Over the years, I've used the 2" component of Epsilon Lyrae as a resolving/colimnation test during the steady skies of the summer. I can see some of each component's image spikes barely touching each other and assume the components are resolved. Using the Dawes limit equation (stars of 6-7th mag.) of 4.5 divided by the primary in inches, the scope should resolve down to .45", in which both components are merged but two sources of light are easily detected. Using this measurement, I can resolve down to 1". The scope has a 1/20th wave configuration primary mirror (as the ad stated back in the early 1980s) but I'm using only medium grade occulars. So my question is, is the scope's resolving power poor,fair or good? Anyone's comments on the above will be greatly appreciated. Mr Q

You have to factor in seeing conditions. The double double is actually pretty easy to split in any scope over about 3" as seeing doesn't bother it all that much. Enlarge the stars from seeing and they still split. In fact it may be easier to see since the disks are bigger.

But when you get down below 1" of arc then seeing can be the great equalizer. You have to get adept at catching fleeting glimpses in 0.1" when the atmosphere lets you see the split. No matter how good the scope if the atmosphere isn't cooperative then you won't see it. I've split stars in the sense that I could see a intentation in the two airy disks at 0.4" many times in my 10" f/8 but usually only for a tiny fraction of a second at a time. Nights the stars move slowly around rather than fuzzing up and getting clear usually are best for this. Sort of like looking at them under water.

Eyepieces aren't all that critical unless you are dealing with a wide magnitude variation or two very bright stars causing glare. Push the power to 50 per inch and beyond. I do most of my tight double star work or wide magnitude variation doubles using my 5.5mm movie camera objective (1950's era) (handles glare very very well) and a 2x barlow for 725x! If the night can't support that, most won't, then go for easier ones.

Oh yes -- accurate collimation is very very important. Doesn't take much error to really mess things up. Faster the scope the more critical this is.

For newbies I've attached a shot of the double double (Epsilon Lyrae).


Rick

Attached Images

DoubleDouble2.jpg (11.6 KB, 23 views)

Interesting. So from what you said, I should consider my scope adequate for splitting close doubles and certainly good for extended objects. Planets I have observed show enough detail to satisfy me but since most of my observing is on deep space objects, including planitaries with some detail, it seems I don't really have a problem with my scope in what I have been using it for.
What about the 1/20th wave configured mirror? Is it actually enhancing my images or just a waste of quality for what I observe? Mr Q

Who manufactured your telescope? A true 1/20th wave wavefront error is very, very rare indeed. I would take that figure with a large grain of salt.

Dave Mitsky

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Actually, I've never even heard of a lambda/20 grade mirror. Even the planned Thirty Meter Telescope is looking at lambda/10, and that's extremely tough and expensive to get. All commercial scopes I know of are around lambda/4 or 5 (diffraction limited). The ad was almost certainly exaggerating; a lambda/20 accuracy, even if obtained, offers no tangible benefit over lambda/10, simply because the effect is too small.

The scope is a Mead DS-10. I remember when I decided to buy it back in the early 1980s that the ad(s) in magazines stated the "1/20th wave" mirror and i (wrongfully ?) assumed it had one? Maybe the ad stated "1/20 wave performance" or something to that effect. I tried several Mead sites on the web but could not come up with any specs on this scope. I would like to look at (on the web) some ads Mead had back then to answer my question. Any advice on where to look? Mr Q

I strongly doubt that Meade has ever sold a telescope with a true 1/20th wave primary, especially one with a fast focal ratio of f/4.5.

Dave Mitsky

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Chance favors the prepared mind.
De gustibus non est disputandum.
Never attribute to malice that which can be adequately explained by stupidity.

I finally found a web sight where I can look at/download old telescope ads. The both ads (one from Crown Optics, the other a year latter from Meade) clearly stated that the primary and secondary are 1/10th wave configured. The 1/20th figure? Must have been a figment of my imagination over all these 25+ years! If anyone is interested in wave figures, visit astronomics.com and check out their glossary of astronomical telescope terms. Some simple(?) explanations , Mr Q

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Meade DS-10 (10" newt)
10x50, 10x70 binos

Back then the definition of what a wave error was was very vague allowing all sorts of very erroneous claims. Even 1/10th wave is way beyond any Meade mirror I ever saw from that era. Their mirrors weren't bad but the ones I saw never came up to true 10th wave by today's standards of what that means. Makes a big difference as to the color your measure it in as well. It's much easier to achieve 1/10th wave in red light than blue for instance. This is why major observatories using Adaptive Optics do so an IR frequencies. At IR frequencies you have a lot more leeway than you do in green light our eye is most sensitive to.

Rick

I agree. Most people don't realize that there are absolutely no consumer protection standards for the spec claims made by amateur telescope companies.

Dave Mitsky

__________________
Chance favors the prepared mind.
De gustibus non est disputandum.
Never attribute to malice that which can be adequately explained by stupidity.

No "official" standards true, but that doesn't mean that unscrupulous companies can get away with wild claims like they used to. The man reason? the internet!

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