Now i am assuming that my G11 mount is doing what it is suppose to, by this i mean, when i finish using it for the night i send it to park position, and it is suppose to point directly south and shut down.
Again assuming the scope is in the right place, and every thing is as it should be, i decided to take a 20 minute exposure just to see where, and if it was pointing to the right place, the SCP.
This is the result, it looks pretty close to me, what do you think.
I'd say it proves nothing Leon. You could take the same phot with your scope mounted on a tripod. If you G11 is a 'goto' there is in my opinion only one valid test for polar alignment, and that is 'goto' accuracy. Guiding will cover up small errors in polar alignment, but a 1 star alignment followed by some goto operations will tell you how well polar aligned you are. A good polar align will repeatedly place a target on a webcam chip, or be close to central on a DSLR sensor. Drift aligning is good, infact mandatory, but after drift aligning, a sure test is goto accuracy. All your image shows is that the scope was ponting south. Where was your polar axis pointing? How parallel to the Polar axis is your OTA?
I was about to say 'Looks spot on Leon" until I read Dougs post.
I know my mount is pretty close to SCP because after I do a 3 star alignment, everything I GoTo is in the FOV on my monitor. I have never drift aligned as I haven't needed to.
Hmmm, Doug, I see what you mean, that put a pin in my bubble didn't it, but after saying that, it's go-to operation is pretty good actually, and the drift align i performed a couple of weeks ago also appears quite good.
A faint Star at the meridian will stay in the 9mm recticle square for well over 15minutes so i expect that i am pretty close to the pole, maybe not spot on but close enough for what i'm doing.
Hi Doug, I had a think about what you said, and it is true, one can take a image of the SCP, with a tripod, or for that matter with any scope pointed in the right direction, if one can find it and align to it.
I assumed my accuracy with the equipment that i was using, being a G11 mount and Takahashi FSQ 106, which is mounted with double Clam Shell Tube Holders, so i can assure you it is rigid, and well aligned to the polar shaft.
After a successful drift align, and some imaging i found it to be pretty well right and have produced some good results without guiding.
This led me to believe that i must have been fairly close to the SCP.
The G11, as you probably know, will, according to it's accuracy of the drift align park it's self in the true south position after each imaging or viewing session.
It was this position of the mount that had me wondering, how close to the pole is this thing pointing, hence the image above.
After slewing and moving across the sky during the evenings, it always lands exactly to that SCP position, and to the image i posted.
So i suppose it was that which led me to believe that the mount was pretty much pointed in the right direction, but i may be wrong and missed something here, and am eager to hear from other members and the opinions.
To my ill-informed way of thinking, I believe that the question is, Leon, is the centre of all those concentric circles of stars located exactly in the centre of your image? Point the camera 10 deg away from the SCP and you'll still have an image of concentric circles, but the centre will not be in the centre of the image. Having said that, I suspect that the error in determining if the centre of those circles is in the centre of your image is greater that the errors you can tolerate in your imaging work.
Or am I just talking rubbish??
Eric
Quote:
Originally Posted by leon
Hi Doug, I had a think about what you said, and it is true, one can take a image of the SCP, with a tripod, or for that matter with any scope pointed in the right direction, if one can find it and align to it.
I assumed my accuracy with the equipment that i was using, being a G11 mount and Takahashi FSQ 106, which is mounted with double Clam Shell Tube Holders, so i can assure you it is rigid, and well aligned to the polar shaft.
After a successful drift align, and some imaging i found it to be pretty well right and have produced some good results without guiding.
This led me to believe that i must have been fairly close to the SCP.
The G11, as you probably know, will, according to it's accuracy of the drift align park it's self in the true south position after each imaging or viewing session.
It was this position of the mount that had me wondering, how close to the pole is this thing pointing, hence the image above.
After slewing and moving across the sky during the evenings, it always lands exactly to that SCP position, and to the image i posted.
So i suppose it was that which led me to believe that the mount was pretty much pointed in the right direction, but i may be wrong and missed something here, and am eager to hear from other members and the opinions.
I would guess, if you locate the "centre" of the arcs and see how close this is to the centre of the image, it would at least indicate that you can't be far wrong! Not a precision way of checking it, but a good start?
Technically speaking, your Gemini system has calculated where the scope should be pointing, whenever you do an Additional Align, the Gemini updates it's Pointing Model and therefore makes slight adjustments to where the SCP is.
By doing what you did, an accurate Drift Align, then building a Gemini Pointing Model, you are effectively removing any major errors, the fact you hit your target each times proves that.
Leon, I do not have a G11, I use an EQ6. Having said that, and admitting thereby that I have no hands on with a G11, my point is that any mount (go-to) can only know where it is if you tell it. John G implies that the Gemini can learn from your successive star aligns (I think), but if I have understood him correctly, then it is only masking out polar errors.
