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UltravioletPhotography

Astronomy Filter Sizes and Adapters


enricosavazzi

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EDITOR'S NOTE: These posts were split off from Example pictures taken with PrimaLuceLab UV filter in order to provide a reference to mounting Astronomy filters for photographic use. Thank you, Enrico, for this useful information.

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Thanks Enrico & Alex

So the glass size is the same as the BaaderU2 ?

Col

The glass disk itself is slightly smaller in the PrimaLuceLab U. Both 2" filters come mounted in metal rings with threaded attachment. A short time ago I started writing a page for my web site about re-mounting astronomy filters, then my time was caught up in other things and did not manage to finish. It may be interesting for others to read a part of what I wrote, or the site administrators may wish to move it to a FAQ (with the understanding that, eventually, I will publish these materials in this or a similar form on my web site).

 

Astronomy filter sizes

 

Two sizes of mounted astronomy filters are commonly used: 2" and 1¼". The most common question in this context is how to mount a 2" threaded astronomy filter on a camera lens that accepts a standard, metric filter size. There is a considerable confusion in the information available on the web on how to mount 2" filters on camera lenses. Dozens of sites and bulletin boards repeat the statement that a mounted 2" astronomy filter can be adapted for use on a camera lens via a 52 mm (or whatever other filter diameter the lens accepts) to Series VII adapter. This is simply wrong. The Series VII female thread is much too wide. There is no way to screw a mounted 2" filter into a Series VII adapter, even with generous amounts of thread tape or glue. In addition, although a female Series VII thread is wide enough to admit the non-threaded portion of a 2" filter ring, a typical Series VII female thread is too shallow to completely contain a 2" filter ring and a Series VII retaining ring to keep the filter in place.

 

The 2" size specification for astronomy filters means something completely different from thread diameter, or diameter of the glass blank contained in the filter ring. It means instead that the outer diameter of the metal filter ring is 2", or 50.8 mm. The actual diameter of the filter ring is actually slightly less than 2", since the filter must fit within the inner 2" diameter of the eyepiece tube (or in other fittings, like a filter wheel, where similar space constraints apply).

 

A Series VII female thread does accept a non-mounted optical filter with a 2" diameter, or 50.8 mm. You also need a Series VII retaining ring to keep the filter in place (unless you glue it in).

 

Although adapters are available to attach a Series VII accessory to a 52 or 55 mm camera lens, you need a second adapter, this one with a Series VII male thead and a 52 mm or 55 mm female thread, to mount a lens shade with standard thread at the front of the filter. With some luck, you may find a slightly out-of-specifications 55 mm lens shade that screws sufficiently deep into a Series VII thread, but this is a hack, rather than a reliable solution suitable for daily use.

 

2" astronomy filters have a male 48 mm x 0.6 thread. You are reading correctly. The pitch is 0.6 instead of the standard 0.75 used in 48 mm camera filters. This means that the male thread of an astronomy filter will usually seize after being screwed into a standard 48 mm x 0.75 socket for about one turn. Still, one turn is better than nothing, and with normal care the filter is unlikely to unscrew and fall off.

 

2" astronomy filters differ from standard camera filters in one more respect. Standard camera filters (with the exception of filters in very shallow mounts designed to avoid vignetting with wide-angle lenses) carry a female thread at their front, of the same size as the rear male thread. This allows the easy stacking of two or more filters, as well as attaching a lens cap or a lens shade at the front of the filter. In 2" astronomy filters, instead, any of the following can apply:

  • Some 2" filter rings have an MA48 x 0.6 female thread at their front. This is similar to what you should expect of a camera filter.
  • Some 2" filtes rings have a different (often smaller than MA48) front thread diameter.
  • Some 2" filter rings have no front female thread at all, and the retaining ring that keeps the glass in place is friction-mounted, or glued, or is a C-spring held in a shallow groove. Sometimes, there is a threaded retaining ring, but smaller than the nominal filter thread and screwed in from the rear, with the glass resting on a machined shoulder at the front of the ring.
  • Some 2" filter rings have a short front thread that is filled by the retaining ring. If there is an exposed portion of female thread, it is too shallow for safely attaching anything at the front of the filter. This is the case of the PrimaLuceLab U filter.

