Most people, who do imaging and astrophotography, do so with some sort of equatorially mounted scope and camera, but it is possible, in a limited way, with a less expensive Alt/Az mount (such as a dobsonian mount). Since I've now been using the Starlight Xpress MX 5 camera along with the STAR 2000 self guiding interface on my Dob Driver controlled scope since March, I thought I would give an evaluation of the system, as well as, my experiences with it.
My system consists of an MX 5 16-bit CCD camera, the STAR 2000 Interface, a Celestron 8" Starhopper controlled by Tech 2000's Dob Driver II, a PentiumII 300 laptop computer, 5 Pentax screw mount lenses, and a Minolta bayonet lens, all with the appropriate adapters from Adirondack Video Astronomy (the M42C adapter, Pentax screw mount adapter and a Minolta adapter; I have only tried the Minolta adapter with a 50mm lens once, and didn't have time to finish the setup and focusing so I can't really comment on it), a short tube 80 scope, a modified Logitech Quickcam VC and a Celestron Micro-guider eyepiece.
The imaging system was fairly easy to hook up to the Dob Driver II (via the ST-4 compatible "CCD link" port) as well as to the computer (via the parallel port for the camera and the serial port for the autoguider). The only configuration problem I experienced was not having a 9-pin serial port available on the computer (my serial mouse was using it). This was quickly remedied by using a 25-pin to 9-pin serial adapter. I've since added another serial port and parallel port for use by the quickcam which I will attempt to use for guiding of planetary images in Astroart. The upcoming USB interface for the Starlight cameras should eliminate the need for the second parallel port. Since I've moved to the laptop, I no longer use the desktop computer and using the QuickCam is just not possible without adding more ports via a USB connection.
Once all the connections were made, the software was the next step in the process. Since I was only going to be doing piggy-back imaging, I figured the "train auto-guiding" settings would probably be sufficient. After my initial trials with the system, only minor changes were made in order to smooth out the tracking (in addition to swapping the E/W direction in the program defaults). I lowered the hysteresis values to 2 and set the RA/DEC pixels to 5 and 10 respectively. Now I was ready to image, or at least I thought I was. I've been doing some reading of the Starlight Xpress group messages on Yahoo and found that the higher valeus should work better in controlling the scope at higher focal lengths.
I had planned on using some older Pentax screw-mount lenses to start with (28mm. 50mm, 100mm, 150mm and 200mm). Later I thought I would try the Short Tube 80. First, I made sure that I had the proper adapters from Adirondack Video Astronomy (the M42C, Pentax screw, and Minolta adapters mentioned earlier) and then started imaging. As I worked on achieving focus with the lenses, I quickly realized that all the lenses were coming to focus short of the infinity mark (which I was told is normal since the adapters are purposely manufactured short to make sure they are all able to focus). Unfortunately, no matter how much I tried, the stars remained bloated blobs within an outer concentric ring. The reason, infra-red light (to which the camera is sensitive) was showing up as an out-of-focus image. Thanks to John Cordiale of Adirondack and Wil Milan of Arizona, a solution was found. An infra-red blocking filter would be needed with these camera lenses.
I was able to attach a filter from Edmund Scientific Industrial Optics (the filter is referred to as a "hot mirror" (catalog # K43-843) and blocks all light between 750nm and 1250nm). Since this was written, Edmund has come out with an IR Blocking filter wich is supposed to work better. It's listed as:SN53-711. I first tried gluing the filter to the back side of the M42C adapter (the part which goes into the MX 5 camera and faces the chip) with epoxy but after the first night, the glue had cracked and the filter came loose. The solution, thanks to friend Joe Hupka, was to use a glue called "GOOP". This time it worked. The only additional step in the gluing process was to glue a thin (.020") ring down first to the M42C adapter so that when the C-threaded portion of the Pentax adapter was screwed into the other side of the adapter, it would not touch the mirror and cause stress on the glue (which might be why the epoxy did not hold the first time). Once dry, the glue seemed to be pliable so that expansion/contraction of the adapter and filter would not cause the glue (or the filter) to crack.
Now I was ready to image. Adjusting focus was achieved by using the focus window in the camera control interface. Varying the exposure from 1-5 seconds allowed me to get close to focus by checking the max value readout. I fine tuned focus by using continuous mode exposures and watching the image on the screen come in to focus. I have not really used the focus mode much, but I guess I should give it a try.
