TELESCOPES FAQ- Frequently Asked Questions

Large telescopes give big, bright views, with lots of detail. A big scope will show you objects you could not possibly see in a smaller instrument. You have probably heard many times that with astronomical telescopes aperture matters, not magnification. But why? Brightness and Resolution.

Brightness: Lets take a 4" telescope that is often seen at sporting goods stores; it has 12 square inches of light grasp. You might expect than an 8" telescope would have twice the light gathering gathering power, but it actually has over 300% more! Now think about the fact that a 20" telescope has over 400% more light gathering ability than a 10" scope. Take a look at this Aperture Comparison Chart, to help you visualize how much difference aperture really makes.

Resolution: Just as with brightness, the level of perceivable detail rises steeply with increased aperture. Again it is amazing how much difference a few inches makes. For instance, a 6" APO refractor with absolutely perfect optics ($9,000+), is blown away by a 10" Dob with 1/10 wave optics ($1,600) in the amount of detail that can be resolved! Darren Drake authored a good article that illustrates the differences here: http://www.astromart.com/articles/article.asp?article_id=473

A note on magnification: A well made telescope mirror can offer 50x of magnification per inch of aperture (sometimes more on a night of really good "seeing"). A 10" mirror theoretically tops out at about 500x, a 20" mirror at 1000x, and a 30" at 1500x. The 4" sporting goods store telescope from the above example would, at best, allow us 200x, a far cry from the 650x prominently displayed on the front of the box. Because of claims like these, a beginner will often ask "how much power does your scope have?"; rather than the all important question of aperture.

I notice your scopes are very "fast" or short f/ratio, how will that affect my viewing?

First, we understand that telescopic f/ratio (Focal Ratio) can be confusing for beginning astronomers and experienced photographers. In astronomy f/ratio is simply the Focal Length (the distance that the mirror comes into focus) divided by the primary mirror diameter. So, if we had a mirror that came to focus 100 inches away from its front surface, and that mirror was 25" in diameter, we would say that the mirror was a f/4 mirror (100 / 25 = 4 ). If the same 25" mirror was ground to come to focus 150" away from its surface it would be called a f/6 mirror (150 / 25 = 6 ), so far so good. If both of these mirrors are perfectly made, they will both give great views (note to photographers: a f/4 scope is NOT brighter than a f/6 scope, but a f/4 scope will have a MUCH wider field of view!), so why the constant concern? Size!

In our above example the f/4 scope would work out to be somewhere around 8 feet tall and the f/6 scope 13 feet tall, a 5 foot difference! So, if the telescope is smaller and the field of view is wider, why not make make all mirrors f/4 (or even f/3)? Difficulty and Coma.

DIFFICULTY: Making a f/3 mirror is exponentially more difficult than a f/6 mirror. Even some of the most famous names in the telescope mirror business have put out some truly awful large aperture fast mirrors. You see them, and the scopes they are installed in, come around again and again on the used market. Some opticians have even quit making large mirrors and have scaled down to smaller, easier mirrors and f/ratios. Take a look around the web and see for yourself how few mirror makers offer large, fast mirrors.

COMA: As a mirror f/ratio gets below f/6, an effect called Coma starts to occur. At the edge of the field of view, the stars start to look like seagulls. The lower the f/ratio, the stronger the effect. Lucky for us astronomers, two easy remedies exist for this. One is the TeleVue Paracorr, a parabolic corrector made exactly for this purpose. The other is the OCS or Optical Corrector System by Denkmeier, this comes with the binoviewers they manufacture for reflector telescopes. All Webster telescopes are designed and balanced from the very beginning to utilize these solutions.

Do I want a wood or metal telescope ?

Most likely a telescope that is a little bit of both. Really! Telescopes have been made of metal or wood for years. And from these years of experience we have learned that a hybrid of both technologies is actually best.

In some places on a scope, wood would not be our best design material. For instance, the truss clamps could split from repeated clamping and un-clamping, and wood truss poles would be difficult to find in 16 foot lengths (let alone the fact that we need them hollow!).

