Month: January 2013

Reader:  Thanks for the previous reply. I have more detailed information for you this time, and another question. My supervisor tells me that our lenses which I have to test will each be mounted in its own metal cell. Our test needs to align them to a common optical axis and test them working as a system to produce an image of a test target. The target will be back-lit and use white light. The goal of this test is: determine the correct spacing between each lens which will result in the very sharpest image of that target. This has been calculated already using software, but we need to convince ourselves that the designer was correct! Finally, one of our other divisions has an alignment telescope which we can borrow from them to help with this testing. It is a Davidson Optronics D-275 Model Telescope and it’s in good calibration. How can I use this instrument to align those lenses and help make this test work?  Thank you!

Tony:  Yes, you can use that alignment scope for this test. First, you need to ask your lens designer if it is safe to assume that these lenses when mounted in their respective cells do not suffer from any “wedge” problem. If they do not have any significant “wedge” issues, then the optical axis of each lens will be co-linear with their mechanical axis. (Note that it is not a given that this will always be the case. I have seen purchased lenses which met all the procurement specs, but had bad wedge issues, because this had not been specified during procurement; a bad mistake. As a result, their optical axes were skewed with respect to their mechanical centerline axes.)

Next, make a cross-hair reticle to mount on each of the lens cells. This can be as fancy a reticle as you can afford to make. Or simple. You can have these made and mounted in your machine shop, if you have the time, and you need them to be perfect. Otherwise, make your own reticles with mono-filament nylon fishing line from a sporting goods store. Use a line which is around 10 mils thick. Mount two pieces of this mono-filament nylon line on each lens cell so that they cross at the mechanical center of each lens. These lines do not have to be at exactly 90 degrees to each other. The important thing is that their intersection (crossed-hairs) is as close to the exact center of the lens as possible. Mount them under some tension, so they do not flop or flex. This concept assumes that the lens does not exhibit any wedge! When they all have cross-hair reticles mounted, continue on.

Before you begin to mount these lenses on your test bench, first set up the D-275 Telescope behind the place where you expect the image plane to be, so that it’s looking directly at the center of your test target, and so that its line of sight is passing directly through the center of your image circle in the image plane. Mind you, this is not easy to do, it will take some effort and time, but it is critical. So do this part well!  What this will accomplish is the line of sight of that D-275 Telescope will represent (take the place of) the actual optical centerline of the lens system you will be testing. (Remember, you can focus the Davidson D-275 scope anywhere in space from infinity down to 16 inches away from the front of the lens barrel; so take this into consideration when setting up the scope.) Note, from this point on, do not move the D-275 Alignment Telescope in any way! It is now your reference line of sight. When you are sure you have this condition set up, then continue on.

As you insert each lens and cell into your test bench, refocus the alignment scope as needed until you see the reticle on that lens cell in sharp focus. Move or remount each lens cell until that cross-hair intersection is located exactly on the center of the thin black internal reticle of your alignment scope. You may have to use a bright light source to illuminate the lens reticles to see them well. Repeat this step with each lens & cell until they are all mounted on your test bench.

When you have all the lenses aligned to the line of sight of the D-275 scope, you can be fairly certain that their optical axes and mechanical axes are co-linear and aligned to your test target and image circle. Note that when you have to remove the Alignment Telescope, (I assume you will have to remove it to proceed with your image quality evaluation), it would be ideal if you had it mounted in such a way that it could be replaced back on that test bench and returned to the exact position in space where it was before. That’s in case you need to repeat the alignment. Otherwise, you will have to realign your line of sight again! This may be tricky to do, but it’s well worth the effort to get it right the first time! (Experience speaking here!)

Of course, if your lenses need to be in very close proximity, so that the reticles interfere with lens surfaces, then you will have to cut them off prior to final adjustments of the lenses. This is where you would appreciate a “cleverly made” reticle which can be removed and reinstalled any time. We do not always have that luxury. You may need to remove them anyway, to prevent them from optically perturbing your image evaluation. It depends on the details.

Then, just move those lenses back and forth, but only in their axial direction, until you find their optimal spatial positions for the best image results. I am assuming that you have a mechanical means for precisely adjusting the axial position of each lens. You are going to need that!  I have omitted some details in this overview. Since your company has done other optical testing, it’s fair to assume that you will have the ability to work through some of the detailed engineering steps, which you are likely to encounter in this work.

Good luck with your testing. I hope this information will help you.  T.D.

Reader:  Speaking about telescopes, I have a question you might be able to answer. I recently looked at some Internet photos of the Hobby-Eberly Telescope at McDonald Observatory, which is in Texas. The photos of the primary mirror which is a segmented mirror in the shape of a hexagon do not show any opening in the center. It appears to be pretty solid, except for the segmented pieces. Question:  How do they get light to the Cassegrain focus, which should be behind the primary mirror? Is there some trick they use which is not obvious? Is this telescope not a Cassegrain scope? Your “About Me” page says you worked at astronomical observatories. Thanks for any light you can shed on this (no pun intended).

