From: "Milton Aupperle" <milton@outcastsoft.com>

Date: July 11, 2007 12:31:00 PM MDT

To: Astro_IIDC@yahoogroups.com

Subject: Re: guiding with Astro IIDC

Hi Folks;

Just as a follow up on this issue.

Phil and I have been conversing off list and the problem appears to be

resolved. The issue was a cabling / connection problem. He's waiting

for a clear night to test out the guiding aspects, but at least Astro

IIDC and the PC Utility can talk to the GPUSB and get the mount to

move properly.

On a side note Darryl Robertson the developer of "Equinox" and

"Equinox Image" posted a "Equinox Image Autoguiding Top 10" list on

the Equinox Yahoo Group at:

http://tech.groups.yahoo.com/group/mpj-equinox/

Most of those items apply to any Guiding Application including Astro

IIDC, however there are few other points that should be mentioned.

11) Check that your mount is in good working order and that it has no

looseness or play on the Ra/Dec axis. Also check to make sure it's

locked tight on each axis before starting guiding.

12) Check that your Guide Cables are firmly seated in the sockets by

pushing and pulling on them. Loose cables produce weak / erratic

signals and cause all manner of strange behaviors.

13) Before doing a calibration, make sure you the mount and Software

are on speaking terms by using the little Left/Right/Up/Down button

interface to move the mount around. There is no point calibrating if

the buttons don't move the mount.

14) Set the ST-4 Auto Guide rate in the Hand Controller so that is

appropriate for your guiding focal length your using. Generally

speaking you should be using around 0.75X to 1.25X guide rate for

every 400 mm of focal length (i.e. about 2x for 200 mm, 0.5X for 800

mm etc.). The reason for this is that at slow guide rates and short

focal lengths, a significant (i.e. 0.5 to 1 second) amount of time may

be required for corrections to occur. By the time the first correction

is applied, it may have moved even further or less than it did. If we

keep the response time short, it can move the mount in small

increments. Also the smaller the pixel size, the higher the apparent

magnification. The reason there is a range of values is because of

pixel size. With smaller pixel size, you get more apparent

magnification than with bigger pixels. For example, my C8 2000 mm

focal length scope using a 0.63 focal reducer with a Flea 640x480

camera (7.4 Micron pixels) has almost the same magnification as a 400

mm refractor with a 2x barlow and a DMK 640x480 camera (5.6 Micron

pixels).

15) If you have low turbulence conditions, make your guide star(s) as

small as possible by focussing sharply and reducing exposure times or

gain / brightness. The ideal star is 2 pixels or less in diameter and

a big bloated star is not as good a choice as one would think. The

Centroid Calculation uses star brightness as part of the weighting, so

a big blob will be far less sensitive to star movement than a < 2

pixel star. Also more stars in the field of view for tracking will

usually improve guiding accuracy than a single star will.

16) Under more turbulent conditions where stars actually can disappear

from view for brief periods, use a big blob of a star (say at least 10

pixels) and then check mark the "Solar / Lunar tracking" option for

guiding. The "Solar / Lunar tracking" uses edge detection and then

calculates the centroid from the off the high contrast disk edges and

it needs a large disk with lots of contrast between background and target.

Hope This helps..

Milton Aupperle