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Re: TMB Designed APM 152mm f/8 Achromat

Started by 1953, 11/02/2007 08:10AM
Posted 11/02/2007 08:10AM Opening Post

Building an OTA is not that difficult, especially if the lens cell fits over or into a standard aluminum tube. You can save a TON of money too. You already have the most expensive part, the AP focuser.

What diameter tube does the cell fit over or into?


"Don't sweat the petty things and don't pet the sweaty things" 8O
Posted 11/02/2007 10:01AM #1
MANY discussions on flocking in the archives. MOST concede it is an easy way to get excellent results, when the right material is used.

David Hawkins said:

The lens is in a collimatable cell, but I don't have the OTA end ring that mounts rigidly to the OTA (that the cell then uses to collimate against with push-pull set screws). So, I'm not sure of the I.D., but an O.D. of 160mm should work.

If I build an OTA, I will need to machine an OTA adapter for the AP focuser, an OTA end ring, and some baffles. I would then need to machine up some sort of sliding dew slide, which is probably the easiest part.

How do y'all feel about baffles? I know the TeleVue NP127 I used to have did not have any that I remember, but the TMB-APM 180mm ED-Star I owned a while back was full of baffles, Markus really likes them. Or, will Krylon Ultra-Flat Black paint and some quality flocking paper at the end of the OTA be enough to control stray light and help contrast (if possible)? If so, machining some baffles will not be necessary and will cut cost and time down.
Posted 11/02/2007 10:19AM #2
If you choose a standard 7" O.D. aluminum tube Barry at D&G has both tubes and endcaps that will take your AP focuser assuming it's an early 90's vintage or younger. Then you just need to have the inner cell made.

With regards to baffling, well take you pick. Some people let the focuser tube openning set the stop, others control it with the baffleing. If you control it with the baffling then only one stop is necessary to set the fully illuminated field at the focal plane. Whichever way you go, the primary duties of baffling is to first allow the full apeture of the objective to be used, second : eliminate stray light and third control the size of the fully illuminate field at the focal plane.

Personnally I use extensive baffling. It's not hard to make and you can even use good poster board if you want to.

I also find that heavily baffling the first few inches of the tube is very important for effective control of stray light. I've built several OTAs around the 6" F10 Jaegers lens using 7" O.D. tubing. I use 7 Baffles with the first 5 baffles spaced 3/4" apart. These 5 are all the same size, being 6" I.D. with an O.D slightly smaller then the I.D. of the tube. The seventh is approximately 2/3'ds down the tube and is sized to give me ~1-1.5" of fully illuminated field at the focal plane, depending on what the focuser of choice will pass and what size hole mandrels I have. The sixth one is located between the 5th & 7th and sized the same as the 7th one. All of the baffles are spaced and glued on to 3-4 stringers. I then paint them flat black AND wrap the whole assembly with black flocking paper. Finally I slide the whole thing into the tube so that the first 6" I.D baffle rest up againt the back face of the lens cell. I get jet black interiors with NO stray light, even from strong of axis sources like street lights or the moon. It really makes a difference for daylight viewing.

If you use binoviewers, you also have the advantage of designing the tube assembly with enough in-focus ability so that you don't need an OCS type of system.

Hope this helps.


"Don't sweat the petty things and don't pet the sweaty things" 8O
Posted 11/07/2007 01:04PM #3
You've got a great lens...designed, my the accounts of many, a legend. Why not build a very special OTA for it. I have recently visited some of the old Alvan Clark refractors and love the look of the tube -- wide in the middle and tapered at each end, classic white, bolts all around it (you could put faux bolts). Would be really impressive seeing your lens mounted in something looking like this...would be a big draw IMO.

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