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Help with orthogonality
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Roland Christen
In a message dated 4/24/2007 1:01:09 PM Central Daylight Time,
lmarchesi@... writes: I had some significant time with my new mount (AP 1200) and the newThere is always some confusion as to what this 2 Star thing is supposed to do. It is not a calibration routine. It does not calibrate anything in the mount. It is simply one more of dozens of polar alignment routine options. This one in particular can be hard to converge, especially if you happen to choose the wrong two stars. As such, this polar alignment option is probably the weakest of all. You should never conclude from this alignment routine that something is amiss with your orthogonality. In fact, I am tempted to eliminate this routine because of the confusion it seems to cause everyone who first uses this mount after using other commercial Alt-Az mountings. The best way to judge the orthogonality of your setup is to GoTo the same star on both sides of the meridian. Choose a star near the zenith, then center it on your crosshairs. Note the RA direction. Then choose the meridian delay feature (1 hour delay either E or W) of the keypad to GoTo this same exact star again on the other side of the mount. Note where the star ends up versus the RA crosshair. Now you can see your orthogonality error - the error is exactly 1/2 of the distance from the crosshair and where your star ended up. Now you can also see how much to shim the mounting ring, either front or back) to bring the star 1/2 way to the crosshair. Once you have shimmed it, you can go back and forth to the same star on either side of the meridian to do a final tweaking if desired. The other thing I want to point out is that with a normal refractor and normally well machined rings, you will never have any orthogonal errors that are too large. You should have very close GoTo accuracy if you are properly polar aligned. If you do not wat to do accurate polar alignment, you can still do precision GoTo by first picking a bright star in the area that you want to observe, center it, press Rcal and then enter the faint object you wish to observe and press GoTo. This way you can always find any object in your telescope field, even if not polar aligned. Rolando ************************************** See what's free at http://www.aol.com. |
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Roland Christen
In a message dated 4/24/2007 4:35:40 PM Central Daylight Time,
drichey@... writes: I beleive it is .8º for CNP. So how does one do the alignmentThe mount can certainly go to Polaris. No problem. When properly polar aligned, any mount can go to Polaris. If you have orthogonal error larger than 0.8 degrees in your telescope, then no equatorial mount will be able to access Polaris. The mount will point to Polaris for sure, but the scope will point elsewhere. I assume that every amateur knows that the direction that a telescope points to is not necessarily the direction that the mount points to. It is up to you, the user, to align the two. Every basic book on astronomical instruments will point this out. Rolando ************************************** See what's free at http://www.aol.com. [Non-text portions of this message have been removed] |
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primeclash
I had some significant time with my new mount (AP 1200) and the new
camera this past weekend (finally!). One thing that is more challenging than I expected is polar alignment. I did the 'daytime polar alignment' routine using a carpenter's level and the Moon. The mount/pier were reasonably level as well. Then, when I do the 2-star calibration I seem to be 'oscillating' back and forth in azimuth. What surprised me most is that the 'cone of error' seems very large, as in half a finder scope's field of view. I am not sure how much of that is how far I am from the pole versus non-orthogonality, but it does make pointing the scope and camera at a specific target somewhat challenging. I have a 6-inch refractor with (IIRC) CNC-machined rings on a Losmandy 17-inch dovetail with 'riser' blocks, then attached to a Casady saddle on the 1200. In the manual for the mount, it says to 'simply shim' the rings until I am orthogonal. I apologize for being dense, but does the shimming happen to the contact point where the rings mate to the dovetail plate, or where the dovetail mates to the saddle, or both? What is a good shimming material ... soda can, 'aluminum' foil? One other thing I should mention is that I don't yet have a permanent setup, so I'd like a solution that is repeatable each time I set up. I recognize that there is software out there to model pointing, but I'm the kind of person that will be 'bothered' by this. I'd rather get the scope as orthogonal as possible before I consider T-Point or its equivalent. Regards, Louis Marchesi New London Twp, PA |
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spcrichey <drichey@...>
I have exactly the same problem, only an AP900. I use a 6" TV refractor with a Robin
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Casady saddle and Parallax rings (rings and saddle new). Spent significant time on star drift alignment, got it perfect for 5 minutes both S and E. My finds are still off. I guess I also have an orthogonality issue. Am waiting for weather to try the AP Ortho check routine. Should one be able to center Polaris in a guiding EP at 60 power. After the alignment (drift method) I cannot. Regards, Dan Richey --- In ap-gto@..., "Louis Marchesi" <lmarchesi@...> wrote:
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Roland Christen
In a message dated 4/24/2007 7:48:26 PM Central Daylight Time,
mphammick@... writes: Taking your point one stage further, there is a potentially flawedYou are correct. Rolando ************************************** See what's free at http://www.aol.com. |
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observe_m13
Uh, no you should not be able to center Polaris. Polaris is not at the
