rings would not be as solid as metal, but at AIC George from AP told me that the nylon tipped metal bolts were harder to procure and I should see no difference. I find myself tightening those things as much as possible, but so far he is right and I have not noticed any difference. Good job AP!

Anyway, just one data point.

Chris




Congrats on getting your guiding to work well with A-P guide scope guiding a 4000mm focal length scope. Can you still achieve round and reasonably small FWHM stars at 30 minute guided exposures (for narrow band imaging)?

Peter


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orion69
scholastic sledgehammer


Reged: 05/09/10

Loc: Croatia
Re: The end of OAG? new [Re: vpcirc]
      #6151647 - 10/22/13 12:51 PM

Quote:

Now we're getting back to opinions again, for many people OAG is a PITA, but I'm sure you guys have mastered it so it's not for you. Here's my issue. I'm buying a 14" Planewave. To add the combination rotator focuser is an extra $3500. Now if I add a MMOAG it's another $900. Why on God's earth would I do that if I can achieve what Aquatone is achieving great success with AP's new design?




It was a joke.
As I said before in another threads, if it works for you, great.
If not, there is always OAG.


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WadeH237
Pooh-Bah


Reged: 02/24/07

Loc: Snohomish, WA
Re: The end of OAG? new [Re: vpcirc]
      #6151687 - 10/22/13 01:15 PM

Quote:

How do you know how much is flexure, and how much is the construction quality of the OTA, the PE of the mount, and how much is caused by setup?




This is not that difficult if you collect guide logs for every session. The guide logs capture the guide commands sent to the mount. But more importantly for this discussion, they also capture the positional error of the guide star centroid from the proper pointing position.

With this data, it is possible to determine with pretty good accuracy how well the guider tracked the guide star. Any tracking errors in the main image that are not represented in the guide error data are the result of flexure.

I think that you can safely rule out periodic error as a source of error when you are guiding. If the rate of error in your PE is enough to exceed what the guider can correct, then you've got bigger problems than flexure.

The biggest practical concern is probably determining the difference between loose parts, shifting mirror (on an SCT), and actual flex of the OTA or parts.

In my experience, if you have an SCT, you will have mirror shift that you will not be able to resolve. I've heard that some larger OTAs may actually have flexure because the OTA and mount actually flex due to weight (I've never imaged through my C14, but have heard that it might have some of this). Assuming stout dovetails and rails with tight connections, the biggest correctable source of flexure for my has been in the focuser, usually on the guide scope, where the drawtube sags a bit. Interestingly, it's this last one that the AP solution specifically addresses.

The good news is that OAG or self-guiding will either handle or avoid flexure from any of these sources. I'm interested in hearing how the SBIG differential guiding solution works. I was really excited about it a few years ago, but finally gave up waiting and went to OAG.

-Wade

PS: For an interesting experiment if you have a guide scope, go through your data to find an all night session on a single target with good guiding. Then, stack the images with no registration. It's interesting to see what flexure looks like when it adds up over the whole night. Note that this assumes that you are not dithering.


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elbee
professor emeritus


Reged: 05/02/09

Loc: Arizona
Re: The end of OAG? new [Re: vpcirc]
      #6151702 - 10/22/13 01:22 PM

Quote:

I'm buying a 14" Planewave. To add the combination rotator focuser is an extra $3500. Now if I add a MMOAG it's another $900. Why on God's earth would I do that if I can achieve what Aquatone is achieving great success with AP's new design?




you wouldn't.

but for every user making these choices there should be an understanding that what you try may not work or may be harder to implement than you thought. the best answer will always be the one that works for you at the expense level and difficulty level you are willing/able to tolerate. if you find a rigidly mounted guide scope does not work, then you would like to be in a position to "try" something else (=$$$).

it's a difficult hobby and the more options that make it attainable to users with differing goals and budgets, the better. i don't have a permanent setup and i swap cameras between different scopes (all refractors). always used an external guide scope; no OAG, no rotator. but if i ever do get a permanent setup, there's going to be a large reflector in it with OAG and rotator - personally, i would not waste time with anything else. and that would be my advice to you for your upcoming planewave (i have an inherent distrust of mirror support stability :-) but that's just one man's opinion.


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freestar8n
Post Laureate
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Reged: 10/12/07

Re: The end of OAG? new [Re: WadeH237]
      #6151711 - 10/22/13 01:25 PM

Quote:

Then, stack the images with no registration. It's interesting to see what flexure looks like when it adds up over the whole night. Note that this assumes that you are not dithering.




