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Adventures with a 200 Year Old Dollond Achromat

classic double star optics refractor
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#1 Rustler46

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Posted 15 February 2018 - 04:03 AM

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.

 

Best Regards,
Russ


Edited by Rustler46, 15 February 2018 - 04:15 AM.

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#2 Benach

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Posted 15 February 2018 - 05:44 AM

Drool, envy etc. Wow what a scope! BTW, afaik Dolond didn't invent the achromat, that is true. But John Dolond made the first achromat.


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#3 zjc26138

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Posted 15 February 2018 - 07:23 AM

So awesome!


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#4 Rustler46

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Posted 16 February 2018 - 03:11 AM

Drool, envy etc. Wow what a scope! BTW, afaik Dolond didn't invent the achromat, that is true. But John Dolond made the first achromat.

Imagine how I felt as a 16-year old to have that little gem dropped in my lap - quite floored to say the least. But I do regret not having done much with this little refractor in the years since. But at one time it was used to see the rings of Saturn.

 

Here are a few images of the little Dollond refractor:

 

Dollond Achromat-00376.jpg

 

When fully extended the telescope is 28-3/4 inches (730 mm) long.

 

Dollond Achromat-00380.jpg

 

Dollond Achromat-00385.jpg

 

The above image shows the little eyepiece dust cover partly swung into place. I amazes me the skill of these early craftsmen.

 

As to who was first, there is some uncertainty as to whether others had made a working achromatic refractor telescope before John Dollond. In the patent lawsuits, there was a claim that optician George Bass had sold some achromats prior to Dollond. The inventor of the achromat, Chester Moore Hall, in trying to keep his secret, had the crown and flint elements made by two different opticians. Each of these further sub-contracted to Bass make the lenses for them. Upon finding that these were for the same client, Bass put them together into a single achromatic lens. Now the secret was out, and eventually revealed to Dollond.

 

But in a landmark patent ruling, the judge said in effect:

 

It's not the person who keeps his secret locked away in a desk drawer who should benefit from the invention. But it should rightly be the one who brought it out so that the masses can benefit from it.

 

That person was John Dollond. It was his son Peter who vigorously and successfully fought in the courts to retain the patent. As a result for 14 years the Dollonds had exclusive legal right to produce the achromatic refractor. Also the English had a lock on the manufacture and marketing of the flint glass needed for the achromats. As a result the best and largest flint glass blanks were reserved for the domestic market. It wasn't until some years later (in the early 1800's) that Guinand and Fraunhofer (on the continent) came up with large flint glass enabling larger achromatic telescopes. Thus began the era of the modern achromatic refractor.


Edited by Rustler46, 16 February 2018 - 03:27 AM.

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#5 deSitter

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Posted 16 February 2018 - 12:53 PM

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.

 

Best Regards,
Russ

Great story! PS dear CN, please implement auto-resizing so people can post pictures without having a degree in computer science.

 

-drl



#6 Rustler46

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Posted 19 February 2018 - 10:01 PM

Here is an update on the Dollond adventure. The first photo shows another Dollond patent - the attachment between successive draws. Rather than having the next draw attached by threads located at the end of previous draw, the attachment threads are some distance inward from the end. This provides for two positions supporting the draw - the threads and the collar at the end of draw. These are at opposite ends of the short brass section on the left in the photo below. Using this mechanical improvement results in less wiggle and sagging from draw to draw. 

 

Here is a photo showing that arrangement and the two lens Schyrle erecting system, with lenses removed.

 

Dollond Piggyback-00465.jpg

 

The photo shows what the section looked like before cleaning. But once the brass tube is inserted into the threaded section, it is locked in place when the two lenses are threaded onto the ends. Then the entire erecting section is threaded onto the Huygens eyepiece section forming the last draw. Showing attention to detail is the fact that each lens in the erector has different sized threads on its mount. So it is not possible to put the two lenses at the wrong end of the erector - well designed to say the least.