It is also necessary that our mirror support frame be solid American steel. There is no way that wood or aluminum could offer anywhere near the rigidity of steel in this most critical area, and still be open to maximum airflow.

Strength vs. Weight: Even a softwood like Spruce is over 2X stronger per pound than aluminum in bending, and over 3X times stronger in buckling. Pound for pound, one would have to go with carbon fiber over foam core ($$$$ and it can weaken in sunlight) to beat the performance of wood.

Vibration: Metal scopes "ring" and oscillate. This ringing shows up in the eyepiece as distortion of the image. Strike a metal bar and you feel it ring for several seconds, strike an equal size wooden dowel and it is instantly dead after the initial strike. Beginners are often shocked that the best telescope tripods are not the chrome plated imports, but are the "old fashion looking" wooden ones! A metal scope will ring for a second or two after you let go of the focuser, so you are always waiting for the image to "dampen down". A small breeze can keep the scope ringing, never actually settling down. Telescope manufactures have been jumping through hoops for years trying to get metal scopes to dampen as well as wood ones, but it has proved to be an impossible feat because of the nature of metal itself.

Durability: Metal scopes need a padded case to keep from getting dented and scratched. Even if you could get the manufacture to give you some "touch up" paint, just like your car, you can never really get the touch up to match the original finish. Large telescopes are always getting banged up by the ladder, no matter how careful you are. Anodizing aluminum telescope parts is the best insurance against a beat up looking scope, but anodization is expensive and many manufactures just paint their parts to save money.

Modifications: Metal scopes are difficult to add accessories to because of the nature of the metal structure itself. If you want to add a laptop computer shelf, or an eyepiece rack to a wooden scope, you just drill some holes and screw them on. On a metal scope, if you drill through the frame you weaken the structure, if you drill through the thin side panels you need to weld on a backer plate of some sort to keep the screws from stripping out. The metal fabrication techniques needed to keep the scope looking "stock" is outside the abilities of most scope owners.

It looks like you don't use any stain on your scopes.

Our family believes that the true beauty of wood is often best displayed with just a protective clear coat. We have nothing to hide behind a layer of stain, as all of our woods are hand matched before any assembly begins. As an added bonus, if you somehow scratch one of our scopes, you can just touch it up with some clear epoxy, lacquer or polyurethane. Small scratches go unnoticed, as lighter, unstained wood is never exposed. Any wooden accessories you fabricate as time goes by are easily added to the scope without the worry of matching stains. We do have a huge inventory of stains at our disposal, so if you have a preference for some color of stain (even crazy blues or reds), we will of course accommodate your request at no additional charge.

I already have a mirror, will you build me a scope?

Our top concern here is that if a mediocre mirror is seen in a Webster Telescope, our reputation could be damaged. We often recommend that you sell your mirror on Astromart and just order a complete scope. If you are in possession of a TRULY great mirror, with INDEPENDENT test results to back it up, we may, at our discretion, build you a Webster Telescope. As of this writing, Optic Wave Labs is providing free, independent mirror testing. We would advise any serious astronomer to take advantage of this opportunity.

That being said, we will fabricate any parts you need to make your own scope, you just won't get a "Webster Telescope" name plate on it. We think that is a fair deal for everyone.

I need to shorten my truss poles a few inches for astrophotography, do I have to buy a 2nd set of poles?

Not with a C series Webster Telescope! Simply remove the pins below each mirror box clamp, now the trusses can be seated as far down as need be. After you find the sweet spot for your camera, measure the distance from the ends of the trusses to the back of the pole clamps. drill a pin hole in each truss so that the pin can be inserted INSIDE the truss at the measured point. Reinstall the pins in their original places. That's it! Now you can instantly switch between the two positions with no tools at all. Get a new camera? Drill a new hole.

You guys always seem to recommend TeleVue eyepieces, what about Meade, Pentax, ect..

