I have an old (nearly 50 years old)alt-azimuth three-inch (7.6cm) refracting telescope that someone else made. They ground the lens, created the cylindrical body and added a rack and pinion style focusing slider. It is mounted on a homemade tripod.

This has been poorly stored for several years in damp conditions and has developed mould on the main lens elements. The main lens elements comprise a convex and concave combination (presumably to reduce achromatic aberration?)


I've cleaned the main lens elements in soapy water and removed the mould using a soft cloth.

How would you now test this lens? Would you reassemble the telescope and check resolution against known double stars or are their tests I can apply to the lens before reassembling the telescope?

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    $\begingroup$ Sometimes doublets will have marks to show how one lens was rotated to best match the other. This wouldn't matter for perfect lenses, but for imperfect (real world) lenses it's important. Did you notice any such marks? I don't know if the telescope maker would have made any, but it's possible. $\endgroup$
    – uhoh
    Dec 9, 2019 at 16:57
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    $\begingroup$ Let me suggest searching: "star testing telescope". Or, if that's too basic, then "optical bench test refractor objective". If you had separated the lenses (3, 2 or single lense objective ?), you will have to collimate them. Else, the forum "cloudynights.com" has a wealth of information :-) $\endgroup$
    – user31179
    Dec 9, 2019 at 20:02
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    $\begingroup$ ebv make your comment an answer. That about what I was going to say. $\endgroup$ Dec 10, 2019 at 2:22


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