Astrobits

I think that if anyone is going to try this they will need to grind their own mirror or commission one from a reputable source. A finished 6" for 25euro from China is unlikely to be of sufficient quality judging by my experience ( admittedly 25+ years ago ) of Chinese mirrors in that bracket. I do not know if we could even purchase a mirror blank for that price and the coating alone would also exceed 25euros. Nigel

Once you have a fast spherical mirror your work is about halfway to a parabolised surface, hence a fast parabola would likely be about twice the cost of a spherical one. Figuring an aspherical corrector plate would be another ball-park probably doubling the costs again. The corrector plate needs both surfaces to be polished, Either 1/2 the correction on each side or full correction on one side and optically flat on the other. Float glass does not have a good enough surface without attention. I have tried to find bits of float to use as flats for Newtonians and had no success. Nigel

Thanks Peter, I have found an article published in the BAA Journal of 1989. Here's the reference: https://adsabs.harvard.edu/full/1989JBAA...99..292F Search "lensless schmidt" for more articles. Nigel

I seem to remember from long ago that a design for a photographic scope consisted of a spherical mirror with a sub-aperture mask placed at the centre of curvature. IIRC the mirror was 8" (200mm) dia and the sub-aperture mask was 4" (100mm). This, of course, results in a slower effective system ( so less SA ) than the unobstructed main mirror but gives a much larger field of view. I can't remember the name of it now. Anyway, an idea to play around with. Nigel

FWIW I was once told that Horace Dall ( he of the Dall-Kirkham, Dall null test, Dall tube ( for fluid flow, nothing to do with astronomy ) and probably many other inventions ) would not bother going out to his 15" D-K unless the night would allow the use of a magnification of 1000. I knew Horace and have no reason to doubt this. Nigel

I have been giving this some thought and have concluded that, as allworlds says, it won't work. My reasoning is this: A telescope can be considered to be composed of two components:- the objective and the eyepiece. The objective produces an image at the focal plane and the eyepiece examines that image. If that image is not perfect then the eyepiece simply shows us a less than perfect image and I'm sure that we have all seen less than perfect images in our observations. Placing an astigmatic lens before the focal plane will distort the image which we will see in the eyepiece. Therefore, any correction must come after the focal plane. Unfortunately, the focal plane almost exclusively occurs within the barrel of eyepieces and after the position where filters are attached, and sometimes between the glass elements of the eyepiece. It is clearly, then, not practical, or possible to have the correction filter anywhere other than after the eyepiece. Adaptive optics are correcting an incoming, distorted, wavefront in order to produce as perfect an image as possible at the focal plane. Nigel

I have been considering getting some glasses from online suppliers using only my astigmatism prescription ( i.e inserting zero power in the order for the glasses ) and purchasing the biggest, cheapest frames. These only cost a few quid and are plastic so cutting them down to fit into any recess in my eyepieces will not be costly if I make a mistake. If I put the same corrections into both eyes on the glasses order form then I have two lenses to cut up and there is a possibility that I could get two goes from each lens depending on the sizes. A second question occurs to me. Could I put the correction lenses into a filter holder and screw them into the barrel of the eyepieces. i.e putting the correction lens before the eyepiece rather then between my eye and the eyepiece. This I will also try sometime. I will report back when I get around to this project unless anyone else does it before me. Nigel

Interesting idea. I have looked up the thermal expansion of wood and concrete and some of the numbers are as follows: Concrete between 7 and 13 x 10^-6 wood ( pine) 5 x 10^-6 So concrete is about twice as bad as wood for thermal stability. However, another consideration is the change in dimensions with humidity where the wood is between 4 and 8% depending on the direction of the grain, i.e. about 10000 times as great as the thermal expansion. I could not find an equivalent number for concrete but would expect it to be much lower. Concrete needs bulk to be strong, try breaking a slab a few mm thick, so the mass will need to be allowed for in the balancing and the overall weight of the telescope. Nigel

As a follow-up, I made a new one and personally delivered it to the buyer, a 450 mile round trip--- on Christmas eve. Nigel

I don't trust any of them. I once ( over 20 years ago now ) sent an 8" dob via courier. It arrived destroyed. The tube was made of galvanized steel, powder coated, with the ends turned to form a safe edge so not sharp. It was packed in a triple layer cardboard box with polystyrene foam and a double layer of the cardboard at the ends. The package had been dropped from a considerable height as the tube had almost cut through the end of the packaging. Worse was that the mirror had completely detached from the backing plate. The mirror had been fixed to the MDF backing plate with Silicon glue. The glass/glue and glue/MDF bonds has not failed. It was the MDF that had failed with chunks of the MDF ripped from the main piece and still stuck to the back of the mirror. The mirror had hit the secondary, spider and tube destroying everything. Insurance? No, it contained glass. It doesn't seem that anything has improved since then. Nigel