Not only does TeleVue make, in our opinion, the best eyepieces in the world, they offer something that no other eyepiece manufacturer in the world does; SERVICE. If you scratch a lens in your TeleVue eyepiece, you can just order a replacement lens or send it to New York and get it cleaned and repaired. As of this writing NO OTHER EYEPIECE MANUFACTURE OFFERS SERVICE OF ANY KIND. Don't take our word for it, call any other eyepiece manufacturer and pretend you have damaged the coatings on a $500 eyepiece. Ask to buy a replacement lens. Ask if you can send it in for repair. YOU CAN'T.

Don't waste your money on eyepieces that can't be serviced. Accidents happen, especially at star parties, because everybody wants to look through the big scope, yours.

Collimating any reflecting telescope is EXACTLY the same process, no matter what focal ratio it is. It is not any harder to collimate a f/3.6 telescope compared to a f/7 telescope.

We think this wife's tale got started because it IS more important that you DO collimate a fast telescope, because naturally the tolerances are closer.

We recommend you collimate every time you transport your scope. It only takes a minute. We use super fine stainless steel threads on our collimation knobs to make precise collimation a breeze.

Are large telescopes more affected by "seeing"?

Seeing is defined as the distortions (or lack of) in the atmosphere. The old wife's tale was that the atmosphere was made up of 8" cells and that allowed small scopes to "beat the seeing", by looking right through them. This, of course, has never been proven by any actual research. More likely the rumor got started because small scopes cool down so much faster than large ones. Thus it would appear that the small scopes see right through the "bad seeing", while large ones are confounded by it, when in reality what is being seen is a thermal issue with mirror mass.

What can be said is; that with a mirror of any size, cooled within 1.5 degrees or so of the ambient temperature, the atmosphere will limit the MAGNIFICATION. If a 6" scope can produce a stable image at 250x so will a properly cooled 30" scope on the same night. But, on a night with great seeing, your 30" telescope will easily exceed 1000x and leave the smaller scope in the dust.

No, all telescopes of every size are equally affected by light pollution. The darker the skies, the higher the contrast of the image in any size scope. A large scope will allow you to "punch through" light pollution more effectively using a filter. There is never a time where more aperture does not result it more resolution of the image.

Enhanced coatings offer slightly higher reflectivity over semi-enhanced coatings, but at the expense of a slightly rougher surface profile, and a slight change in the mirror's figure. This results in some additional light scatter and a lowering of contrast. Semi-enhanced coatings are built up of two layers (aluminum and protective overcoat). Enhanced coatings are made up of many layers of coating. As these coatings tend to build up at the edges and curves, the actual figure of the mirror can be compromised. This is why mirrored flats and diagonals can be coated with enhanced coatings generally without problems. But note, even a $5000 laboratory reference flat with enhanced coatings is only guaranteed to spec in the 90% inner zone, the outer 10% zone is often considered a wash. Most mirror manufacturers advise AGAINST enhanced coatings. Some batches of enhanced coatings are un-strippable, and result in the mirror needing to be completely re-polished. Here is a chart showing the effective additional aperture gain by using enhanced coatings:

1. A straight vane causes diffraction in a 2% area. A curved vane’s diffraction is equal to its curvature, thus a 90 degree curve in a vane equals a 90 degree swath of diffraction.

2. Mechanically, all curved spiders are inferior to straight ones, thus the spider material has to be more than 14 times thicker to equal the strength of standard three vane spider. This results in much more physical material in the light path.

3. Because the vanes are curved there is an increase in the overall length of vane material in the light path of up to 66%.

4. If curved spiders were of any real advantage at all, professional observatories would be using them (the idea was first proposed back in 1931).

If you are looking for the faint companion of a bright double star, the curved diffraction pattern of a curved vane spider will often bury it. With a straight vane spider, you simply move the faint companion star out from under the diffraction spike and observe. At a recent mirror making symposium, the speaker was asked why curved spiders were seen on some beginner telescopes. The answer was "They allow the inexperienced to think they are getting something for nothing". Much laughter ensued.

