During some of my observing sessions in the summer of 2008, on the top of the Santa Cruz mountain range south of San Jose, I have primarily used my Orion 10" Dobsonian scope in lieu of my much better C-11 NexStar GOTO instrument: because of the dust. The roadside where I have observed for nearly thirty years has recently been graded, and the dust is probably like the surface of the Moon! Cars go by occasionally, and one guy seems NOT to like amateur astronomers, even ones like me who have permission from the landowner: he careens past me at about 40 mph, spraying billowing dirt clouds that settle all over my Uranometria atlas pages as if they've been sprinkled with talcum powder. I rush to cover up my scope but usually the damage is done.
After three years of hard use, mostly at this site, my Orion SkyQuest 10" Dobsonian has begun to show its age. On Monday 4 August 2008, I struggled to detect something -- anything -- of the planetary nebula Abell 72 in Delphinus, a very faint and tenuous shell of gas that just barely shows up in the Palomar Sky Survey image. The picture below, from Stathis Kafalis' Abell nebula page, seems to have been significantly enhanced:
I worked on Abell 72 for nearly two hours, after the object had transited but with it positioned in the darkest part of my sky; and during a period of time when heavy ground fog had formed below the mountain, cutting off almost all light pollution. Despite that, I could see very little of the nebula. After about 90 minutes of struggling, I made this sketch:
I have rotate the sketch to match Stathis' picture, and marked the same region. I could discern only part of the outer shell of the nebula, with fugitive averted-vision glances, using my highly efficient Orion 7.5 mm Lanthanum eyepiece [160x, 1.6 mm exit pupil, 15.5' FOV] and the SkyGlow anti-streetlight broadband filter. I could also BARELY detect a trace of it with my 5 mm Stratus eyepiece, no filter [240x, 1.1 mm exit pupil, ~17' FOV] and was, frankly, a little disappointed. There was some general, unresolved glow in the region of the 8th-magnitude star SAO 106544 (which is actually very blinding in a scope of 10 inch aperture, and detracts from the ability to see the faint Abell planetary.) That general glow was visible faintly with the O-III filter; but Abell 72 did not stand out at all, even at lower power.
On my next observing session, when I first set up the scope and trained it on Polaris to align the two finders, I looked into the focuser -- and saw nothing! The secondary mirror had come loose, and had twisted on its axis at the attachment to the spider, and wasn't pointing at the eyepiece at all: apparently, my car had at last hit one too many ruts in the rough mountain road, and the secondary had suddently lost its adjustment. In the little snapshot below, you can see a few of the hundreds of deep ruts and potholes in the last mile or two of the road to my mountaintop site. There are so many that they are impossible to avoid (especially when encountering a vehicle approaching from the opposite direction.) I would NOT like to take delicate astrophotography equipment up to this site!
It took me about twenty minutes to get the alignment of the optics into some semblance of order, but this was exactly the motivation I needed finally to work on the telescope! I knew that the mirrors were, by now, VERY dusty; also, two years earlier the flocking paper that I had installed on the inside of the tube assembly had pulled away and some of it had ripped off: see this article (the subsection titled "Oh: it's just an easy, foolproof modification!") for a discussion of that scary event.
Frankly, I HATE cleaning optics. In fact, I've written an essay suggesting that you should avoid doing it. Sometimes it does more harm than good. Every time you take an expensive, precision first-surface mirror out of a telescope, you risk ALL. One false move, and that's the end of it.
My old friend and observing mentor Rich Page taught me many years ago how to clean the optics in the 10 inch scope that he helped me put together. I added these instructions to my "Eyepiece" software program, and have put them into a web page that covers many of the basics for beginning observers.
The instructions seem fussy, but if you deviate from them (eliminating the distilled water; not bothering with the pure alcohol rinse; using a different method of cleaning the surface that risks scratching it; or allowing droplets of water to dry on the mirror) you get inferior results.
A good cleaning job can result in the mirror looking like new, with no damage to the aluminum coating. A bad job can COMPLETELY WRECK IT. So, the dust coating has to be really heavy before one should risk it. I felt that since I now had to realign my scope from scratch, knowing that I could definitely see dust on the primary if I shone light in the tube assembly (and suspecting that the views of faint nebulosity were not quite up to par), I could take that risk.
I've cleaned telescope mirrors, corrector plates, objective lenses, eyepieces, and filters many times in the past thirty years: and not once have I ruined anything. So it was time to take one more chance, and hope that the roll of the dice would not come up bad for me.
