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Building a Holcomb Reflector


Intro - How I had secret crazy cravings for a giant refractor, but could not afford it.

I sometimes wonder if telescope making isn't a kind of madness. A decent sized one can consume months or years of spare time. Your non-astronomy friends will look at you with wide uncomprehending eyes. Your astronomy friends will question the rationality of your design and decisions. The house gets taken over by an ever expanding monstrosity and promises are made that this is definitely the "last one".

So this is my story about building a long-focus newt on a mounting that hasn't been seen much since the 19th Century

It began when a friend kindly gave me his old home-made 8" f/8 newt. it was apparent it needed an optical overhaul, and if I was going to do that, then I needed to practice polishing and figuring a mirror.

I had an old 8" mirror blank to practice with, but then it needed to be ground first. And that started to sounded like making a new telescope. Hmmm! :-) Attached File ES.jpg 34.05KB 43 downloads

After procrastinating, I decided that:

Even though this was a practice piece, I didn't want to waste time making something that was horrible to look through.

  • It had to be on a budget.
  • I've only hand-tools. Plus, I'm a bit cack-handed.
  • I'm mainly a planetary and lunar observer with restricted access to the sky.
  • It had to live outside. Big telescopes and I usually end up with squashed fingers or cracked elbows.

I came across the idea of a the long focus newt. The more I read about them, the more I was intrigued.

Planning - or Why you shouldn't make a newtonion reflector

The biggest help for me was the creation of an Excel spreadsheet. It allowed me to create a virtual model of the telescope's geometry

One of the criticisms of reflecting telescopes when used on the planets, is the size of secondary obstruction on performance. The rule of thumb seems to be that the diameter of the secondary should no be larger than 16% - 20% of the primary mirror.

The role of the secondary is to fold the lightpath. Most EPs will work with an image plane that approximates the diameter of the field lens, and it's relatively simple process to calculate the theoretical diagonal size.

Using Excel I was able to create a table showing diagonal sizes for various mirror diameters and f-ratios. Making some modest assumptions about focal plane dia, back-focus, etc: Attached File exceltable1.jpg 67.88KB 28 downloads

What the tables show is that you can have a "planetary" telescope beginning at about 10 inches in aperture, provided you start from traditionally modest focal ratios. A smaller image plane prvides even more wriggle room.

Of course, for my 8" blank to meet the criteria, I was going to have to go long. But go long enough and you can figure the primary spherical. This simplifies testing and manufacture. The envelope of tolerance for errors is less stringent that in a shorter focus instrument. I felt that my chances of producing a decent mirror that would behave like a proper telescope were much higher.

So it seems that the magic of a long focus newt is a combination of optics that have a more generous tolerance for errors because of the long focal ratio, and that a long focal ratio lends itself smaller diagonals. But a "planetary" newt is not necessarily a long focus one.

Having finally made the decision, I got straight onto making the mirror. Attached File grinding1a.jpg 29.34KB 22 downloads

The Mirror - Shortcuts make long delays

Polishing and grinding was fairly standard. Complicated by a chip on the front face of the mirror that took time to grind out. Time was spent on doing the full number of laps and checking the sagitta carefully. Going thru the fine grinding was routine, with attention being paid to doing the full number of wets (And maybe a couple extra for luck!).

Polishing was also straightforward. And not! :-) polishing out the pits went by the book. By the time I had a fairly even polish, cursory testing revealed that the mirror was a shallow bowl shape, but in my efforts to deal with this, things rapidly turned into an oblate sphere with a rolled edge.

There were much shenanigans with a sub-diameter lap which out of shame will not be mentioned, but eventually the surface was wrangled back from something resembling a mud pie, to something resembling a sphere with a narrow turned edge.

One of the things that books on mirror making don't really emphasise is how much muscle you can put into this. Because of my injured elbow, I found applying the necessary pressure a bit tough, and had much success with a weight stack. I also found Gordon Waite's videos useful for seeing polishing done "live".

The end result is a mirror that I feel is satisfactory for now (However I have discovered that like home renovations, telescope mirrors are never quite finished... )

The tube - You too can turn junk into gold.