 

This arrangement of lenses is shown in the following graphic:

 

Screen Shot 2018-02-20 at 1.18.21 PM.png

 

More to come.


Edited by Rustler46, 20 February 2018 - 09:49 PM.

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#7 Rustler46

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Posted 19 February 2018 - 11:09 PM

During previous testing of the Dollond refractor, it was mostly supported by tube rings on the mahogany section. The three draws when extended hang an additional 20 inches beyond the last ring. While I had placed a foam block under that last draw to help support it, the arrangement was less than ideal.

 

So not wanting to risk damaging the wooden section by the additional torque placed on it by the unsupported brass draws, I wanted some means for extra support. This was complicated by the fact that the draws needed to extend and retract for focusing.

 

Here is the piggyback arrangement for supporting the refractor:

 

Dollond Piggyback-00470.jpg

 

The orange dovetail plate is riding atop the Celestron-8 optical tube. On the left are the two tube rings with copper strips protecting the refractor. On the right of the dovetail is the support for the extended draws. The little wood block has a countersunk 1/4-20 bolt holding it to the chrome S-support. In the background is the Telrad finder with electrical connection for the dew heater on the angled glass plate.


Edited by Rustler46, 19 February 2018 - 11:59 PM.


#8 Rustler46

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Posted 19 February 2018 - 11:23 PM

Here is a closeup of the support:

 

Dollond Piggyback-00471.jpg

 

Notice the notch over the counter sunk bolt, giving clearance for the knurled end ring of the supported draw. Also the curved upper surface of the block matches the shape of the brass tube preventing excessive sideways movement with different orientations of the telescope.

 

It's not real pretty, but is what can be accomplished with my limited technical skills.


Edited by Rustler46, 20 February 2018 - 12:40 AM.


#9 Rustler46

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Posted 19 February 2018 - 11:57 PM

This is what the whole thing looks like before the brass draws are fully extended:

 

Dollond Piggyback-00474.jpg

 

Here can be seen more of the entire arrangement. On the upper left is the dew heater for the Dollond refractor's objective. This was crucial in my moist marine environment, since I don't as yet have a dew shield for the little 40 mm lens.

 

Notice the DewBuster dew control system, which I highly recommend. The gray cable plugged into the left end of the Dewbuster controller leads to the ambient air temperature sensor, seen in front of the orange dovetail, below the brass draw. Connected along the top left edge of the controller are power leads to the two temperature controlled dew heater strips (located on the left end of the C-8 OTA). The next two connections on the right lead to medium heat (un-controlled) power for the Telrad and refractor dew heaters. The power cords are managed by wrapping the excess lengths around the camera piggyback mount. The power cord for the DewBuster (with red/black PowerPole connectors) can be see below the controller to the right. Below that is the Velcro strip used to secure the supported draw to the wooden support block.

 

More to come.



#10 Rustler46

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Posted 20 February 2018 - 12:33 AM

Now here is the whole shebang with the draws extended and secured to the right side support using a Velcro strap.

 

Dollond Piggyback-00477.jpg

 

The final draw on the right is unsupported, allowing for focus adjustment. The Velcro strap is not ideal, but works OK. I need something that can be cinched up tight, but that can be easily loosened and removed. I have found that the pointing direction of the refractor is governed by the adjustment of the tube rings on the main mahogany section. Any sag of the extended draws doesn't change the pointing direction. It only affects which part of the field is shown in the eyepiece. Yet the effect is relatively minor. What is centered in the FOV remains near the center despite any shift caused by the draw tube sag.

 

Now I'll be waiting for some clear nights for further testing. I would not be able to do much observing without use of the G-11 mount's go-to function. The lack of a star diagonal makes viewing quite difficult, let alone being able to point the 'scope to a desired direction.

