Measuring eyepieces

[Tweedledee]

Anyone who knows a bit about eyepieces will also know that the manufacturers specifications are not always that precise, ie a 25mm 60 degree eyepiece may actually be 26mm 58 degree or a 24mm 62 degree. Some eyepieces especially wide angle ones, have weird non-linear distortions where the magnification varies slightly across the field of view, thus skewing the published specification figures. Apparently some manufacturers even lie about the specification of their eyepieces to make them seem more attractive. Most people will not even spare this a second thought, and certainly, it will not change anyones life or their viewing pleasure because it doesn't make much difference if the figures are slightly out. How would one find out anyway?

Well the rest of this post will probably bore everyone to distraction, so maybe you'd be best to check out some other more popular thread now.

The best way is probably to do a several star drift tests and take an average. That would need to be done with a star on the celestial equator, or if at a different declination then the calculation would involve extra trigonometry. Inacuracies would creep in if the star did not drift precisely across the longest diameter of the field of view. So that is something for me to check out on a suitable night when I'm not looking at anything more interesting.

One afternoon recently I was at a loose end, so I fixed a tape measure horizontally across my garden fence, as you do. I then set up the the ST120 at the other end of the garden at the same height as the tape measure and measured the distance from the objective to the fence. I then used each of my eyepieces in turn in the focuser to look at the tape measure. I noted down the measurement of the visible part of the tape measure from field stop to field stop in the widest part of the field of view to the nearest millimetre. Afterwards I fed the numbers into a spreadsheet I'd made and using the required trigonometrical formulae let it calculate the measured fields of view.

 

Not unsurprisingly, the numbers varied a bit from the manufacturers specifications. They were not all skewed in the same direction, and generally the higher powered (shorter focal length) wide angle eyepieces indicated that the manufacturer had understated the field of view and/or had understated the focal length. The opposite effects occurred with the longer focal length (lower powered) eyepieces.

 

These changes were greater than could be accounted for by any measurement errors or inacuracies introduced by me. The details in the attached spreadsheet at least show these relative trends, which is also born out by the second spreadsheet which calculates the numbers from the measured field stops, that I measured with a caliper. I don't think that these measurements alone can produce any conclusive results apart from that relative trend. The star drift timings should give a more conclusive result.

These results do bear out some of my web gleanings about the Explore Scientific 9mm 120 degree eyepiece. Various different American observers have been physically checking the FOV of this eyepiece to be anything up to 144 degrees! My own telephone discussions with the ES technical department also confirm this situation. My figures so far seem to show that it is certainly greater than the stated 120 degrees. The apparent field of view of this eyepiece is quite stupendous whatever the actual figure turns out to be. It is certainly a very unique eyepiece.

If anyone has an understanding of what I'm talking about and can shed anymore light on this, I'd be very interested.
 

 

 

[Daz Type-R]

I understand what your saying Pete and it does make an interesting read. Not a lot I can add to this really other than would it not be manufacturing defects that affect the FOV in any given ep? Liken it to computer or smart phone memory, my IPhone for example is classed as 32GB, yet it only has 28.6GB, my wife's works IPhone (same make and model as mine) has 31.4GB, yet they have the same components ETC. Could slight changes in any one of the elements introduce these variances across EP's, if so, I suppose manufactures have to give an average, as a EP selling as a 120-144 degree ep doesn't sound as good.

[M.]

Interesting.

I did a university piece that focused on 'uncertainties' it was to calculate the specific heat capacity of water and the variables that influenced the end result were countless.

That said, this is a very wide tolerance :/

Well thought out bit of work you have done there.

[Tweedledee]

I understand what your saying Pete and it does make an interesting read. Not a lot I can add to this really other than would it not be manufacturing defects that affect the FOV in any given ep? Liken it to computer or smart phone memory, my IPhone for example is classed as 32GB, yet it only has 28.6GB, my wife's works IPhone (same make and model as mine) has 31.4GB, yet they have the same components ETC. Could slight changes in any one of the elements introduce these variances across EP's, if so, I suppose manufactures have to give an average, as a EP selling as a 120-144 degree ep doesn't sound as good.

I don't think you can call it a defect as such, but I think you are correct that minute deviations in the positioning of lenses will easily create differences in specifications. This variation could potentially be greater with a larger number of elements. Your final sentence may also be very true

Edited July 7, 2015 by Tweedledee

[Tweedledee]

I wonder if other variations and errors can creep in to the measurements when focusing different eyepieces on close objects, that would not occur with an object at infinity.

[Doc]

I guess no one is learning to drive any more in Mansfield then Pete 

 

Fantastic piece of work Pete and I understand fully what you are trying to do but coming from an engineering background everything is made within tolerances, so of course you are going to get some eyepieces with a greater tolerance and some with a smaller. 

 

When I worked in the cigarette manufacturing business we had machines that cost over £3 million pounds, they made 3 million cigarettes in 7 hours. Every cigarette was checked for weight, moisture, hardness, lightness, the list goes on and every cigarette made was different, never was one made the same, but as long as it was within a tolerance it was deemed OK.

 

Good work though Pete.

[Tweedledee]

I think there are errors creeping in to my figures that I have not considered, as I feel in general that the tolerances would not vary quite as radically as my figures show.

[Doc]

I agree with you Pete. The companies involved should have way better testing equipment to test these measurements, you have invented a great way to show this but compared to the proper kit that they must have your figures are always going to be different.

[Smithysteve]

That's a great piece of work Pete, showing interesting results!

I like your use of a tape measure on the fence!

It was a good idea of yours to check these things out, though with respect to eyepieces I have not heard of anyone else doing it this way before. So good on you.

Some of the results, like the Meade etc show quite a difference in manufactures claims and actual FOV. It's quite a bit lower than claimed , so not brill!

The 9mm explore scientific 120 deg is quite an understated beastie! It has a much larger FOV than stated, so that's very good! I must have a look through that monster at some point! And see, literally, if my brain can take it! 😀

I would not be surprised if the focal lengths of individual eyepieces differed slightly in each build too, affecting magnification and the FOV across your tape measure etc.

All good stuff!