73 replies to this topic

[Rushwind]

I'm not trying to start a flamewar; I'm trying to get it.

So a Newtonian exhibits coma (and astigmatism?) off-axis. Fine. So you add a Schmidt corrector, and it *still* exhibits coma and astigmatism (a bit less perhaps)? What's the benefit? Is it that you can use a spherical primary?

What benefit is acheived (other than having a cool and different optical configuration than your neighbor) by creating a SN?

ie, given a 6" f/5 Newt, say, and a 6" f/5 SN, what would you expect to be different in the image quiality produced by these two OTAs? I would assume that the fast Newt had a parabolized mirror. Note that the SN OTA is likely to cost about twice as much.

What gives?

Jimbo

[jrcrilly]

What benefit is acheived (other than having a cool and different optical configuration than your neighbor) by creating a SN?

Reducing coma by half is a significant factor, especially for imaging. It's also much easier (read less expensive) to produce a well-figured spherical mirror at F/5 and below than a well-figured paraboloid. The SN's are twice the cost of the least expensive Newts at similar apertures - but they are less expensive than would be a Newt of similar quality even though the SN can perform better.

[Rushwind]

OK, thanks for the clarification. When I mentioned price, I was simply comparing the Meade 6" f/5 Newt (comes with LXD75 mount for $470) to the Meade 6" f/5 SN (comes with LXD75 mount for $800).

From the spot diagrams in Telescope Optics, it wasn't clear that the corrector was reducing coma by as large a factor as half.

OK, so it's probably worth it to pick up a SN, all other things being equal, as long as the price was more or less right.

Thanks!

Jimbo

[BillC]

Every so often, I slip through the fence, and wonder away from the bino section.

This time I would like to point out that coma is not only a product of focal ratio, but that it is also linear. For example, coma is much worse in a 12-inch f/4 than it is in a 4-inch f/4.

Just a thought.

Cheers,

Bill

[David Knisely]

I'm not trying to start a flamewar; I'm trying to get it.

So a Newtonian exhibits coma (and astigmatism?) off-axis. Fine. So you add a Schmidt corrector, and it *still* exhibits coma and astigmatism (a bit less perhaps)? What's the benefit? Is it that you can use a spherical primary?

What benefit is acheived (other than having a cool and different optical configuration than your neighbor) by creating a SN?

ie, given a 6" f/5 Newt, say, and a 6" f/5 SN, what would you expect to be different in the image quiality produced by these two OTAs? I would assume that the fast Newt had a parabolized mirror. Note that the SN OTA is likely to cost about twice as much.

What gives?

Jimbo

I don't know. An f/5 Newtonian with a halfway decent Paraboloidal mirror and a Paracorr would perform about as well or perhaps better than the Schmidt-Newtonian, and at a lower cost (although the Newtonian would now be f/5.75 instead of f/5). In addition, with the regular Newtonian, you wouldn't have the problems of dew on the corrector plate or lack of good ventilation for fan-assisted cooling. Clear skies to you.

[matt]

Wouldn't the price of the Paracorr equalize the price of the newtonian and Schmidt-N? But you have a point Dave.

Also, diffraction spikes-haters should love the fact that a Schmidt-N does not need a spider to hold the secondary.

Not that I care in the least which one might be considered "best design".

[jrcrilly]

The Meade SN6 goes for $220 shipped on AstroMart. This is less than the price of a Paracorr (even on AstroMart). Obviously, Paracorr wins when the aperture (and OTA weight) goes up.

Hi, William.

The cost advantage remains at larger apertures; a 10" parabolic mirror of comparable quality at F/4 would cost more than a complete 10" SN optical tube, and with a Paracorr would still be slower than an F/4 SN. It'd take a paraboloid at F/3.5 to achieve F/4 with a Paracorr. Prices go up quickly below F/5.

[Rushwind]

Here's a followup question; if I'm going to use the OTA for photography, and specifically 35mm film photography (so I need as wide and flat field of view as possible), do you think that I'll still need a Paracorr if I am shooting through a 6" f/5 SN?

It's clear to me that I'd need a Paracorr for photography with a 6" f/5 Newt, but I'm not sure whether the Schmidt corrector is going to correct the edge-of-field stars enough to obviate the need for a Paracorr on the SN.

Jimbo

[jrcrilly]

do you think that I'll still need a Paracorr if I am shooting through a 6" f/5 SN?

It's clear to me that I'd need a Paracorr for photography with a 6" f/5 Newt, but I'm not sure whether the Schmidt corrector is going to correct the edge-of-field stars enough to obviate the need for a Paracorr on the SN.

Jimbo

Hi, Jimbo.

The Schmidt corrector is to correct for the spherical primary. The reduction in coma is because spherical primaries have less of it than do paraboloids.

I've never shot 35mm so I don't have any experience there. I can say that S&T did a review on the LXD55 not long after it came out and the review included a couple of 35mm shots taken with them. I don't remeber noticing coma in those.

