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Posts Made By: Steve Lathrop

February 19, 2003 02:44 PM Forum: Equipment Talk

Should I consider an SCT ;-)?

Posted By Steve Lathrop

"Perhaps the central obstruction and contrast degradation is a myth; for if it weren't, how then could these pictures be so contrasty and sharp?"

I don't know if it's a myth or not. I've never looked through an SCT either. I do know the pictures you are looking at on the web are not sharp. The originals might be, but not the ones you are looking at. Unless you have the freedom to zoom in to view microscopically small detail, your computer screen can not display sharp images, because its resolution is too coarse. With that exception, everything you see on a computer screen is inherently soft, and this can hide a multitude of sins.

To illustrate, the Legault website's most impressive image to my eye, by far, is the image of Saturn, and I think we would all agree it looks like a supurb image. On my screen that image is 4 inches across. I can see the Encke division, which shows that this is a high-resolution image. But I can barely see the Encke division, which shows that this is not a sharp image. A high resolution sharp image of Saturn displayed at that size should make the Encke division stand out much more.

Contrast is another issue. Moon pictures, particularly, almost always display harsh contrast. This is not the same as good contrast. The key is not whether the blacks look really black, and the whites look really white, but whether subtle, nearly identical shades of grey can be distinguished. If they can, especially in the highlight and deep shadow areas, then that's good contrast.

Conveniently, you have an excellent point of reference for both sharpness and good contrast with your TV-102. Just set it up next to someone's SCT on a night of good seeing and you will be able to find out all you need to know about these subjects in a few minutes.

You may also find out that the SCT has virtues that have nothing to do with sharpness and contrast, for which you could excuse some shortcomings in those areas. I'd like to find out more about the Celestron 9.25 myself.

Steve


February 20, 2003 09:08 PM Forum: Equipment Talk

travel scope

Posted By Steve Lathrop

One think you might check out is to stop thinking about carry-on luggage and start thinking about buying your scope a ticket. That's what professional cello players do, and I think the airlines might cooperate.

I asked Southwest about this not too long ago, and the person I was talking to on the phone said it would be OK. The problem of course is knowing whether everybody down the line will get the memo.

You might even be able to call your scope your baby and get a child discount or something!

Steve

February 22, 2003 01:55 PM Forum: Equipment Talk

Vixen 140 NA refractor or 6" chinese refractor?

Posted By Steve Lathrop

Just curious.

Do folks with experience find that a 5- or 6-inch long-focus achro is a better planetary scope than a 4-inch APO, such as my TV-102? Would I be blown away by the difference if I were looking through a 6-inch f15 D&G?

Steve

February 25, 2003 02:23 PM Forum: Equipment Talk

Questions about FOV and degrees viewed

Posted By Steve Lathrop

You should not just stick with a standard Plossl FOV until you have had a chance to try the alternatives and decided the Plossl FOV suits you better.

Probably only a small minority of experienced observers would prefer the Plossl view to the views obtainable through high quality wide-field EPs such as Naglers, Panoptics, and Radians..

One way I have heard the difference expressed, which seems to match my own experience, is that the wider fields are more emotionally involving. For me, subjectively, a narrow apparent FOV makes the observed object seem as if it is sort of detached, far away, down at the end of a tube. More a picture of the object seen from a distance than the object itself. The wider FOV makes it seem more real.

Al Nagler, who developed the 82 degree AFOV Nagler line of eyepieces uses the term "spacewalk" to market them. My own experience is that you have to use Nagler EPs just right to get that effect...but you can, at least with some of them.

There are other advantages. If you want to do 300x observing of planets off an alt-azimuth mount you are probably going to use an observe-then-track technique as you chase the planet across the sky. The wider the FOV, and the sharper at the edges, the longer you can observe before you have to swing the scope.

Also, there are some very neat views that just can't be had with a narrow field. Sometimes I like to put a 22 Nagler in my TV-102, which delivers only 40x, but 2 full degrees of real field, and look at Jupiter or Saturn. Seeing these planets not as giants, but as tiny little worlds out in the vastness of space gives a different perspective. Once when I was looking at Saturn this way through a 13mm Nagler at 67x, I happened to catch the moons arranged so that four of them were sorted by brightness and apparent distance from the planet simultaneously. The view produced a stunning illusion of immense 3-dimensional space, with the whole tiny system adrift in the dark, and the moons being illuminated by the light of the planet itself, because the brightness of the moons diminished with distance from the planet. Just an illusion, but unforgettable.

