Astronomy Hacks: Hack 9 - Choosing the Best General Purpose Telescope

[powerwindows1985]

This is a very lengthy chapter, but extremely informative.

Synopsis:
1)Scope Characteristics:
Apeture: diameter of primary mirror or objective lens
Focal Length: distance from the optical center of the mirror or lens at which it brings an object located at infininity to focus. Focal length is a determining factor of how much a scope can magnify (Mag = Focal length of Scope divided by Focal length of Eyepiece).
Focal Ratio: the ratio of the focal length to apeture (Focal Ratio = Focal Lenght divided by Apeture). This determines wether a scope is fast (ie f/5) or slow (ie f/8). The Focal Ratio determines the length of the optical tube assembly (faster scopes = shorter tubes). According to the author faster scopes require better (more expensive) eypieces to provide a good view from edge to edge.
Mount: There are two basic types of mounts - Altitude/Azimuth and Equatorial. The former is easier to use, but is not designed to track objects. the later is heavier and more expensive, but is designed to track objects.
Portability: This is very important. A scope that is harder to transport will be used less than the one you can carry easily.
Cool Down Period: This is the time it takes the optics to adjust to outdoor temperatures. Refractor cool down quickly (a few minutes), small dobs in about a half hour, Schmidt Cassegrains one to two hours, and Maksutov Cassegrains two to three hours.

2)Scope types
Refractors: Advantages - Durability, portability, fast cool down, pristine image quality, usable for terrestial viewing, and ideal for astro-photography.
Disadvantages - small aperture (big lenses are more expensive than big mirrors), Inconvenient eyepice location (imaging trying to look at an object straight up with one), and they cost more.

Dobsonian Reflectors: Advantages - Price, Stability, Portability (at least the small to mid size ones), fast cool down, and high image quality.
Disadvantages - Frequent Collimation, lack of tracking, and not designed for astro-photography

Equitorially mounted reflectors: Advantages - same as dob plus tracking abilities. Disadvatages - highe price, and more weight. The author admits that he is biased against this type of scope.

Catadioptrics (Schmidt Cassegrain, Maksutov Cassegrain, Schmidt Newtonian, and Maksutov Newtonian).
Sct's are most popular. They are very portable and versatile, but also cost more.
Mct - the author considers this type of scope unsuitable as a first and/or only telescope. It is best used for high power, narrow-field observing.
Mak Newt - the author considers these astronomies best kept secret. They have excellent image quality, but are very expensive.
Schmidt Newt - very similar to Mak Newt but not widley manufactured.

3)Choosing a telescope: Avoid department store telescopes, avoid any scope promoted based on magnification, by from a retailer that specializes in astronomy, try out different scopes (attend a star party), if in doubt by the next size up, and don't blow your entire budget on the scope (you will want accessories).

The author, like most I have met on this forum are partial to the Dobsonian becuase you get a lot of scope for less money.

There is a lot more information in this chapter, and I recomend anyone who is shopping for a scope to read it.

Next Hack - Equip yourself for Urban Observing (Grab n Go Scopes)

[Vinnie]

That's all pretty good info, Keith. I don't necessarily agree with everything there, but then we all have individual opinions, and all in all it is good solid advice.

I will expand (and certainly not disagree) on one point, however, and that is "Cool Down". This could well be the single most common stumbling block for beginners. I wish I had a dollar for everytime I have fielded a conversation or post that goes like this:

-"I just took my new Newt (or SCT or Mak) out and I can't focus it, or get a clear view"

-"Well did you let it cool down?"

-"What's that?"

As you have correctly stated the scope needs to adjust to ambient temperature so that all the materials in its makeup (glass, tube etc) all reach a stable state of expansion and contraction, but even more importantly *so that any thermal currents in the tube itself become steady and dissipate*

The words "cool down" are actually rather misleading, as this can also mean "warm up". A good example would be if you take your scope out from an air conditioned room into a typical Northern Aussie summer night, where the temperature inside is maybe 25 degrees C and its 40 degrees C in the back yard.

How do you know if your scope has "cooled down" sufficiently? well you can't really time it, but the indicators that you have given are probably realistic. As you try to focus on fainter objects, the "cool down" factor becomes more pronounced.