Now the hoary dilemma of making actual telescope arrived! How was I going to make a solid non-bendy tube for this beastie. And what about a mounting for it? I knew from my refractor that long telescopes were prone to picking up pendulous oscillations from the gentlest of summer breezes! This baby needed to be rock solid. And possibly made from rocks too!

The tube had me stumped for a little while. I would have quite happily used uPVC, but culvert was incredibly expensive. A wood tube could be made, but again, the cost was just as expensive. Eventually, from an online auction, I snaffled a piece of uPVC the right diameter at a bargain price, but too short. I decided that I would create a split-tube held with struts.

My favourite handtool in this endeavour was a brace and bit. It was much better than a drill in chewing out thirty two 20mm holes in the pillow blocks that would hold the aluminium struts

This designed turned out to have plenty of rigidity. It may even be over-engineered.

There are a whole host of other details to mention. The secondary is based on an idea I gleaned from R F Royce. Forget fiddly counterscrews - make your secondaries his way - http://www.rfroyce.c...cv_8/holder.htm

In my design, the threaded rod is held in place by two nuts sandwiched between two brass plates. This can be made with handtools and requires no special dexterity to make. One of the wonderful advantages of this is that if you want to use a wider diagonal, just unscrew the old one and thread on the new one. Simple and easy. Attached File spiderHuba.jpg 24.42KB 9 downloads

The mirror cell is a sandwich construction made from 3mm aluminium treadplate. Instead of push-pull screws, the cell sits on a central pivot and has 3 pull-screws as featured in an article I found on Gary Seronik's website.

The helical focuser is nothing more than a shower waste with the grid cut out. I was going to have a push-pull tube inside for rough focusing, but didn't need it in the end. Simple, cheap and it works.

The Mounting - Your neighbors will always see it as a giant ray-gun

In the very early stages, I truly, seriously toyed with the idea of creating a modern version of one of Herschel's smaller reflectors. A non-conventional telescope requires non-conventional thinking. Well, it certainly allows you to indulge in it! http://en.wikipedia....elTelescope.jpg

Like a small cannon in a carriage, his telescopes were bodily turned to get azimuth and then ropes and pulleys provide the altitude and fine azimuth control. Although it looks ridiculous (Ropes! Pulleys?!), it seemed obvious to me that this would have been an incredibly stable and solid mounting. Sadly though, it needed a flat, level surface like a terrace or patio to really work, and I did not relish the thought of smashing up and flattening out the backyard for one telescope!

In my internet travels I came across a website by Mel Bartels, that talked about an American instrument maker, Amasa Holcombe. Holcomb's clever solution to mounting the long reflectors of his day was to make the tube of the telescope part of the mounting. The nose is held up and guided about the sky by the means of extending arms. These work with nothing more than rope and a disc brake. Here is my interpretation on this:

The legs consist of a telescoping inner and outer section. A cord, is tied to the top of the inner section and runs down the length of the leg. It's wrapped around a pulley which is fastened to the bottom of the outer section. The cord them doubles back up the leg until it is wrapped around a bobbin attached to the top of the outer section. The bobbin is attached to a large disk. This disk is clamped by an adjustable brake-pad. Attached File holcomb1.jpg 22.69KB 29 downloads

When set up, gravity keeps the leg compacted - the telescope is effectively suspended on the loop of cord sitting in the pulley. Winding up the bobbin shortens the length of this cord loop, effectively pulling the telescope up. Friction by the brake-pad keeps the mounting from collapsing back down.