 

The little Dollond refractor, with its 40 mm achromat lacking any modern lens coatings, is limited to brighter than 11th magnitude stars. Since there are 6 lenses in all (2 in objective, 2 in erector & 2 in eyepiece) with 12 air-glass surfaces, that magnitude limit would undoubtedly be further compromised. With around 96% transmission at each surface, only around 60% is transmitted unimpeded (0.9612 = 0.61).

 

I'll be using a 70 mm aperture mask on the Celestron-8 to give a comparison with the smaller classic achromat. That 70 mm aperture leaves the SCT unobstructed, providing a 12.9 magnitude limit at f/18.

 

I have assembled a list of some candidate double stars to be examined. With the magnification available, doubles with separation less than 15 arc-seconds will be difficult. Only if the components are of equal brightness will separations approaching 10 arc-seconds be easy to discern. I'll report my results.


Edited by Rustler46, 20 February 2018 - 04:39 AM.

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#11 Rustler46

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Posted 20 February 2018 - 04:20 AM

Here's the 70 mm off-axis mask for the Celestron-8 that I just made from poster board. That is the maximum size that fits between the secondary mirror and edge of aperture. At least that was my intention. I'll need to see an out of focus star to determine if anything impedes on the full 70 mm circular aperture. Since the poster board is quite flimsy, I've glued on some reinforcement strips and a small grab handle.

 

70 mm Off-Axis Mask for C-8-00480.jpg

 

Now waiting for a clear night.

 

I've been researching how to determine the magnification of the spyglass. It seems like the exit pupil divided into the 40 mm entrance pupil gives a measure of magnification.

 

Does anyone know if that is true? If so, in what practical way can the exit pupil be measured?

 

It seems like another way to measure magnification would be measuring the linear size of an object of known angular size (i.e. the Moon). The problem with that is the focal length is only approximately known (~28 inches). Any ideas??


Edited by Rustler46, 20 February 2018 - 05:29 AM.

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#12 Rustler46

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Posted 20 February 2018 - 09:59 PM

I can see from the above photos that an improvement could be made by turning the chrome S-bracket 180°, extending the support block further back around 2 inches (5 cm) toward the eyepiece. So I made that adjustment and like the placement of the support - it is more balanced on the brass draws. But I'll need to make another wooden support block since the old one no longer captures the knurled end ring as before. That is real handy to have to prevent the inner draws from moving back & forth while adjusting focus with the last daw. So it's back to the shop for some more wood working.


Edited by Rustler46, 20 February 2018 - 10:33 PM.


#13 Rustler46

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Posted 23 February 2018 - 09:21 PM

I can see from the above photos that an improvement could be made by turning the chrome S-bracket 180°, extending the support block further back around 2 inches (5 cm) toward the eyepiece. So I made that adjustment and like the placement of the support - it is more balanced on the brass draws. But I'll need to make another wooden support block since the old one no longer captures the knurled end ring as before. That is real handy to have to prevent the inner draws from moving back & forth while adjusting focus with the last daw. So it's back to the shop for some more wood working.

So after several tries, I've made the support for the brass draws of the Dollond achromat as shown below:

 

Dollond Piggyback.jpg

 

The wooden block was lengthened to 8-inch to support the entire 2nd draw. The knurled rings at each end of that draw are captured in some saw cuts on the block. This will prevent the inner draws from collapsing inward when trying to focus with the outer draw. Only the outer draw has a slight amount of sag possible. The entire support now seems very sturdy. Using the tube ring screws I will need to fine-tune the pointing direction to be more nearly collinear with the C-8. But there should be enough give in the draw attachment point on the mahogany section to accommodate any change without resizing the support dimensions on the right end.

 

Now I have a short list of wide doubles that can be used to test the performance of the Dollond achromat and C-8 at 70 mm aperture:

 

Screen Shot 2018-02-23 at 5.32.59 PM.jpg

 

The Washington Double Star catalog listing (WDS) is useful since that catalog is in the Losmandy G-11 database for go-to operations. Otherwise using my iPad-1 (that's a classic itself) with SkySafari to control the mount is an option. So I'll be giving this little achromat some more tests next opportunity. As mentioned fixed magnification is an issue. I might see if holding a low power 1-1/4 inch eyepiece behind the Dollond eyepiece might work to increase magnification.  The adventure continues.