Read why professional observatories don't use curved spiders: http://www.opticsinfobase.org/abstract.cfm?id=27568

We've seen lots of great looking ultra-light telescopes over the years, and have designed a few ourselves. You always have to give credit to the clever designs you see using bean-thin poles, piano wire and Styrofoam. But for a successful commercial telescope, we believe you need to offer much more durability. Our scopes are built Detroit tough, with Dovetail joints, American made steel mirror cells and over sized trusses. A grown man can literally stand on our mirror boxes without worry of a dent. But even with all this strength, a Webster Telescope is still lighter than most other scope designs that are not nearly as large in aperture.

Webster Telescopes are designed to be used at public gatherings where kids are going to be touching them, ladders are going to be banging them, and people are going to be admiring them.

Actually, none. We know that this is not the answer you wanted, but, in our opinion, using a truss Dob for a solar scope is just too dangerous. We know that you can purchase full aperture films that are made for our scopes, but the larger the sheet of film, the more chance of it becoming damaged, possibly during use. Because of the portable nature of our scopes, they may not be 100% light tight as a solid tube would be. Your eyesight is just too much to risk.

We Highly recommend you purchase the Coronado PST hydrogen alpha solar scope. For less than the price of a good eyepiece, you get one great telescope that will show you the prominences and surface details that you would not be able to see through a film filter anyway. The PST is over the top great, and it gives you something to do during the day (besides sleep).

First, you have to understand that you will be using a lot more power than you think. Even if you start simple, you soon will have quite a collection of electrical gadgets. You will need power for the cooling fans, eye piece dew heater, secondary dew heater, ArgoNavis, ServoCAT, 12v hairdryer (to clear eyepieces you left out uncovered), and maybe even a laptop computer (and maybe even a heater for the laptop computer). That's a lot of power hungry stuff.

Many astronomers start out with gel-cell batteries because they are cheap, or because they came inside one of those "jump paks" or "power tank" portable power centers. They may work out OK for a small telescope, but you are going to need more for your big Dob. Gel-cells can be permanently damaged if discharged completely, and really only deliver about 1/2 of their rated amp hours. So you can only count on about 10 amp hours out of your "20 amp" cell. Many Gel-cells survive less than 100 charging cycles.

What you really want is a Deep Cycle Marine battery. You can run them down to zero and simply charge them back up without damage. Deep Cycle batteries can usually survive 2500 charging cycles. Go to K-mart and get one for about $60 that is rated for a true 80 amp hours. Don't buy a Automotive battery, or a marine starting battery, they will be damaged if run completely down. The only other thing you need is a Float Charger (NOT a Trickle Charger, they can overheat your battery). Float Chargers cost less than $15 (we've got them if you need them), so even with the charger you have spent less than the price of a "jump pak", and now have probably 10 times the power!

Don't forget, if your truck will not start out in the middle of nowhere, you can just use your Deep Cycle to jump start (or possibly replace) your truck battery.

A great tip from Gary at ServoCat: Boot your laptop on its own, internal battery (just in case your laptop balks at the voltage drop from all of the DVD, hard drives, WiFi ect, all turning on at once). After you are at the desktop, tap into your Deep Cycle Battery and drop the internal battery out of the laptop. No sense in charging a battery with a battery! Thanks Gary.

Because you will never lift the individual components by themselves. Our scopes come standard with wheels, so you never have to worry about how much each piece weighs. You roll the scope into your SUV, across the grass, out of a trailer, anywhere you want to go. It does not make sense to carry each piece around separately. Because of our lightweight designs, the primary mirror is the heaviest component.

Our D series telescopes use slightly different hardware to hold the trusses, and use optics from the legendary Carl Zambuto.

Frequently Asked Questions:

Why would anyone need such a large telescope?

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I notice your scopes are very "fast" or short f/ratio, how will that affect my viewing?

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Do I want a wood or metal telescope ?

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It looks like you don't use any stain on your scopes.

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I already have a mirror, will you build me a scope?

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I need to shorten my truss poles a few inches for astrophotography, do I have to buy a 2nd set of poles?

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You guys always seem to recommend TeleVue eyepieces, what about Meade, Pentax, ect..

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Are large telescopes more affected by "seeing"?

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