Posts Made By: Jim Richberg

Herb, I wonder if it might be worth making a "public service announcement" on the Astromart classified board about the merits of running computer virus protection software. I have received the "Klez" email virus six times in the last three days, which wouldn't be so exceptional (it is the latest fast-spreading attachment virus) except that I've *never* received so much as a single virus in my years of being on the 'Net previously. I keep a very low cyber-profile (no news group postings, etc.), and the only thing that has changed of late is that I've been posting and answering more Astromart ads, including a flurry last week (just before the virus broke out). If you do correspond to someone about a classified ad, this places your email address in other party's Microsoft Outlook or Outlook Express address book, where the Klez virus can find it. (FYI, Klez sends itself to every name in the infected user's address book, but randomly picks a second name from the address book to put on the email's "from" line to make it harder to attribute the source of infection.) The next time I get one of the infected messages I may keep it and take a closer look; I understand Klez is sloppy and leaves the real sender's email path at least one place in the header information.

My virus protection software (Norton Antivirus) caught all six infected files and deleted them before the email messages were downloaded, but I wonder how many other Astromart users are getting infected and spreading this unknowingly. There is free software that will clean up the Klez virus, and there are a plethora of free virus scanners/protection packages out there.

Has anyone else been hit with Klez this week?

Jim

I don't want to beat a dead horse/angle for the big fish (insert your favorite metaphor here...) and I *have* read with interest the previous central obstruction threads. My question is, bearing in mind my average viewing conditions (East Coast humidity and light pollution) am I likely to notice much difference in practice if I replace the 4" secondary in my 16" Dob with one small enough to get below the 'magical' 20% central obstruction (CO) threshold?

I know the arguments (less vignetting, higher contrast, etc.) and physiological effect that supposed kicks in at 20% CO or less, but is this difference likely to be noticeable when starting from 25% CO? The 100% illuminated area will just about vanish (less than .1", compared to the .8" dia of the 4" secondary's fully-illuminated region), and I'll have to do major modifications (move the primary back in the tube, move the trunions, and bore a new hole for the 120mm high efficiency fan that blows across the mirror face) just to keep *any* fully illuminated area to the view with the smaller diagonal. Moving from 4" to a 3.5" doesn't require any scope mods, but it only reduces the obstruction from 25 to 22%, which I'm virtually certain is an imperceptable difference (except to my wallet-- those big secondaries aren't cheap even second-hand!).

I've read the theory and crunched the numbers, now I'd welcome opinions and views (informed or otherwise?! ). I'd be especially interested in hearing from anyone who did move from one of the Meade/Coulter style 'oversized' secondaries to a smaller one falling below the 20% CO threshold. I'm inclined to think it's not gonna' be worth it for me to do this for those rare nights of perfect seeing, but I'm willing to be persuaded otherwise.

Thanks! Jim

I find the argument made by Rick for going with a high quality diagonal mirror--greater than 1/10 wave and no turned edge-- in the 25 vs 20% obstruction thread to be pretty compelling. Now where do you guys find such critters in sizes over 3" minor axis? Most of the high grade ellipticals I've seen listed have been smaller than 2", and most of the ones I've seen larger than that have been spec'd at 1/10th wave max.
Thanks,
Jim

In light of the ongoing rain locally, I have nothing better to do with my astronomy time than think about things like my long-range dream to build myself a permanent observing site. I know that a big chunk of the seeing-degrading turbulence is located within the 12 meter layer of the atmosphere closest to the ground. Some on this forum have built observatories on the second or third floor of their home, which seems to solve the turbulence problem but at the expense of an off-setting effect from viewing from/over a heated structure and roof. Has anyone ever observed with a telescope from a three story detached tower or platform? I'm thinking of something like a short forest fire watch tower. It shouldn't be too expensive to build one out of lumber. You could even get fancy and have a free-standing central hub of pillars to give greater rigidity and vibrational isolation to the part that the scope would sit on.

So, has anyone ever seen/tried such a beast?
Jim

A new Yahoo group (Dob-fans) has been formed. Lest we get sucked into the great small refractor vs. Dob black hole , let me hasten to add that we're talking about thermal management/active cooling issues on reflector mirrors.

I've thrown a few posts in to start the ball rolling.