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#14 deSitter

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Posted 24 February 2018 - 05:47 PM

 

I can see from the above photos that an improvement could be made by turning the chrome S-bracket 180°, extending the support block further back around 2 inches (5 cm) toward the eyepiece. So I made that adjustment and like the placement of the support - it is more balanced on the brass draws. But I'll need to make another wooden support block since the old one no longer captures the knurled end ring as before. That is real handy to have to prevent the inner draws from moving back & forth while adjusting focus with the last daw. So it's back to the shop for some more wood working.

So after several tries, I've made the support for the brass draws of the Dollond achromat as shown below:

 

attachicon.gif Dollond Piggyback.jpg

 

The wooden block was lengthened to 8-inch to support the entire 2nd draw. The knurled rings at each end of that draw are captured in some saw cuts on the block. This will prevent the inner draws from collapsing inward when trying to focus with the outer draw. Only the outer draw has a slight amount of sag possible. The entire support now seems very sturdy. Using the tube ring screws I will need to fine-tune the pointing direction to be more nearly collinear with the C-8. But there should be enough give in the draw attachment point on the mahogany section to accommodate any change without resizing the support dimensions on the right end.

 

Now I have a short list of wide doubles that can be used to test the performance of the Dollond achromat and C-8 at 70 mm aperture:

 

attachicon.gif Screen Shot 2018-02-23 at 5.32.59 PM.jpg

 

The Washington Double Star catalog listing (WDS) is useful since that catalog is in the Losmandy G-11 database for go-to operations. Otherwise using my iPad-1 (that's a classic itself) with SkySafari to control the mount is an option. So I'll be giving this little achromat some more tests next opportunity. As mentioned fixed magnification is an issue. I might see if holding a low power 1-1/4 inch eyepiece behind the Dollond eyepiece might work to increase magnification.  The adventure continues.

 

This is awesome!

 

-drl



#15 Rustler46

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Posted 24 February 2018 - 07:37 PM

This is awesome!

 

 

-drl

 

Thanks for your comments. It as been quite an interesting experience for me. For once I have been wanting to explore how much can be seen by a smaller aperture. The 40mm Dollond achromat is a joy to look through, knowing that its kindred refractors were such an improvement over what was previously available. Even the larger reflectors of that era were saddled with speculum metal mirrors that soon tarnished and needed polishing & refiguring. So as larger flint glass blanks became available in the early 1800s, the age of the great achromat refractors began. An early example was the Great Dorpat Refractor, which at some 9-1/2 inch aperture was the first of these grand refractors.

 

I'm eagerly awaiting the next night of clear skies to test my lesser apertures. The 70mm SCT is very easy to look through having variable magnification and a nice 2-inch star diagonal. But having 3 mirrors in the light path makes for a mirror-image view. So comparing the views in the two telescopes will be a mental challenge. While the Dollond is a joy to look through, it can be a pain in the neck, lacking the star diagonal. Yet the go-to capabilities of the mount give an ease of use never enjoyed by astronomers 200 years ago.


Edited by Rustler46, 24 February 2018 - 08:35 PM.

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#16 Rustler46

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Posted 27 February 2018 - 04:48 PM

Thanks for your comments. It as been quite an interesting experience for me. For once I have been wanting to explore how much can be seen by a smaller aperture. The 40mm Dollond achromat is a joy to look through, knowing that its kindred refractors were such an improvement over what was previously available. Even the larger reflectors of that era were saddled with speculum metal mirrors that soon tarnished and needed polishing & refiguring. So as larger flint glass blanks became available in the early 1800s, the age of the great achromat refractors began. An early example was the Great Dorpat Refractor, which at some 9-1/2 inch aperture was the first of these grand refractors.

