Cheap Astronomy – Using Microscope Eyepieces For Astronomy

An important part of astronomy is having a selection of eyepieces at one's disposal, as it is important to e.g. lower the magnification to locate an object, increase magnification to see details of an object (e.g. an planet), or generally adjust the properties of the optical system due to various parameters (size and brightness of object, aperture and focal length of the telescope, desired exit pupil, and so on) to optimize the astronomical observation of an object.

One can spend a lot of money on eyepieces, as even cheap (but reasonably good) eyepieces, that cost next to nothing in production – like simple Plössls, that are thrown in for free with various entry-level telescope sets – are rather expensive if one buys them alone. Buying a scope with included good eyepieces is a good start, but it only gets you so far – so having microscopy as another source of (usually) cheap eyepieces is good.

A short interlude:

If I write here about microscopes, I mean "monocular" microscopes, and not stereo-microscopes – the later are a different beast, and AFAIK not such a good source of cheap eyepieces. (And just in case: a monocular microscope with an binoviewer is not an stereo-microscope.)

Mechanical and labeling differences

But of course, while there are a lot of similarities between eyepieces for microscopes and for astronomy, there are unavoidable differences. The first difference between an eyepiece for an telescope and one for an microscope is the outer diameter of the eyepiece barrel. There are three established sizes for astronomical eyepieces:

• 0.965 inch (24.5mm)
• 1.25 inch (31.75mm), aka "1 1/4 inch"
• 2.0 inch (50.8mm)

The 0.965" size was common long ago for (smaller) Japanese telescopes, and is found today mainly on cheap telescopes. The 2" size is typically found on the larger and on the more expensive telescopes, which leaves the 1.25" eyepiece size as the semi-standard for entry-level and for some of the compact telescopes.

On the other hand, the majority of microscope eyepieces come in the standard 23.2mm size. More modern sizes for microscope eyepieces include the 30mm (and 34mm) standard, while some manufactures use even other sizes. Buyer beware. Typically older microscopes come with 23.2mm eyepieces, while 30mm would be found rather with newer microscopes (but some new microscopes still come with the old 23.2mm standard).

Another interlude:
Don't buy toy microscopes

It has to be said: There are many different microscopes that one can buy (research, medical, metallurgic, or student microscopes, to name a few), and then there are these obnoxious toy "microscopes" (with little plastic "boxes" and some toy-gadgets like toy-tweezers, and prepared glass slides). Typically a proper microscope is heavy (easily a kilogram or two), and typically the distance between microscope lens and microscope eyepiece is about 160mm, some 6 inch (with larger specimens going into the tens of kilograms and requiring an table of its own). As a "rule": anything that is substantially lighter or smaller is typically a toy, giving you only an hint of what an "real" microscope could do – so do not buy toy microscopes. (There are exceptions to this "rule": good small microscopes for mobile use – but these tend to be expensive.)

Furthermore, while it is tempting to buy such a toy for cheap and take it apart (which can be instructive in itself), it will however not help you get useful eyepieces for astronomical use out of it, as the eyepieces in these toys are typically not suited for astronomy at all.

So back to proper optical gear…

A 23.2mm microscope eyepiece can be usually fitted in any piece of astronomical equipment that accepts 0.965" inch (24.5mm) eyepieces. While one can simply mount an 23.2mm eyepiece in an 0.965" telescope (if the screw to hold the eyepiece is long enough) and ignore that the eyepiece is slightly offset from the axis, it is advisable to pad the diameter. I have made good experiences with adding four layers of electrical tape to my 23.2 eyepieces, which works fine for me (so far) – and the tape can be removed if necessary.

Usually the mechanical stop on the microscope eyepiece is larger than 24.5mm and will hinder the eyepiece from taking a plunge into a 0.965" scope, but check this first – before you drop your eyepiece inside the telescope…

Same as above, 30mm microscope eyepieces can be used instead of 1.25" eyepieces. (But as the selection, quality, price and suitability for astronomy of 1.25" astronomical eyepieces is usually better than that of 30mm microscope eyepieces, I find it not so interesting – but do try it if you have a chance!)

On a side note, there seem to have been telescopes which would accept directly 23.2mm microscope eyepieces – but such instruments are rather rare, and therefore don't really matter for Cheap Astronomy.

