In a message dated 10/24/04 11:54:12 PM, twade@bmi.net writes:
I use the eFinder as a guide scope. I typically keep the corrections
to every 4 seconds. This supposedly prevents the STV from chasing
the seeing. I use the drift method for polar alignment; however, it
is difficult in determine how accurately polar aligned I am. How
accurate of a polar alignment do I need when using a Pentax 67 with
lenses ranging from 45 mm to 300 mm. I plan to expose E200 from
60-90 minutes at f/5.6. As a reference, in October, I was getting
average corrections (i.e. AEX and AEY) of 1-2" with instantaneous
reading ranging from 0.0 to 3.0". Keep in mind, the average values
were being average over a 64 second period. The seeing that night
was pretty poor given the instantaneous readings. I saw no field
rotation with my 20 minute exposures. Are these values common? Do
these values point to poor polar alignment? What values are
acceptable to prevent field rotation?
Any suggestions would be greatly appreciated.
Thanks in advance,
Wade
Wade:
When doing your drift aligning, how long are you going before the selected stars move off the reference point? If you go as long as 10-15 minutes you are very well aligned.
The 4 second exposure/iteration time you mention indicates you are using relatively dim guide stars.
What kind of brightness reading are you using for these stars?
While advertising says the STV will guide with dimmer stars than the ST-4, I've found it guides better when using the efinder with brighter stars. By 'brighter' I mean brightness values 1000 and up.
This will allow you to choose shorter exposure times, like 1/2, 1 or 2 seconds. To level out the seeing, in the Manual Track mode you can set it to not correct every exposure, picking one in every 2,3,4 or more exposures.
With the 45 to 300mm focal lengths you mention, you can have much larger corrections than you've indicated and they will not show up at all in the images. I routinely image at 1086mm and guide with the STV and efinder with good seeing averages in the under 1 arc second range. In bad seeing it might be in the 2-3 arc second range. Using E200 and with the 1086mm (AP155) focal length I could actually get into the 7 arc second range before it would show up on the film, although it never gets that high.
One problem I initially had with the STV was tracking oscillations, where the track graph looked like a sine wave or even 'saw blade'. This usually happened when I allowed the ATV to Auto calibrate and Auto track. Through experimentation, I learned to tame the oscillations by using Manual calibrate and Manual tracking, setting the parameters myself.
Kent Kirkley
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