Here is the way the EQ6 works. When I do a 1 star alignment and subsequently park, the mount goes to what should be south based on it having been pointed at a known target. But this assumes that it is in fact polar aligned! Question: If my mount was pointing East instead of South, and I did a 1 star alignment, would park set it pointing south? Answer: No. So park depends on me setting the thing up correctly in the first place, because it will drive the Dec axis the required amount to go-to 90deg from where I have said it was, and it will rotate the RA sufficient to bring it to zero hours, again based on where I have said it was.
When I drift aligned, rather than watch a star for just 15 minutes, I used K3ccd tools and a web cam. I turned off Dec drive corrections and tweaked till the star trace oscillated +/- over the center line. There will be a little drift due to atmospherics and various mechanical abnormalities, some slight imbalance etc. I found that chasing these was a waist of time, so I reasoned that they were red herrings and any real Dec drift due to mis alignment would never come back to the zero line without correction. After watching the trace vary +/- over an hour satisfied me that statistically speaking the trace would remain the same over 48 hours ( if possible), there is no cumulative drift even if there is some sporadic drift now and then. In other words my mount is Polar aligned...period. As a result, whenever I start an observing session, I initiate a 1 star alignment. The target star is always well within the fov of the imaging chip, DSI or Toucam to start off with. Once centered, The mount will place any go-to target as near as anything to center. The biggest error I get is if I need to cross over the meridian,(cone error) but even then the target remains on chip.
Now that I am satisfied that my mount is polar aligned, if I were to do the same test as you have done, (all I would see is a %$#@ pine tree) I would need to center the OTA to the mount, not touching the mount to get the star trails centered on the sensor. I venture to predict that any go-to mount that is truly polar aligned will faithfully bulls-eye any target after a 1 star alignment. 2 and 3 star alignments are for field setups where time prevents accurate Polar alignment.
cheers,
Doug
Yes, you are correct about the Gemini refining any errors associated with a polar alignment.
I have always done an accurate Polar Alignment using the Drift method, my aim is to have a star stay within the central X-hairs of a high powered guiding eyepiece for at least 20 minutes or so with only drift in RA.
On startup, the Gemini uses the Counterweight Down position with the OTA pointing at the Pole, the more accurate this is, the better, although saying that, it does not really need to be absolutely perfect. From that position, the Gemini will go to a selected bright star, you then center that star using a X-hair and synchronise the system. From there on in, you go to at least 4 to 5 stars on that side of the Meridian and do what is called an Additional Align, the Gemini is then building a Pointing Model, all the time it is giving you A and E numbers (Azimuth and Elevation), these numbers are in Arc Minutes and are the distance the telescope is from the Pole. This is for a Cold Start.
You only need to Synchronise on one star for a Warm Start, then GOTO whatever object you want.
You then select other stars on the other side of the Meridian and continue to Additional Align, all the time the A and E numbers should be falling. You can use the Hand Controller or the current ASCOM driver or Gemini Control Center to show you the detailed results.
So, in reality, if you get A and E numbers of 0 and 0, you are within 1 Arc Minute of the Pole, if need be, you can use the PAC (Polar Alignment Correction) feature to get closer.
I have A and E numbers of 0 and + 2 on my mount and find that my Autoguider has no trouble correcting these.
well this has been a very enlightening thread, thank you gentlemen for your frank discussions of a benign image I thought? Man I have some learning to do with my G11
In the start I drifted and then moved to the STV which can act as a super sensitive guiding eyepiece but now I have found TPoint and Automapper.
If you have a goto mount, a CCD camera, TheSky CCDSoft TPount and Automapper (which is free) then it can determine your misalignment in arcseconds. In about half an hour,I can get to within a few arc seconds of the pole. Of course there are refracted pole issues too but for now lets just say that by taking a couple of dozen frames, letting the software compare the images with its own maps and determining the offset, I have beaten polar aliging for good!
This way always a challenge because a big frame like the STL/FSQ gives will show field rotation very quickly if less than well aligned.
Yes TPoint has had rave reviews alright. Must be a real blessing for those having mobile imaging situations, but of no cost effective value to a permanent setup me thinks.
cheers,
Doug
Well guys this has certainly been a very informative thread, and i thank you for your imput.
Tonight i tested my theory, and all that fiddling and imaging the SCP did actually pay off.
After manourvering the mount to get the SCP centered in the scope, it was time to see if it was all worth the effort, and i can honestly say it worked a treat.
Started with a cold start with the Gemini and did a 4 star pointing model, and it landed smack in the middle of the field of view each time.
The A and E readings were (A was 0, and E was minus 0) so i dont expect it will get much better than that.
Then decided to take some un guided images and, at one minute the stars were perfectly round, any more and a slight trailing was noticed.
So, there you have it, we all work in mysterious ways to get the same results.
Monte, when I purchased THE SKY 6 from software Bisque they shipped it via UPS and I finished up being charged Import, customs + local freight+ GST. It cost me a lot more than the quoted price (adjusted to AUD). Just wondering how/from where you obtained your copy of T-Point? The current AUD is about $299, and I gues by the time UPS declare it at customs it would cost around $380- $420 or so. That makes it a very expensive package. I wish they would take a leaf out of Diffraction LTD and make their products downloadable.
what do you think.