So lets say you set up your 8" Dob and come back half an hour later. Look at the moon and the focus is almost, if not perfect. Now swing to say, Jupiter and it is fuzzy. Fair chance that you haven't allowed enough cool down time. OK 20 minutes later Jupiter becomes clear, so you swing onto a faint star and it is a fuzzy ball. Well then that's almost certainly lack of cool down. You have already eliminated most other possible problems except for maybe a slight collimation tweek, by virtue of the fact that you got good focus on the brighter objects.

Refractors usually cool down the quickest, because the objective lens group is right up front and open to the air. (but beware there are designs that are exceptions eg the Petzval Refractor has one lens group at the front and a second group about three quarters of the way down the tube, although these days the only companies that I know of still making Petzvals are Tele Vue, Takahashi and Vixen, and are unlikely to be encountered by a beginner, as they are quite expensive.

The open tube design of the Newt (Dob mounted or not) and some Catadioptrics is slower than a refractor as the primary mirror is at the opposite end of the tube and so there can be a significant thermal current residual in the tube between the opening and the mirror. It therefore follows that with an open tube design, that cool down will be faster with the tube vertical, but warm up will be faster with the tube horizontal

And yes, the Catadioptrics like SCT's and MCT's and Mak Newts are the slowest, because the corrector plate seals off the objective end of the tube and the mirror is still at the other end, so there is nowhere for the air in the tube to conveniently "mix" with outside air, if you like.

I better finish before I end up writing a book LOL

[powerwindows1985]

Thanks for expanding on that vin. There was a lot of info in that chapter, and I found it quite difficult to narrow it down to a reasonably lengthed post. What's more, they tend to lose me when they start talking focal ratios and speed (focal length and magnification I understand).
I also would be interested in some of your differing views. I forgot to note that the first thing the author states is how varrying the opinions are.

[Vinnie]

Hi Keith,

f/ratio is focal length divided by aperture, this you know. Longer focal length gives higher mag with a longer EP, this you know, but OTOH this longer FL will reduce the field of view. Here's an easy way to picture this. Get a cardboard tube from inside a roll of paper towel and look through it. Now cut it in half. You will see a wider field.

Speed is a term that goes back to camera optics. In a camera, the lower the f/ratio the faster the shutter speed the can be used. Or the Higher the f/ratio the slower the shutter speed that must be used. To my way of thinking, in a telescope, f/4 or f/5 is fast, say f/6 to f/10 is mid range and greater than f/10 is slow, but there is no rule, this is just an opinion.

There are a lot more factors that come into play with varying f/ratios in different types of scopes, and I would have to write a book to work through it all, and probably get half wrong but here are some examples

In a refractor, the faster (shorter FL and lower f/ratio) a scope the more prone it is to CA (chromatic aberration) or the slower it is, the less the CA. This is because the further the EP is from the objective the less the light has to be "bent" and so the more different wavelengths will arrive at the same point.

A fast Newt will be more sensitive to Coma with many EP's than a slower one for a similar reason

So why don't we just all have slow scopes (ie ones with larger f/ratios and longer FL's? Well for starters, it makes the thing heavier, physically longer and so needs a bigger and more expensive mount, second is that narrowing of FOV, but personally I don't mind the narrowing in say an f/12 Mak because the trade off is that I can use a longer EP to get good mags, which I find more comfortable, and the compact nature of the scope means a lesser mount is required than say a similar aperture refractor (this is an opinion)

But faster is an advantage for AP: faster means less exposure time means less tracking time means more accurate tracking

The Maks and SCT are slower by virtue of how they work. Incidentally, the major difference between the Schmidt and Maksutov designs is the shape and thickness of the corrector.

The Mak corrector is thicker and so more expensive to make in larger sizes, and also therefore takes longer to "Cool Down"

The Catadioptric Newts, Mak Newt and Schmidt Newt do make very useful compromises. Here's one that I disagree with. Currently Meade have a range of Schmidt Newts that are fast (around f/4 or f/5 I think), but less coma than a fast Newt with almost all EP's and no spiking, and don't cost the Earth. Given that you can buy a 10" Schmidt Newt with mount for around the same price as a 9.25" SCT OTA (no mount), I don't see the price objection.