[matt]

Here's a followup question; if I'm going to use the OTA for photography, and specifically 35mm film photography (so I need as wide and flat field of view as possible), do you think that I'll still need a Paracorr if I am shooting through a 6" f/5 SN?
Jimbo

It lloks like a good choice because the 6" SN has a nice, wide, sharp flat field visually, so photographically it should be the same.

And no, the paracorr would not be of any use on it.

[deSitter]

Compare an SN10 side by side to an Orion XT10, and you will get it

The SN10 has one big problem that can be fixed with barnyard engineering - the tube is partially open, which is worse than wide-open (Newt) or closed (SCT). It takes quite some time for the boundary layer on the mirror to disperse. Some impatient owners install computer fans to the rear end to speed it along.

-drl

-drl

[Rhadamantys]

Here's a followup question; if I'm going to use the OTA for photography, and specifically 35mm film photography (so I need as wide and flat field of view as possible), do you think that I'll still need a Paracorr if I am shooting through a 6" f/5 SN?
Jimbo

It lloks like a good choice because the 6" SN has a nice, wide, sharp flat field visually, so photographically it should be the same.

And no, the paracorr would not be of any use on it.

I will hazard a slightly educated guess and say that it could even be worse...

Hint: If you really want to cancel coma and astigmatism with a 10in f/4 Schmidt Newtonian, just move the Schmidt corrector away from the primary mirror, a distance 80in should do the trick. The only drawback will be the not so compact OTA.

[Rushwind]

...and a curved focal plane.

Jimbo

[Rhadamantys]

and a curved focal plane.

Nothing that a small positive lens with good anti-reflection coatings near the captor can't cure... As a bonus, the OTA gets 5mm-10mm shorter

[RRaubach]

The major advantage of the S-N design is a relatively flat, well corrected, wide TFOV. For visual use, introduction of a Paracorr into a Newtonian system introduces a lot more air/glass surfaces that need good coatings. My choice of a SN-10 was driven by the need to receive the maximum number of photons at the retina for a given aperture without significant distortions.

Rodger

[sixela]

or the image softness of a fast and cheap Newtonian reflector. Somebody mentioned that making a decent Schmidt corrector lens is cheaper than making a truly great reflector mirror.

Colour me puzzled. First, there's nothing that would make images "soft" in a newtonian, and these days in 8" even both Synta and GSO make pretty decent mirrors (certainly as good as the Meade primaries tend to be).

Secondly, you still need a good mirror in that Schmidt-Newtonian. These days, I doubt it's the actual mirror figuring curve that limits mirror performance for the mass-produced mirrors, at least at these sizes.

Not to diss the Schmidt-Newtonian - it's a fine design, and certainly with design trade-offs that are different from those of a straight newtonian.

But I feel you're overstating the "problems" with straight newtonians (certainly at 8", where there are quie a few scopes with mirrors that aren't much faster than f/6).

[sixela]

The major advantage of the S-N design is a relatively flat, well corrected, wide TFOV. For visual use, introduction of a Paracorr into a Newtonian system introduces a lot more air/glass surfaces that need good coatings.

And in what way does a Schmidt corrector not have air-to-glass surfaces, and how does it not need a good figure and excellent coatings?

[sixela]

The front corrector plate of an SNT allows a mirror mount with no mirror vanes to cause diffraction loss.

And te central obstruction is larger, which causes much more diffraction loss than spider vanes (and curved vanes distribute the energy evenly in all directions, before we take another tangent).

Coma will be half in a well designed Schmidt-Newtonian.

Granted.

With a well insulated tube, internal air currents should be the same as a refractor.

And cooling behaviour is worse, much worse, so you get to enjoy your currents for longer .

The fact that Meade put their faith more in the Schmidt-Newtonian design (and compound designs in general) says something for the all around practicality.

If you believe they have all the answers, yes.

Sealing off the tube to where a cooling fan is not necessary

I'm sure all those SCT owners that have a Lymax fan will now chuck it over the fence.

I know that there is more capability in Newtonians than mass manufacturers get out of them.

Mass manufacturers get a lot out of them these days, and as I said before, there's no indication that the mirrors from Meade and Celestron are typically much better than the GSO/Synta mirrors these days (though undoubtedly the chance of getting a lemon may be less large).

The thing is that Schmidt-Newtonians can be f/4 and take more magnification before image breakdown compared to a fast Newtonian.

Granted, it will show less coma - and that is one of the reasons for the design. But an f/4.46 with a Paracorr is also a fine beast, and f/6 newtonians aren't *that* long in focal length either.

If you want very wide fields of view for your aperture, then yes, it's a fine design. It all depends on what you want to view.

And coma is not "image breakdown". The amount of coma in the centre of field of a well collimated newtonian is zero.

Look, it's pointless: you made a fine choice, which I'm not going to diss, but you're over-rationalising by dissing other designs, and while it's a fine way to resolve cognitive dissonance, it's not based on reality.

[chazcheese]

Well said Sixela . I was trying to form a response, but you put into words what I was thinking. There is no need to diss other scopes as there is not any one scope that can do it all. They all have their good points and bad points. i/e I have a long refractor for the moon, planets, and double stars, a dob for faint fuzzies, and a SCT for public sar parties.