Tony:  You asked the right person this question, I suppose!  As it turns out, I worked at the Hobby-Eberly Telescope (we referred to it as the HET) at McDonald Observatory. While there, I was their on-site optical engineer for that scope. So, I definitely know the correct answer to your question.

The HET is not a Cassegrain telescope. It is a Prime Focus type of telescope. Therefore, there is no secondary mirror at all. There are mirrors up near the Prime Focus, but their purpose is to reimage the pupil of the scope on a corrector surface (also a mirror.) More on this shortly. The primary is segmented as you said, incorporating 91 individual mirror segments, each of which is a hexagon shape, one meter across the flats. So, the resulting primary is enormous, as you might imagine. That’s what makes the HET a wonderfully useful telescope for doing spectroscopy of faint objects, which is its main reason for existence!

The figure of each of the mirror segments is spherical, and they all have the same radius of curvature, so when they are adjusted correctly, they all fit into the same global sphere. Of course, the resulting large spherical mirror will suffer from spherical aberration. This aberration is corrected very nicely by a “corrector mirror” which has an aspherical surface. Its figure is “just right” to cancel the spherical aberration. This is located up close to Prime Focus. The image at Prime Focus is analyzed by a spectrograph, which is how data is acquired by the astronomers, for whatever research they happen to be doing. When I worked at HET, there was a low-resolution spectrograph located in the Tracker Assembly right at Prime Focus, and two other systems, a medium-resolution and a high-resolution spectrograph located in the basement below the scope. Light was fed to these spectrographs by means of fiber optics originating at Prime Focus and terminating at the appropriate spectrograph optics below. I don’t know if that configuration has been changed, now. I believe the HET is being used for a new research project.

Hope this helps you get a better understanding of the Hobby-Eberly Telescope. Thanks for your question.

Reader:  Hello. I am an optical engineer by trade and a newbie amateur astronomer. I bought a new Schmidt-Cassegrain telescope (SCT) recently. It is a “….” (brand and model deleted by T.D.)  I collimated this scope twice, using a bright star, per the manufacturer’s instructions. I have 2 questions for you:  First, why does everybody in this hobby refer to this process as “collimating a telescope?” My training in optics engineering tells me that this has very little to do with a “collimated” lens! And second, although I followed their instructions exactly and it appeared that I accomplished the goal of this adjustment, I am not happy with the images I get. They seem a bit “soft” to me; if you know what I mean. I expected this big SCT to deliver nearly perfect (diffraction-limited) images!  Am I doing something wrong? Help!

Tony:  Nice to hear you’re getting into astronomy as a hobby; it’s a good hobby. I can answer both of your questions. First, your question about the use of the term “collimating” is a good one and you are correct. For those readers who may not be familiar with this technique, the process known as “collimating a Schmidt-Cassegrain telescope” is a misnomer, in the field of optics, because it’s actually a procedure for “aligning” the secondary mirror of the SCT to the primary mirror. It is truly an alignment task. It’s only necessary to do because, in the case of many mass-produced amateur SCT scopes, their secondary mirrors (usually spherical figures by the way) are heavy, but poorly supported in the center of the scope’s corrector plate. Because of their inadequate mechanical support, they tend to get misaligned to the primary mirror (also a sphere) when the scope is jostled in transportation, or by rough handling.

But, you are correct; the term is a misnomer, in the sense that we “optics” guys use it. A telescope used for astronomy is normally “collimated” anyway, by virtue of the fact that it makes images of objects that are located at infinite conjugate. So it’s the reversed version of a “Collimator” which you might use in a lab. When you adjust that secondary mirror by tilting it, you’re just aligning it to the optical axis of the primary mirror which normally does not tilt.

Your second question requires a more delicate answer! In my career, I have examined a lot of telescopes used by amateurs (as well as some big-boy, professional observatory scopes!) Mass-produced amateur scopes (especially SCTs) often do not provide the exquisite, diffraction-limited imagery you might expect as an engineer working in the optics field! They are still good, in the sense that they don’t suffer from chromatic aberration, and they offer fairly large apertures for modest cost.

But, for one thing, they have an obvious central obscuration (the secondary) in their pupil. That means they suffer from apodization, by definition. This means their MTF (Modulation Transfer Function) is altered such that energy is stolen from the higher frequencies and added into the middle-range frequencies. (You can look this up in a good optics text.) The resulting apodized image is a peculiar animal. It just does not look like a classical-diffraction-limited image does. But even the big boy Cassegrain scopes used by professionals at observatories have the same issue. They usually have a secondary mirror component which is their central obscuration.

The other fact is, because they are mass-produced in a factory, and their manufacturers need to make a serious profit, their optical elements may not always be perfect. They can have some zonal defects, even if they are nearly perfect spheres! The images which result will vary a lot from one specimen to the next, even from the same lot of products made the same week! I observed this to be a fact, in my personal experience, in a previous occupation.

So, you may be adjusting your new SCT in the correct manner, and not doing anything wrong. If you continue to be uninspired by the imagery you are getting, contact the manufacturer of the scope and discuss this performance matter. Also, you might look through similar scopes which other amateurs are using, and do some comparative observations to see where your scope falls in the area of imagery performance. You might find you are doing as well as the rest are. Hope this information helps you!