Although the mounting thread of a 2" filter is supposed to be always M48 x 0.6, the diameter of the glass disk is not standardized, and varies by several mm across different filter brands, series and types. It can be as much as 8 mm smaller than 48 mm.

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One added complication is that threaded astronomy filters are designed with the male thread facing towards the subject, which is the opposite of photo filters. For coloured glass it's not a problem, but filters with dichroic coatings (the Baader-U comes to mind) have to be reversed-mounted, either with a reversing ring or by reversing the filter in its cell. The only source of 2" or 1.25" reversing rings that I have been able to find is a company called Blue Fireball, and their products are not cheap.
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When I got my Baader filter, rather than construct some fancy adapter, I just cannibalized the 49mm ring from an old secondhand skylight filter, transferring the glass disk to that. My Baader is now labeled TIFFEN SKYLIGHT, but what the heck...
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When I got my Baader filter, rather than construct some fancy adapter, I just cannibalized the 49mm ring from an old secondhand skylight filter, transferring the glass disk to that. My Baader is now labeled TIFFEN SKYLIGHT, but what the heck...

Same here. I used a 52 mm B+W Pro filter ring because it is machined from blackened brass, has a wider-than-usual inner shoulder (enough to keep in the 2" Baader U glass) and a retaining ring screwed in from the rear, but other than that, the principle is the same. The 2" Baader U comes with a stick-on label that can be peeled off the original ring and reattached to another ring, or at least mine did.

 

I have about a dozen UV-pass filters of smaller sizes mounted in 25, 28, 30.5, 34 and 37 mm filter rings from eBay. They are much cheaper than the empty filter rings from Edmund Optics. The main thing to watch out for is that they have a screw-in metal retaining ring (cheaper ones don't). Black rings (as opposed to "silver", or natural aluminium) also help better against reflection, but sometimes you order based on a picture that shows a black ring with a screwed in retaining ring, and get a silver one, or one with crimped-in glass instead of a removable retaining ring.

 

Edit - It may be a coincidence, but it is remarkable that the 2" PrimaLuceLab U ran out of stock within weeks of us starting to write about it.

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  • 4 weeks later...

One added complication is that threaded astronomy filters are designed with the male thread facing towards the subject,

 

Sorry to "step on" your thread Alex.

 

renaud, I have seen this stated elsewhere but am fairly sure that is incorrect.

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renaud, I have seen this stated elsewhere but am fairly sure that is incorrect.

 

Well, I have to confess that I was just repeating what I read here and elsewhere. I don't practice astrophotography and I have no idea how astronomy filters are normally mounted in a telescope. As I said, I don't think it makes any difference for dyed glass. However, the common knowledge around here is that dichroic filters have to be reversed for photographic use (I have just checked that the UV/IR filter sticky states that for the Baader-U filter). Is there an astrophotographer here who can solve this question?

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enricosavazzi

Well, I have to confess that I was just repeating what I read here and elsewhere. I don't practice astrophotography and I have no idea how astronomy filters are normally mounted in a telescope. As I said, I don't think it makes any difference for dyed glass. However, the common knowledge around here is that dichroic filters have to be reversed for photographic use (I have just checked that the UV/IR filter sticky states that for the Baader-U filter). Is there an astrophotographer here who can solve this question?

I don't do astrophotography, but I am fairly sure that most 2" astronomy eyepieces have a female 48 mm thread at the end of the barrel (2" outer diameter) that inserts into the 2" (inner diameter) eyepiece holder of the telescope. A filter screwed into these eyepieces is oriented the same way (i.e., light enters at the "front" of the filter) as the same filter mounted at the front of a camera lens. The same should apply to 1-1/4" eyepieces.

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