 

 

I'm eagerly awaiting the next night of clear skies to test my lesser apertures. The 70mm SCT is very easy to look through having variable magnification and a nice 2-inch star diagonal. But having 3 mirrors in the light path makes for a mirror-image view. So comparing the views in the two telescopes will be a mental challenge. While the Dollond is a joy to look through, it can be a pain in the neck, lacking the star diagonal. Yet the go-to capabilities of the mount give an ease of use never enjoyed by astronomers 200 years ago.

 

Last night was clear for a few hours, though challenged with a bright gibbous Moon. But having an extended object like the Moon helped in adjusting the achromat to be co-linear with the C-8. After that Go-To's were close enough to get the subject in the Dollond's FOV. Here's an expanded list of doubles for testing that old refractor:

 

Screen Shot 2018-02-27 at 1.19.52 PM.png

 

I was not able to resolve the Trapezium with the refractor. In the 70mm SCT the little quartet was an amazing sight. The bright moon light made seeing the fainter secondaries quite difficult with the Dollond. Anything much fainter than 6th magnitude required averted vision just to detect the secondary. With the exception of the Trapezium, none of the doubles examined had a separation that challenged the resolving power of the refractor. The most satisfying view with the Dollond was of the bright, wide double Kappa2 Tauri (magnitudes 4.2/5.3 @ 340").

 

The 70mm SCT was by far easier to use, not only because of the star diagonal, but also the the higher magnification - around 40X. The old achromat had another characteristic making viewing difficult. If my eye was not placed correctly behind the eyepiece, there would be some flaring off the edge of star images. These could at times be misinterpreted as the location of a faint, close secondary star. So not only was viewing a neck breaker, but eye placement was important. But after an hour or so, thick fog set in, bringing an end to observing. Nevertheless I'm coming to terms with the difficulties of using the old achromat. After all I'm using it in a way never intended by the Dollonds.

 

I'm looking forward to a moonless night for more tests. 


Edited by Rustler46, 27 February 2018 - 09:43 PM.


#17 Rustler46

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Posted 28 February 2018 - 12:39 AM

I've been researching how to determine the magnification of the spyglass. It seems like the exit pupil divided into the 40 mm entrance pupil gives a measure of magnification.

 

Does anyone know if that is true? If so, in what practical way can the exit pupil be measured?

 

It seems like another way to measure magnification would be measuring the linear size of an object of known angular size (i.e. the Moon). The problem with that is the focal length is only approximately known (~28 inches). Any ideas??

According to Amateur Astronomer's Handbook (J.B. Sidgwick, 1961), page 53, magnification is given by:

 

Magnification = Entrance Pupil (aperture) ÷ Exit Pupil

 

By pointing the telescope at a white target I used a millimeter ruler to measure the exit pupil to be around 2-1/2 mm. So the magnification of this old achromat is given by:

 

Magnification = 40 mm (aperture) ÷ 2.5 mm (exit pupil)

Magnification = 16X

 

While that would be a nice useful magnification for a hand held terrestrial spyglass, it's a bit low for resolving double stars. That explains some of the difficulty it has in resolving double stars compared to my Celestron-8 at 40X. The C-8 separates the resolved components 2-1/2 times more than the old Dollond achromat. But the comparison was meant to be just that - a comparison, not necessarily fair.

 

While I was fiddling around with the eyepiece, I unscrewed the eye lens cell and was able to dust both sides as well as the inside of the field lens. This should help improve the view.


Edited by Rustler46, 28 February 2018 - 03:33 PM.


#18 Rustler46

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Posted 28 February 2018 - 07:37 PM

 

I've been researching how to determine the magnification of the spyglass. It seems like the exit pupil divided into the 40 mm entrance pupil gives a measure of magnification.

 

Does anyone know if that is true? If so, in what practical way can the exit pupil be measured?

 

It seems like another way to measure magnification would be measuring the linear size of an object of known angular size (i.e. the Moon). The problem with that is the focal length is only approximately known (~28 inches). Any ideas??

