7" f/12 iStar refractor 19: Dewshield, finish & power.

The last remains of the self-rolling plastic, which I had been intending to use for a lightweight dewshield, were woefully insufficient. I had "wasted" it on an over-sized dewshield for my 6" f/8 Celestron and no more was available.

Searching online suggested the coiled plastic sheet was called "Easy Bagger." The clever idea being to reverse the plastic roll. So that it holds a bin bag wide open for easy stuffing with soft materials. [Autumn leaves, shrubbery or grass clippings, etc.] Reversing the roll allows it to contract back to its tightly coiled 3-4" diameter for storage.

The leaf bagging aid only seems to be commercially available in the US after endless searching online. I had bought my original roll from a Danish supermarket for a fraction of what the US dealers were asking on Amazon. With postage and taxes added, ordering even a single unit from the US would be prohibitively expensive! I shall just have to use a roll of lightweight camping mattress as a dewshield until something else turns up. There is always cardboard tube, of course, though it would need protection from moisture.

A Danish gardening shop had a different form of bag filling aid called "Pack-Bag." The plastic was thicker and almost flat without the desirable auto-coiling but was still worth trying. It was also only available in dark-ish, matt green though other colours are listed on the large, sticky package label. [Which fortunately peeled off quite nicely] Dark-ish green is fine for its raw material value as it will hardly be noticed in the dark. The lightly embossed, matt surface finish kills a lot of reflection even without a coat of matt black paint.

The glue I used on the plywood rings is supposed to be able to fix plastic so I might give that a try. For the moment I have the plastic rolled tightly in a pipe to see if it takes on a natural curve over a reasonable period of time. Less spring and more curl would be highly desirable. The forecast is wet for a few days so the dewshield is not urgently needed. I want the dewshield to slip on and stay put without the ugly clamping or tying usually required for foam. Failing that I have enough aluminium sheet left from the baffle material for a slip-on dewshield. I just need to decide on a neat way of cutting it and jointing it into a smooth cylinder. My straight bladed shears leave a jagged edge no matter how carefully I cut. I don't have access to a large guillotine nor a roller.

In the end I settled for an aluminium dewshield made from the remaining half of the roof valley flashing. Cutting a straight line involved a steel straight edge and a craft knife. I kept scoring the line until I was able to fold along the line using broad-jawed pliers. Reversing the bend on the line cracked off the unwanted strip leaving a nice clean line. I then spent an hour slowly bending the dewshield material around a 6" PVC pipe to avoid kinks. This went well and my patience was eventually rewarded with a smooth curve.

I then wrapped the new dewshield around the existing stubby one on the OTA. Applying a tourniquet of strong cord tightened the two together until I was able to drill and pop-rivet the overlap. I left the original doubled edge on the flashing. So that I had a nice firm edge to rest the OTA on its nose. Though it needs even taller storage now and just clears the ceiling between the joists of my shed.

The difference in scale: The Vixen 90mm dewshield is for a  refractor of exactly half the aperture of the 180mm dewshield on the right.
 
There was just sufficient material in half of the original flashing material to make a 2.2 x aperture dewshield length. I think it looks infinitely better than the stumpy one. A pause in the rain, if not the gales, allowed quick snaps of the new dewshield. It will eventually be keyed with a sander and painted to match the rest of the telescope. It is 25cm in diameter x 39cm long. [10" x 15.3"]

I have decided to paint the entire telescope, pier and mounting mid-to-dark grey in imitation of the classical instruments of the past. It adds a timeless, serious and solid quality to the appearance while hiding a multitude of sins. [Such as unconventional or mixed materials.] The Fullerscopes MkIV mounting really needs repainting as does the pier. The telescope itself is still "raw" from construction and needs a unifying, preferably non-modern looking finish.

Historical Cooke 8" refractor from South Africa being sold at auction. Note the massive scale of the mounting and the sheer size of the wormwheels!
  
I am not fond of white for a telescope tube due to its cosmetic "fragility" over time. Darker colours are far more forgiving of regular handling in confined spaces in poor light or darkness. Though lighter coloured instruments are fine when fixed to the mounting in a permanent observatory. The galvanized finish on the 7" telescope main tube has persuaded me towards more traditional colours than my previous choice of Hammerite dark blue.

With a focal length of 2160mm [180mm x 12] the power soon rises with short focus eyepieces:
2160 / EpFl= Magnification] The following magnifications are available from my modest collection of [secondhand] Meade 4000 Plossls mm. Further powers are available by using a 2x Barlow lens as listed below. The main problem is remembering them all without resorting to a torch and a printed list. Though keeping a list in the eyepiece case is at least a regular reminder.

 Ep mm   Magnification  
   32      =     67.5x
   26      =     83x
   20      =   108x
   16      =   135x  [32mm x 2 Barlow]
   15      =   144x
   13      =   174x  [26mm x 2 Barlow]
     9.7   =   222x
     8      =   288x  [16mm x 2 Barlow]
     6.4   =   337x
     5      =   432x  [10mm x 2 Barlow]
     3.2   =   675x  [6.4mm x 2 Barlow]

Another cycle ride of 38 miles around the rural charity shops. Now I have three more candidates for an external fitting, focuser back plate/main tube end cap. I had been pressing a saucepan base inside the tube until now. While it worked perfectly it looked rather ugly and unfinished to my critical eye. Now I have three more, much heavier, aluminium saucepans to choose from. They each fit over the shortened end of the main tube perfectly. Each has its own particular shape but I prefer the one with the sharpest curve at the base and parallel sides. It looks as if it were specially made for the job rather than added as an obvious afterthought from a cut down cooking pot. I make absolutely no apologies for my choice of donor materials to save expense. The extra weight will further aid the balance of the OTA. Or at least help to balance the longer dewshield.

Click on any image for an enlargement.

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