Stellarvue (SV) and Our Own - Vic Maris (Vic)
SV - How many telescopes do you think Stellarvue has built over the years?
Vic - Many many thousand over the past 21 years! Next year the annual number we make will be much lower as we are liquidating our Access (import) telescopes now and we are meticulously hand figuring every apo triplet here in our optical shop. I cannot tell you the exact number since each objective takes months to complete to this standard. It takes a very long time to hand figure .99 Strehl objectives with six smooth surfaces but the satisfaction we get from doing these makes the effort worth every moment.
SV - Simply speaking, what is an Apo?
Vic - Basically a refractor with no false color. Technically, "an objective corrected parfocally for three widely spaced wavelengths and corrected for spherical aberration and coma for two widely separated wavelengths". There are many other definitions depending on which dictionary you are looking at but this is generally accepted to be the definition. Some dictionary definitions omit the spherical corrections and I have seen some definitions requiring three or more elements. A two element objective using a super ed or fluorite objective that is has a long enough focal length and is well corrected can meet the generally accepted criteria. So you can have a two element apo. However most two element refractors have more false color than well made triplets and the triplets can be made much shorter and remain free of false color.
SV - What are the most common questions you’re asked from customers?
- The most common has to do with thread sizes,
getting an imager's camera back spacing right, letting them know that certain software programs are not necessarily showing right right info (as when someone has perfect stars across the field but cannot enjoy it since their software is showing a non-existent problem). Alex and I spend hours each day helping folks who hit a dead end. Solutions are often easy and mostly overlooked. We get them back on track by listening, reviewing their images and explaining things like the importance of everything being tight, the need to adjust back-focus as needed and the effects of cool down, atmospheric refraction and why it is not a good idea using a cheap star diagonal! 😊
SV - It looks like there is a small scratch (or a few very small dust particles) on my lens, does that hinder the performance of my telescope?
- All telescope lenses have these and no, they are far below anything that can be seen or imaged at the eyepiece or in the camera. In terms of micro scratches or digs, we inspect each surface using a microscope to make sure they have a scratch/dig ratio below military spec. Part of the process in making high end apo refractors is using test plates to measure the precise curves. These can leave extremely small marks that have no impact on performance but result in a much more accurate lens.
Dust particles are unavoidable. We blow out each tube in a clean room, and clean the lenses on our flow-bench using Hepa filters. We assemble them carefully and do a final inspection to make sure they are not there. But then they are shipped. Some very small particles of dust that withstood the compressed air will dislodge from behind baffles and other places and migrate during shipping. The amount is far less than what you will have two years down the road as you uncap the cover, switch eyepieces, etc. A little dust never hurt anyone. As
Fraunhofer said, "Look through the instrument, not at it!" So put your flashlights away and plan out your observing session like a real astronomer! 😎
SV - What is Strehl ratio and why is it so important? Does it matter if I'm not imaging (why)?
Dr. Karl Strehl first defined this more than a century ago. It is a way to measure aberrations in an optic compared to a literally perfect optic. His definition was "the ratio of peak diffraction intensities of an aberrated vs. perfect wavefront.
It is basically the amount of energy in the airy disk of a perfect optic (represented as 1.0) compared to less than perfect optics (0 - .99). So if you can imagine a lens so perfect that it places 99% of the light precisely where it should be, you have a .99 Strehl objective. Such an objective will have a very intense airy disc and extreme contrast. Many mass produced import objectives we have tested over the years (all but three) have ranged from .7 - .92 Strehl. Our new objectives are hand and machine figured here in Auburn to .98 - .997 Strehl.
Strehl ratio is only part of what we look at as w
e optimize optics here in our shop in Auburn, California. We also work on specific aberrations that impact both visual and photographic performance. We evaluate and correct for spherical errors, zones, astigmatism, coma and trefoil to a level that is five times better than what we were doing before this process was developed.
Lately, Alex has been trying to make all lenses come in at .99 Strehl or higher. You can immediately see the difference when you look through a highly accurate set of optics at .98 or higher - perfect airy disks with 98 - 99% of the light going precisely where it should.