Once focus is achieved, it's time to image. This is usually best after a few minutes of tracking with the Dob Driver II, otherwise it seems as if the scope is always making large adjustments, therefore "chasing the star". My original images were done with the Dob Driver II hand pendant set to "track" instead of "guide" because the guide setting did not seem to be making large enough changes to the scope to keep up with the star. Obviously, more experience with this is needed. Maybe longer tracking and training the Dob Driver is needed so that the "guide" setting can be used. Time and experience will tell.
The final aspect of the process was to determine how long an object could be imaged before field rotation would become a problem. I was somewhat successful with Tech 2000's ROTOCALC software, but again, I will need to do more work with it, as well as, determine how much field rotation can be tolerated before trailing of stars occurs. From what I have imaged so far, the times vary from 1 minute around the celestial pole (with a 28mm lens) to 6 minutes along the ecliptic (with a 100mm lens). This is one aspect which will need to be constantly checked from image to image. Since I've build my dob wedge, I no longer have to worry about field rotation. I've been able to image for 20 minutes now without any rotation creeping in. Tracking has been outstanding since I moved to the dob wedge. I've been able to do 10 minute unguided exposures with my 28mm lens piggybacked on my scope (while this may not seem likea major accomplishment, consider that the scope, as an Alt-Az, was incapable of precise tracking and had a lot of field rotation). I'm looking forward to seeing what kind of film exposures I can do this summer.
As you can see from the photo at the left, I have the system set up to image through the Short Tube 80 while using the 8" electronics for tracking. I used a 10mm Kellner eyepiece (which came with the ST-80) and an eyepiece extension tube to make a parafocal eyepiece for focusing the camera (the EP is pulled out of the tube about 2mm to be parafocal). My initial focusing results were really good. I only had to do minor tweaking of focus in the STAR MX5 software. A problem with the ST-80 (and sometimes with the lenses, is the alignment of the imaging lens/scope with the main scope (which has the guiding eyepiece for setting tracking). The camera bracket introduced a good bit of flexure in the alignment process (not because it is a flimsy mount, but because the ST-80 and camera are attached at only one point) so I figured a set of tube rings would be a better mount for the scope. The rings have really made a difference in imaging. One "improvement" I made in alignment and the flexure problem for piggybacking the camera lenses, was to place a sturdy sheet of steel in between the CCD camera and the camera mount. Since Tech 2000's camera mount had 3 small thumb screws for adjusting camera position, I was now able to utilize them (because of the steel plate in between the camera and mount in aligning scope and camera to the same point in the sky. My results so far have been ok with this scope (considering its limitations), probably because of a need for greater accuracy (not to mention patience), especially on my part. More work with software settings will be needed to insure that imaging with this arrangement (as well as through the main scope) will be possible.
I have lately been doing some planetary imaging not only at prime focus, but also with a 2x barlow (yielding a focal length of 2400mm) and will be shortly trying a Tele Vue 3x barlow at 3600mm (and maybe even trying to stack both barlows for 6x and a 7200mm focal length). The shorter exposures needed for the planets have worked well with my setup. I'm really anxious to try some longer focal length deep sky imaging, but a few things will be needed to help this become a reality. I will be adding an Infra-Red blocking filter from AVA to help clean up the ST80 images which also seem to be suffering from bloated or "seagull" stars. Also helpful will be a True Technology Flip Mirror Filter Finder (which is manufactured to allow the insertion of filters for tri-color imaging as well as serve as a flip mirror.) The final addition I will be working on is building a dob-wedge which will help to eliminate a good bit of my field rotation. If I get it to work I will post the specs and diagram for anyone interested in trying to build one (it has been posted). The biggest drawback will be that I live at 40 degrees N, meaning the scope will have to be attached to the wedge at a 40 degree angle. Difficult, but not impossible. The last statement really got me into a lot of trouble last summer. The wedge needed to be at 50 degrees instead of 40. I realized that when I went to polar align and I was no where near polaris. A few pieces of wood helped that situation, but I've since made new legs to the proper length. See my Dob Wedge page for more pics.
If you have any questions, please feel free to contact me.
Thanks to Gary Honis for the above picture of me and Ralph DeLong up at our Eckley site. You can see Gary's 20" Starmaster and ETX-125 in the background.