 |
THERMAL EFFECTS IN GUIDING AND FOCUSING COMPOUND MOVING MIRROR TELESCOPES
BACKGROUND
The Schmidt Cassegrain is Telescope (SCT) is a very popular compound moving mirror telescope in the United States. And why not? It promises compact design, portability, large aperture, and a plethora of accessories from multiple sources - all at an affordable price. With the introduction of electronic periodic error correction, those with inexpensive worm and wormholes were able to get results approaching much larger and more expensive precision drives. Permanent periodic error programming further increased performance of low-cost mounts. Finally, integrated goto electronics allows even those with minimal knowledge of the skies to find and center objects in the eyepiece or imaging camera. The larger Schmidt Cassegrain can also provide the image scale needed to do a nice job of recording the more plentiful smaller objects.
Indeed, the Schmidt Cassegrain is capable of taking some rather nice images. Just visit the web sites of amateur astrophotographers and one can find very fine images taken with these telescopes. My better SCT images were always taken under steady skies, stable temperatures and little to no wind. I learned long ago to check focus often. Optimal imaging times frequently require waiting until the second half of the night and sometimes a couple of hours before dawn. It is quite possible that during the course of an exposure, the skies will become less steady.
The following image illustrates the effects of seeing deterioration
during an exposure. The effect, while subtle, is seen especially
in stars which are slightly bloated and have a red ring around them.
The red image was taken last and due to increasing scintillation the red
image is slightly smeared out. This causes the red rings around the
stars. Under normal, steady skies, this effect is totally absent.
A "seeing" deficiency is not simply a slight atmospheric bloating of the
star images, but rather, is caused by a random moving about of the normal
image at perhaps 30 Hz or so. This scintillation appears as a larger
image when integrated over a second or two.
|
One can find images taken by amateurs that are not as nice as they would
like. One might conclude the better images are a result of good fortune,
good equipment, experience, and imaging talent. One path to good
results is to understand the limitations in oneself, equipment, available
seeing, and sky darkness. Let's explore at least one of the aspects
we can control now, the telescope - more specifically, the SCT. For if
we fully understand the equipment we are using, we will know how to get
the most from it.
CHOOSING A GUIDING METHOD FOR A SCHMIDT CASSEGRAIN TELESCOPE
Guiding is useful in correcting tracking errors resulting from alignment errors and periodic errors. Both new and experienced astrophotographers will likely find the separate guidescope quite intuitive and much simpler to use than the alternative - the off-axis guider. Separate guidescopes are very nice because one can find a nice bright star in typical suburban sites and start guiding rather quickly. If an autoguider is employed, fewer and simpler autoguider calibrations are required because the autoguider orientation remains rather constant. For those getting started in SCT imaging and experienced imagers needing short exposures such as those used for stacked CCD images, I would recommend a separate guidescope.
Personally, I have not had very dependable results using a guidescope
on an SCT where exposures were over 20 minutes. After that, guiding
errors start to appear. When I did get better results, it was during
rather specific conditions that are described in the recommendations section
below. The use of the affixes guider is not unusual for larger telescopes
over 10 to12 inches in aperture. There is a good reason for this,
any differential tracking between the two instruments result in tracking
errors in the image known to be guided properly. Differential flexure
and mirror shift is frequently blamed for poor SCT results. I have
long suspected that there are other factors at work that may be equally
and arguably more of a factor than differential flexure and mirror shift.
OPTICAL LEVERAGING IN THE SCHMIDT CASSEGRAIN TELESCOPE
The Schmidt Cassegrain telescope is a really quite versatile. We can
attach several accessories to it and reach focus without remounting the
mirrors. The SCT is exhibiting optical leveraging. A small
movement of the moving primary mirror results in a very large movement
in the focal plane. The optical leveraging can approach 25 to 1.
Small temperature changes that cause expansion and contraction of various
optical components can have rather large effects because they are amplified
by the inherent optical leveraging. To explore effects of temperature
changes one might likely experience while imaging, a controlled experiment
was completed using a popular 8" SCT and 90 mm Maksutov employed as a guidescope.
EQUIPMENT TESTED
Here is shown the 8" LX200 with the ETX firmly mounted on a Losmandy
rail and ring set which was used in the temperature tests reported in this
article. Note that for the measurement described in this article,
the OTA and guider were mounted on a permanent pier without the LX drive
and fork.
 |
I completed testing on an SCT and Maksutov combination that was fixed to a multi-ton isolated structure in a large environmentally controlled building whose air volume was great enough that ambient temperatures were maintained to about a quarter of a degree F. over several hours.
The intent was to isolate mirror shift, flexure, seeing, drive errors,
and other anomalies to determine which had the greatest influence effecting
imaging results. I had suspected (based on earlier tests of jamming
a primary mirror to the baffle tube) that mirror shift might not be the
only cause or even perhaps the primary cause of differential tracking during
longer, one hour exposures.
INITIAL EVALUATION OF THE GUIDESCOPE, PLATES, RINGS, AND EQUIPMENT
The main scope was an 8" LX200 SCT. The guidescope was 90 mm ETX Maksutov. Losmandy plates and rings were used to secure the ETX to the LX200. The ETX had a rather minimal image shift of about 34 arc-seconds. This was quite good and a bit better than several other ETX's I have examined. In addition, a small refractor was also tested in place of the ETX.
The guidescope plates and rings were Losmandy's finest plates and rings.
The plates are the solid (non milled-out) style; the strongest and most
rigid version of this style sold by Losmandy. The rings were the
correct size for the ETX and mounted properly just inside the front corrector.