: I remember one night back in 1985 when my club was observing under a clear and dark sky at our remote Sierra location. I was using a commercial 8" SCT that was purported to be diffraction limited (.8 Strehl). People nearby started saying we where under "Super Seeing" conditions. I disagreed. Saturn looked like the same soft image as always with Cassini's division and not much else showing. So I walked over and looked through the telescope they were using, an Astro-Physics 6" Apo.
I recall being less than eloquent. I shouted, "Holy Crap"! I had never seen Saturn so detailed, with razor sharp lines and vivid planetary detail. The contrast was stunning. I went back to my SCT. Same soft image. I packed it up and that was the last time I ever observed through it. Realizing we were under exceptionally steady air we all decided to turn all of our telescopes at Saturn. Nearly all of them showed the same lack of detail as my SCT. The exceptions were two reflectors using very high Strehl John Hall mirrors and a 22" Obsession using a very smooth Galaxy mirror. The image in the Astro-physics refractor was dimmer but every bit as detailed and there were no tube currents. I rated it the best on the hill. Literally every other telescope on the hill including every commercial telescope could not reveal we were under magnificently steady skies. Accuracy matters.
: Many aberrations can make imaging more frustrating than it already is. For example, if a telescope has glass with long range inhomogeneity or if during polishing the element was improperly mounted, the result can be trefoil. Stars will appear as triangles. No well versed imager would accept this. Astigmatism is another error that needs to be corrected for and is common. Better optics make better astro-graphs.
SV - What are the differences between field-flatteners and reducer/flatteners, and do customers need both? (why have one vs. the other, are they just for imaging?)
- Apochromatic refractors are made to look through. To convert an extremely sharp visual lens into a camera lens you need a field flattener that will project the view not into a 1/4" opening in your eye but a larger, flat camera sensor. Flatteners are only used for imaging. Visually, one is not needed. When observing visually the field will be flat if you use a well corrected eyepiece, such as our Optimus eyepieces. Cheap eyepieces will always have issues but this is not the fault of the telescope.
A flattener turns the telescope into a telephoto lens that has the same focal length as your telescope. So our 102T f-7 (714 mm focal length) Apo Triplet becomes a 714 mm f-7 telephoto when a flattener is used.
A reducer/flattener is like a flattener in that it projects the image onto a flat camera sensor but it also reduces the focal length. Use our .74X reducer/flattener on the 102T and you end up with a 535 mm f-5.25 telephoto. This means the image will be brighter, the objects will be smaller and the area of the sky you image will be larger.
Which one you need depends on what you want to image. A reducer may make objects too small, likewise, a flattener may not capture everything you want included in your shot. So you need to look at your telescope's focal length with and without a reducer/flattener, the size of your camera's sensor and what you want to image. Richard Wright of Software Bisque wrote an excellent article on this and more in Sky and Telescope magazine's online site:
SV - If they can buy just one eyepiece, what would you recommend?
Vic - I wouldn't. What I would recommend is at least three. One eyepiece alone may show magnificent wide fields of view but planets will be the size of a pinhead. So I would recommend at least three and preferably four:
- Low Power/Wide Field. This is the eyepiece to use when finding things in your telescope and the one to use when wanting to view extended objects like the Andromeda galaxy. Get under a dark sky with this and start touring the Milky Way looking at thousands of stars. My recommendation, without hesitation is the Stellarvue Optimus 20 mm.
- Medium power, This is the deep sky eyepiece. When you see a small nebulae in the wide field eyepiece, replace it with this eyepiece to increase the size, detail and contrast of the object you are viewing. Use the Stellarvue Optimus 9 mm or 13.5 mm for viewing the deep sky!
- High Power, So you want to look at planets, split close double stars and see craters on the moon down to two miles? Switch to high power. Now this eyepiece will vary depending on seeing conditions. If the air is still use our 3.6 mm Optimus but if it is a bit turbulent use the 4.7 mm Optimus.