Source: Tom's Guide US | Keywords: celestron, telescope | Themes: Digital Entertainment, Digital Cameras
5. Celestron's Telescopes
Celestron has four basic SCT telescope lines; the telescopes actually have optics produced by the same process, but the mounts and aperture ranges differ. There is the single (SE) or double (CPC) arm fork mount, and two grades of equatorial mounts. One is made in China (CG-5), while the other is a precision German equatorial mount (CGE, also very heavy); the grease used is $700 per container! A page on improving the CG-5 mount can be found here. These telescopes are known as optical tube assembly (OTA) designs, and most are available without a mount. Additionally, some of Celestron’s reflectors are available in a Dobsonian mount.
Celestron’s Factory
Like other manufacturers, Celestron’s less expensive mounts and telescopes are produced overseas.
Celestron engineering is done at their Torrance facility.
The premium mirrors and coatings, though, are all produced at Celestron’s Torrance factory. Alignments and calibrations are also done in house.
Verifying optical quality.
The heart of any reflecting telescope is the mirror; Celestron buys the blanks, and grinds and polishes them to better than 1/6 wavelength (total accuracy, equivalent to 1/12 wavelength at a Newtonian mirror) before multi-coating in a state of the art vacuum chamber. QC (quality control) verifies each stage via visual inspection and, usually, a functional test.
QC check in a laminar flow hood
The basic idea for a reflector is to convert the mirror’s spherical shape to a paraboloid. In an SCT, both mirrors are ground to spherical shape, and the corrector lens is aspheric. The steps are roughly listed below, and are very similar to what you might do to grind your own. Some of the big advantages Celestron has are the mechanical grinding apparatus, the temperature controlled tubs, and the optical benches. These would be expensive for any of us to construct! Obviously, some of Celestron’s exact sequence and methods are trade secrets, but this is the general process:
- Inspect blanks, and core the blank with a big drill press. The SCT design as shown in the diagram has a hole in the mirror for the focus point.
- Bevel the blank edges with a belt sander.
- Grind. Celestron uses diamond pellets on a lap base, but with a spinning wheel. The blank is on an eccentrically rotating arm, cooled with a constant spray of water. Without this setup, it’s just another reason it’s hard to grind your own as consistently.
- Polish. Again, they use a machine, and pre-heat the slurry used.
- Inspect. This is done in a laminar flow hood, looking for inclusions, scratches, pits, and other inconsistencies.
- Figure. The figuring tub is where the mirror is polished and (re)shaped. The polish actually embeds in the pitch (refined tree sap) and figures the mirror. There is a separate room for making the pitch laps. Celestron uses two different kinds of pitch to obtain the correct hardness, before cutting the grooves. The pitch is poured onto the polishing tool.
- LED Ronchi test: Using a pin point light source, a diffuser, and a grating (Ronchi screen), the light passes through the grating, bounces off the mirror, and back through the grating. The pattern tells you how close you are to a smooth parabola. Celestron has a precise stand for doing this testing.
- Near blacklight inspection (for the corrector). For the corrector and secondary mirror, the process is slightly different. Match plates are set on the corrector so the technician can determine how to lap. This repeated sequence is then:
- Figure
- Check figurewith match plate, looking at Newton rings. The plate and blank are sealed together and inspected. This is a basic interferometry test. -* Lap again. Repeat.
- Coatings. On SCTs, the primary mirror is aluminized, and the corrector plate is coated. The primary mirror has multiple coatings, including TiO2 layers, that increase hardness.
As a former electron microscope user, I thought this process was most impressive. Celestron’s large vacuum chamber, big enough to contain several large 6’+ men, is used to apply the XLT coatings. This chamber uses three vacuum pumps to get down to 10-6 torr; one is a high vacuum pump, another high vacuum pump is presumably a diffusion like pump, and the third is a cryo-pump. The area in back of this chamber holding the pumps, and to one side, housing the sputterer controllers, is bigger than the chamber itself.
The Celestron large ultra-vacuum chamber sputter coats lenses.
The sputterer-Celestron calls it an “E-gun”-vaporizes the solid mixes in the bottom of the chamber, including the hafnium and magnesium fluoride that become XLT Starbright. The mirrors are mounted on a rotating circle holder near the chamber top. Sample microscope slides are included with every chamber run to validate the thickness and evenness of the coating, which is measured with a precise spectrophotometer. A similar, but much smaller chamber is used to aluminize the primary mirrors. This XLT coating is considered as one of the best. Here is a graph of the transmission versus wavelength, showing that the XLT coatings are fairly uniform for both visual and CCD work. More details are here.
Whole system light transmission versus wavelength for StarBright XLT coating.
