What filter size fits a Pentax 100 SDUF telescope?
Asked 3/3/2019
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2 answers
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I want to use a Pentax 100 SDUF telescope as a long focal length lens and add a clear protective filter to the front. The front cell has threads, but there are no markings showing the filter size. My rough measurement of the thread spacing/diameter is about 107.1 mm, and I have not been able to find a definitive specification online. What filter size should fit this telescope, and are screw-in filters for it commonly available?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
1
Often the millimeter size of the lens is written on the back of the lens cap. All of the spotting scopes currently being made by Pentax have the front lens size as part of their model number. If this applies to your Pentax 100 SDUF, then the filter size should be 100mm.
I went to B&H website (bhphotovideo.com) and discovered that they don't sell a 100mm circular screw-type filter. Since B&H pretty much carries everything, I assume nobody currently makes a 100mm UV filter. I have found one made by Olympus on eBay for $89. Here's the link https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2334524.m570.l1313.TR0.TRC1.A0.H0.X%22100mm+uv+filter%22.TRS1&_nkw=%22100mm+uv+filter%22&_sacat=625&LH_TitleDesc=0&_osacat=625&_odkw=100mm+uv+filter
Originally by user76522. Source · Licensed CC BY-SA 4.0
user76522
7y ago
0
Generated from our catalog & community — verify before relying on it.
Based on the community answer, the most likely filter size is 100mm. Pentax commonly used the front objective size in the model name for similar scopes, so “100” in 100 SDUF likely refers to a 100mm front size. Also check the back of the lens cap, since filter size is often marked there.
That said, telescope front threads do not always match common photographic filter standards, so a physical test or manufacturer spec is still best if you can find one. Availability may also be limited: 100mm screw-in circular filters are uncommon, so even if the size is 100mm, your choices may be sparse.
If your goal is purely protection, be aware that adding a front filter can sometimes introduce extra reflections or reduce image quality slightly, especially on a telescope used for imaging.
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UniqueBot
AI7y ago
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