Here is a chronicle of the events:
The Existing Mirror Condition
I had never removed the mirror from the cell, though I've had the tube assembly all apart to install flocking paper. On this occasion, in the harsh light of day, I was able to see the appalling amount of junk that had accumulated on the surface:
Above, the dust particles stand out with the mirror at an angle. Below, a closeup of the region underneath one of the mirror mounting brackets, showing relatively little dust:
There are four brackets arranged around the periphery, with rubber or plastic pads that are not supposed to touch the surface but to act as "protectors" in case the mirror ever tends to tip off the back of the cell. As you can see, some dust managed to work its way even under the pad, but the surface is still much cleaner than the fully exposed mirror.
I might comment, incidentally, on the main primary mirror cell. It is extremely hard to remove from the main metal optical tube assembly, and is fitted very tightly to the end of the tube, and attached by numerous short bolts into tapped holes in the outer periphery. The mirror cell is by no means one of the "deluxe types" that are recommended to very picky amateur telescope makers; but it's a far cry from the rudimentary cell that I bought many years ago for my 8" Dobsonian. There are three small round (about 1/8th inch thick) cork pads underneath the 1-inch thick mirror: the back of the blank rests upon them. The cell's outer mirror holder is only a very tiny bit larger than the outside of the blank, which fits into it perfectly (with four very thin pads glued to the metal, just touching the outer edge of the mirror blank.)
The mirror therefore cannot shift more than a fraction of a millimeter even though it is not held down tightly and therefore is not stressed. Four mounting brackets consisting of rubber pads held in place by metal plates are screwed down (very loosely) to act as safety positioners (one should be able to insert a sheet of typing paper between the rubber and the mirror surface: but even doing that is risky and might mar the mirror.)
Below, I show how one of the pads is arranged. I am removing the long mounting bolt.
When replacing these pads, BE SURE NOT TO SCREW THEM DOWN TIGHTLY, OR YOU WILL STRESS THE OPTICAL FIGURE! I have quite a bit of experience doing this on various mirror cells, but you might prepare for a bit of trial and error: if, when reassembled, the mirror shows stress in a high-power test, take the mirror cell out and ADJUST THE PADS AGAIN to make sure that there is no direct pressure onto the front of the mirror.
Frankly, in my opinion the four mirror protector front pads are TOO BIG. They do cover a significant part of the surface. My old cell had but three brackets, not four; and they were each about half the size. But, a 10 inch mirror is heavier than an 8; and the telescope had to be shipped safely all the way from the maker in China. So at least the mirror is very safely supported from the front, and cannot possibly fall out of the cell. A really obsessive-compulsive telescope modifier might want to change them; but I am only too happy to leave the original design quite alone, and not risk any consequences.
Cleaning the Mirrors
Follow the directions given in my article. If you would like to confirm them, there are other similar instructions on the Net that will show the recommended procedure. Lenny Abbey has written a useful article, here. I differ very slightly, in that I don't use "DustOff" or other air in pressurized cans. Sometimes the propellant will condense on the mirror surface, causing a film that is next to IMPOSSIBLE to remove.
Another article that can serve as an alternative useful reference is this Holowiki page, quoting the advice of the optical expert Dr. Clay Sherrod. But the article goes into many other options that might prove confusing to the first-time experimenter.
My process is relatively simple (as documented by some snapshots done by my cooperative wife Regina.) After thoroughly cleaning my sink, and shaking out a towel that had been carefully washed and dried, I suspended the mirror in clean VERY lukewarm tap water to which a few drops of an unadulterated (non-perfumed) detergent had been added:
I let the mirror soak for a while allowing the water to drain very slowly while being continuously replenished by the trickling tap. Then, carefully, I DRAGGED WITHOUT PRESSURE clean cotton balls -- ONCE ONLY -- across the surface of the mirror, always keeping the cotton and mirror surface completely under the water. I did not allow myself to be tempted to use any of the cotton balls a second time, even though they looked perfectly clean TO THE EYE: for tiny particulates, dragged across the aluminized surface, can scratch it easily.
When I had cleaned the entire surface, one strip at a time, I did it again at 90 degrees to the first cleaning. Then, I let some of the water out and drizzled onto the mirror a mixture of 99% isopropyl alcohol and distilled water; followed that by several rinses of pure distilled water; and then more of the alcohol-distilled water mix, and so forth: until a full gallon of pure distilled water was used up.
By now, the water was pretty easily running off the mirror, though a few stubborn little droplets still remained. Before they could dry, I used a hair-dryer that I keep ONLY for drying my corrector plate or telescope cleaning -- a device that is otherwise hardly ever used and therefore NOT coated inside with dust -- set on warm/low, to direct a stream of air and drive the blobs of water to the edge. A few that remained were sluiced up by touching JUST THE DROPLET, not the mirror surface, with the edge of a sheet of Kodak lens-cleaning paper.