The final steps, with QC after each, are as follows:
- Matching process. This makes sure the corrector plate and mirror are optimally aligned. In a collimator, technicians perform another Ronchi test (150 lpi grating) and a knife test, as well as other tests. They make index marks on each pair to ensure optimum alignment. Mirrors that don’t match within specification (which is rare) are discarded at this point.
- Assembly of mirrors and components into the tube.
- Alignment. Celestron has a green laser optical bench with a long path to measure alignment and collimation, complete with calibrated targets.
For those familiar with telescope making, there was nothing too surprising in this process. I was very impressed with the coating steps, though, and how quality control, particularly at the end, is very structured and integral to the whole process. QC steps are carried out by someone other than the technician. Most of all, I was jealous: I would love to have use of the factory to build my own-just for a few days!
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GoTo mounts are for lazy people who don't really understand the essence of observing the sky. It's certainly a fun thing to create if you are an engineer, but if you're considering using one, take my advice and buy a book with pictures of the Messier catalog, it's a lot cheaper. A true amateur astronomer never uses those revolting, despicable, hideous GoTo mounts. Get a real standard equatorial mount. Get a good sky atlas. Get a pair of quality binoculars. Know your sky! That's astronomy...
A sensitive topic to be sure. But the point of the article was to encourage those who have an interest in astronomy but never bit the bullet. That is, they don't own a scope because they aren't willing to learn all that needs to be done to find stuff. OK, even lazy. And our readers totally grok computers.
Many amateurs started by using a GoTo. IMHO, the more astronomers the better. Talking to denizens of LAAS and reading cloudyskies.com, there is little sense of elitism. All amateurs are welcome, even and esp. those with GoTos.
I didn't have room to talk about the wedges that covert yoke mounts to equatorial mounts, but they exist for many yoke types. That way you can have the best of both worlds.
Finally, one of the books I recommended does just what you suggest: tries to teach the night sky so you can find stuff without using (or even using) a GoTo. Hope this addresses your comments.
Doug
I think the guys who say to get a star chart and spend 6 months learning how to find and track things are missing the point and living in the 19th century.
The fact is that these new generations of scopes make astronomy so much more accessible to people who find astronomy interesting but don't have the time or ability to invest. They also make it simple to introduce new people to astronomy by quickly showing them lots of interesting things.
Plus, as a computer geek, there is nothing more fun than plugging your telescope into your laptop, hooking up a camera and driving it around from your computer. Do an easy DIY project like adapting a webcam to use with the scope and you have yourself tons of fun *and* you learn the night sky, you just don't waste months of time trying to figure out how to see anything.
I totally agree with smurfdog - you spend more time observing and imaging objects than trying to find them - and most of them are very dim anyways. THe manual method of finding objects is definitely 19th century stuff.
Science is ever changing and self-correcting. To memorize the sky from Earth is one thing and to explore the universe is another. Technology makes Astronomy possible for all who are willing to learn and grow. Why not use both to your advantage.
Doug,
I really praise the point of your article. I also love to teach everything I know about the topic to anyone who's interested in the subject, like most amateurs.
Like you said, there's no to little sense of elitism in those groups. But I also know this: Like all the really rewarding things in life, Astronomy requires persistence and solid interest.
I decided to comment your article since I disagree on some things you wrote and that contrast may help people who read it, look at the picture from another angle.
Trying to find Deep Sky Objects or the planets (very easy with little experience and knowledge) in the sky, is where most of the fun is! Having a computer doing it for you is like you own a Porsche and let a chauffeur drive you. The only difference here is that, unlike the chauffeur, the computer has no fun at all!
I didn't find anything interesting on cloudyskies.com, apparently is just a domain for sale. Did you mean www.cloudynights.com?
If your interest is solid, you've got nothing to fear, understanding the sky is a lot easier than most people think.
Diogo.
Diogo et al,
My bad! Yes of course I meant www.cloudynights.com. It's a great site for advice, and mostly cogent tips on purchasing and use.
I think we can agree that for some, starting with GoTos is a great way of getting into astronomy. Then you can progress to star charts and maps, relying less on the GoTo. I want to encourage people to begin this exciting hobby.
Amateur astronomers are one of the few science disciplines where amatuers can and do make professional level contributions!
And even though I understand how the SkyScout works, it is unbelievable to actually use it and watch it ID an object, or have it guide you to some random star in its database...Awesome product. For me, this really helped me better learn the night sky. It's like using a crutch (or a chaffeur) then weaning yourself away from it and driving yourself. Much less pain, and kick butt fun.
Hope this answers your concerns.
Clear skies!
Doug
Doug,
I am a beginner in the field of astronomy and astrophotography and I would like to purchase some equiptment. Im not exactly sure what types of telescopes and astrophotography equiptment would be best for a beginner and it would be great if you could offer me some advice. I have done a lot of research on telescopes preferably under $500 and CCD cameras along with laptops. If you have any advice that would be great! Thanks.