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Equipment Discussions >> Mounts

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jmiele
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AP900 Question
      #4477820 - 03/27/11 02:31 PM

I'm having less than expected success with pointing accuracy when I flip. With good Polar Alin on spot on until I flop
Sound like an Ortho issue to anyone? Or something else. I'm sure I'm pulling some newb move and missing something. Any input welcomed. Thanks in advance... Joe


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skybsd
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Re: AP900 Question new [Re: jmiele]
      #4477943 - 03/27/11 03:32 PM

Hello,
Which polar alignment method did you use?

Regards,

skybsd


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M13 Observer
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Reged: 12/09/06

Re: AP900 Question new [Re: jmiele]
      #4477956 - 03/27/11 03:40 PM

Quote:

I'm having less than expected success with pointing accuracy when I flip. With good Polar Alin on spot on until I flop
Sound like an Ortho issue to anyone? Or something else. I'm sure I'm pulling some newb move and missing something. Any input welcomed. Thanks in advance... Joe




If you are truly polar aligned with visual or CCD drift method then yes, your mount and telescope are not orthogonal. If you are not well polar aligned then it could be all polar alignment error, both polar alignment error and non-orthogonal (most likely the case), or all orthogonal misalignment. The AP mount manual has nice write-ups on how to do both.


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jmiele
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Re: AP900 Question new [Re: M13 Observer]
      #4478023 - 03/27/11 04:33 PM

Ortho misalignment seem more unlikely than polar to me. But I guess still possible. I've used several of the methods in the manual. Last night was best and I used the polar scope, however, the polar scope I guess could also be suspect. I will do a complete camera aided drift alignment tonight but to be honest, I shouldn't need a drift aligned scope for desent pointing accuracy. Any meade or celestron with a 2 start alignment can do a fair job with pointing over most of the sky at low power. I'll post tonights findings.

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DeanS
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Re: AP900 Question new [Re: jmiele]
      #4478030 - 03/27/11 04:36 PM

Sometimes I get lucky with polar scope alignment and it hits targets after a flip, most of the time not. At home here I do PemPro and get it very close, at star parties I just recal on a star after a flip and it is fine until I flip again.

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jmiele
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Re: AP900 Question new [Re: DeanS]
      #4478695 - 03/27/11 10:46 PM

Ok so polar aligned and PEMPro'd (drift) to within 20 arc seconds both alt and az. Pointing is great till I flip and then it's out by several eyepieces at x85. Am I being to picky? Also, I remounted the saddle, rings and dovetail before starting out tonight. Joe

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BlueGrass
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Re: AP900 Question new [Re: jmiele]
      #4478779 - 03/27/11 11:43 PM

Joe,
Based on what I've read in the GTO manual, it seems you've covered the primary items except you haven't mentioned if you've double checked the site lat / lon, time etc.

"The accuracy of your clock and location settings and especially the orthogonality of your entire system can play a role."


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jmiele
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Re: AP900 Question new [Re: BlueGrass]
      #4478824 - 03/28/11 12:15 AM

I will check again but I thought is was on the money. One other note I didn't mention because I thought it was related to the PC side of things - when I connect to the SkyX ( I didn't tonight or the last time I reported issues) is reports I'm on the east side of the mount when I'm on the west side. I thought is was a software setting and just ignored using the Sky until I get this worked out. I'll check the clock again... Joe

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gnowellsct
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Re: AP900 Question new [Re: BlueGrass]
      #4478835 - 03/28/11 12:22 AM

AP hand paddles have a routine for very accurate pointing, requires you to go to a star near the object you're chasing and then fine tune. That IS their preferred technique for dealing with pointing accuracy issues when crossing over the meridian. You get the extra step but accuracy can be less than an arc minute's error.

Orthagonality is an issue on the AP mounts because the company has not introduced software to deal with it. Software Bisque has software that will model the mount and compensate for orthagonality and many other variables. So if you use your AP900 with a laptop you'll get better pointing accuracy if you build a pointing model.

Argo Navis has a modeling capability as well. On my non-go-to AP900 I get all-sky pointing accuracy of about three to five arc minutes with no meridian flip issue.

One of the factors in meridian flip is that the backlash on go-to systems is usually dealt with via computerized compensation. That allows the worm gear to be set with a looser tolerance which in turn increases reliability by decreasing binding. I'm sure the AP go-to backlash must be very small. But I don't think you can get rid of it entirely. If the backlash compesnation is not perfect (and it never is) the meridian flip will introduce some pointing accuracy issues.