Also I really don't understand the recent trend to be anti Newt on a GEM. Its still the cheapest way to get aperture combined with tracking ability.

I am not really picking your info apart, my comments really are mostly opinion, and it is still a very useful synopsis for any beginner.

I hope I haven't muddled you up worse over the fast/slow thing

[Vinnie]

I should add, Keith, the two pearls of wisdom in the whole synopsis that really stood out for me are:

"Don't buy from department stores" and

"Don't blow your whole budget on a scope (you will want accessories)"

[powerwindows1985]

Thanks again Vin. This is great info. The not buying from a department store is something I have seen on just about every web site I have been on. But I don't need a web site to tell me that, I have learned that on my own. A couple of years ago my son got one for selling boy scout popcorn. You could see the moon and that was about it. The mount was so light a light breeze could have knocked it over (and here in SD winds are over 20 MPH more often than they are under). You couldn't align the finder scope, so it was impossible to find anything. The only thing this scope did was spark interest. Had I not done some research and bought a better one, he would lost interest rather quickly (as I did when I was his age).
I don't remember where, but I read someplace that buying a telescope should be no different than other hobbies; you wouldn't buy golf clubs from a department store, why buy a scope from one. (or something along those lines).

[First Scope]

A very simple way of explaining fast or slow to a complete newbie is to use the distance factor....

Lets just say that an F10 has a further distance to travel in the optic path than the F5 Does. The longer time will make that the slower scope. As a in a foot race, the longer time goes to the slower runner.

The light will travel faster from entering the tube to your eye in the shorter F5 than it will in the longer F10. That makes the F5 the ''faster" runner in the race.

While this may not be 100% accurate I have found that most will accept it and not be turned off by the complexities that astronomy has to offer the neophyte<G>

BTW I am very partial to fast scopes. Also to Newts on GEM's. Or Newtonian Reflectors on an German Equatorial Mounts for PWR. You do have to spin the OTA to accommodate the EP sometimes...no biggie

I have an 8" F4 discovery on a G5 (EQ mount) that is a super scope...I wasn't always so happy with the scope until a friend showed me what it could really do!

He simply put a wide field of View (FOV) eyepiece with an O3 filter in the scope and showed me the entire Veil Nebula at one time! That is tough to do with a F10. I developed a whole new appreciation for that cool little scope!

High magnification is not always the measure of what you get to see!

Plus fast dobs use shorter ladders <G>

PWR, IMHO...the very best way to determine the best scope for u to get?

Join a club, go to star parties and look through as many different scopes as you can! Decide what you really want. Then Measure your vehicle and your bank account!

Figure out what will work for all three and your there!

(Leaving out Astrophotography..You'll probably find it is a mid size dob... Say 8" F6 or so)

I think everyone on this forum and every place else will agree.... the absoulute best telescope is... THE ONE YOU WILL USE!

[Vinnie]

Interesting and informative post, Terry

Its good to see someone else who is pro Newt on GEM. Since the "Dob Revolution" it has become quite fashionable for many of the "Gurus" to be aghast at the words Newt and GEM in the same sentence, and I think that is a shame. As you say, rotating the OT is no biggie, and there are some quite simple DIY mods around that can make it very easy to do.

I haven't owned a Newt for some years, but I did have several on GEMs (I've never owned a Dob). The reason that I moved away from the Newts, though, was purely to do with transportation. Refractors and SCT's and Maks take a lot more bouncing around, and I travel a lot into more remote areas by 4wd.

Also I still do a lot of planetary so I prefer the slower f ratios, but I have a couple of quicks for DSO work, where as you say, mag isn't a big deal

Most beginners these days do start out with Dobs, and for good reason ($$$$) but a Newt on GEM is still the cheapest way to get aperture and tracking together.

[First Scope]

Hi Vinie,

Thanks for the acceptance <G>

I think you will see a lot more sophisticated Newtonian scopes on the horizon with the advent of technology. Better mirrors, glass and ways to mount them are coming out every day...

IMHO...The last and final scope for me will be something like a 10" Tech Mak on a mountain mount...

with that all the other scopes would go LOL