• My series on making a Newtonian telescope
  • How Leon Foucault Made Telescopes

Guy's Math & Astro Blog

Guy's Math & Astro Blog

Monthly Archives: January 2018

A 6″ Dob for Young Relatives

31 Wednesday Jan 2018

Posted by gfbrandenburg in astronomy, Math, Optics, Telescope Making, Uncategorized

≈ Leave a comment

I just finished a 6″ f/8 Dobsonian telescope as a gift for my great-nephews, one of whom I discovered is VERY interested in astronomy and happens to live in a place with pretty dark skies – about the middle of Maryland’s Eastern Shore. The mirror is excellent, and the mechanical parts all work very well — in my opinion. Let’s see what the recipients think.

I finally succeeded in putting in the mirror yesterday afternoon in a very stiff and cold wind right outside my house, estimating where the mirror should go by aiming at a distant chimney. This takes patience because it’s trial-and-error, no matter how much you calculate beforehand!

Later that evening, after the nearly-full Blue Moon came over the trees at DC’s Chevy Chase Community Center where we have our telescope-making classes, fellow ATMer and all-around interesting person Jim Kaiser helped me collimate it by pointing the scope at the illuminated curtains in the windows of the CCCC. We then verified that the Moon actually did come to a focus with the eyepiece nearly all the way screwed in. If you want to focus on closer things than the Moon or galaxies, you need to screw the eyepiece out towards you, the observer in the cheap but effective helical focuser that was lying around the shop.

This scope incorporates a couple of innovations by me, and a bit of artistic whimsy.

First small innovation: I made the secondary diagonal mirror holder so that no tools are needed at all: you just rotate the part holding the elliptical mirror and turn a little thumbscrew to collimate it quickly and easily, while you watch. Here is a sketch of how I made it.new type of secondary holder

 

Second innovation can be seen near my right hand (to your left) atop the cradle: two 1/4″-20 machine screws with simple homemade knobs on top, going through threaded inserts (T-nuts would work too), which push against a piece of lumber in the shape of prism with an isosceles right triangle at each end. I call this the tube brake, which can be applied or released quite easily, whenever needed. Small springs (almost impossible to see in this photo) pull this brake up against the corner of the tube, while the machine screws press it down. If you want to change the position of the eyepiece because a taller or shorter person has arrived, no problem. A few CCW turns of the wooden knobs releases the brake, you rotate the tube to the desired position, and then you lock it down again with a few clockwise turns. If you add or remove a heavy eyepiece or a finder or whatever, same procedure, except this time you can slide the scope up and down inside the cradle.

The artistic whimsy is partly seen in a photo Jim took of me after we got it collimated but before we rushed back inside: lots of colors, thanks to several tons of paint cans salvaged by fellow ATMer Bill Rohrer from being thrown away by a third party who lost his warehouse lease, and also because smurf blue is the favorite color of one of the boys. The altitude bearing is made out of the Corian countertop that my wife and I got rid of a few months ago when we had our kitchen remodeled. (30 years ago we did it ourselves, mostly. This time we hired professionals. They are SOO much faster and better at this than us!)

So that my young relatives can keep this thing looking good, they also get four or five quart or pint cans of paint – the ones I used on the scope. Free, of course. The more we get rid of put to use, the better. They can repaint anything that gets scratched, you see?IMG_9416

You can also see some wood-cutting fun above and below. This retired geometry teacher had a lot of fun figuring out how to lay out and cut out stars with 5, 6, and 7 points, as well as a crescent moon and a representation of Saturn seen with its rings edge-on. I guess you could show Saturn’s rings a 30 to 45 degrees to the viewer, if you instead carved it out of solid wood or did wood burning, but I just had a hand-held jigsaw and a Dremel knockoff. And plus, this is supposed to be a scope that is USED rather than just admired for its artsy parts.

I designed what I wanted onto two sheets of paper and then taped them to the plywood. This worked, but it wasn’t the most wonderful plywood, so on many of the pull strokes, the wood splintered a bit. So that side got to face inside.. Painting all those little nooks and crannies was tough!

design artsy astro cutouts

(The purpose of the cut-outs was simply to make the telescope lighter. It’s got a very heavy and sturdy base. Each square inch of plywood removed saves about 7 grams. Also, more holes means more hand-holds!)

 

 

Under-corrected Commercial Mirrors

21 Sunday Jan 2018

Posted by gfbrandenburg in Uncategorized

≈ Leave a comment

I have tested three different Newtonian mirrors from 12″ to 16″ diameter over the past few years that were labeled as being by Meade. One had been refigured by someone else before I tested it.

Each of the mirrors initially looked nice and smooth, and the Ronchigrams looked pretty close to the theoretically-perfect image generated by RonWin or similar software.

However, when I did a zonal test, in every single case, the mirror turned out to be seriously under-corrected, in some cases by about 50%. In other words, if a perfect mirror should have the outer zone (say the outer inch) to “null out” at 0.236″ from the location where the central zone nulls out, then the mirrors I tested might null out at only 0.118″ instead.

I took care to repeat the measurements several times in each of these cases, and in one case the owner also took a set of readings; his and mine agreed pretty closely.

I don’t know if it’s my skills at reading Foucault/Couder shadow zones that are suspect, or if I’m correct. Anybody else have similar or opposite stories or experiences?

Make your own telescope!

18 Thursday Jan 2018

Posted by gfbrandenburg in astronomy, education, Optics, Safety, Telescope Making, Uncategorized

≈ Leave a comment

Tags

achromatic, doublet, Gaithersburg, lens, make your own telescope, PVC

This weekend, I’m hosting a small workshop at the Community Museum in Gaithersburg, MD, where interested persons from 8 to 88 years of age can make their own telescope in an hour or two. We will be using surplus but high-quality achromatic primary doublet lenses as well as inexpensive eyepieces, along with PVC tubing and some really cool tripods to hold it steady.  We will some basic optics experiments to help explain how these gizmos work, and will have spray paint and colored tape to decorate the tubes.