I did exactly the same thing with the diagonal mirror (which is glued to a metal shaft with one end formed to the proper 45 degree angle), taking care NOT to risk putting any pressure or stress on the adhesive bond. The diagonal did not 'break away' from the metal post -- thank goodness! -- or I would have had a real problem on my hands: finding an adhesive to reattach it that would not shrink unevenly while drying, stressing the back of the glass. Take great care with these glued-on diagonals!
Regarding the coffee-maker seen in the picture above: don't drink any before or during the mirror cleaning process. It will make your hands shake...
Finished, Clean Mirror
The finished result was excellent. Only a very few tiny dry spots remained, each perhaps a half-millimeter, or even smaller, in diameter; I decided they were inconsequential and did not wet and re-dry the mirror to remove them. Chances are, you will always find one or two when you finish; they'll have no significant optical effect. I've found that only FACTORY CLEANING PROCEDURES can do better, leaving a surface that looks as fresh as when it was first aluminized.
Off-hand, I'd say that at least 95% of the dust has been removed, with no scratches or streaks. The human eye probably can't see a difference in the optical result between this three year old, cleaned mirror, and an identical brand new one that has been freshly coated. But, each mirror cleaning WILL tend to degrade the surface, so the more you clean, the less reflective the mirror will become. A cleaning every five years, if you keep the scope in a dust-free environment (which I unfortunately cannot do!) is more than adequate; so therefore the optics will last for many, many years. I will probably be too old to use the telescope by the time the mirror coating is deteriorated enough to require stripping and replacement by professionals.
Re-Assembly and Collimating
Before re-assembly, I took the trouble to patch the torn flocking paper, which had left about a square foot on the interior of the tube uncovered. The Orion Dobsonians of this series do NOT have very good internal tube flocking, and some black felt, gummed flocking paper, or better flat-black paint can improve the contrast.
In order to make the patches, I had to reach half-way into the length of the tube, a difficult job: but one which I managed to accomplish without removing the spider by working from the open rear end before the primary cell was reinstalled.
For the first time in fully aligning a Newtonian from scratch, I now had the benefit of a laser collimator. I had purchased one on close-out from Orion, and am glad I did! Normally, when one reassembles a completely stripped down Newtonian, it takes a long period of time to get the optics properly aligned. A special agony is the problem of getting the diagonal set right. But, not so with the laser!
The first job was just to get the blasted mirror cell back on the metal tube. It is useful to have a helper; but my wife had gone to the store and I was on my own. I worked at it for about twenty minutes, as the telescope tube 'shrinks' very slightly and goes a bit out of round with the cell removed. Finally I managed to squeeze it onto the end of the tube and get all the screws in. (Before doing that, I had used my caliper to set the mirror cell adjustment absolutely level, to know that the mirror was pointed properly and not skewed off to one side.)
Here, below, is the laser collimator, being used on my Orion Star-Blast reflector. For that scope, a Barlow lens worked fine. Unfortunately the Barlow did not seem to function right -- with the laser collimator -- in the Orion SkyQuest 10" Dob, so I used the laser straight into the focuser, not as shown in the picture below.
To begin, make sure the primary is pointed straight toward the main opening of the tube assembly. Without the diagonal installed, one peers down through the hole in the center of the diagonal holder of the spider. My previous adjustments with the caliper were very close, and the small round white spot in the center of the primary appeared to be positioned about right.
Install the diagonal and "ballpark" aim it at the opening of the focuser.
Insert the laser collimator into the focuser and turn it on.
Watch the red dot that bounces back from the surface of the primary, looking down the front of the tube assembly off-axis so that the red light doesn't bounce right into your eye.
Adjust the position of the diagonal until the red dot hits the center of the primary. Tighten down the diagonal's three Allen screws. Check again, and if the red dot is still centered, shut off the laser.
Knowing that the primary mirror is pointing correctly, you need only worry about the angle adjustments of the diagonal (the offset is already pre-established by the existing dimensions of the Orion instrument: the position of the spider, the focuser, and the length of the metal post that holds the diagonal mirror.) I put the tiny plastic sight cap that came with my telescope into the focuser, and made sure that I could see the full reflection of the primary.
Finally, I replaced the sight cap with the laser collimator, and turned the three knobs on the primary until the bright red spot, reflected in the 45 degree mirror in the collimator, "disappeared into the hole" in the center: PERFECT collimation and axial alignment!