According to Amateur Astronomer's Handbook (J.B. Sidgwick, 1961), page 53, magnification is given by:

 

Magnification = Entrance Pupil (aperture) ÷ Exit Pupil

 

By pointing the telescope at a white target I used a millimeter ruler to measure the exit pupil to be around 2-1/2 mm. So the magnification of this old achromat is given by:

 

Magnification = 40 mm (aperture) ÷ 2.5 mm (exit pupil)

Magnification = 16X

 

While that would be a nice useful magnification for a hand held terrestrial spyglass, it's a bit low for resolving double stars. That explains some of the difficulty it has in resolving double stars compared to my Celestron-8 at 40X. The C-8 separates the resolved components 2-1/2 times more than the old Dollond achromat. But the comparison was meant to be just that - a comparison, not necessarily fair.

 

Perhaps a fairer comparison would be between what can be seen by the old 40 mm Dollond achromat and by my 15X80 binoculars. Although the latter has significant advantages with light-gathering power and resolution, the magnifications are very close. The resolution limits for the achromat and binoculars are 1.45 and 2.90 arc-seconds respectively - much less than what can be seen with the low magnification. So in this case the determining factor in seeing double stars is not in resolution capability, but having in enough magnification and light-gathering power to see the resolved components. My skills will be tested using the binoculars since they will not have the benefit of go-to pointing or tracking. But the use of two eyes is a huge plus. We'll see. 


Edited by Rustler46, 28 February 2018 - 07:40 PM.

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#19 deSitter

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Posted 28 February 2018 - 08:37 PM

 

I've been researching how to determine the magnification of the spyglass. It seems like the exit pupil divided into the 40 mm entrance pupil gives a measure of magnification.

 

Does anyone know if that is true? If so, in what practical way can the exit pupil be measured?

 

It seems like another way to measure magnification would be measuring the linear size of an object of known angular size (i.e. the Moon). The problem with that is the focal length is only approximately known (~28 inches). Any ideas??

According to Amateur Astronomer's Handbook (J.B. Sidgwick, 1961), page 53, magnification is given by:

 

Magnification = Entrance Pupil (aperture) ÷ Exit Pupil

 

By pointing the telescope at a white target I used a millimeter ruler to measure the exit pupil to be around 2-1/2 mm. So the magnification of this old achromat is given by:

 

Magnification = 40 mm (aperture) ÷ 2.5 mm (exit pupil)

Magnification = 16X

 

While that would be a nice useful magnification for a hand held terrestrial spyglass, it's a bit low for resolving double stars. That explains some of the difficulty it has in resolving double stars compared to my Celestron-8 at 40X. The C-8 separates the resolved components 2-1/2 times more than the old Dollond achromat. But the comparison was meant to be just that - a comparison, not necessarily fair.

 

While I was fiddling around with the eyepiece, I unscrewed the eye lens cell and was able to dust both sides as well as the inside of the field lens. This should help improve the view.

 

You have to be some distance from the exit pupil to accurately measure it. I tape a scale across the eyepiece and look at it with binoculars from some feet away.

 

-drl



#20 Rustler46

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Posted 01 March 2018 - 12:43 AM

 

 

I've been researching how to determine the magnification of the spyglass. It seems like the exit pupil divided into the 40 mm entrance pupil gives a measure of magnification.

 

Does anyone know if that is true? If so, in what practical way can the exit pupil be measured?

 

It seems like another way to measure magnification would be measuring the linear size of an object of known angular size (i.e. the Moon). The problem with that is the focal length is only approximately known (~28 inches). Any ideas??