The legs are attached to a yoke. This connects to the telescope though a universal joint. This is not complicated, and is made from wood and brass. Attached File universalJoint.jpg 26.9KB 20 downloads

The brake discs are made from brass sheet. These are attached to the bobbin by woodscrews. The brake pads are just offcuts of brass bar (One is aluminium) with self-adhesive furniture slides used for the brake pads. Anything could work here. A bolt with a wingnut adjust the tension of the brakes. Attached File holcomb2.jpg 32.51KB 21 downloads

This is the tail-end of the telescope. In an early prototype I made a mini alt-az base that the base of the scope rested in and could pivot around. Attached File base1a.jpg 19.02KB 17 downloads

Eventually I realised that I could simplify this considerable by making a wheeled tail-piece. This meant I could pick up the nose of the scope and "wheelbarrow" it around my garden if I needed to move it. This wheeled bases does need to be anchored in place, otherwise it will slide away on you. Attached File base2a.jpg 14KB 18 downloads

This weird and unlikely idea works as well as any other mounting that I've tried. Better than some! But lets not get romantic about it. Some of it's flaws are:

  • The tail of the telescope needs to be restrained or it will slip away. In my case it is tethered to a stake, but it could abut against a wall, rock, have a handbrake, etc.
  • Keep the base of the triangle "wide" or the telescope will topple. You may also want to have some means to prevent the legs from splaying out suddenly (I dig mine into the ground). This means the zenith is inaccessible.
  • Getting a ladder underneath can be a little tricky - but can probably be improved by careful placement of where the legs attach to the tube.
  • It's not a mounting for sweeping or sight-seeing. (But then, this isn't that kind of telescope either). It forces you to think about what you want to see and plan accordingly, so maybe not such a bad thing!
  • I haven't got the balance between the tension required on disk-brake and the force required to turn the bobbin right. This is an area that could be improved.

But for this I get an elegant self-supporting mounting that has built in fine guiding and vibration dampening time of less than a second. Setup for observing only takes the time to remove the tarp covering the scope. There is little risk of oscillation or vibration as the mounting has parts that only slide, not rotate. Gravity is working as our friend here. In fact, the universal joint on my scope has a bit of play in it, yet the scope "settles" and works fine.

To be honest, my telescope is probably at the hefty-end of Holcomb's design. I believe that for shorter telescopes (or lighter) this mounting would be absolutely perfect. In fact, I'm going to use this when I refurbish my friends 8" f/8. Although finished for now, I see this as a prototype instrument that will continue to be worked on and refined.

The verdict - After all this effort it'd better work!

This telescope was finished late Nov 2014. How does it stack up?

The Holcomb mount completely surprised me with how solid and stable it was, given the lightness of it's construction. It seems to soak up reverberation. Positioning a stepladder is probably the only awkward bit. This is the first telescope that I've had where I can actually hang onto it and not have the image jump about in response. Locating an object does require some care. After initially walking the front legs to the correct azimuth, it's a simple matter to winch up to the correct altitude. A large, accurately aligned finderscope with a w i d e FOV is indepsensible. I bodged mine from plumbing parts and a cheap imported 70mm Chinese spotter I bought online - it works brilliantly.

I feel that perhaps the uPVC tube is too heavy. If money was no object I might try this with a complete skeleton tube from aluminium.

I tested the function of the mounting on the Moon, as my mirror is unsilvered at the moment. Tracking at over 300x is smooth with almost zero backlash, wiggling, etc. The type of cordage you use on the legs is important here. Woven polyester rope may have too much stretch in it, meaning things may "bounce" slightly, so go for twisted cord. The process of tracking, although not as intuitive as bumping and nudging a dob, is easy to pick up. If you overshoot, you wind back the relevant bobbin and things smoothly come back into view.

If you like to scan about, do a bit of sightseeing, etc, then this mounting will annoy you as being too cumbersome. But for deliberate study of a planet or the Moon, this mounting works surprisingly well.

At the end of the night, I leave the telescope standing up, covered. But if I had a shorter, lighter tube, it would be an easy matter to lower the scope to the ground, strap the legs against the tube, and wheelbarrow it to shelter.

How about optically? I am pleased enough with the result. The mirror is not perfect by any means, yet I am thrilled with the images it's plain-glass surface (I'm hoping to get the mirror silvered soon by a fellow CloudyNighter) has provided me. I'm sure star-testing and a more critical eye will reveal some sobering truths - but then that's just an excuse to warm up the pitch lap: who can resist! :-)

Even though it's the sort of child only a parent could love, I'm quite chuffed with the whole outcome and importantly, it's given me the confidence to tackle the refurb of my friends old newt.