Finally, there are some objects which are so large, such as the Andromeda Galaxy and the Veil Nebula, that only a wide-field EP in a short-focal-length scope will allow you to see them whole.

Steve

February 27, 2003 07:13 PM Forum: Equipment Talk

Best Eyepiece for Viewing Planets.

Posted By Steve Lathrop

You have a scope which gives you something over 175x with an 8mm EP? If so, the 8mm Radian, which I do not own because it gives too little magnification in my TV-102, gets some of the best reviews in the Radian line. I do own several shorter Radians, and they have all been terrific on Saturn, Jupiter, and the Moon.

However, you might also consider trying the 7mm Nagler T6. In my scope this EP is simply fantastic. Extremely sharp and wonderful contrast. Immediately after I got this EP I started to notice that there were times when it would show me some planetary details that I hadn't noticed in my higher-power EPs. It also works the other way, of course, higher power being what it is, but it startled me to see a lower-power EP at 125x outperforming another very good EP operating at 175x or 240x, even some of the time.

Steve

February 27, 2003 07:21 PM Forum: Equipment Talk

About FOV

Posted By Steve Lathrop

Keep in mind that the amount of sky you can see varies with the square of the FOV. So the Nagler in your example will show 44% more sky than the other.

That said, you probably won't see it all at once, because with Naglers the AFOV is so wide you have to move your gaze around to take it all in.

For me, a 68 degree AFOV is just about as much as I can take in comfortably.

Steve

March 2, 2003 12:28 PM Forum: Equipment Talk

buying a scope for school

Posted By Steve Lathrop

This same question generated quite a few contributions last december. If you go to Search Forums and enter "School" you can read the opinions, but there is no clear consensus.

My .02-- short refractors let you find stuff pretty easily, they are durable, portable, familiar in concept, don't need much adjustment, and are particularly good on bright objects like the moon and planets, which are easier to see from most school locations than dim galaxies (which kids won't find very spectacular to look at under the best of circumstances). I think these are all school-type virtues, particularly in the lower grades.

Steve

March 3, 2003 07:56 PM Forum: Equipment Talk

Binoculars For Eyeglass Wearers

Posted By Steve Lathrop

Consider Fujinon 10x70s. Plenty of eye relief for glasses (I use mine both ways), wide actual field, wide apparent field, extremely rugged, diopter adjustment > +4/-4, and spectacular views. Also, stars look particularly colorful through these.

There are a few drawbacks. Individual focus makes them a poor choice for terrestrial viewing, except at infinity; they show some pincushion distortion and some coma at the edge of the field; big strong people can hand-hold them, but most people can't; and they are middling expensive.

Steve

March 8, 2003 08:51 PM Forum: Equipment Talk

Aperture to resolve Cassini Division into gap?

Posted By Steve Lathrop

Maybe I'm confused, (or misremembering, I basically missed Saturn this year because of the weather) but in my TV-102, on nights of extremely good seeing, the Cassini division appears wider at the outsides of the rings than it does in the middle, and it tapers visibly between these two extremes. It would be hard to convince me I am not seeing resolved width in cases such as these.

In normal weather conditions, not this year, I may be spoiled with regard to seeing. I live at the end of a 4-mile-long peninsula, with water essentially all around. Maybe unusually good seeing combined with good optics makes some of the textbook numbers too conservative?

Steve

March 9, 2003 06:05 PM Forum: Equipment Talk

13mm Nagler or 14mm Pentax with XT8

Posted By Steve Lathrop

Don't know about the 14 mm Pentax.

The 13 mm Nagler T6 is one of my favorite EPs (super-sharp, great contrast, comfortable to use), and probably MUCH smaller than you suppose, not much larger than a Plossl. I think the older-version 13 mm Nagler was much bigger, but have never seen one.

Whether my experience, with a TV-102, is relevant to what you would see using the same EP with a Dob...?

Steve