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Three weeks after ordering through the UK distributor my 7" [180mm] f/12 R35 iStar objective lens has arrived. At first it seemed to be made of lead encased in cast iron. It's remarkable weight totally eclipses my 6" f/8 Synta/Celestron! After several attempts with different scales I made it 4.9kg or 10lbs exactly. The latest spec 7" f/8 is claimed to weigh 4.65kg. I had allowed 5kg for the objective in my balancing experiments so that was close enough.The term "yard cannon" takes on a whole new meaning at this scale! 10" diameter dewshield, 8" main tube x over 7' long overall.
First light was with a bare 8"x 6'6" tube about 7" too long for focusing with an eyepiece in the Vixen focuser. I had to resort to holding eyepieces in my fingers at the end of the open main tube. Not having any useful tube rings I had to prop the OTA on my stepladders to observe the very low, gibbous moon. Once it cleared the 10' high, eastern hedge the Moon was perfectly placed for such a humble, altazimuth mounting! You couldn't ask for better! Except, perhaps, a set of functioning tube rings and a mounting on which to hang the strangely massive OTA.
Despite the shiny [unpainted zink] of the inside of the main tube the moon looked fine at low powers. I worked my way up from 32mm to a 20mm Meade 4000 Plossl for about 105x before I completely ran out of focus in the depths of the tube. The Moon was nicely white with just a hint of green or magenta on the limb depending whether I was inside or outside of focus. At focus there seemed to be no fringing at all. Though it is far too early to judge anything else at this stage I was impressed with the lack of false colour and the sharpness of the lunar features. There was not the slightest wish for a minus purple filter so typical of my using my 6" f/8. I routinely move the Baader "Fringe Killer" filter from one eyepiece to the next on the f/8. It would have made far more sense to fit a 2" filter to the star diagonal to save all the palaver.
Most people use a laser collimator but here I was holding eyepieces in my hand like a complete amateur. I lasted a couple of hours before the Moon was no longer at a comfortable angle for stepladder mountings. I also missed the comfort of my 2" dielectric star diagonal!
The first job this morning was to shorten the main tube. By holding up a scrap of paper to focus the moon last night I found the focal plane seemed to be right on the end of the focuser when fully racked in. Wrapping the main tube with a length of lining paper ensured the circumferential pencil line was perfectly square. I decided to cut off 8" when 6" or 7" would probably have done, leaving a tube length of 70". The hacksaw wanted to jam as I progressed around the tube resting on two folding B&D workbenches. The benches have been superb for working safely on the OTA with the jaws wide open. The thought of working on a flat bench with the terrifying risk of the tube [and objective lens] rolling right off onto a concrete floor does not bear thinking about!
Once shortened, I now needed a focuser extension tube to reach focus with an eyepiece when fully racked out! Easily achieved, as I have several 2" extenders in my collection which came free with secondhand focusers purchased online.
BTW: The galvanized, steel tube is very stiff but only 0.6mm thick and comes from Lindab's LRTR complete range of industrial extractor tubing in 2m lengths. Available in sizes from 4" up to at least 12" from memory. This is not the more common spiral tubing but has a neat, flattened, longitudinal seam. The stiffness is probably on a par with 3mm aluminium but marginally lighter. Sawing the end off removed the small flange which I had been hoping to keep for greater radial stiffness. However, once the focuser back plate was pressed firmly into the main tube everything was perfectly stiff again. I am now wondering whether I can find a donor saucepan which will fit snugly over the end of the tube instead of inside it. This would make a much neater job and hide the raw end of the tube. The difficulty is in measuring the inside diameter of the bottom 1" of tapered saucepans, often with decorative spun rings.
Once shortened, I now needed a focuser extension tube to reach focus with an eyepiece when fully racked out! Easily achieved, as I have several 2" extenders in my collection which came free with secondhand focusers purchased online.
