Can any of the Astrophotography gurus tell me the degree of accuracy needed when setting up in order to cancel out field rotation, when using long (say 20 minute) exposures? Would it be +/- 1 degree, +/-30 minutes, or what? I shall be using my 300D both with camera lenses, and through the scope, so there will be wide field and tele shots. I have a Vixen GP equatorial drive
There is only one answer that I can give you, learn to Drift Align.
That will get you within 1 or 2 arc seconds of the Pole, this, in my opinion, is the only way you will elliminate any sort of trailing or drift in your photos without an autoguider. The longer your star stays centered on your cross hairs the better, try and aim for about 20 minutes with the star centered and the only movement you should detect will be in RA, caused by the Periodic Error in the Worm.
There are a few variables with field rotation such as length of focal length and location of object in the sky to name only two. As JohnG pointed out, drift aligning is the most common way of accurate polar alignment which will in turn resolve your field rotation concerns. CCD or Web came drift aligning will provide greater accuracy than using a illuminated recticle eyepiece as they are pick up drift quicker than the eye can perceive. You can also confirm/correct polar alignment by building an accurate all sky pointing model using software such as TPoint or MaxPoint, however this is typically well beyond most amateurs (generalised statement).
Drift aligning isn't difficult, just time consuming if you want to get high accuracy. 1 or 2 arcseconds from the pole is achievable, but expect to spend a few hours getting there. For a permanent set up, its worth spending the time, but for road warriors carting the equipment around to a dark sky, I wouldn't expect to get this level of accuracy all the time. I'm presently at 15 arcsecs azimuth and 17 arcsecs altitude from the polar (confirmed with MaxPoint) with a permanent set up. I could spend that extra hour to refine it further, but in all seriousness there is no huge gain especially with auto-guided exposures no more that 30 minutes each.
Thats ok JohnG, I knew what you meant.
1 to 2 arc seconds is still achievable. But, you will be limited by your seeing conditions and patients. The latter in particular. I spent five hours getting down to 15 arcsecs azimuth and 17 arcsecs altitude polar alignment error. I had fun doing it though as I was testing the new PEMPro 2.0 beta drift alignment proceedure which makes drift aligning a breeze certainly with a goto scope. Automatically slews to the area of the sky for either azimuth or altitude adjustment, finds a star and starts telling you how much you have to adjust. Too easy.
The thing that really frustrated me is that you don't know how many turns (or fractions of a turn) on the azimuth or altitude adjustment controls corresponds to arcsecond movements. Towards the end I was making adjustments so tiny to get things settled. I have subsequently come across someone that has actually measured the adjustments on the Titan so I now know - Altitude = 60 arcminutes is approx one full turn; Azimuth = 37 arcminutes per millimeter (very fine). Also when you lock down the adjustment controls you experience shifts (due to the mount design) so you need to tweak some more. Sent me crazy - now you can understand why accurate polar alignment under 1 arcminute took five hours!
Something to note for Gemini users who are using the integrated polar alignment features of this telescope control system. The A: and E: values after each additional alignment are not displayed as arcseconds or degrees, but in arcminutes. Hence if you are using this system, the best you'll get is 1 arcminute from the pole - under normal circumstances this is probably good enough anyway. The only reason why I went that extra mile was because its a permanent set up and worth the effort. I wont have to redo the alignment, though I do check it quarterly.
Geez Jase, my poor 59 year old brain is not up to that.
Bit heavy for me, must admit that I have A and E numbers of 0 and 2 on the Gemini, so that puts me within a couple of arc minutes of the Pole and photographing I use an ST-4 autoguider so that is really close enough for me. Unfortunately I will have to dismantle my setup soon to move, will do a better job of drift aligning when I settle into a new location.
It is still something I recommend people learn, it was taught to me over 35 years ago and I use it with my portable setup.