According to Amateur Astronomer's Handbook (J.B. Sidgwick, 1961), page 53, magnification is given by:

 

Magnification = Entrance Pupil (aperture) ÷ Exit Pupil

 

By pointing the telescope at a white target I used a millimeter ruler to measure the exit pupil to be around 2-1/2 mm. So the magnification of this old achromat is given by:

 

Magnification = 40 mm (aperture) ÷ 2.5 mm (exit pupil)

Magnification = 16X

 

While that would be a nice useful magnification for a hand held terrestrial spyglass, it's a bit low for resolving double stars. That explains some of the difficulty it has in resolving double stars compared to my Celestron-8 at 40X. The C-8 separates the resolved components 2-1/2 times more than the old Dollond achromat. But the comparison was meant to be just that - a comparison, not necessarily fair.

 

While I was fiddling around with the eyepiece, I unscrewed the eye lens cell and was able to dust both sides as well as the inside of the field lens. This should help improve the view.

 

You have to be some distance from the exit pupil to accurately measure it. I tape a scale across the eyepiece and look at it with binoculars from some feet away.

 

-drl

 

Good idea - I'll give it a try. When I measured the exit pupil, I held a millimeter ruler at the eyepiece and lined it up with the exit pupil. So likely that was too close. Next I'll follow your suggestion to get something more accurate. Thanks!



#21 Rustler46

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Posted 13 March 2018 - 10:14 PM

Perhaps a fairer comparison would be between what can be seen by the old 40 mm Dollond achromat and by my 15X80 binoculars. Although the latter has significant advantages with light-gathering power and resolution, the magnifications are very close. The resolution limits for the achromat and binoculars are 1.45 and 2.90 arc-seconds respectively - much less than what can be seen with the low magnification. So in this case the determining factor in seeing double stars is not in resolution capability, but having in enough magnification and light-gathering power to see the resolved components. My skills will be tested using the binoculars since they will not have the benefit of go-to pointing or tracking. But the use of two eyes is a huge plus. We'll see. 

I haven't forgotten about further testing of this old achromat. But I've had few clear nights recently. Also I've been finishing my new binocular observing chair for use with the above mentioned 15X80 binoculars. So at next opportunity, I'll be comparing the 16X, 40mm achromat to the 15X80 binoculars. I expect the binoculars will give better performance in splitting doubles. While it has slightly less magnification, it has 4 times the light gathering power as well as the use of two eyes. Such binocular vision is reported to enable a bit deeper limiting magnitude. I'll let you know how that turns out.


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#22 Rustler46

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Posted 31 March 2018 - 12:14 AM

I haven't forgotten about further testing of this old achromat. But I've had few clear nights recently. Also I've been finishing my new binocular observing chair for use with the above mentioned 15X80 binoculars. So at next opportunity, I'll be comparing the 16X, 40mm achromat to the 15X80 binoculars. I expect the binoculars will give better performance in splitting doubles. While it has slightly less magnification, it has 4 times the light gathering power as well as the use of two eyes. Such binocular vision is reported to enable a bit deeper limiting magnitude. I'll let you know how that turns out.

So here's a bit of an update on the Dollond adventure. While I've finished the binocular observing chair, the binoculars are another story. I'm working on adjusting the collimation so that both eyes can be used when observing. Its collimation is so far off that the second eye is useless. So when I've used the binocs one side is capped.

 

When I finally got an opportunity for some observing with the Dollond achromat there was a bright gibbous Moon. With just a 40mm aperture, that little spyglass is already light challenged. So the bright sky made seeing the fainter double stars nearly impossible. But two brighter pairs were seen well enough to add to my observing log:

  • 15 & 17 Canum Venaticorum - Double star - 5.9/6.3 magnitudes @ 180 arc-seconds, you can drive a whole bunch of trucks between 'em, a lot of black sky there, nicely shown, with these magnitudes it's punching through the bright moonlight
  • Alpha Canum Venaticorum - Double star, Cor Caroli - 2.8/5.5 magnitudes @ 19 arc-seconds, a beautiful, beautiful double star, brilliant white primary and a dim secondary; despite the difficult neck angle, I'm counting this one as resolved, the little 5.5 magnitude secondary is sometimes kind of hard to see, but it's bright enough to be visible