A 16" f3.5 parabola is very steep and a polishing tool greater than 1/2 dia will struggle to get the figure. You should try using the 1/2 dia lap and see how that progresses. You will almost certainly need even smaller laps for final stages of figuring. Nigel

Epoxy would be the best. Go for the normal and not the quick set to give you more time to align the flange. Just a thought, check the position of the internal part of the flange before cutting. It is possible that it is longer than the outside might suggest. A longer interior could help to assemble and align the components at the original manufacturing stage. Nigel

I live in Somerset and you're welcome to come and see my 16"dob to get some ideas. I am planning to build another dob ( or 2 ) just as soon as I get down to that project on my to-do list. Nigel

I think that I would saw the tube flush with the flange. When onto the lathe work, cut out the remains on the tube, do the necessary mods that you want and perhaps make a small extender to account for a now shortened tube. Peter posted as I was writing this🙂 Nigel

Not exactly. The power cuts were scheduled when I lived there and the thieves took advantage of the cables not being live in the areas affected. Much safer to cut power cables when the power is off. Nigel

I note that you mention "dobsonians", i.e. more than one. For better views on those 'less than perfect' nights you could try using a smaller mask, off axis, which could give you a relatively long focus unobstructed set-up and allow more comfortable eyepieces to be used ( cost: £0 unless you buy more eyepieces to suit 🤩). A 75mm dia mask on a 200mm f/6 dob will give you focal ratio in the region of f/16 and should avoid those diffraction spikes. Although I am a dob fan, if you really want to purchase something else then I would suggest a smaller refractor which would be much more portable than the dobs for when you go on holiday ( especially abroad?). Nigel

I made my own tube drills for cutting elliptical flats some 25+ years ago. I turned and bored ( at a local college workshop ) a steel bar to length and diameter and sent them to a diamond coater for tipping. The longest would cut 150mm depth of glass which allowed me to cut 4-5 blanks in one go. I found that where the cutter exited the glass there would be some chipping of the edge of the blank so the stack of glass sheets were stuck together with a special optical wax. This way only the top and bottom edges of the stack would be chipped, with the interior surfaces unblemished. Edge chipping would not occur when cutting circular blanks of course. I still have those tube drills somewhere. Nigel

A couple of Christmases ago I was given a puzzle of the Moon and Stars. The Moon was on one side and the Stars were on the other side. Having done both ( do one, break it up then do the other ) I decided that I would like it fixed permanently. This was done with a propitiatory puzzle fixer. To display it was another puzzle as it is round and would need to be turned and rotated at will. I therefore decided to design and make a suitable two sided frame for it. I used a mix of Sapele and Ash woods and this is the result: Nigel

Reminded me of observing the total eclipse in 2002. We went to the Kruger National Park in South Africa for the observation, along with hundreds of others. The Park Authorities had selected a large grassy area in the open Park as the site for the viewing, no fences or other barriers just a few armed Park Rangers wandering around. Didn't see or hear any wild animals while we observed the spectacle. Plenty of wildlife later as we drove through the park on the way home. Nigel

Cheap and n...y, batteries don't last. Not had a problem with it turning itself on while in the box though. Nigel

PIC Nigel

You could also try RS: https://uk.rs-online.com/web/c/engineering-materials-industrial-hardware/knobs-levers-handles/clamping-knobs/?pn=1&applied-dimensions=4291251431 however, they appear to only do round ones in metric. You might be able to get a 6mm one to fit onto a 1/4" head. Nigel

Way back in the 1970's I was told by Jim Hysom ( he of AE ( Astronomical Equipment) fame ) that he had dismantled and tested some Russian binoculars and every surface was !/10th wave. 😳 Nigel

In the 1939-45 war the Russians got the Zeiss factory, equipment, designs and slave lab workers and continued to produce the pre-war Zeiss designs to Zeiss specs which is why the early Russian optical kit is so good. When the Berlin wall came down and Russia opened up we started to see the stuff that was not previously 'export' quality and some of it was pretty dire. Nigel

I once ( nearly 50 years ago now!) purchased a |12" Cas set but used it as a Newtonian by making a large flat. As far as I was concerned and those who also used it it performed admirably. 12" was pretty big for those days so I don't know how well it would have stood up to today's critical eyes🧐 Nigel