Deep-Sky Observing with Refractors

This is a question to you experts in optical matters from an avid deep-sky observer (viewing, not imaging) who is, however, "optically challenged," having never been inclined to study the physics or other esoterica of telescopic optics: I understand that apochromatic refractors are preferred over achromatic ones because they bring most colors to focus at the same point and thereby avoid the halo of purple or other color around planets and the moon. But what about viewing clusters, nebulae and galaxies? Assuming good quality optics and workmanship in other repects, does the apochromat have a significant advantage over the achromat as to these objects also? Or could it be that this question doesn't arise nowadays because reflectors are preferred over refractors for deep sky observation? And what about observing double stars; does the apochromat have an advantage over the achromat here too? I thank anyone who can enlighten me on these questions.

Hi Herbert,
I have a Takahashi TOA-130, a 130mm (5 inch) diameter apochromat refractor. It's used almost exclusively for visual observing, including a lot of public outreach at Denver Astronomical Society monthly open houses (think mini star parties). The scope consistently gets unsolicited comments from members of the general public about how good its images are compared to other scopes.

While most folks who come to the monthly open houses are usually interested in the more popular observing targets--the moon and planets for the most part--the scope does get pointed at the occasional deep sky object and some of the prettier doubles. Even in the light polluted skies of Denver, it does as well as one might expect for a telescope of its aperture.

The Tak performs even better at dark sky sites, although to be fair, there is nothing like aperture when it comes to the dim fuzzies.

Still, in very informal comparisons with achromats of similar aperture at some of the open houses, even the achro owners have positive comments about the Tak's general image quality.

Ron

Herbert, Yes, they retain an advantage. It is not as obvious that they are superior because you cannot easily see the false colors. But they are still there. They result in a smearing of detail, and so forth across the object--even in the extended object. In short, that false color is throughout the image, just more obvious on the edges.

So, yes, apos still win, not counting the false color fringing.

Herbert: You ask a tough question because there are so many variables to consider. I have done a few side by side comparisons between an ED-80 and a 100mm F6 Achromat. Both these scopes have a 600mm focal length.

I was particularly interested in how they would perform on DSOs because I was taking a trip and could only take one. The comparisons were conducted from my urban San Diego backyard under light polluted skies, mag 4 at best...

My conclusion went something like this...

I looked for faint targets, M65-66 were about as faint as I could see under the circumstances and I was able to see them in both scopes with the same eyepieces. It seemed that smaller scope had better "contrast", probably something to do with the light pollution and less light gathering...

On clusters, the 4 inch seems brighter but again I don't really see anything more. I have done some informal sideby side testing under darker skies and the results were about the same, I think the 4 inch is probably brighter but specifically it seems everything I could see in the large scope I could see in the ED-80.

The 100mm scope gathers 56% more light, this is like the difference between an 8 and a 10 inch scope. It is there but not substantial. When the better optics of the ED-80 and probably better baffling are included into the equations, well, it just doesn't seem like there is much difference on DSOs.

This is not what I expected nor is it anything more than a comparison of two scopes with Orion optics, it is simply a report of what I observed.

These comparisons convinced me that the 80mm is more than adequate as a quick look easily transported scope and I no longer lust after a 100mm F6 APO...

Jon

Hi Herbert. I think the answer is a conditional "yes." The condition being that all other things (aperture, quality, etc.) are equal. This may be overly simplistic, but here goes. Lets say hypothetically you have 1000 photons of across the entire visible spectrum. To yield maximum resolution, contrast and detail a "perfect" refractor would focus all 1000 photons at one point. For the apochromat, perhaps 950 photons focus at one point. For the achromat perhaps 750 photoms focus at a single point. The weightings may be off, but the point is there's more visible light in focus in an apochromat than in an identically sized and f.l. achromat.

Change any other variable, however, and all bets are off. A so-so apochromat (miscolimated, poorly figured or something else) might yield less resolution, contrast and detail than a much better quality achromat.

Regards,

Jim