Optical changes
Mirror/Optical system changes:
· R Replaced terrible default mirror cell mounting system w/a steel spar system. Default setup (which is the most negligent part of default design IMO—even worse than the cheap 2” focuser!) requires moving the face of the primary mirror within fractions of an inch of three steel screws and bolt posts, no two of which could be seen at any one time! Fraught w/ disaster, and my second hand scope already had a few scratches on outer edge of mirror from it, and I think I put another one on it the one and only time I tried to use it with the default mounts). I moved the mounting posts and screws to the outside of tube, so that there was no metal near face of mirror. This made the mounting posts new position larger than the diameter of the particleboard disk, though, and necessitated using “spars” that I attached radially. Was tricky to line them up, but fortunately this only had to be done once.
Downside of my spar system was that they protruded too far out for the box, and hence I had to “spread” the rear end of base by ½” inch. That meant that box now marginally too wide in spacing for the trunions and added a “lip” to inside of one to give trunion a ledge to rest on. (Law of unintended consequences)
Drilled out secondary assembly to allow direct access to alignment screws
Tied the secondary alignment wrench to a cord that gets looped around my wrist before I adjust position of secondary mirror. Precludes dropping on primary. Did the same thing w/my red and white light flashlights that I use for collimation. (I find it easier to check precise location of laser dot/secondary alignment w/white light; otherwise use red)
Replaced the 2” plastic focuser w/JMI DX-1 and extension tube. Didn’t know about the Featherlights at the time… could have gotten one on Astromart for less than I paid for a new JMI from Adorama, but they appear aperiodically. Don’t find myself using the finer speed—probably could have gotten by w/a lesser focuser (DX2. Have switched from JMI’s extension tube to others (2”, 3”) with brass non-marring compression rings. Since my Paracorr has a short enough focus to need to be in the JMI without an extension tube, I have replaced the two steel retaining screws with one brass and one plastic compute case screw—I tend to tension only the plastic one, to minimize set screw markings on the Paracorr. (Too bad that the Paracorr itself doesn’t have a non-marring system for the eyepieces you put in it, but given the angle—a constantt 40* or above the horitizonal—there is little chance of one falling out and I can get away with minimal tension on a screw)
Fan behind mirror. Started w/80mm computer fan, intended to go to a spare high flow (100 cfm) 120mm fan and got as far as drilling the extra vent holes in disc before reading S&T and becoming convinced that the critical cooling interface was the boundary layer in the inch above the mirror face rather than trying to cool the whole 40+ pounds of glass. Ran off of 12v default; hooked the computer power supply jack into disk.
Variable speed 120mm fan mounted to side of tube at level of mirror face. Originally I put four 1 ¼” ID 90* shunts on as air intakes—the element missing from any of the published articles or Internet discussions of this type of venting is impact of ambient light being introduced at mirror face—the worst possible place! (Ideally I’d like to say that my scope gets used exclusively from dark country skies… as a practical matter, though, it sees 95% of its use from my streetlight-polluted suburban sidewalk—which is another way of saying I get a lot more use out of the ‘scope than I had initially envisioned. Light is enough of a killer without creating extra paths into the light path!). This fan is variable speed, generating 67-99 cfm. Both fans are isolated by sorbethane/silicone caulk from OTA. I’m agnostic about the merits/need for exhaust holes (which I would shunt also)—may add if see the need. UPDATE: discovered that the fan mounted on the inside of the tube did intrude marginally into the light path, and tested airflow and found it inadequate. (Simple geometric calculation suggests that the four shunts were limiting the fan to at most 20 percent of its nominal airflow.) Scrapped the shunts, enlarged their holes to a single hole 5” dia, mounted the fan on the outside of the tube, and covered it on in a plywood box open on the bottom (a super shunt). Fan now delivers its full airflow with no intrusion into light path; the box appears sturdier than the PVC shunts (one of which I had knocked off when wedging the scope into my car.). Added a second potentiometer in-line for fan, since rarely even need the 67 cfm setting. Usually move about half that much air
Put temperature probe from an indoor-outdoor thermometer accurate to 0.2* F on side of mirror face farthest away from fan, and insulated it to maximize likelihood it measures temp. of glass near mirror face than the air. Persuaded by the 12/2001 Sky and Tel article on the disruptive effects of as little as 2* F temp difference. This lets me check the temp diff. And see whether to turn on:
ded added Jim Fly’s “Catseye” collimation system—sweet! Also have an “EZ Collimator” that I used for collimation before getting the Catseye (which is even more accurate—see Mels Bartel’s Newtonian discussion for the impact of having your laser spot even 1-2mm off the mirror’s actual center during collimation on a Dob of this size and focal length.). Now use it to check secondary mirror collimation before set up. (Holds well—rarely need to tweak).
I opted to keep the stock spider and secondary mirror holder when I was advised by Astrosystems that their unit probably wouldn’t work, since Meade mounts its spider vanes so close to the front rim of the OTA. I haven’t really had any complaint about any excessive damping time from the thin spider vanes. Although I liked the idea of “tool free” secondary collimation adjustment, the fact is now that I have the scope ‘dialed in’ I rarely have to make any secondary mirror adjustments, and when I do I got a nylon cord affixed to the allan wrench and the other end looped around my wrist (hence no danger of dropping it onto the primary.) UPDATE: eventually put in an Antares Optics 3.1" secondary with enhanced coatings and interferometry-backed 1/15 surface wave, 1/100th RMS quality. Also switched over to a "tool-free" (thumbscrew-based) Astrosystems spider and secondary. In order to do this, I extended the mounting points for the spider against the OTA by nearly 3" above the rim of the tube by adding angle iron "spars" that protruded above the page. (see the images on the main Frankenscope page for details.)
UUP
Used vinyl report folders to make a shroud extending 10” beyond front of scope. Use velcro to secure it. (Velcro mesh tended to pull off/stick to ‘hooked’ in cold, so reinforced adhesive on scope rim w/staples). Since black paint tended to flake off the vinyl over time, I put contact paper on the folders—also rendered them a bit stiffer.
Used same vinyl folders as source of shroud that can cover the light gap where mirror cell bolts onto OTA. These two changes make immense difference—when observing on the sidewalk, a car can drive down the street five feet way from me and I don’t notice any change in the darkness of the FOV if the scope is pointed reasonably high in the sky. Before this fix, traffic generated temporary “white out” and killed my night vision. Also made a "streetlight killer" backboard block I can install to block the light.