Once you have built a pointing model you will, with T-point or Argo Navis, get statistical feedback on which variables are affecting pointing. On my older AP900 mount the NP (orthagonality) variable plays virtually no role in pointing accuracy (as reported via the computer's analysis of the variables). Whatever variation is there must be below the resolution of my encoders which means very very small.

On the clock: last night a friend of mine was getting frustrated because Argo Navis was reporting Saturn below the horizon. We checked wires and built several models, even tried reversing the az and alt polarities (thinking we were somehow in southern mode instead of northern). Finally I thought to look at the date. Well, the unit was at its default setting of January 1, 2000 so it's no wonder that the Saturn was reported in an impossible position.

We used to shim for orthagonality on the Losmandy group but the practice disappeared with Gemini when computerized compensation came in, and became even less significant for die hard dsc users when Argo Navis became available. It is a problem that is easily solved computationally and only with difficulty mechanically. You have to remember too that the orthagonality issue is not just the mount but how the scope sits on the dovetail and the rings etc. It is a compensation that must be made for each scope (if it is large, which it likely will not be).

If you dig out the Argo Navis pdf instructions, around p. 115, there is a table explaining the main pointing variables that the computer compensates, and these are the main pointing variables most corrective programs consider, although there are others.

regards
Greg N


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jmiele
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Re: AP900 Question new [Re: gnowellsct]
      #4478857 - 03/28/11 12:36 AM

Thanks Greg. I have the SkyX and Tpoint but not using yet. If these issues are normal for these mounts thats fine they are minor and I can work through them. I'm just wanting the make sure I'm using it properly. I think Dean also mentioned syncing on a nearby star following the flip before going to the next DSO.

I will say the backlash is low. Also, the RA tracking is under 5 arc sec Peak to Peak with PEC off - amazing. These two items are more important to me than pointing because as you said I can do some models and improve it. Any thoughts on why the SkyX is reporting things backwards?
Thanks again for the reply... Joe


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RodShea
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Reged: 11/01/09

Re: AP900 Question new [Re: jmiele]
      #4478999 - 03/28/11 05:03 AM

I have had very similar problems w/ two different scopes and their plates, etc, on a 900. I TPointed the system, which worked very well, but is time consuming to do each time you set up, and needs a computer and maybe other stuff...

TPoint confirmed that, yes, the problem was, at least in my case, orthogonality. I then gritted my teeth and followed the routine in the instructions where you GoTo the same star from either side of the meridian, and shim until orthogonality is controlled. It was less intimidating than it sounded in the book. I shimmed between the dovetail plate and the clamshell or rings, depending. From Tpoint and some trig, I has a sense of the shims needed, but, in the end, it was trial and error. I did not really need Tpoint to do the shimming.

In an evening, I was able to get good pointing with a medium eyepiece on either side of the meridian, w/o having to Tpoint. With more work, I probably could have done better.

Pointing on either side of the meridian, using TPoint, was quite good after a 20 to 30 star modeling run. In both cases, essentially, the target would be at the center of a high power eyepiece. Using Tpoint after the shimming did improve the reported error by about 50%. It also helped subsequent Polar alignment when using routines that use the scope itself.

Hope this helps,

Rod


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rmollise
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Re: AP900 Question new [Re: BlueGrass]
      #4479032 - 03/28/11 05:58 AM

Quote:

Joe,
Based on what I've read in the GTO manual, it seems you've covered the primary items except you haven't mentioned if you've double checked the site lat / lon, time etc.

"The accuracy of your clock and location settings and especially the orthogonality of your entire system can play a role."




If a mount is aligned, it's aligned. How is time/location going to affect alignment on one side of the Meridian vice the other side?


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Sharkman
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Reged: 08/12/06

Loc: Nelson, New Zealand
Re: AP900 Question new [Re: rmollise]
      #4479173 - 03/28/11 08:38 AM

Make sure the mount is correctly synced -- with the counterweights pointing down, and on the opposite side of the mount from the scope. Once configured that way, there's a command on the keypad to re-sync. You can also bypass the issue by resuming from park-1.

Make sure the "N/S" switch on the mount is set correctly for your hemisphere.

Make sure no cables are dragging while the mount flips.

Make sure the clutches are tight, so neither axis slips during the flip.

Make sure the mount is accurately balanced on both axes.

The clock and lat/lon is more important in allowing the mount to correctly determine local sidereal time, the location of the meridian and the right place to flip -- I can't see how it would impact alignment.

