(UPDATE  - September, 2002.  This article still contains the truth, but there have been more offerings in the realm of stepper controlled focusers.  Even nicer, a number of telescope control programs have appeared that can control the focuser and perform the best focus at the push of a button.  This is exceptional for automated astronomy.)

How important is focus when surveying for supernovae (SN) using an automated system?  Maintaining focus is essential.  What happens when a telescope is not focused?  The light from a single star is scattered over more pixels than it would be if it were focused.  This means that your limiting magnitude drops to an unwholesome value.  As I have mentioned before that many SN are discovered at 16th magnitude or so. Now wouldn't it be a terrible shame if you had taken an image of that galaxy, saw nothing, and there it was the next day in the IAUC funny papers with a SN discovered by someone else?

In addition to decrease of limiting magnitude, the out of focus blob that would be a SN can merge with a galaxy's nucleus.  It is not uncommon for SN to be within 4 or 5 arc seconds of the host galaxy's nucleus.  So not only do the dim stars disappear, resolving power is lost.  So, the combined loss of dim stars and resolving power can easily mean that all the work and planning you have done to find supernova has been defeated by the simple fact that the air got colder, as it almost always does every night.

Okay, problem defined.  Now what are the solutions?  If you work visually or stay up all night looking at each observation as it comes in read no more.  You can constantly correct your focus.  But if you are like me and the ever increasing number of people who wish to start their telescopes, let them do their magic, then analyze results the next morning then read on.  This little article is for you.

The bad boy on the block when it comes to focusing is the ubiquitous Schmidt-Cassegrain (SCT).  How lovely and compact they are!  How lovely the first 50 images look after you have focused.  How disappointing are those donuts you see in the images after it has gotten colder by 6 degrees C.  How aluminum they are.  Therein lies the rub.  Aluminum loves to contract when the temperature drops.  Its coefficient of expansion (contraction) is large compared to steel, invar and carbon. So as the temperature drops, the distance between the primary and secondary in your SCT is smaller therefore the focal point moves farther out.  The effects of the pyrex primary mirror shrinking has the opposite effective; this would move the focal point inward.  So, given the evidence it is clear that the culprit is aluminum when the telescope focus changes due to temperature.  Then why do the scope manufacturers use it?  It is light (look at those airplanes flying around) and it is relatively cheap.

Okay, that's the bad news.  Here's the good news:  the expansion is linear with temperature for most telescopes.  This means that for every degree C. the tube shrinks there is X mm. movement of the focal plane outward.  If you made a plot of the amount of change in the focal plane out compared to the temperature dropped, it would be a straight line. There is ammunition in this fact.

Let's say that you had a device that you could use in two modes.  It is a motorized focuser on the back of your SCT.  In the first mode you would focus your telescope (not by moving the primary!) and press a button and your focuser would note its position.  Now wait for things to cool off more than say 4 degrees C.  Then you carefully focus the telescope (the focal plate would have moved out) and hit that button again to teach it a second point.

Since 2 points define a line, this device has literally learned how to change focus as the aluminum tube shrinks in real time.  Now you have (theoretically) a learned association between temperature and focus and whenever you turn on your telescope it will focus itself.  Nice, very nice.

I have recently bought such a device called the TCF (Temperature Controlled Focuser).  Moral stuff:  I have no financial interest in the company from which I bought this..  I paid full price for my TCF.  I don't even know anyone at the company.  They are disembodied voices on the phone the couple of times I spoke to them.

I have found that this device is a real pleasure for my 14" SCT.  Now I get good focus all night long as I sleep in the house curled up with my beautiful wife and our dogs.  (Okay, not everyone will sleep with a dog but we do.)  Once I taught the TCF as described above I haven't touched the focuser on my scope.  When I turn on the unit, it just senses the tube temperature and the telescope is focused.  And that focus stays good all night long.

How do they get away with this?

(1) The learning programming is simple because the contraction of the tube is linear with temperature.  What a mess it would be if this wasn't true.

(2) Virtually all the change in focus of any telescope is due to one component, the shrinkage of the tube, truss or whatever.  Remember, shrinkage of a pyrex mirror forces the focal plane inward, not outward. Again, the function that would describe movement of the focal plane would be complex if it wasn't virtually 100% due to tube shrinkage.

(3) They use a stepper motor.  Analog motors can't replicate themselves very well.  In other words, if you apply the same voltage to an analog motor time and time again, it would not be as accurate unless you had superb electronics controlling the voltage and the motor was exceptional.  The stepper motor has a zero point and the unit can always remember where it was when it was turned on and off.  So, a stepper is a very nice motor for this kind of thing.

Caveat Emptor (Buyer Beware)

(1) This device isn't cheap, about $850 US.  But if you are like me you have already dedicated yourself to survey astronomy while you sleep and the total cost of a mount, OTA, good sized CCD plus observatory has already pushed you well over $25,000US.  What is another $850 US? Okay, so your wife will make you sleep of the sofa for a week.  I think it's worth it.

(2) For people who know steppers, microprocessors and digital electronics, it probably costs MUCH less to build one.  But I know very little, so ...

(3) AND THIS IS IMPORTANT.  I have never seen an SCT that does not have mirror flop.  It is inevitable.  This means that when your telescope moves, so will the mirror. The SCT is made to change focus by moving the primary.  Even the smallest motion of the primary may kill focus. You will gain NOTHING from this kind of focuser that changes with temperature if your mirror is moving when the telescope slews.  I had the entire mirror focusing mechanism removed from my SCT to get rid of this problem.  Mirror flop can also destroy your automated pointing capability, a topic for another day.  So, you must take care of any situation, other than temperature, that will cause focus to change. Generally speaking, mirror flop in SCTs is the major culprit for non-temperature changes in focus.

A word for the non-survey types:  There are lots of people who will still want to use a temperature controlled focuser if they don't spend the night slewing around but stay on one object and do long CCD or film exposures.  Chances are the mirror will not knock around and a temperature controlled focuser can be as welcome an advance as automated tracking.

(4) I have tested the Optec product over a relatively short range of temperatures, from 21C. to 10C.  I have no idea what happens over longer ranges or when it gets below freezing.

(5) You may experience focusing changes over temperature that is an unusual combination of optics and various support materials that does not change linearly.  This can still be overcome by a focuser that is temperature compensated, but you would have to teach it much more than two points.  As it is now, the TCF makes that linear assumption. If the focus change vs. temperature is not a straight line, this device is not for you.

(6) I have not tested to see if the focus at any one temperature as determined by the focuser is the best possible focus.  In other words, is this device just as good as the trained human eye?  I don't know. What I do know is that I find that focus from the beginning of a night to the end is in line with seeing.  My measured FWHM does not change over a night.

Conclusions

If you lose focus as it gets colder your ability to find SN will drop. The loss of focus mean a loss of limiting magnitude and At altitude, such as Arizona in the winter, it possible to go from 21C at the start of a night's run to -15C in the dark before dawn.  I can assure you that without adjustment of focus on an SCT you will go from nice star images to big fat donuts.  You may find that a temperature compensating focuser is a real plus for your survey work.  For me, once I solved my pointing and scheduling problems through off-the-shelf software and hardware, the only problem left to solve is maintaining focus.  The problem has been solved to my satisfaction.  I also hope that this little discussion about focus will help you find more SN or at least add something to your arsenal that increases your chances.  Good hunting!

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