BTW: The galvanized, steel tube is very stiff but only 0.6mm thick and comes from Lindab's LRTR complete range of industrial extractor tubing in 2m lengths. Available in sizes from 4" up to at least 12" from memory. This is not the more common spiral tubing but has a neat, flattened, longitudinal seam. The stiffness is probably on a par with 3mm aluminium but marginally lighter. Sawing the end off removed the small flange which I had been hoping to keep for greater radial stiffness. However, once the focuser back plate was pressed firmly into the main tube everything was perfectly stiff again. I am now wondering whether I can find a donor saucepan which will fit snugly over the end of the tube instead of inside it. This would make a much neater job and hide the raw end of the tube. The difficulty is in measuring the inside diameter of the bottom 1" of tapered saucepans, often with decorative spun rings.
I had bought some matt black, blackboard paint on my shopping trip this morning. So I took advantage of today's 60F warmth to paint the inside of the main tube. A long handled, cranked paint brush made short work of the tube paint job as I was easily able to reach the middle from each end. Never pass up a dirt cheap discount offer of paintbrushes at the supermarket.I also painted the inside of the stumpy dewshield, the baffles and the inside the focuser back plate. One generous coat of T+ was easily sufficient for a nicely black, non-reflective finish. The baffles will do the rest in killing contrast-stealing stray reflections. A perfectionists would probably give everything a second coat to hide the few remaining brush strokes but it will never be seen except by me. I have heard it suggested that adding flour or powdered grout to blackboard paint improves it even more.
The galvanized exterior of the steel, main tube is surprisingly heat absorbent in even the weakest sunshine. Not an ideal material for a dome one would have thought! Though domed silo tops are occasionally converted for observatory use aluminium is much cooler in sunshine. Once I had finished painting I turned the tube at right angles. I was hoping to soak up the late September, afternoon sunshine to speed the paint drying. Schrinlings T+ blackboard paint is water based and very safe even for children. The far more expensive alternatives at the DIY store contained Zylene. Which had me rather worried what the solvent off-gassing might do to my objective lens or its coatings over an extended period.
The T+ paint went a very long way and had a slight jelly-like consistency which completely avoided the usual drips. The perfect thickness and low odour made it very pleasant indeed to work with. Cleaning my hands and the brushes in water afterwards was completely effortless. So different from the stench of turps and scrubbing to get one's hands clean with the usual oil-based paints, followed by cracked cuticles. The T+ paint dries to a matt finish remarkably quickly [even at a mere 15C, 60F] and is claimed to be waterproof after 48 hours. I may leave re-assembly of the OTA until tomorrow to be sure the paint has fully hardened and released whatever drying agents are involved. I also need to consider how shortening the main tube affects the baffle assembly. The supporting threaded rods are now far too long and moving the nuts along too time consuming to do it twice. I still have my full size drawing to ensure the baffles are arranged correctly.
I used my full size drawing to confirm the placement of the baffles in the shortened tube and cut off the excess of threaded rods. After refitting the objective I double checked the focal plane on the sun. A couple of sunspots suddenly popped into sharp relief. The blackening of the OTA innards and baffles has greatly improved the contrast on distant trees. No sign of the purple wash, which plagues my 6" Synta f/8 on terrestrial subjects, which is very pleasing.
The T+ paint went a very long way and had a slight jelly-like consistency which completely avoided the usual drips. The perfect thickness and low odour made it very pleasant indeed to work with. Cleaning my hands and the brushes in water afterwards was completely effortless. So different from the stench of turps and scrubbing to get one's hands clean with the usual oil-based paints, followed by cracked cuticles. The T+ paint dries to a matt finish remarkably quickly [even at a mere 15C, 60F] and is claimed to be waterproof after 48 hours. I may leave re-assembly of the OTA until tomorrow to be sure the paint has fully hardened and released whatever drying agents are involved. I also need to consider how shortening the main tube affects the baffle assembly. The supporting threaded rods are now far too long and moving the nuts along too time consuming to do it twice. I still have my full size drawing to ensure the baffles are arranged correctly.
I used my full size drawing to confirm the placement of the baffles in the shortened tube and cut off the excess of threaded rods. After refitting the objective I double checked the focal plane on the sun. A couple of sunspots suddenly popped into sharp relief. The blackening of the OTA innards and baffles has greatly improved the contrast on distant trees. No sign of the purple wash, which plagues my 6" Synta f/8 on terrestrial subjects, which is very pleasing.
Click on any image for an enlargement.
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