Yes, but you need to make sure it isn't field rotation, which will also show trailing. If the trails are long enough you can tell the rotate around the guidestar and are not due to flexure.

But any steady marching of the stars from one exposure to the next while guiding is definitely not due to periodic error or a guiding problem. It's either field rotation or flexure *somewhere*. People usually blame the guidescope itself - but with a mirror scope it is the mirrors themselves changing slightly.

There is a separate issue with a short guidescope and that is the actual centroid accuracy you can achieve. People talk about short guidescopes being fine for centroids - but it is rarely shown in terms of small fwhm in long exposures. With OAG you get the benefit of greater centroiding accuracy on the pixel scale, along with no flexure.

Many people who have "tried" oag did not go all the way and use a planetarium FOVI to pre-plan the guidestar and dial it in. That is standard practice nowadays and avoids the hunting that people associate with OAG. Some people with high end mounts, such as Peter here, don't bother with that and instead just use longer guide exposures and always find a guidestar.

Frank


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Aquatone
sage


Reged: 03/23/06

Loc: Moraga, CA (Bay Area)
Re: The end of OAG? new [Re: Peter in Reno]
      #6151750 - 10/22/13 01:49 PM

Quote:

Congrats on getting your guiding to work well with A-P guide scope guiding a 4000mm focal length scope. Can you still achieve round stars at 30 minute guided exposures (for narrow band imaging)? Peter




You raise a key point that exposure time is a critical variable. I am currently doing 10 minute RGBL subs but with a higher QE KAF-6303E CCD. (As galaxies are my main interest) I have not gone out as far as 30 minutes, partly because I am getting slightly egg shaped stars at that exposure length using *both* SBIG internal guide chip and the AP guide-scope. I am fairly certain this is due to mount alignment and pier issues. I do not have a permanent pier but rather a sturdy 10" ATS tripod, and George at AP suggested that it could be due to something as simple as me using the metal rather than hard rubber pads on the feet, introducing a possible slight slippage on the (hard) ground as the telescope mass distribution changes. I will be changing to hard rubber for more friction. I also have an AP1600GTO with absolute encoders arriving later this week, which will be a significant step in further removing the mount from the equation.

I will try and post some pictures of the current guidescope configuration when I get back from work later.

I know that on/off-axis guide methods are most appropriate at this long focal length, but demonstrating I can guide an almost 4,000mm cassegrain with a guide-scope and that it is possible, has become a kind of perverse (and fun) challenge. At the least it really identifes weak points in the overall pier/mount/telescope assembly.

BTW: I certainly have imaged for 30 to 45 minutes using the new AP guide scope mounted on the focuser of my much shorter 1200mm focal length AP160 refractor with perfectly round stars. That is a very good match and really makes an OAG unnecessary in this less demanding configuration. As has been earlier pointed out, Roland has advocated focuser mounted guide scopes on refractors for some years.

Chris


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SL63 AMG
scholastic sledgehammer
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Reged: 12/21/09

Loc: Williamson, Arizona
Re: The end of OAG? new [Re: Aquatone]
      #6151773 - 10/22/13 02:16 PM

I image at 2857mm FL. I have tried using a 250mm VarioFinder on both a MiniMax and Max-F as well as direct mounting a Stellarvue 650mm FL CF OTA to the top of my RCOS. I have also tried both of these options on my ASA N10 astrograph. I have tried them both on an AP1600GTO CP3 mount and an ASA DDM85X-SL mount.

My experience is that using an MMOAG or QSI camera with a built in OAG are both much easier with which to guide than using guide scope. I believe this is due to at least two issues. I believe guiding at the FL of the main OTA works better for long FL telescopes than guiding at some fraction thereof. Also, it is likely there are issues with differential flexure due to these guide scopes being mounted on the rings of the OTA versus the OTA itself.

In any case, my experience has taught that guiding with an OAG is simple as is finding a guide star, particularly when you have rotator.

Having a rotator has the side benefit of giving one an opportunity to frame an object in an aesthetically pleasing position.

Elbee images with refractors and has great success using short focal length guide scopes mounted directly to his main imaging refractors. This shows that for every system there is a solution that works and no particular solution should be put down versus another without careful examination of the equipment and the circumstances of its deployment.

If the SBIG product solves all or many of these issues for imagers, that's fantastic. Having more options in an expensive hobby is always welcomed.

I'm glad to see the discussion has changed to one of comparison of products under different circumstances rather than that of putting down one method over another.


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