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Subject: Long post on drive rates
From: dicicco%cfa4.DECNET@cfa.harvard.edu
Reply To: mapug@shore.net
Date: Sun Apr 09 07:00:18 1995
Unless you're really interested in the intimate details of drive rates or
long messages, you might want to skip this one. If you read on, don't say
you weren't warned.
Replies, both public and private, to my post about Mogens Winther's drive "problems" with his 16" LX200 got my interest piqued again about drive rates and periodic-error correction (PEC). So I did what every good theorist should -- Friday I rolled the observatory roof off, put a micrometer-reticle eyepiece in the LX200 (8" f/6.3), and prepared to do a star test. Seeing whereas sunset was still more than 2 hours off I was about to walk away, but decided to punch up the position for Sirius for fun. It ended up nearly in the center of the field (sometimes I really do think these computerized scopes make things too easy!). Furthermore, Sirius was then near the meridian, so I started the test. You should note that some months ago I had gone to some effort to very accurately train the smart drive of this instrument, and the PE was around 5 arc seconds.
Within a few minutes I could see that the scope was tracking fast (moving west of the star). But I let the test continue for nearly 4 hours. Ironically, during this time I checked my e-mail and found Mogens' post answering some earlier questions. He confirmed my earlier suspicions that his CCD camera was attached to his scope in the "normal" way and that his drive was tracking fast as well. After 220 minutes I stopped my test. The scope had moved 384 arc seconds (6.4 arc minutes) ahead of Sirius. Taken at face value this is about 1.75" of drift per minute, but not all of it is due to the scope -- refraction, especially near the end of the test, caused some of the displacement. Even so, the drift rate is still at least 1.5" per minute. This does seem high, but it confirms earlier suspicions I had when testing both Meade and Celestron PEC systems -- namely that the drive rate was being affected by the PEC. (Those test were done in a way to accurately show the PE, and any drift was somewhat masked by the method.)
As much as it pained me to do it, I erased the smart-drive training and reset the scope on Regulus, which by now was near the meridian. Guess what? For more than an hour the star never budged (apart from the now-apparent 30" PE "wobble") from the crosshair -- the drive rate (which was running at the default 60.1 Hz) was dead-nuts on! By the time 3 hours past, refraction caused the scope to drift about 30" ahead of the star. (BTW, since Regulus has a much higher declination, its refraction drift is quite a bit less than Sirius for these tests at my location.)
Okay, so we have a limited test of two scopes that shows the PEC causes the scope to run a bit fast. Having access to the PEC learn, as Mogens suggested, doesn't appear to be the answer, since my scope was carefully trained on the stars and it drifts. Moreover, I just don't see this as an issue, apart from fueling the fire for a possible nit picker. No manufacturer has ever claimed that its drives are capable of long-exposure, *unguided* photography. Even if PEC eliminated PE *and* the drive rate were perfect, you'd still have to guide in most cases because of refraction. The thought of having on-board computers correct for refraction has crossed my alleged mind during flights of fantasy, but refraction isn't a fixed value, it depends on air temperature and barometric pressure as well as the better- known distance above the horizon. As a bit of further proof that guiding is *always* necessary, Byers is making a computer-controlled tangent drive that has very little PE. I haven't tested one, but an astrophotographer friend in Nevada has one and he says the guide star stays "glued" to the crosshair. Nevertheless, apart from backing off of the eyepiece for a few seconds at a time to sip coffee or shake off the stiff neck that comes with this form of self abuse, he has to guide (or let the ST-4 do it). Guiding simply goes with the territory when making exposures longer than few minutes at most! While it might be possible for Meade or other PEC manufacturers to produce PEC and a perfect rate, it's not something I'd put high on the list of priorities.
The results of my test do suggest some ways to best use the LX200 for photography. In the case of piggyback work, I'd quit the PEC. Even with the longest telephotos, the PE wobble will not be visible, and the near-perfect drive rate without the PEC would allow for rather long unguided exposures. When imaging through the OTA, I'd opt for the PEC, it would make guiding easier whether it were done with an autoguider or an eyeball. Furthermore, with an autoguider, the PEC allows using longer integrations and the use of fainter guide stars.
I know it's my opinion that having a perfect drive rate with the smart drive isn't an "issue," but this is an area in which I can at least claim some experience. As an astrophotographer/telescope maker, I've always had a special interest in drives. As a teenager in the early 60's I was stringing together trains of lathe change-gears trying to coax a 1 r.p.m. motor into cranking the polar axis at sidereal rate through a 100-tooth worm wheel. Fortunately, the June '64 S&T had plans for a transistorized drive corrector that made it possible to fine-tune the drive rate. The problem here was using visual methods to set the rate amid the huge PE most gear systems then had. (Only those of us approaching geezerdom [or already there] are likely fully appreciate this point!) Gears, especially commercial units, got better, but it wasn't until the mid 1980's and digital electronics and stepper motors that the next "leap" in drive technology came along. I had a Takahashi Sky Patrol mounting that used this technology. With a 144-tooth worm wheel and a worm that appeared to turn once in 10 minutes, it looked all the world like a solar-rate drive. But after using it for more than a year, I got called away from an exposure for nearly a half hour one night. I expected the exposure to be ruined, but surprisingly the guide star remained on the crosshair, which shouldn't have been the case for a solar rate. Back at home I set up an elaborate test along the lines of the garbage-bag twist-tie technique mentioned by Jason Ware. After three days the drive was running within a second or two of the exact sidereal rate! Today we have PEC and errors in the 5" or less range.
Despite all these advances, I still have to guide for the reasons mentioned above. The work has gotten a lot easier, but it's still required.
BTW, Mogens said I shouldn't try sending any neighborhood kids in exchange for his scope. Too bad! But I do have a free offer -- how about one large, very dumb, barking dog? Now that it's spring and the nighttime temps only near the freezing mark, I sit an enjoy the sky more while the scope is happily autoguiding away on some asteroid field (as opposed to retreating to the adjacent "warm" room). The damn dog is ruining the aesthetics of the experience!
Sorry to ramble on for so long.
Dennis di Cicco CCD Astronomy / Sky & Telescope