If you are interested, here is the necessary information:

=======

Gaithersburg Community Museum's photo.
JAN21

Make Your Own Telescope

Public

 · Hosted by Gaithersburg Community Museum
Interested
Invite Friends

Share
  • clock
    Sunday at 1 PM – 4 PM
    3 days from now · 36–50°Partly Cloudy
  • pin
    Show Map
    Gaithersburg Community Museum

    9 S Summit Ave, Gaithersburg, Maryland 20877
  • About
  • Discussion
3 Going · 65 Interested

Share this event with your friends
Share
Details

Make your own refracting telescope in just an afternoon! This workshop will be led by veteran DC amateur telescope-maker Guy Brandenburg. He will show you how to make a small functioning telescope that can either be held in your hands or mounted on almost any camera tripod. All of the materials needed will be provided, and no experience is necessary. This workshop is open to anyone from the ages of 9 through 99, but a parent would need to accompany any child from 9 – 11. You might get a little dirty, so don’t wear your best clothing!

You will also see how various types of telescopes such as reflectors, refractors, and catadioptrics are put together and operate, using actual examples, including the type made and used by Galileo around 1609.

$30 City of Gaithersburg residents/$35 non-residents. Space is limited to 15 so pre-registration is required. To register go to RecXpress at https://online.activenetwork.com/gaithersburg/Start/Start.asp. It’s activity #49690.

For more information or if you have trouble registering call the museum at 301-258-6160 or museum@gaithersburgmd.gov

 

Is This A Sketch of the Evolution of Galaxies Over Time?

15 Monday Jan 2018

Posted by gfbrandenburg in Uncategorized

≈ Leave a comment

The answer is, of course, no. Though they do bear some resemblance to the amazing astronomy graphics you come across these days at websites like Astronomy Picture of the Day, or graphs that astronomers show to their peers, these are merely wonderful doodles made by a fellow-volunteer at the registration desk at last week’s American Astronomical Society at the Gaylord convention center at the National Harbor in Oxon Hill, MD. Sorry, I have forgotten his name and I also didn’t get permission to post. (He was an undergrad or grad student but I forget where.) So I definitely need to pay him royalties if he ever tracks me down.

(He won’t get very rich though: even 12% or 37.5% or 99.99% of zero income from this blog is still zero…)

All of us volunteers and paid staff were very impressed by his doodles.

I’m sure that the reaction of a group of Freudian psychoanalysts would have been quite different.

Photos from Chabot

14 Sunday Jan 2018

Posted by gfbrandenburg in Uncategorized

≈ Leave a comment

IMG_9111

IMG_9099

IMG_9102IMG_9097

A bit of weight-saving astro whimsy on a scope for my Great-nephews

14 Sunday Jan 2018

Posted by gfbrandenburg in Uncategorized

≈ Leave a comment

I was inspired to try making some Astronomical cutouts in the side panels of a 6″ f/8 Dob I’m making for my Great-nephews. The tube is smurf blue, as requested. I’ve never tried cutting out decorative holes in plywood before, so don’t laugh too much.

A visit to Oakland’s Chabot Science and Amateur Telescope Making Class

10 Wednesday Jan 2018

Posted by gfbrandenburg in Uncategorized

≈ Leave a comment

Three highlights of my visit in late

December 2017:

🏵the weekly Friday-night telescope-mirror-Making workshop there …

🏵visiting some of their big, ancient refractors, one of which is the smaller cousin of the 26″ refractor at the USNO in DC …

🏵viewing some of their collection of old & modern donated telescopes, which they are endeavoring to put into service again and into the hands of interested observers …

I’ll post pix in a bit

Subscribe

  • Entries (RSS)
  • Comments (RSS)

Archives

  • November 2022
  • October 2022
  • August 2022
  • July 2022
  • June 2022
  • May 2022
  • April 2022
  • February 2022
  • January 2022
  • December 2021
  • October 2021
  • September 2021
  • August 2021
  • July 2021
  • June 2021
  • May 2021
  • March 2021
  • December 2020
  • October 2020
  • September 2020
  • August 2020
  • March 2020
  • February 2020
  • January 2020
  • December 2019
  • November 2019
  • September 2019
  • August 2019
  • June 2019
  • May 2019
  • January 2019
  • November 2018
  • October 2018
  • September 2018
  • August 2018
  • May 2018
  • March 2018
  • January 2018
  • November 2017
  • October 2017
  • September 2017
  • August 2017
  • July 2017
  • June 2017
  • May 2017
  • April 2017
  • February 2017
  • December 2016
  • September 2016
  • June 2016
  • May 2016
  • April 2016
  • March 2016
  • February 2016
  • January 2016
  • December 2015
  • November 2015
  • August 2015
  • July 2015
  • April 2015
  • March 2015
  • February 2015
  • January 2015
  • December 2014

Categories

  • astronomy
  • astrophysics
  • education
  • flat
  • History
  • Hopewell Observatorry
  • Math
  • monochromatic
  • nature
  • optical flat
  • Optics
  • Safety
  • science
  • teaching
  • Telescope Making
  • Uncategorized

Meta

  • Register
  • Log in

Blog at WordPress.com.

  • Follow Following
    • Guy's Math & Astro Blog
    • Join 48 other followers
    • Already have a WordPress.com account? Log in now.
    • Guy's Math & Astro Blog
    • Customize
    • Follow Following
    • Sign up
    • Log in
    • Report this content
    • View site in Reader
    • Manage subscriptions
    • Collapse this bar
 

Loading Comments...