Re: [APML] Ideal STV Guiding Errors?

["Taras R.Hnatyshyn" <tarashnat@earthlink.net>]

Wade,

No matter how well you or the STV guide, it will have little effect
on field rotation. Field rotation is a function of polar alignment
error and exposure duration. The longer the exposure, the more the
field will rotate during it. Any drift alignment which shows no drift
for over 5 minutes should be adequate for these lengths of exposure.
At 45mm on 6x7, depending on your mount, you may not even need guiding,
providing your mount doesn't have a nasty periodic error or wobble.
I have shot some 50mm on 6x6 for up to an hour with no trailing on an
LX200 without any guiding.

According to Covington, guiding tolerance = arctan(1/40F) where F is
the focal length of the system and assuming a 1/40mm blur on the film
acceptable. This means that for a trail 1/40mm to appear on your film
you need a drift of 17 arcsec at a focal length of 300mm and 115 arcsec
at 45mm. Also according to Covington, you get 3 to 5 arcsec of random
variation due to atmospheric turbulence over the course of any long
exposure.

Given the above, I would think that you should be fine for these
focal lengths, given a good polar alignment. And remember, perfect
guiding cannot cure poor polar alignment.

Taras

On Oct 25, 2004, at 12:53 AM, Thomas W. Earle wrote:

> 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
>
>
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--
                         <end of message>
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* Taras R. Hnatyshyn                     tarashnat@earthlink.net *
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