I also gave a look at the Moon itself:

  • Waxing Gibbous Moon (12.4 days old) - Centered in the FOV there's a fair amount of dark sky around the outside edge, estimate FOV is around 0.7°; there seems to be a yellowish-green look to it; not far from the terminator are the craters Schickard, Nasymth & Phocylides in the south; also Aristarchus and Sinus Iridum in the north; a beautiful view, nice and sharp

The yellowish-green cast is likely from the crown glass used in the telescope. Yet none of the bright stars show any spurious colors. But I am hoping for some clear nights once the Moon gets out of the way. I'd like to give this old achromat at least one other try on some double stars without being challenged by moonlight. After that, I'll be ending the Dollond adventure. Despite giving some real nice images it's just too tough on my old neck without a star diagonal.


Edited by Rustler46, 31 March 2018 - 07:05 PM.


#23 Rustler46

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Posted 04 April 2018 - 02:53 AM

Well, my adventures with this old Dollond achromat are just about coming to an end. I had an opportunity last night to view some more double stars with the Dollond spyglass riding atop my old Celestron Super C-8 Plus. Between dusk and moonrise there was over an hour of fairly dark skies along with good seeing. Having only 40 mm of aperture that old spyglass was never meant for nighttime viewing. Intended for hand held, daytime use its 16X power would make it difficult to hold steady enough for clear views. But riding on a Losmandy G-11 go-to mount gave it solid support for examining double stars. But without a star diagonal, views of many celestial objects were hard on my 72 year old neck. In any case here is a list showing how the Dollond fared alongside the Celestron-8 with a 70 mm off-axis aperture stop (at 40X magnification).

 

Screen Shot 2018-04-03 at 11.43.49 PM.png

 

The visibility of various doubles in the Dollond achromat was limited by 3  factors:

  • Separation of components
  • Brightness of components
  • The elevation of the double star

The smallest separation visible was limited by the low, 16X power of the achromat. But components much fainter than 6.5 magnitude were often invisible next to a brighter primary component. If the elevation of the star was too high, the lack of a star diagonal put much strain on my neck and limited the ability to hold still enough for clear views. Of course if the elevation was too low, then the usual factors involving increased airmass came into play.

 

One real surprise was Beta Monocerotis. After viewing this one with the 70mm Celestron my initial feeling was that it would not be seen in the Dollond achromat. But I decided to give it a try anyway. Much to my surprise there they were - two tiny stars separated by just 7 arc-seconds. Helping to achieve that was the brightness of the components (4.6 & 5.4 magnitudes) and the relatively low elevation of the pair. Prior to Beta Monocerotis the smallest separation achieved was on Mizar at 14 arc-seconds. But given the limitations of the little Dollond achromat, I'll soon be de-mounting it from atop the C-8. It has been fun exploring the capabilities of this old spyglass. 

 

All that remains of my "adventures with a 200 year old Dollond achromat" is further inquiries as to previous owners. The fact that it is still in good working order after 208 years attests to the care given by previous owners as well as the quality of the original manufacture. As yet I haven't discovered much except that it may have passed through Cecil Joe Hindley's antique shop in Ferndale, Humboldt County, California some time prior to 1962, when I acquired it. It may have come from the estate of the Falk family.

 

I hope you have enjoyed sharing in these adventures.


Edited by Rustler46, 04 April 2018 - 02:38 PM.

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#24 memento

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Posted 04 April 2018 - 05:11 AM

Russ, awesome thread and infos! I love how you use the modern stuff like C8, mount, DewBuster together with the Dollond. Thomas



#25 Rustler46

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Posted 04 April 2018 - 02:42 PM

Russ, awesome thread and infos! I love how you use the modern stuff like C8, mount, DewBuster together with the Dollond. Thomas

I'm glad you enjoyed the adventure, Thomas. It seems only fair to give equal advantage to the old achromat by means of modern contrivances. We have come a long way in 200 years.


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