Mike, does your GP-E have the polar alignment scope (PAS) attachment? If so, you should be able to get around 3 arcminutes from the pole using this. From there you could choose to drift align or leave it as is (depending on what you intend to image and your focal length). If you intend to image objects close to the pole, you're accuracy must be higher than imaging near the celestial equator.
Thanks for your comments JohnG and Jase. At the moment, it all seems too hard, and I shall satisfy myself with "near enough is good enough". I'm fairly confident of being able to get to 30 mins without too much problem, and when I have become familiar with imaging and processing, I shall give consideration to a higher degree of accuracy. I do have a PAS on the Vixen, but I have never been able to pick out Octans through the PAS. I can identify Octans with binoculars, but the PAS is hopeless, it's just too dark to see anything. With the supplied light fitting, all that can be seen is a bright red glare, no stars. I'm only interested in "pretty pretty" at present, but if I go down the serious road (note that I said IF) then I shall reconsider what I have to do.
I pitched my posts at the theoretical side as I thought you wanted to calculate specifically how much drift/rotation would be present for a given focal length etc etc. Don't be scared by the data I've provided - its certainly not too hard as you put it.
I guess the question you need to ask is - do you see field rotation in your images now? If you're using short focal length camera lenses, then your statement near enough is good enough is probably fine. Perhaps start imaging with the shorter focal lengths as they are more forgiving than longer focal lengths (tracking, seeing and polar alignment). As you advance you can progressively improve your polar alignment accuracy and use longer focal length instruments.
Jase
I don't know why the PAS should be sick, it's clear enough in daylight, but from feedback from other users of the PAS, the comments have been similar, throw the illuminated finder away.
To be honest, I have only taken about half a dozen serious shots, and they were at the Qld Astrofest last year. These were only wide angle images with a 20 or 22mm lens, and as a first attempt, I was quite happy. However, they were not 100%, and I want to do better. Hence my question.
Attached is my effort to date, which combines 2 images stitched together, each approx 6 minutes at f 4.5 with an ASA setting of 800. A bit of coma is evident, or it may be bad tracking, or whatever.
I used my Canon 300D with a Tokina 12-24mm f4 zoom (AT-X Pro DX, to give its full title) set at 21mm. I had removed the scope from the Vixen mount, because piggy-backing on the rear of the scope at 21mm, gave a better view of the scope than it did of the sky. I didn't do any dark frame subtraction, or any real post processing, other than tweaking the contrast and doing a Photostitch, then cropping slightly.
I also found that when I had done a 3 frame stitch, that I had managed to repeat one of the images on the wrong end, so instead of A B C, I had managed an A B A, if you see what I mean. One of my Club Members pointed that out, but I don't think anyone else noticed that I had re-arranged the Milky Way.
Hi Mike,
Those images a great. You don't need to worry about accurate polar alignment i.e less than a few arcminutes from the pole when using short focal length lenses. As I mentioned previously, when you feel comfortable at using longer focal lengths you can then begin to work on your drift alignment technique.
The A: and E: values. These are Gemini telescope control system specific. Gemini allows you to build a basic pointing model. As you perform additional align after additional align on bright stars across the sky the model builds a map of the sky. After each align the hand controller gives you A: and E: values represented in arcminutes. These values are Azimuth and Elevation (latitude) errors from the celestial pole. The lower the numbers the better your polar alignment. Once you have a good pointing model, you can use one of Gemini's polar alignment features to manually adjust azimuth and latitude. Its a nice feature for those road warriors that cart their equipment around as it can minimise set up time, but it isn't a substitute for drift aligning - especially if you need the accuracy.
, got arc seconds on the brain for some reason.
But, you will be limited by your seeing conditions and patients. The latter in particular. I spent five hours getting down to 15 arcsecs azimuth and 17 arcsecs altitude polar alignment error. I had fun doing it though as I was testing the new PEMPro 2.0 beta drift alignment proceedure which makes drift aligning a breeze certainly with a goto scope. Automatically slews to the area of the sky for either azimuth or altitude adjustment, finds a star and starts telling you how much you have to adjust. Too easy.
