When astrophotography fever struck me many years ago, one of the things I bought was the Lacerta MGEN autoguider to use on my Skywatcher ED80 functioning as a guide scope. I wanted to be able to take longer subs with my telescope, and realised that with polar alignment that is not very true, I would need the guider to do a lot of work.
I have to confess that I had only used it a few times and never realised until recently that some of my early failings were as a result of the bad settings I chose for the guiding parameters. I will talk about this later on, as it ism something I read on an astronomy forum, and was something I noticed personally on the guider recently.
Anyway, the guider comes as two parts: the camera and the guider controls.
The hand controller has adapter points for (left) 3.5mm Canon EOS-compatible shutter output, RJ-45 guide camera port, power port, RJ-12 ST4 mount guider port, mini USB for updating firmware.
The ICX279AL-E sensor is a high sensitivity monochrome CCD from Sony. The sensor measures 4.5mm diagonally and has 752 x 582 pixels. It has a 35mm equivalent of 9.7x crop factor.
Now as my setup is very stable particularly with very accurate polar alignment in declination (DEC), I decided to just guide in RA and disable guiding in the DEC. When I did drift alignment to perfect my polar alignment, there was hardly any drift perceivable in declination even after 13 minutes of looking through a 5mm reticle eyepiece.
As I had read before that in order to not correct for the movement of stars as a result of the seeing, I had to average out the frames over about 3 seconds. I ignored this rule initially and set my RA guiding to correct after each frame. As I required 1 second exposures to detect the guide star, that means the guider was calculating whether adjustments were required every second.
I ended up seeing the guider alternating between adjusting in one direction and then the opposite direction constantly for the 4 minute exposure. The exposure ended up with double stars throughout i.e. the guider was overcompensating for the seeing.
As a result, I changed my settings to adjusting after every 3 frames the guide camera sees i.e. 3 x 1 second = every 3 seconds. This stopped the see-sawing and the guider was now only guiding every now and then to correct for the periodic error of the worm gear, which was great.
Below are the settings I used on my Lacerta MGEN. As you can see, I have had to boost the gain to maximum (9) in order to detect a guide star. I used a threshold of 8 to reduce the background noise seen on the hand controller screen.
I set for the guider to respond to changes in the star position after every 3 exposures of the guider camera, tolerance of 5 pixels, aggressiveness 70% and mode 2 (correct to original position of star)
As you can see, I’ve set the DEC guiding to off by selecting the guider to respond to 0 exposures. What I could do is to set this to a very large number, perhaps the maximum of 99. This will allow for very gentle corrections in declination, also in mode 2.
With the guider on, all the 4 minute subs I managed had pinpoint stars, despite some wind blowing about during that night (which would have ruined all unguided exposures).
In the end, I decided to test how well the guider could do with my CGEM. I did 2 shots at ISO 400 8 minutes and 1 shot at ISO 400 20 minutes. The 100% cropped images are below. As you can see, the 8 minute sub has nice and round stars, and the Cigar galaxy is just visible.
With the 20 minute sub, there is a slight oval appearance to the stars now, but the Cigar galaxy details are becoming more visible.
The image below also shows the stars from the 20 minute sub, which is egg-shaped. To counter this, I think I should have set the declination guiding back on but using many more frames in between adjustments.
Anyway, here are the guided images stacked in Deep Sky Stacker 3.3.4. I used about 25 light frames for the Cigar Galaxy, most 4 minute subs at ISO 3200 and 6400, with a few at ISO 400 (8 and 20 min subs) and 1600.
Do bear in mind that the image quality is not the best as there was significant skyglow present that night (mostly averted by my Astronomik CLS-CCD filter), and I was using high ISOs. In future, I will use longer subs and ISO 800-1600, to capture better detail with less grain in the images.
But I am pleased that I have finally cracked the guiding thing. BTW, it is not easy guiding using an off-axis guider on a 1600mm F8 telescope (as the guiding is at 15,500mm effective focal length), and number of stars visible are much less. I used the Orion thin off-axis guider for the guider, and will review this when I have the time.
At this focal length, it was actually possible to visualise the minute drift in declination in my system (DEC guiding disabled). While I was doing the periodic error correction training the other day, which took over 48 minutes to complete, I was able to see the star drifting down in declination on the hand controller screen a couple of pixels.
During one of my recent nights while imaging, I decided at the end of the session (due to my camera battery running out), to sit there and view an RA guided star towards the east in the guider liveview screen and adjust the declination. This is definitely a very sensitive way of detecting drift, as it will detect it at a subpixel level. And I managed to adjust it such that the star remained solidly in place with no shift in RA or declination for a good 20 minutes. I don’t think I need to worry about drift in declination anymore for 20 minute subs.
With the correct settings, the Lacerta MGEN auto guider will allow your guiding to be spot on even for exposures at least as long as 8 minutes. You just need to find the right settings for your setup and conditions. I am very pleased with it now actually, as it is doing much better than what I could before. There will be many more astro images to follow.