Microscope eyepieces are labelled in "magnification power" (e.g. "15x"), which is a measure by how much the focal image of the microscope is magnified. You can calculate the focal length from this formula:

focal length (in mm) = 250 / magnification power

Or the other way round:

magnification power = 250 / focal length (in mm)

And here, for your benefit, is a focal length conversion table for some common eyepiece magnification powers:

Eyepiece magnification power	Focal length
4x	62.5 mm
5x	50.0 mm
6x	41.6 mm
7x	35.7 mm
8x	31.3 mm
10x	25.0 mm
12x	20.8 mm
15x	16.7 mm
16x	15.6 mm
20x	12.5 mm
25x	10.0 mm

Other microscope accessories

One interesting variant of microscope eyepieces are "projective eyepieces" (or "Projektiv" in German). These are specially designed for what is called "eyepiece projection" in astronomy, where the image is directly projected to a camera (directly onto film, and nowadays directly onto a CCD or CMOS camera sensor) without any further optics. The magnification of such an "Projektiv" is expressed as a ratio (e.g. "3,2:1" or "4:1"). I haven't tried these myself, but suspect that such eyepieces might be interesting for astrophotography. (There a "K" type or "Kompensationsokular" versions out there, which are probably not suited for astrophotography – but I may be wrong.)

Another interesting type of microscope accessories in 23.2mm are binoviewers. Though due to their weight possibly a bit more difficult to adapt to an telescope, these might be interesting for astronomical use. However such binoviewers are more susceptible to dirt, miscollimation and general mistreatment, and are a bit more delicate than eyepieces. Either you have to pay more for one with guaranteed quality. Or if you get a cheap one with unknown quality, then be prepared to sell it again if isn't up to spec (and it is too difficult for your set of skills to clean and align it). Of course you can use an microscope binoviewer only with microscope eyepieces (unless you mechanically modify the binoviewer), and you now need two of each microscope eyepiece you want to use in it.

As to microscope Barlows, I'm afraid I haven't seen any such device in the wild… They are rumoured to exists, but are probably the domain of microscope aficionados, and not Cheap Astronomers. Some microscopes have an build-in zoom capability (or you can add a tube for zooming), but these tend to be rather heavy instruments – nothing I would mount to any of my cheap scopes.

Some notes on microscope eyepiece for astronomy

Using microscope eyepieces for astronomy has several drawbacks and there are some things you should keep in mind:

• The 23.2mm size limits the possible FOV.
• 23.2mm is slightly smaller than the 24.5mm of the 0.965" size, and padding the diameter is recommended (e.g. electrical tape works fine for me).
• You can't easily mount 1.25" astronomy filters.
• Microscope eyepieces can usually be quite easily fitted with reticles. Though the reticles are not illuminated, the reticles available are more interesting for microscopy than for astronomy, and the sizes of the reticles differ quite a bit between different makers (and even different models!)
• Low-power eyepieces are typically of a simple type type like Huygens, Ramsden or Kellner. While microscope Huygens eyepieces are superior to the Huygens eyepieces supplied with cheap scopes, they still offer the same small FOV.
• Anything shorter than 42mm (or "6x") is rare, with the occasional "5x" or "4x" being advertised. As the FOV is restricted to 23.2mm, these low-power eyepieces don't make too much sense anyway.
• High-power eyepieces (focal length shorter than 17mm or "15x") are difficult to come by, the highest power I have seen was my 10mm (or "25x"). This is due to the fact that one can easily change lenses on an microscope (and has typically a selection of different powered lenses mounted in an turret) – unlike the typical astronomical telescope, where one changes only eyepieces instead.
• Microscope eyepieces have typically lenses/elements that are not coated (but have typically less lenses/elements anyway).
• Microscope eyepiece have typically more internal reflections than astronomical eyepieces (which can be alleviated somewhat with flocking and somesuch DIY measures).
• For microscopy Kellner eyepieces are considered "wide field" (WF)
• Some combinations of microscope eyepiece and astronomical telescope can have excessive field curvature. Then you will have a sharp image "on axis" (in the middle), but out of focus image on the edge (and vice versa).
• Some microscope eyepieces are special types, which are usually marked (e.g. "KPL" or "CPL") – with typically P ("phase contrast"), C ("corrected" "compensating eyepiece") or K ("korrigiert" "Kompensationsokular"). These are made for special microscope lenses (to compensate an undercorrected color aberration of some microscope lenses), and will not work properly in astronomical telescopes. Every manufacturer uses its own codes (and the LOMO "C15x" is not an "compensating" type, but an symmetrical type, from the Russian/Cyrillic "симметричный"). And moreover some speciality eyepiece are not properly marked!
• Bottom line: some microscope eyepieces work nicely in telescopes, some less so. Some work better in telescope A than in telescope B. You have to try them out.

