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Carton 100mm f/13 Refractor


johninderby

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Carton 100mm f/13 Refractor Project

The final few little bits arrived yesterday so I was able to get the project completely finished at long last. Now it's ready for some proper testing. I'm really pleased with the outcome of this "classic" long focal length refractor project. Where ever I take it I know I'm not going to see another one like it. :p

The basic ingredients:

  • Carton 100mm f/13 achromatic objective and cell from Lyra Optics
  • Countercell custom machined by Skylight Telescopes
  • Baader Steeltrack 2 speed crayford focuser
  • Lumicon 50 Super Finder with Omegon erect diagonal
  • Modified William Optics finder bracket with Synta type base and Baader mounting shoe
  • 4" aluminium tubing for OTA
  • 5.5" brass tube for dew shield
  • CNC tube rings with TS carry handle & ADM dovetail bar
  • Brass thumbscrews and thumbknobs by Epsilon Telescopes
  • ProtoStar Flockboard
  • LensCoat Hoodie lens cap

The OTA tube was cut from a length of 4" 16swg aluminium tube and sanded smooth. The Baader mounting flange was fitted at one end after first making sure the tube was cut off dead square. At the other end the countercell that had been custom machined for me by Richard at Skylight Telescopes was fitted by drilling and taping three holes through the countercell. Rather than have three exposed screw heads on the outside I fastened the screws from the inside so that all that showed on the outside was three small holes. The dewshield was cut from a length of 5.5" diameter brass tubing. It took many hours to get the brass polished smooth as it was an exceptionally hard brass that required polishing compounds usually used for steel. I decided to use small brass thumbscrews to hold the dew shield in place rather than just having exposed screw heads, and of course it makes it easy to remove the dew shield.

The OTA was painted with Candy Apple Red paint which is a fairly involved process. First two coats of acid etch primer was used to give a good bond to the metal. Then three coats of metallic silver base coat was applied followed by a further three coats of the transparent red.Then three coats of two pack clearcoat was applied and left to harden for a week before rubbing it out. The countercell was painted in satin black to match the focuser and tube rings.

The OTA is flocked with ProtoStar Flockboard fitted in short sections with baffle rings made from the flockboard between each section. A ray traced diagram of the OTA was used to get the baffles in the right positions and the right sizes. It's so black inside that I can't even see the baffle rings.

The finder is a Lumicon 50mm Super Finder that can be used with an erect diagonal or straight through as it has a drawtube so switching from RA to straight through is a simple as removing the diagonal, extending the drawtube and re-fitting the eyepiece. Such a simple solution I'm surprised other manufacturers haven't caught on.

I came across the LensCoat Hoodie lens covers intended for camera lenses and thought one would be perfect for this scope. It has a hard plate in the end to protect the objective and simply slips over the end of the dew shield and once on it definitely isn't going to come off accidentally. It will also fit over the objective when the dewshield is removed. Highly recommended if someone is looking for a lens cap for their scope and they come in quite a few sizes.

Replacing the thumbscrews and the thumbnuts on the tube rings with brass ones, while not necessary, does make a big difference to the look of the scope and was well worth the added expense. Rob at Epsilon Telescopes did a great job and the brass thumbnuts on the tube rings in particular are a real work of art.

I have to say thanks to Glen at Lyra Optics and Richard at Skylight Telescopes for their help and advice with this project.

John

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Although it may be very decorative indeed in the daylight, it's how it performs in the dark that counts.

I've just come in from a 1/2 hour first light with the scope. Despite the clear forecast for this evening cloud banks are moving in and spoiling things, but at least I got a chance to try it out.

Jupiter was simply stunning. The contrast and surface detail was far better than I've ever seen in a scope of this aperture, or even one a fair bit larger. The subtle detail in the equatorial belts in particular was mesmerizing. Oh boy is this ever a specialist planetary scope! It performed every bit as well on the moon showing lots of subtle detail and shadings on the surface. The straight wall really stood out well tonight. And as far as CA is concerned........what CA. :confused:

I hadn't collimated it at all but I did a star test anyway. The collimation was spot on with text book airy discs, which perhaps shouldn't have been too surprising given how well it had performed so far. I suppose a long focal length and paying attention to detail during assembly helps.

An obvious comparison would be with my Lyra Optics 102mm f/11. It definitely outclasses it optically with better contrast and detail. I had expected it to be a bit better but not this good. I'd have to say the Carton 100 actually performs better than it looks.:p

Can you tell I'm pleased with it? :icon_salut::D:D

John

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stunning scope and super report too! one question, when observing near the zenith do you kneel or lay on your belly? :)

Zenith....Whats that? ;)

Actually it's not too bad with the Berlebach tripod fully extended. I can get away with a low stool. It's nice and solid as well with no vibrations even at full height.

I still can't believe how good the scope was on Jupiter. Visually it's definitely a better planetary scope than the old FLT-98 was. :(

John

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to be honest, it's so long since I saw it that I had to look up Zenith myself :)

blimey - fine praise indeed! can you compare it to how your old 6" f8 used to perform? I'll eventually get to do a side by side on a good refractor and my 6" f11.

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While the FLT-98 would be the choice for imaging, for visual observing a specialist planetary scope like the Carton has it's advantages. It would be close to the old OO 1/12PV 6" f/8 but I feel it would still have the edge.

Skylight Telescopes sells their version of the Carton 100mm f/13 starting at £1,295.00 and I've got a few extras such as the brass thumbscrews and knobs and a better finder that would add a bit more.

http://skylight.myshopify.com/collections/frontpage/products/the-skylight-f13-refractor

John

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impressive John but yours is better on a number of levels.

also impressive how good newt optics are in terms of value for money if you can be bothered with collimation/contrast improvement mods lick flocking etc - I honestly prefer them so your comments are music to my ears!

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