Here’s the story on this old achromat:
Back in 1962, when I was 16 years old, I had completed a home-made 6-inch Newtonian reflector. When my mother’s co-worker heard of that, she offered to give me an old spyglass. Thinking I didn’t need a spyglass when I had a real telescope, for a while I didn’t take her up on the offer. But when I finally did, was I surprised! Here was a beautiful old spyglass with a mahogany main barrel and brass fittings, three brass draws and nice heavy leather case. On left side of the brass eyepiece was written in beautiful script “Dollond London”. Being somewhat familiar with the history of the telescope, I knew this could be a real gem. John Dollond didn’t invent the achromatic refractor, but he and his family held the patent on it for some 14 years, beginning in 1758. While John was long dead by the time his son Peter had manufactured my spyglass, by then the name Dollond had become synonymous with achromatic refractors.
Now in 2018, 56 years after acquiring this telescope, I’ve done some more research on it. I’ve come to believe it was from around 1810, making it 208 years old. My research is chronicled in the following DPReview Astrophotography Forum thread:
Since I’m new to Cloudy Nights, it will take a bit of time before I figure out how to post photos of my old refractor. But I’ll eventually post some photos here, too. While I was able to view the Moon and a few double stars by propping the telescope against the edge of my travel trailer, the views were quite shaky. I knew it needed to be supported on an equatorial mount. So now I’ve gotten the spyglass riding piggyback on my 1981 vintage Celestron-8 on a Losmandy G-11 go-to mount. It took quite some effort to get the two telescopes pointing the same direction. But looking at Sirius helped make the process easier.
I doubt that previous owners had been able to use it much for astronomy, aside perhaps from the Moon. But having it go-to enabled has been fun. I'm hoping to send some photons down the tube from objects never before seen by that little achromat. While it is just 40mm in aperture, I was hopeful that some of the brighter, wider double stars would be within its reach. With that aperture it should, under optimum circumstances, resolve doubles of around 3 arc-seconds. But having a fixed, low magnification really limits what it can resolve.
But the views are very sharp, with no trace of color fringes. I don’t know its magnification, but would guess it is around 25X. It gives an erect image, with an actual FOV of around 3/4°. The apparent FOV is very narrow - perhaps 15-25°. From the aperture of 40mm and length of over 28 inches, it is somewhere around f/18. There is a two lens Huygens eyepiece and another two lens erecting system. These latter two lenses can be removed if desired to give a bit brighter view. Those four uncoated air-glass surfaces would reduce the brightness by 15%. I have given the lenses a careful cleaning, and didn’t notice any mold on the lens surfaces. There is a slight greenish cast due to the color of the crown glass used (think Coke bottle).
With just one evening’s observations, I’ve gotten a sense of what this little achromat can do. Of course brightness and magnification are challenges. But with bright enough components, I can pick out 14 arc-second pairs, such as Mizar in Ursa Major. Another interesting double star was 40 and 41 Draconis, magnitudes 5.7 and 6.0 at 19 arc-seconds. One other real challenge is the lack of a star diagonal. So to avoid neck-breaking angles it’s best to view objects fairly low in the sky. It has been interesting to compare what is shown by the little achromat with the views given by my C-8 at 40X. But I think next time it would be better to use an off-axis mask on the SCT, giving 40 to 70 mm unobstructed. That would allow seeing how much of a role magnification plays in splitting these doubles.
While this little telescope was never intended to be used much for astronomical objects, it does give a sense of what it was like for astronomers of yesteryear. Yes, things have really changed in 200 years. We have it much easier, with access to much more capable and less expensive telescopes. Add to that the improvements in glass, optical design, lens coatings, electronics, etc. - boy are we well equipped! I have been trying to discover who might have owned this Dollond telescope over the last 200 years. I've owned for the last 50+ years. But the previous 150 years? - I'm still investigating. It is amazing it has survived for so long. In any case, I’ll keep y’all posted on further developments with this little achromat.
Edited by Rustler46, 15 February 2018 - 04:15 AM.