My experience

It is my personal experience with microscope eyepieces (in my two telescopes) that the Huygens eyepieces from microscopes (one CZJ, the other possibly from PZO) are far superior to the Huygens eyepieces supplied with cheap telescopes – but they are still Huygens eyepieces, with small FOV. So if today I had to choose (at comparable price and focal length) between an simple&cheap microscope Huygens, and an simple&cheap astronomical Plössl, I would take the Plössl (and after all, decent Plössls are not that expensive if you look around). And the same is true for the better microscope eyepieces I have (like the 17mm symmetrical, or the 10mm ortho): If you have to choose (at equal price) between an 10mm microscope Abbe ortho, and an similar astronomical Abbe ortho, then take the astronomical version.

When on the other hand you start out with astronomy (and have maybe a scope which accepts 0.965" eyepieces), and can get microscope eyepieces cheaper than comparable astronomical eyepieces (e.g. three microscope eyepieces for the price of one astronomical eyepieces), it can make sense for the Cheap Astronomer™ to get some microscope eyepieces instead. And if you already have an microscope with some eyepieces anyway (or plan to get one), than I would highly recommend to any self-selected Cheap Astronomer™ to give microscope eyepieces a try on the night sky.

BTW, I got my microscope eyepieces through two routes: I bought an old Carl Zeiss Jena (CZJ) "horseshoe" microscope, which came with several eyepieces. And then I bought microscope eyepieces (like the 10mm Abbe ortho and the 17mm symmetrical) when I had the chance.

Buying a microscope that comes with several eyepieces is a nice way to increase one's eyepieces collection. The included eyepieces are typically of simple type (Huygens, Kellner) and of lower power (maybe 40mm to 17mm, sometimes 12.5mm), and make nice "overview" eyepieces. If you have bought an microscope with eyepieces, you can either keep the microscope – most of them are fine instruments which can be handy sometimes – and use the eyepieces for both astronomy and microscopy. Or you go (after you have tested all eyepieces with your telescope and determined which ones to keep) and sell the microscope again with only one or two eyepieces (keeping the useful eyepieces) – and usually you still get an reasonable price.

What I found recommendable and affordable are the microscopes from former communist production – companies like Carl Zeiss Jena (CZJ), LOMO, PZO or Meopta. One advantage is that the eyepieces from communist production are typically not of the "corrected" type (or only very slightly corrected) and will work nicely in astronomical telescopes.

Furthermore, affordable scopes are/were sold by Bresser (like the Biolam – which used to be from LOMO production, if I am not mistaken, but now is supposedly from the Far-East). And of course the Chinese (and other Asian nations) are increasingly getting better at producing goods like microscopes (but older ones can be quite awful).

And if you can get an affordable used microscope from "western" production – from companies like Zeiss Germany, Leitz, Olympus, American Optical or Bausch&Lomb – then by all means try them (though at least Zeiss Germany has an tendency to supply "corrected" eyepieces).

And don't pay too much for an microscope! 20€ to 30€ is a good price here for a simple microscope (e.g. an "horseshoe") with three or four eyepieces (and three or four microscope lenses), with 50€ being way too excessive. And this is Cheap Astronomy after all, where we stay cheap, even if we stray into other optical realms.

tl;dr

In conclusion it might make sense for an Cheap Astronomer to get one or two simple Huygens microscope eyepieces in the area between "4x" and "7x" (something between f=63mm and f=37mm). Furthermore the mass-produced LOMO "C15x" (f=17mm) is not that expensive and an really nice eyepiece (an "symmetrical" eyepiece, which is a simple version of an Plössl) – I love using it. Maybe an "20x" microscope Kellner "WF" eyepiece would be a good addition for astronomy. And finally if you can get an high-powered microscope eyepiece (like the "25x" Abbe ortho 10mm I have) for an low price (lower than an comparable astronomy eyepiece!), you should consider buying it.

If you have a telescope that accepts 0.965" eyepieces and you have some 23.2mm microscope eyepieces (or can get some cheaply) then by all means do try them in your telescope! The same goes for 30mm eyepieces in 1.25" scopes.

Furthermore you will need to test the eyepiece with your telescope – some microscope eyepieces work better than others – depending on both what eyepiece and what telescope.

Don't pay more for microscope eyepieces than you would for comparable astronomy eyepiece – microscope eyepieces make only sense when you can get them cheaper than astronomical ones (e.g. as bundle, or together with an microscope). Or of course when you want to actually use them for microscopy. Don't shop around for expensive microscope eyepieces if you want to use them exclusively for astronomy – then rather get good astronomical eyepieces instead.

Microscope "projective eyepieces" (or in German so called "Projektiv" eyepieces) might make an nice addition for Cheap Astrophotography when you do eyepiece projection – though I have no experience with this.

Microscope binoviewers might be a cheap route if you want to add binoviewing to you Cheap Astronomy inventory.