by Gary Honis
I began imaging with the GOTO Starmaster three years ago with an Olympus digital camera that was limited to a maximum of 32 second exposures. I am now using a 6 Megapixel Canon 10D digital SLR that has very low noise, high ISO (3200) and can do bulb exposures. Since 1987 I had been doing astrophotography with Meade LX200 scopes wedge mounted but began doing deep sky imaging with the big Alt-Az dobsonian after seeing the success of a fellow GHAAS member, Tom Orff doing CCD work with a dob. Alt-Az imaging is limited because of field rotation but there are sections of the sky that can be imaged without rotation for a few minutes. This is plenty of time to acquire enough signal in 1 to 3 minute exposures with the 20" F/4.5 scope. The exposures can then be combined by using stacking software to align, brighten and reduce noise in the image. A Denkmeier 2 inch OCS and T-ring is used to couple the Canon 10D to the Starmaster focuser for prime focus imaging.
In September 2003, Rick Singmaster of Starmaster Telescopes and Victor McKeighan of Sky Engineering (Sky Commander), helped with providing upgrades of the Starmaster's GOTO Skytracker system to allow for responsive guide corrections for astro imaging.
GUIDE RATE:
The standard guide rate of the Sky Tracker is fine for visual use at the eyepiece but it is too fast for making guide corrections of the drive system. Victor asked which guide rate I would prefer for prime focus CCD imaging. I made a study of the guide rates of drives by Losmondy, Astrophysics, Celestron, Vixen and Meade and requested a drive rate of 7 arcseconds/second. Victor programmed a new chip with this guide rate and imaging tests with this rate have gone well. The new guide rate is also very useful for keeping planetary images well centered while imaging with CCD imagers, digital cameras and web cams.
The slew and track rates of the Skytracker remain unchanged and the drive system operates very smoothly. For visual use, the tracking rate is fine for centering objects, even at high power, by tapping the hand controller buttons.
An added advantage of the Skytracker upgrade is an improvement in the quiet operation of the Starmaster's drive.
ROLLER
BEARINGS:
Rick sent me a double roller bearing to add to my Starmaster and also one to replace my scope's single altitude roller bearing. Having two double roller bearings on the scope eliminated a slight side-to-side movement when the drive system was moving in altitude.
UPGRADE:
Click on photos for larger images with details:
Placement of new roller bearing to replace teflon
pad.
Hole drilled in rocker box for new roller bearing.
Two double roller bearings mounted.
Small easy glider pads added to side of rocker
box.
MANUAL GUIDING WITH 102mm SHORT-TUBE REFRACTOR:
Click on photos for larger images:
OLD MOUNT: Bogen quick release camera plate
OLD MOUNT: 102mm guide scope, 3X barlow, 9mm reticle
eyepiece w/ adjustable led brightness/strobe rate
NEW MOUNT: Bogen/Manfrotto 410 Geared Head with
slow motion controls
NEW MOUNT: 102mm guide scope, 3X barlow, 9mm reticle
eyepiece w/ adjustable led brightness/strobe rate
An imaging session with the Canon 10D digital SLR involves the following steps:
1. Mount Canon 10D in Feathertouch Focuser using 2 inch Denkmeier OCS.
2. Release altitude clutch and balance scope in altitude using velcro strap soft weights hung from the upper cage. If imaging object in the Eastern sky, balance scope so that it is a little front heavy; for an object in the Western sky, balance scope so that it is a little back heavy.
3. Focus 10D on bright star near object to be imaged using a right angle magnifier (I use an Olympus Varimagnifier).
4. Frame image in 10D viewfinder using Skycommander hand controller.
5. Roughly center guide star near object to be imaged in the 102mm guide scope and precisely center within cross hairs of reticle using the adjustable cross hair reticle eyepiece.
6. Use Canon's Remote Capture Software to control and capture exposures directly to notebook using USB cable connection.
7. Use homemade electronic shutter release cable to trigger exposures. (also see homemade mechanical shutter cable). UPDATE: Completed a parallel cable to trigger the shutter using the computer parallel port with DSLR FOCUS software.
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