Thursday, May 6, 2010

My 8" F/4 travel scope: Overview and mirror testing

Here is my home-made travel scope, which I made for airline travel. It's about 16.5 pounds in total. The mirror box, after removal of the altitude bearings, fits in a backpack (with a cardboard and plastic cover to hold it in place) while all the other parts can be tossed in a suitcase (ideally mostly in Ziploc-style bags). The secondary mirror cell bolts to the inside of the primary mirror box, in a corner, for transportation. Assembly requires one Philips screwdriver, and the rest is one home-made knob and a bunch of three-lobe knobs going on carriage bolts. Originally, I wanted it all to be no-tool assembly, but using Philips screws saved some weight and space on the focuser board and on the strut joint.

This is a low-cost 8" F/4, made from a mirror that had a crack in it on one side, below the front surface, so I got both the primary and the secondary for about $40. Moreover, a large chunk of the primary was duct-taped over, presumably to cover up the area distorted by the crack. I ended up testing the mirror by using an LED flashlight covered with aluminum foil with a pinhole, holding the flashlight at a little more than twice the focal length, while putting my eye at about twice the focal length. When the two distances are just right, I could see a big blob reflected on the mirror--I think that was the image of my pupil illuminated by the pinhole light. As I moved my head around, the blob moved. Moreover, as the blob got closer to the area with the crack, the blob distorted in weird ways. Monitoring where the blob was distorted and where not showed me where the figure of the mirror was distorted, and I masked off the area--quite a large area--with black felt. And, yes, it works just fine with the area masked off.

The mirror box is made of some hardwood that I had found lying around and used a friend's table saw, but it could have been done with a jigsaw if one had a good blade. Besides some simple hand tools, I used a power drill (with a hole saws and a 1.25" spade bit), a Harbor Freight jig saw that was on sale, and one elongated hole got cut with a friend's small router, though it could have been done with a rattail file, too.

The next couple of posts will highlight various parts of the design. Please feel free to ask questions, e.g., in comments. The design is very simple and I made it with very minimal woodworking skills and tools.

In the interests of full disclosure, I admit that I am including some Amazon Associate links in these posts, but only for products that I actually bought from Amazon (sometimes the link may point to a different quantity--I don't keep that good track).


  1. Hello, where I can buy the mirror.

    Thanks, good job.


  2. Look on's classifieds. Two users who often have mirrors, and with whom I've dealt, are MSO and MASILMW. PM them and tell them I sent you.

  3. I asked Hubble Optics, and here's what they said: "We will make some 8" f/4 in about month, and it will be priced at $260 + S&H. We will list some on ebay too." Their web page is and their ebay store is

    As far as I can tell, they've got a solid reputation in telescope making circles.

  4. By the way, there are more detailed build instructions here.