pole, it is beside it. Rick. --- In ap-gto@..., "spcrichey" <drichey@...> wrote: refractor with a Robin Casady saddle and Parallax rings (rings and saddle new). Spentsignificant time on star drift alignment, got it perfect for 5 minutes both S and E. Myfinds are still off. I guess I also have an orthogonality issue. Am waiting for weather to try theAP Ortho check routine.After the alignment (drift method) I cannot. |
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spcrichey <drichey@...>
That's what I thought. I beleive it is .8º for CNP. So how does one do the alignment
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routine in the AP manual using Polaris and another star?--- In ap-gto@..., "Rick K" <JunkMailGoesHere@...> wrote:
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Woodwind
Rolando,
You have put your finger on a phenomenon which so often seems to afflict us all at one time or another. Flawed leaps of logic - which can leave one totally bewildered by our inability to achieve an aim - and no apparent solution until someone points out the obvious. Taking your point one stage further, there is a potentially flawed assumption that anything bolted to a mount is pointing in the right direction and indeed that any series of things bolted to a mount are pointing in the same direction. Equipment-alignment and polar-alignment are two very different things which are both important. Just a thought. Murray chris1011@... wrote: In a message dated 4/24/2007 4:35:40 PM Central Daylight Time, drichey@... writes: > I beleive it is .8º for CNP. So how does one do the alignment > routine in the AP manual using Polaris and another star? The mount can certainly go to Polaris. No problem. When properly polar aligned, any mount can go to Polaris. If you have orthogonal error larger than 0.8 degrees in your telescope, then no equatorial mount will be able to access Polaris. The mount will point to Polaris for sure, but the scope will point elsewhere. I assume that every amateur knows that the direction that a telescope points to is not necessarily the direction that the mount points to. It is up to you, the user, to align the two. Every basic book on astronomical instruments will point this out. Rolando ************************************** See what's free at http://www.aol.com. [Non-text portions of this message have been removed] --------------------------------- Ahhh...imagining that irresistible "new car" smell? Check outnew cars at Yahoo! Autos. [Non-text portions of this message have been removed] |
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spcrichey <drichey@...>
So, one person says "Uh, no you should not be able to center Polaris, Polaris is not at the
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pole, it is beside it." , and another says "the mount can certainly go to Polaris." I am very confused. Is not being able to center Polaris an orthogonal error, or not? I have a "normal" refractor and "normally well" machined rings, so either I am not well enough polar aligned, or my equipment isn't "normal." --- In ap-gto@..., chris1011@... wrote:
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observe_m13
If your telescope is orthogonal to the mount and the mount is even
moderately well polar aligned, when you point the telescope right at the pole, Polaris will NOT be centered. It will be noticeably offset even in a low power eyepiece field. Polaris can be used as a quick alignment star for visual use of a moderately wide field telescope with goto in most cases. To rough polar align I use the PASILL with great success. In fact, with a bit of care, I manage to achieve very good to excellent polar alignment. One can drift after that to fine tune it further if shooting close to the poles where field rotation really makes a mess of things if you are not well polar aligned. Rick. --- In ap-gto@..., "spcrichey" <drichey@...> wrote: Polaris, Polaris is not at the pole, it is beside it." , and another says "the mount can certainlygo to Polaris." I am very confused. Is not being able to center Polaris an orthogonal error,or not? I have a "normal" refractor and "normally well" machined rings, so either I am notwell enough polar aligned, or my equipment isn't "normal." --- In ap-gto@...,chris1011@ wrote: polar larger than 0.8aligned, any mount can go to Polaris. If you have orthogonal error to accessdegrees in your telescope, then no equatorial mount will be able will pointPolaris. The mount will point to Polaris for sure, but the scope that a telescopeelsewhere. I assume that every amateur knows that the direction to. It is up topoints to is not necessarily the direction that the mount points instrumentsyou, the user, to align the two. Every basic book on astronomical will point this out. |
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primeclash
--- In ap-gto@..., chris1011@... wrote:
There is always some confusion as to what this 2 Star thing issupposed to do. It is not a calibration routine. It does not calibrate anythingin the mount. It is simply one more of dozens of polar alignment routineoptions. This one in particular can be hard to converge, especially if you happen tochoose the wrong two stars. As such, this polar alignment option is probablythe weakest of all. You should never conclude from this alignment routine thatsomething is amiss with your orthogonality. In fact, I am tempted to eliminatethis routine because of the confusion it seems to cause everyone whofirst uses this mount after using other commercial Alt-Az mountings.Noted. I should have followed your preferred methodology you outlined in message 17688. I would have sworn it included the '2-star calibration' but it does not. I started using the 'carpenter's level' technique but then referred to the manual. I'm ripping that page out tonight after I get home from work ;-) Thanks for the help, Louis Marchesi New London Twp, PA |
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Auchter Tom-W11806
We are mixing pointing at the pole and pointing at Polaris. If the
mount is polar aligned and the telescope is orthogonal then you can do the following. When you point the telescope at the pole ie 90 degrees Polaris will not be centered. If you tell the mount to goto Polaris then Polaris will be centered. I have done it many times. Tom |
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