Posts Made By: Steve Lathrop

I'm teaming up with a physics teacher at my local public high school to put together an observational astronomy supplement to the physics course. Questions: what scopes would best fit the needs of beginning students, offer a reasonable prospect of firing the imaginations of at least some of them, stand up to possible misuse, and not break the bank to buy? Portability and the scopes' ability to withstand repeated transport will also be an issue, because we will travel to find dark skies.

We will probably have to get donated funds to do this.

I am particularly keen to hear from people with the experience to compare the performance of any suggested scope to that of a 4-inch apo (Televue 102 in my case), which is the standard I'm used to.

My current thinking suggests a mix of Orion 8" dobs for deep sky, and Stellarview refractors for wide fields, and to ease initial difficulties with finding stuff, but I haven't used these. What can you see through them?

What other choices should I be thinking about?

What issues should I be considering that I haven't mentioned?

Any feedback?

Steve

I live near the ocean, (about 1000 feet from the beach, not immersed in salt spray) and, so far, have done my observing with refractors. One concern that contributed to my refractor choice was that the marine environment might quickly dull the coating on a Newtonian primary. I'm not sure where I got this idea. Does anyone have first-hand knowledge?

Steve

Before you sink the big bucks into that TEC scope, maybe you should try a little game I sometimes play with myself when I'm reading the astro mags.

See if you can sort the moon pictures by scope type without looking at the credits. I just check to see if a picture is really, really, "oh wow!" sharp, and guess refractor. A little soft means cassegrain. Mostly (almost always it seems) I'm right. You might have to spend a little time educating your eye, and learning to make adjustments for the scale of the image, but pretty soon you might learn to see the difference reliably.

I'm not trying to bash compound scopes. Obviously they have wonderful advantages or there would not be so many people using them. I'm just suggesting that for a bright, easy-to-find target like the moon most of those advantages may not come into play.

A TV-102 will easily support 250X, or 300x in great seeing (rare, lately, where I live). The images are breathtaking. It's a lot less expensive than the TEC, and that might let you keep your Meade for the faint fuzzies the TV-102 can't show you.

Just a thought....

Steve

Will somebody who knows more please check me on this line of reasoning. I particularly want to know if my premises are off the mark.

1. With a given magnification and a given aperture, exit pupil is determined: field size, apparent or actual makes no difference to exit pupil size.

2. In a comparison of two EPs, one with a larger field, and one with a smaller field, but both with the same focal length, and hence the same exit pupil size, any given resolvable detail will have a smaller image size in the wide-field EP, because the wide-field EP fits more stuff into the same-size exit pupil.

3. So maybe planetary observers can get larger image scales for tiny planetary details simply by using narrow-field EPs instead of Naglers and Panoptics.

Anything wrong with this?

Steve

Anyone who is interested--I would like to hear from anyone who has a favorite view that you think is better than any picture you have seen of the same subject. This is a way, (perhaps, maybe) to generate information about what different kinds of equipment excel at.

I would like to have as much specific information about the telescope and EP as possible, including make, model, magnification, aperture, focal length, etc. Maybe also a word or two about what makes the view special.

To kick this off, I use a TV-102, and the view of the double cluster through this with a 22 Nagler T4 is far more impressive than any picture I have seen of this object. Contrast of brightness among stars, and different star colors seem to be what separates the scope view from the pictures.

The scope is an APO refractor with 4 inches of aperture, 880 mm of focal length. The EP gives 40x and 2 degrees of actual field.

Steve

An accoustical engineer friend has told me that to get a perceived doubling of loudness, mechanically measured sound energy has to be multiplied by 10.

Anybody know if light perception works the same way? If so, then mathmatically it takes a 12.5-inch aperture to double the perceived brightness of a 4-inch. Does this square with experience?

Please note, I'm not asking about photons, I'm asking about perception. Obviously, it's subjective.

Steve

As the very happy owner of a TV-102, I don't know much about what other kinds of scopes can show. My scope seems to show more on galaxies than I had been led to believe, but let's face it, it's only 4 inches.

I'd like to know what difference I might see if I added an inexpensive 8- or 10-inch dob. Here's what I see now on a few deep sky targets under mag 6 skies:

Whirlpool: both galaxy cores stand out, spiral structure just discernable with averted vision and effort. Seeing structure takes time.

M33: Spiral structure detected like with Whirlpool, averted vision, effort, and time to be sure.

M104: Dust lane evident with averted vision and maybe with direct vision in good seeing.

NGC 4565: Dust lane hard to see, but somewhat evident across core with averted vision

M13: Individual stars resolved across entire cluster with direct vision. Too many stars to count. Many more stars go unresolved. Magnification necessary to show resolution also makes cluster look quite dim.

Question is, would I see a great deal more with an inexpensive Dob?

Any experienced commentators who have compared premium 4-inch refractors with 8- or 10- inch non-premium dobs out there? Comments on these or other objects seen through the Dobs?

Steve

Anyone out there who has had the experience of owning a 4-inch premium refractor, such as AP, Takahashi, or Televue, and then adding an 8-inch Dob?

I'm particularly interested in performance on galaxies and globulars. Can you describe comparatively what you see on M13, M3, M81, M51, M33, M101?

The question I' m trying to answer is whether to buy the 8-inch now, or save my bucks until I can get something in the 14-inch range. If the 8-inch provides an appreciable improvement, then of course I can buy it, use it, and sell it later, as I work my way up (is it up?) to a serious case of aperture fever.

My concern is that when differences in contrast are figured in, I may see brighter images, but not much more detail in the 8-inch, and it would just be a waste of time.

Replies to previous posts have yet to really answer the question for me, so I am trying again with a different question.

Steve

I'm a refractor person who is thinking of getting a Dob.

I have questions about buying a used Dob because of mirror coating issues.

How long do coatings last?

How much does recoating cost, and how do you find someone to do it?

Is a recoated mirror, done right, as good as a new mirror?

If you use your scope near the ocean, does that create bad coating chemistry? (if you have experience with this one, it might help to give specific distances from the water, my home is about 800 feet from the ocean)

Steve

To my surprise I have just discovered that at least the Providence RI division of Southwest Airlines considers my TV-102 carry-on portable if it will fit in an overhead bin that is 38 inches long.

The scope itself, w/o diagonal, is only about 30 inches, but the standard TV hard case is about 38.25 inches long. Does anybody know of a soft case that would provide adequate protection for carry-on?

Any cautions about trying this?

Steve