The Group can be found at:
http://groups.yahoo.com/group/Dob-fans/messages

I have had fantastic luck using a 9-21mm Nikon zoom. Zero ghosting, phenomenal light throughput (exceeding Cemax and rivaling TMB monocentrics) and is nearly parfocal throughout its range. It is sold as a 15-45X/20-56X spotting scope eyepiece for ~$180; this version lacks the 1.25" adapter you'll need to put it in a scope for nighttime viewing, but is fine for use on a PST, since the adapter (at least the one APM provides) is just enough longer than the eyepiece barrel to keep it from coming to focus. Consequently, I put the eyepiece in with it's 1.25" collar against the set screw instead. Nikon makes a newer eyepiece that should equate to a 7-21mm eyepiece (it is their 25-75X field scope eyepiece, available for $230+). Some have claimed that the zooms that Hands on Optics and Apogee sell are OEM's from the same manufactorer as Nikon. I can't vouch for that, but for ~$50 they may be worth trying out. I *can* vouch that the genuine Nikon zoom is all the eyepiece you'll need for your PST.

Clear skies,

Jim

(This is cross-posted from my quick review on the PST Yahoo group.)

I'd been talking to Harry Siebert about his optical correctors and finding an option that offered low magnification and required no modification to the PST-- even one as trivial as unscrewing the blocking filter/eyepiece holder (which can't be done on my PST anyway, as it is cemented in place.) Harry thought about it and came up with a relatively tall 1.3X corrector, and sent me what I believe was the prototype unit. As such, mine looks a bit different than the production units are likely to, but I'll cut to the chase and tell you right now, IT WORKS WONDERFULLY!!!!

I managed to thwart the "new equipment curse" in that the clouds parted today long enough for me to get an hour's use out of the unit, so here are my first impressions.

1. Although I have owned various binoviewers for five years now, I am not a "bino nut" and in fact greatly prefer 'Cyclops mode' viewing. That still may be the case for night time observing, but this OCA worked well enough on my Black Night binoviewers that I may switch my solar observing exclusively to bino.

2. My PST renders particularly sharp images (A more experienced solar observer and I have estimated it at <0.7 Angstrom, and it consistently beat my Maxscope 40), but it has always had "sweet spots" within the FOV that would be in better focus compared to the rest of the field. I got used to moving the scope slightly to center whatever portion of the solar disc I was interested in within this region of best focus. (This was one of the advantages the Maxscope had over the PST, by the way-- a much "flatter" focused FOV.) Harry had said that the OCA would help flatten the FOV. I was frankly dubious, but it proved to be true. An unexpected plus for binoviewing!

3. Light loss: Insignificant. I measured this by looking at the faintest prominences visible through a good (high light transmission) eyepiece in mono mode, then looking for the same wisps through the bino. I know there was light loss, because of all the extra elements in the optical path with a bino, but as a practical matter the drop off is so slight that I was unable to decide definitively whether or not I was seeing one. (Part of this was due to the fact that my bellows-style eyecups offer 100% baffling of environmental light in mono mode, but "leak" light--which reduces contrast-- when used in bino mode because of the different position of the eye socket over the eyepiece. I'm fixing this by modifying my eyecups.)

I guess I shouldn't be surprised by the lack of drop-off in observed light at the eyepiece, given the huge quantity of light we're dealing with compared to night time viewing, when binos in my experience do not detect stars of the same magnitude as mono viewing.

4. At first I thought the magnification of the binos was greater than 1.3X, but after comparing it to monocular views, I think it is in fact 1.3X after all. I did all of my quick testing with my pair of Nikon zooms set at 21mm, which translates into 25X in the binos. I'll do some higher power testing later, where the dimming of the image may become a factor.

5. Since this is Harry's "zero modification" unit, all of the optical correction takes place above the PST, and the unit you'll have to stick in your binoviewer is is much like a very long Barlow. (Mine is 4" long, but the production units are supposed to be considerably shorter) Your binoviewers will be sitting well above your PST; depending on your mount, this might make for some stability or balance issues. Harry plans to shorten the length of the OCA on the production units by folding or bending the light path within the OCA; I opted to have my unit made "straight through" because I'd like to try using it at night (it's lower powered than my current 1.8X OCA), and wanted to avoid the additional light loss from the extra optical elements.

I'll close by reiterating that this thing works, and makes no- modification binoviewing a reality on the PST.