Not trusting the laser to be precisely aligned -- people have warned about that on the Net -- I got out my "artificial star" (which is discussed in detail in this article about upgrading my focuser.)
I set the 'artificial star' reflector up on the fence of my neighbor's house, about 150 feet from my driveway. With 400x (using my 6 mm Expanse eyepiece and 2x Barlow, requiring an extension tube to reach focus) I found that the telescope yielded a textbook "Airy disk" when slightly defocused, with the bright central peak EXACTLY in the middle of the concentric rings. The laser was on the nose.
No signs were visible of any stress of the optics: whew!
Results Under the Stars
Despite fatigue from nearly 8 hours of work on the scope, a lack of enthusiasm, and a late Moon-setting, I decided it was worth the trouble to take the instrument back up the mountain (to my 3,400 foot elevation observing site) to try it out. Stars were crisp and clean. At around midnight (August 7, 2008) I got the star field of Abell 72. Amazingly, I could actually see, with no doubt, that the telescope had MUCH BETTER CONTRAST! With my 15 mm Expanse [80x, 3.2 mm exit pupil, ~49' FOV] and the O-III filter, now Abell 72 was discernible, a brighter region in the faint glow visible in the area shown in my sketch, above. Not only could I see the faint 'half arc' I had drawn days before, but also much more of the nebula was visible; and it was brighter. Sky conditions might not be responsible, as a matter of fact: there was no ground fog now, and my naked eye stellar magnitude limit wasn't as deep.
Thrilled with this result, I now tried to see if I could get the faint galaxy MCG +02-53-005 (or PGC 65491) that is very close to the Abell planetary. On 4 April, I had struggled for many minutes, trying to spot it with averted vision in vain. NOW, however, I was finally able to see it, requiring about fifteen minutes of diligent study at 341x (with my 3.5 mm Baader Hyperion eyepiece) after allowing the field to drift across many times, while covering my head with a dark cloth, and pulling back slightly from the eyepiece to cut out the blinding 8th magnitude star nearby. I had a couple of fugitive glimpses of the galaxy -- a faint grey speck of indeterminate shape -- plus several faint stars that I had not seen or marked earlier on the same chart.
The Hyperleda page for PGC 65491 provides no B magnitude; so I loaded the applet Aladin and superimposed the NOMAD survey and some others, but did not find a measurement that could be converted to visual magnitude (err--at least by a means of which I am aware.)
I continued cross-checking until I did find one, via the NED catalogue, where the page for for PGC 065491 gives "15.4 magnitude", which is not a visual rating (the Deep Sky Browser page claims "16.2p" but it would be brighter than that to the human dark-adapted eye if correctly converted to visual magnitude.) That suggests that this little speck, 1.0 by 0.4 arcminutes in diameter (photographically, not visually!) is probably not much different in perceived brightness than some parts the faint envelope of Abell 72. Now that I could see both, I feel that the process of cleaning and careful alignment have yielded a completely successful result!
(An amusing side topic is that my observing session was cut short before I could continue very much longer. I had been warned that a huge mountain lion was lurking in the area, and was paranoid about the rustling sounds that I could hear from the woods across the road from my telescope site. They continued for about 45 minutes, and did not diminish when I played my audio cassette of vicious dogs barking. I got so upset by the rustling that I could not concentrate, and finally gave up at about 2 am and loaded everything back in the car. As soon as I turned it down the road, with bright lights on, I could see the cause: two big snakes (probably each at least four feet long) were coiled and writhing around in ecstacy, their heads poking up and darting about like puff-adders. I could see that they were not rattlesnakes, but that didn't make me very much more at ease: because, as they coiled and rolled, they were slowly working their way across the road, very near the raised part where I had put up my scope. If I had stayed, sooner or later they would have been viewing companions! Gingerly I maneuvered my SUV around them; they didn't seem to care. I could tell you what the object was that I had been trying to observe, but it might be too suggestive. Oh, the heck with it: believe it or not, it was Abell 69! So, I drove back home, leaving these lovely creatures to their amours, glad at least that I had seen Abell 72, plus one of the faintest little galaxies ever spotted with my 10" Dobsonian.)
Srw
Copyright Statement: The original contents of this article are Copyright © 2007-8 Stephen R. Waldee - All Rights Reserved. All trademarks or copyrights are properties of the original copyright holders. The author Waldee requests that you do not copy these articles elsewhere, particularly into lists, blogs, web pages, databases, or astronomy compilations in any form. The primary reason is that these articles are essentially raw data from our observing logbook, consisting of reports and sketches that have not yet been thoroughly fact-checked, intended as preliminary drafts for future "Faint Fuzzies" articles that will benefit from extensive corrective research.
