APO OR ACHROMAT, DOES IT MAKE A DIFFERENCE?

Started by aaronsland, 09/10/2007 03:50AM
Posted 09/10/2007 03:50AM Opening Post
WHEN VIEWING THE SUN IN H-ALPHA, IS THERE ANY DIFFERENCE IN THE VIEWS OF SAY A $200.00 80MM ACHROMAT VS A $1200.00 APO? AND WHAT IS SO SPECIAL ABOUT THE DEDICATED SOLAR SCOPES?

Nothing in education is so astonishing as the amount of ignorance it accumulates in the form of inert facts.
Posted 09/11/2007 12:06AM | Edited 09/11/2007 12:07AM #1
Michael,

It shouldn't make any difference whether you us an apo or an achromat with your H-alpha filter, since the bandwidth is so tight, except for the fact that apos *may* be less susceptible to things like astigmatism.
I use a 4" f 15 Vixen spec achro with a 60 mm. Coronado filter and BF 15; I use the same Coronado combo with a B.O. 5" f 8, and both work very, very well.

That said, I've also looked through well paired apo/Coronado combos which were very impressive! I've heard, ah, differing opinions as to whether the Coronado dedicated scopes were an improvement over one's own scope.

IIRC, Daystar scopes used long f.l. singlets with spectacular results!

James Adams
Posted 09/11/2007 03:51AM #2
Michael,

Nope, it doesn't. I've used both and am currently using a inexpensive doublet with my filter with excellent results.

The subject of the dedicated scopes is a tough one. They ARE optimized for h-alpha viewing, but as to whether they are worth the extra cost is more a matter of each person's viewpoint and wallet size. Look at it like you would any telescope purchase. Is a $10,000 Apo really 5x better than a $2000 Achro? No to many who find it difficult to drop $10k on a scope, yes to those who can or who want the "best" despite the cost.

The advantage of owning a separate refractor and filter is the dual duty aspect of the set-up. Good for night, good for day. Already got a great night scope and have the funds for a dedicated solar scope? Then maybe an optimized system is a better choice.

What are you thinking of doing?

Tom


Clear, sunny skies.
Tom Masterson
Coronado-Lunt-Daystar Solar Filters forum Moderator
Posted 10/01/2007 10:08PM #3
Michael Frias said:

WHEN VIEWING THE SUN IN H-ALPHA, IS THERE ANY DIFFERENCE IN THE VIEWS OF SAY A $200.00 80MM ACHROMAT VS A $1200.00 APO? AND WHAT IS SO SPECIAL ABOUT THE DEDICATED SOLAR SCOPES?

I don't know specifically about the dedicated solar telescopes, but I can venture a guess: they are designed to correct for spherochromatism at the rather long wavelength of 656.3 nm.

In case I just spoke gobbledygook at you, spherochromatism also goes by the long-winded name of "chromatic variation of spherical aberration." What that means is that the degree to which a refractor exhibits spherical aberration varies by the wavelength of light going into it. (Reflectors are, of course, immune to spherochromatism.) It ordinarily makes sense to correct for spherochromatism near the middle of the visual range, since the eye is most sensitive to those wavelengths (about 550 nm).

For H-alpha viewing, such considerations are moot, since 550 nm light doesn't get to the eyepiece. Only light in a narrow range near 656.3 nm gets to the eyepiece, and if you are going to correct for spherochromatism in a H-alpha telescope, you should correct it for light of that wavelength.

As I said, I don't know specifically that this is what they mean when they say specially optimized. But that's what makes sense to me. If that's the case, then an apo will be better than an achromat, simply because they generally control spherochromatism better than an achromat. But you can make a specially optimized achromat for less than you can make a generally optimized apo.