Testing scopes and mounts - Imaging from Hawaii
Some musings from my weeks of testing in Hawaii:
I just got back from Hawaii where I was able to spend a lot of time testing the guiding function of a 1600 encoder mount with an AP 175mm refractor. The tests were performed under various seeing conditions which ranged from 1.5 FWHM to 7.5 FWHM. Our observatory is located on the west slope of Kohala mountain, which is on the dry side of Hawaii Island.
The sharpest seeing occurred when the prevailing winds came up-slope from the ocean or were parallel to the shoreline. The worst came from trade winds that came over Kohala mountain and curled overhead. The worst seeing was so bad that Saturn was unrecognizable - it was just a boiling mess.
Over a period of 3 weeks I imaged just about every night and gathered a lot of data - both image data and autoguiding data. I also did a fair amount of unguided imaging. A couple of things stood out immediately. No matter what settings of guide exposure, time lapse and aggressiveness i used, the guiding results were almost 100% dependent on seeing. If the seeing was poor, the guide results were the same whether I used 2 second guide exposures or 10 second exposures. Putting long delay between exposures also did not change the results. A larger Min Move did help to prevent back and forth star chasing to some extent when the seeing was bad. But no matter what, when the seeing was 5 arc sec FWHM the guiding was consistently around 0.6 arc sec rms. When the seeing produced stars of 1.5 arc sec FWHM, the guiding at times went below 0.1 rms. In other words the stars were twinkling slightly but perfectly stationary and not doing the hula dance.
One thing i discovered was that for best guiding results the axes should be very well balanced. The gear mesh release mechanism of the 1600 gearbox allowed almost perfect balance with this large refractor. It is much more critical to set the Dec balance accurately than the RA axis. Reason is that unbalanced Dec causes static friction in the worm gear teeth, which makes Dec movements non-linear at the sub-arc sec level. The encoders do compensate and produce the final position very accurately, but the static friction introduces delay in getting there, overshoot, etc, which reduces the moment to moment positional accuracy. Bottom line - balance Dec as close as possible. RA is not affected by unbalance because it is always moving, so there is no static friction to contend with. The only thing I noticed was an increase in the sub-arc sec ripple in the sidereal drive rate. The more balanced the axis, the smoother the RA tracking.
I accumulated on the order of 300 sub exposures of the deep sky object that I was imaging (a mix of 10 minute and 20 minute sub exposures). Out of these only about 5 had non-round stars due to an occasional passing cloud. All stars were round, even when the seeing was an astonishing and atrocious 7.5 arc seconds (seeing is a judge of star size in FWHM, guiding is a judge of the guide star motions in arc sec rms). So, even in poor seeing the dual encoders operated equally in both axes to keep the scope pointed properly, even though the guide star was pulsing like an amoeba. What might surprise long-time imagers is that I plan to use very frame in the final image. I will not be throwing out any of the subs, even though they contain bloated stars.
I did do some unguided imaging using the drift model in the keypad. I was operating at approximately 1000mm focal length and could do up to 20 minute subs with round stars. I used 8 points along the object path, starting from 4 hours in the east with scope under the mount, to 3 hours past the meridian in the west. The object was low enough to the south to allow the scope to start underneath with counterweight up. I experimented with a mix of unguided and nudge guiding with very low aggression. The problem with low aggression is when you are dithering it takes a long time for the guide star to come back to zero. Same is true if you are using long exposure times or long delay times between guide exposures. You have to allow for long guider settling time, and that eats into your imaging time.
The scope itself performed beautifully with very predictable focus with temperature. Even though the lens is a 175mm triplet and the daytime observatory was between 95 - 98 degree F during the day, once the roof was retracted and the sun set, the scope acclimated within the hour to 75 degree air temp. It produced sharp images as soon as it got dark enough to image. I refocused perhaps twice or 3 times during the night until just before dawn when the temp bottomed out at 68F. The nice thing about this triplet design was that the focus did not change between all my narrowband filters.
For those who say that large airspaced triplet lenses have long cooling/settling times, I did not experience that with this scope. But then maybe my observing conditions are not as extreme as some people experience? The AP 175 Triplet lens has thinner lens elements than is normally used. This required a lot of careful polishing at very low pressures when I fabricated them. And that cost a lot of time and money, but the results are worth it. The lens is made with 2 elements of BSL7 and one with S-FPL53, both Ohara glasses and both outstanding quality wise. Why is that important? Because what you get with BSL7 is extremely high internal homogeneity. Some people are enamored with Lanthanum mating elements, but these glass types were never intended for objective grade lens applications and do not have the high internal homogeneity of borosilicate crown glass. Lanthanum is also much heavier and retains heat with longer thermal settling down times.
Unfortunately S-FPL53 is no longer available but fortunately FPL55 and Hoya FCD100 can be used with similar performance using the same borosilicate crown mating elements.
I will do some more postings on mount and scope performance once I get a chance to analyze the data.
Great post and great trip!