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One for Roland/TMB/APO makers

Started by klawson, 07/09/2006 08:10PM
Posted 07/09/2006 08:10PM Opening Post
I know that in designing a standard achromat cell there can be a little slop in the tolerances between the OD of the lens and the ID of the cell. Say .002" to .004". So what about a APO doublet?

The steep inner curves cause the lenses to be very intolerant of lens shifting. I performed some experiments some time back on a couple of optical design programs and was surprised to see how little shifting it took to destroy the high power imaging of 2 element ED APO lens (.0015"/.038mm.).

I have a Meade 178 ED that has the best overall lens correction of any refractor I have owned to date (I've had a few in 30yrs). I love what I see through it. It gives stunning images but I find that from time to time I have to do a "little" minor tweaking of the lens to realign it (on axis colimation is spot on). It has the new lens cell that Meade designed some time ago to "fix" the shifting problem and I have reworked some of the issues concerning it but as a refractor user I could care less to have to do a realignment every month or so.

I can machine a new cell to a tolerance of +/-.0001" if I have to. So my question to you is this: What are some of the "points" to remember in fabricating a cell for a APO doublet? Please forgive my ignorance. If I'm asking too much (trade secrets) I will understand if you don't answer this question.

Seeking to fix a problem,


40 year old restored C14 with recirculating cooling w/Skywatcher DOB GOTO Conversion (CPC14)

Synscan DOB mounted Sky-Watcher Evostar 150mm/1200mm ED Refractor

"Rescued" Craig's List Orion 120mm/1000mm Refractor


Posted 07/09/2006 08:23PM #1
>>I can machine a new cell to a tolerance of +/-.0001" if I have to. So my question to you is this: What are some of the "points" to remember in fabricating a cell for a APO doublet?>>

The main problem is that metal has a much larger expansion co-efficient that glass. So, if you make a cell to fit the glass as close as .0001", it will seize up as soon as the temperature drops 2 degrees F.

In order for an aluminum cell to always clear the glass down to -20 degrees F, the initial clearance at room temperature must be .001" per inch of aperture. Therefore, for a 7" glass, the clearance must be .007". If you just want to go from room temperature to freezing, you can cut this in half, but you must allow for it. If you don't have enough clearance, the glass will seize up and eventually spring or bust in half.

The clearance of the glass to the cell is not really your problem with this doublet. When you have the lens aimed even slightly off the zenith, both elements will naturally slide towards the lowest part of the cell, and they will line themselves up properly, even if the clearance were 1". What is really the problem is the spacer thickness. This will vary ever so slightly, and will throw the collimation off 10 to 100 times faster than any edge misalignment.

Of course, any slight spacer shifting can be compensated to most extent by shifting one of the elements laterally. That is why their advanced cell has the "collimation screws", which are not really for collimating, rather they are for compensating for spacer thickness variation and shifting.

Even if you managed to make a very tight fitting cell, you will still have the exact same problem. It will not go away.

Roland Christen