Note: The ETX front corrector cell should not be used for the rings because
the corrector is not floating in its cell. As a result, placing the rings
over the corrector will cause pinching of the optics in the ETX. This LX200
had a rather minimal image shift of 41 to 54 arc-seconds.
SETTING UP AMBIENT SIMULATION TESTS
The tested combination were moved to a large environmentally controlled building free of air currents (near perfect seeing) such that the scope focus could be set closer to infinity. The LX200 was secured directly to a multi-ton isolated concrete pier and allowed to reach thermal equilibrium over a 24 hour period. The LX200 fork or mount was not employed as this might effect results.
The goal was to eliminate as many variables as possible to determine
if this particular setup has potential to take 1 hour guided exposures.
This was a static test- mirror shift, flexure, seeing, drive errors, temperature,
humidity and other variables were virtually removed. Temperatures were
controlled to within 0.15 degrees C. (as measured and recorded locally).
A target was placed at the opposite end of the building. Both scopes were
adjusted to the same point on the target. A heavily insulated suit, hat
and special gloves were worn to prevent unwanted local temperature rises
of 0.2 degrees C. noted within a few minutes of approaching the equipment.
RESULTS OF AMBIENT SIMULATION TESTS
As little as a 0.15 degree temperature change resulted in a 2 second optical movement in the LX200 and a 1.5 second optical movement in the ETX. A 0.6 degree C. temperature change resulted in 8 arc-seconds of optical movement in the LX 200 however, the ETX did not track the LX200 optical movements linearly. Sometimes the two tracked more linearly than other times. I believe the correlated motion for some periods was coincidental. After about a 2.5 degrees C. change over the course of 4 hours, the amount of misalignment between the optics approached 24 arc-seconds. Clearly, any temperature change greater than 0.6 degrees C. during the course of a single exposure would result in oblong stars. This amount of temperature change is not unusual at all in many locations during the earlier part of the night before the temperature has stabilized.
I also replaced the ETX with a small refractor to ascertain if eliminating one compound telescope would improve results. It did, but it was not much better and certainly not enough to assure dependable results. This is important because even a 75% guiding success rate may temp one to take two shots for every object, because we don't know which shot in four is the going to be the bad shot. It is quite likely oblong stars (trailing) will be noticed with this setup if the ambient varies more than 0.6 degrees C. during the exposure.
I also measured a focal plane change of as much as 0.006" with at 0.6 degrees C. temperature change. Warming moved the focal plane outside of focus and cooling moved the focal plane inside of focus. At the native focal ratio of f/10, this amount of focal plane movement is outside the acceptable depth of focus tolerance. At typical focal ratios used with these telescopes, of f/6.3, the SCT focal plane movement is twice the maximum desired depth of focus tolerance required to maintain the sharpest possible image. If we consider that it is likely we centered the focal plane during the focus process, it is likely we have exceeded the depth of focus tolerance by close to a factor of 4. In this situation, the image has drifted out of focus a bit and it shows in the result. That is too bad because now the image taken under good 2 to 3 arc-second seeing produces a 3 to 4 arc-second result. Thus, it is prudent to check focus frequently in compound telescopes when the temperature is not quite stable.
Editor's note (Doc G): A focal plane change of 0.006 is 0.15 mm. To realize how much this is, note that the depth of focus of an f 10 telescope, for a 10 micron circle of confusion is only 0.100 mm. Thus the focal shift due to temperature change is significantly greater that the depth of field. For an f 6.3 telescope the depth of field is only 0.063 mm which makes the situation even worse.
A test was made using 3/8" black foam insulation wrapped around the
optical tube to slow cooling or heating. The mirror casting end was
kept open to allow the primary to reach and remain at near thermal equilibrium.
The obvious problem with this approach is the optical tube insulation will
also slow down the desirable properties of reaching thermal equilibrium
more quickly. An external fan and filter could be used as needed
to overcome this problem. As a result of the tube insulation,
the amount of LX200 optical movement during a 0.6 degree C. change over
1 hour was reduced from 8 arc seconds to 1.5 arc seconds and focal a plane
change from 0.006" to 0.0013". These results were quite encouraging.
RECOMMENDATIONS:
Decide if your imaging site will produce a 0.6 degree C. or less temperature change over an hour. This may require the start of your image in the second half of the night when temperatures are more stable. If one can live with this restraint, and this is a big consideration, a separate guidescope might be employed. It may be possible to employ external heating to minimize optical tube changes. The idea would be to use some sort of precision thermostat or perhaps trial-and-error to keep the optical tube from cooling during the exposure. The amount of heat added would only be enough to hold the Optical tube slightly above ambient at the end of the exposure. Corrector dew heaters may be a start. Another option is to add insulating foam over the optical tube to slow cooling or heating of the optical tube.
Since the focal plane also moves with temperature changes, it may be
wise to add 0.003" (at f/6.3) to your focus tool (moving the focal plane
backward) to compensate for falling ambient temperatures (unless external
heat is employed). In this way, when we start the exposure, it is at the
fringe of acceptable depth of focus.
CONCLUSION:
I believe I have may have identified and quantified the single biggest
factor that spoils SCT images guided with a guidescope or likely any compound
moving mirror telescope for that matter. This is likely why so few
have obtained consistent results with the guidescope on these telescopes.
We have also found temperature significantly effects the focal plane location
during an exposure. Those that are looking to get optimal performance out
of their SCT will want to watch for relatively small temperature changes
that could spoil their imaging results.
--
Michael Hart
Husen Observatory
mhart@netexpress.net
For high resolution images go to the following web site
http://www.netexpress.net/~mhart/