There's a quick-align process described in the AP900 manual, where you start at park-1, level the RA axis, then go to park-2, level the dec axis, then go back to park-1 and see if the dec axis is still level. If it's not, then either the mount wasn't aligned in elevation or it was and something else is wrong (such as dragging cables, loose clutches, etc).


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Jeff B
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Re: AP900 Question new [Re: Sharkman]
      #4479778 - 03/28/11 01:56 PM

Boy, this is a very timely thread for me as I'm getting used to the AP system on my Parallax HD250 observatory mount. It carries a D&G 11" F12 achromat and I'd bet a weeks pay that I'll have some orthognoality issues, especially with the rotating rings (which may turn out to have built in shims via the teflon support plugs.)

Just waiting on the weather...and waiting....and waiting...and then there's the wind.

BTW,AP tells me they will soon be beta-testing their modeling software for introduction maybe mid-year. however, you'll still need an outboard computer.

Jeff


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LLEEGE
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Re: AP900 Question new [Re: Jeff B]
      #4479821 - 03/28/11 02:21 PM

Quote:


BTW,AP tells me they will soon be beta-testing their modeling software for introduction maybe mid-year. however, you'll still need an outboard computer.

Jeff


My understanding is the pointing model software will be part of the HB software and will be updatable through the flash memory. Only the new APCC (PulseGuide replacement) will be computer dependent. I doubt either will be available any time soon. APCC wont be released until all the ASCOM bugs are worked out. The process is taking much longer then anticipated. AP was previewing the new APCC during NEAF 2009. Two years later, we wait.

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jmiele
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Re: AP900 Question new [Re: LLEEGE]
      #4479975 - 03/28/11 03:40 PM

I think I found the issue. I was going over things for another session tonight and found that if I leveled the saddle and versa dovetail plates - the scope tube was off a bit from them. Thats gotta be it. I removed the plate and rings and the tak bridge plate. The tak ring are only 5 inches apart with that bridge. I remounted the rings over 8 inches apart (reversed the front and back rings as well) and now the saddle, dovetail and tube are all reading the same.

Makes sense to me that being off axis by a degree or two would hurt most when I flip the mount. I will see tonight.. Am I off base here? Hopes to high?

Joe


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DeanS
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Re: AP900 Question new [Re: jmiele]
      #4479979 - 03/28/11 03:44 PM

Sounds like that should fix most, if not all the problem.

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David PavlichAdministrator
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Re: AP900 Question new [Re: DeanS]
      #4480068 - 03/28/11 04:18 PM

This is the reason I really like the Mount section. Gotta' love mechanical things even when they frustrate the bejeebers out of you. I was thinking while reading this, a fork mount on a wedge or a purpose built mount like Mathis Instruments' fork mount solves the flip problem. HOWEVER, once committed to a fork, you have very limited options when it comes to scopes. You have the scope in the fork and perhaps a piggybacked refractor.

I've always liked the old Meade RX400s and thought that a 12" version would be a pretty terrific imaging scope if the electronics stayed healthy. But then, you're committing to a wedge. No flip, but that's your scope. Perplexing problems we have.

David


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drksky
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Re: AP900 Question new [Re: David Pavlich]
      #4480137 - 03/28/11 04:53 PM

Quote:

This is the reason I really like the Mount section. Gotta' love mechanical things even when they frustrate the bejeebers out of you. I was thinking while reading this, a fork mount on a wedge or a purpose built mount like Mathis Instruments' fork mount solves the flip problem. HOWEVER, once committed to a fork, you have very limited options when it comes to scopes. You have the scope in the fork and perhaps a piggybacked refractor.

I've always liked the old Meade RX400s and thought that a 12" version would be a pretty terrific imaging scope if the electronics stayed healthy. But then, you're committing to a wedge. No flip, but that's your scope. Perplexing problems we have.

David




Not to derail the topic even more, but why not build a fork mount that has a dovetail platform in the center instead of have it mount to a scope directly? Then you can have your fork and eat it too...or something.

You wouldn't necessarily be limited by what scope you could mount, although you would certainly be limited by the instrument length.


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RodShea
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Reged: 11/01/09

Re: AP900 Question new [Re: jmiele]
      #4480401 - 03/28/11 06:44 PM

Joe,

That should help a lot. It may not get you quite all the way there, but you'll know that soon<g>. Somewhere in the manual they say something about a 1 degree difference between the mount's mechanical axis and the scope's optical axis leading to a a few minutes error at the equator but hours of error near the pole.

In other words, collimation of the scope, diagonal, camera, or other issues may still make a difference even if your rings are meticulously aligned with the mount's axes. (Ask me how I know this...<g>

Good luck,

Rod


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