Clear and sunny skies!
Jim

The honest answer is "it depends on the PST". Some seem to come in at nearly 1.0A, others as good as 0.6A. I guess it is an artifact of the fact that the etalons are hand-mated, and some turn out better than others. That's probably why the specs are quoted as " <1.0A." although I notice that the Sky and Telescope article did reference them as between 1.0A and 0.6A. (I'm glad, because Coronado was trying to tell me the views I and a number of others had through my PST was physically impossible. )

I owned a Maxscope 40 and bought a PST so that I could stack the two. Repeated side by side tests revealed that the PST was yielding *better* resolution on both the proms and surface features than the Maxscope, and the stacked unit was so minimally improved in resolution that it wasn't worth the dimmer image. Since the stacked resolution is <0.6A. that implies I was basically at that point before stacking.

On the other hand, one of the observers who agreed that my PST was yielding ~0.6A resolution has looked through several dozen PST's in total, and reported that some are barely sub-Angstrom.

Sounds like it's luck of the draw whether you get a serviceable unit or an absolute gem.

Jim

After a couple of months of intensive work, I am basically finished with what I hope will be my 'ultimate' or lifelong telescope: a 16" f/4.5 built around a notionally 1/20th wavefront Obsidian Optics primary with Galaxy Optics' enhanced coatings, and a certified 1/40th wave Antares secondary.

I started with a Baltic Birch structure and 18 point cell/tailgate by James Grigar of Astro Sky Designs, which I customized considerably with, among other things:
--a Clement focuser,
--Protostar spider and heated secondary holder
--Flash 4 Sky Commander with 8000 tic encoders
--powered ground board
--metal mirror sling, with teflon on the sling and mirror edge to avoid lateral friction/astigmatism (the coefficient of friction for teflon-on-teflon is virtually zero)
--built-in accessory table
--locks to allow secure transport of the assembled scope
--SEVEN variable speed ultra low vibration fans for active cooling

The Clement focuser was an integral element of the design, since it allows me for the first time to use my favorite planetary filter without Barlowing. (It takes 1.5" of in-travel beyond my next closest eyepiece, the Nagler 31.)

Here is the URL for the web site I'm building.

http://users.adelphia.net/~jbrspace/

Follow the link to "Natasha" if you want more details about this scope. Comments are welcome, either substantively or about the site itself (it's my first foray into web publishing, so be gentle.)
I'll be adding a quad geared Dob Driver 2 once I decide whether to go with the classic belt drive, rim drive, or new "clutchless" rim drive on the azimuth axis. (I already plan to go with the clutchless setup on the azimuth axis.) Does anyone have any first hand experience with the clutchless drive?

Jim

The human body at its most quiescent (basal metabolism) puts out as much heat as a 75 watt light bulb; with a moderate amount of walking around, you can double this. Unless you are fortunate enough to have a breeze that dissipates this heat away from your optical tube, you may in fact be adding considerably to the thermal distortion of your Dobsonian's optics! With my old standby solid tube Dob, I had nearly 3/4" of Sonotube and similar insulating materials between my body and the optical path. With a truss tube scope, you have only a Lycra shroud-- at best! (Lots of folks seem to like to 'go naked' at dark sky sites to better ventilate their mirrors.)

Point your telescope at a bright star or planet and defocus the optics until the disc fills the whole field of view. Even putting your hand on the edge of the upper cage will cause dramatic currents to form! Now imagine how much more heat your head and upper torso give off... and how, if the winds are blowing from your back, they can be carrying this plume right across the optical path! Kinda' defeats the purpose of using fans to cool your mirrors and making certain that your collimation is dead on...

I would expect this to be especially pernicious in planetary viewing, and as a fairly regular Mars observer, I will be testing various simple thermal solutions out between now and opposition or longer. The first one I put in when I went to my favorite dark sky site was to take an old closed cell foam backpackers' sleeping pad and wrap it around the upper two-thirds of the OTA, securing it with bungie cords. These pads are lightweight, cheap, and thermally insulating enough that you can sleep atop one laid directly on snow and not melt it! I plan to make short segments of this same material to put on the upper cage-- since the light baffle is probably little better at thermal insulation than the Lycra shroud, and may experiment with making an extension to protrude above the upper cage assembly by a foot or so on the side facing the observer to deflect the rising thermal plume.

Has anyone else tried options like this to minimize thermal intereference with optical performance, or are we all collectively fixated on dealing with heat "at the mirror's edge"?



Jim

This is an excerpt from some experimentation I am documenting on the "new posts" section of my ATM home page
http://users.adelphia.net/~jbrspace