4-inch refractors

I have always been a one-scope owner, presently having a 12.5-inch Portaball as my personal scope. Lately I have been thinking about getting a small refractor as a supplement, and I regularly browse the Classifields section. Still being at the "just looking" stage I get more and more surprised about the enormous variation in the prices of 4-inch refractors. Actually I'm thinking about an APO, or at least a semi-APO, and I have the impression that a basic OTA start somewhere just below 1000$. Quite a few are, however, offered for amounts in the 2000 - 3000 range. In some cases just the OTA is offered, in other cases the deal also includes a diagonal and/or a finder. But still I find the differences quite enormous. So my question is: What is the motivation for buying a 4-inch for 3000$ if you can get one for 1000$? Is it a matter of differences in colour correction? And if this is the case, is it possible to see these differences visually in other objects than Vega and Sirius?
I'm a visual observer. Maybe there are some secrets of the imagers' community I don't know about.

Inge S

Welll...bearing in mind I've never owned one but only looked through a few, my impression is that it's partly a matter of color correction, with better correction being needed for CCD work than what will give very pleasant and apparently "free of noticeable false color" for visual work, some is a difference in mechanical quality, some in the f/ratio (more on that in a second) and that you also pay more for a large and most especially flat field, again mainly for photography. Longer focal lengths (usually in the less expensive models) make it easier to get images relatively free of false color where shorter focal lengths require more elements, more exotic glass, or both.

You don't even have to look at the even wider variation in used scopes to see this as its evident in new prices. Look at two TeleVue 4" APOs for example, the TV-102 and the TV-NP101. The former is an f/8.6 doublet with a listed maximum field of 3* and a price from Anacortes of $2250 for the OTA only, the latter an f/5.4 four element, including the Petzval element for a flatter field, has a listed maximum field of 4*, and costs $3685 from the same dealer:

http://www.buytelescopes.com/product_list.asp?t=1&fmanf=3&pricelow=&pricehigh=&sort=name

It's similar in other lines. Tak has less expensive 4" scopes, and they also have the FSQ106 which is really designed as a purpose built astrograph, not to say it isn't also a wonderful visual scope.

Other scopes optimized for photography may have other features like oversized focusers (less important these days but some people still do shoot medium format film, for example) and the like.

The bottom line is that a scope for mainly visual work is likely to be cheaper than one optimized for photography but of course there are other differences as well, sometimes simply coming down to name brand reputation and desirability.

A 4" refractor would be an excellent second scope. I recommend getting one with a relatively short focal length - somewhere around 600mm - so you can enjoy low power, wide field views that are simply unavailable through your Portaball. If you also want the best performance at high powers, an APO would be the best choice. For low power viewing, a good achromat would be fine.

You might also give some consideration to somewhat smaller refractors, especially if your main interest is wide field views.

Clear skies, Alan

Hi Inge

It's not just color correction that is a difference between achro's and apo's(and their corresponding costs), it's the contrast. Impossible to say if the gain in these two qualities is worth the price difference without looking thru a few. If you're set on getting a 4 inch, I would recommend the Tak FS 102 (f/8). I have seen them on A-Mart for as low as 1500.00. I have the FS 128 (f/8) and the contrast and sharpness are wonderful. The color correction in the 128 is excellent (if not quite as good as AP, TMB, TEC) and would be even better in the 102. Even at f/8, with the proper eyepiece, the 102 would give you a very sharp 3 degree field of view.

Hi Inge,

I own a 12" Newt and I've owned a few 4" refractors. If you're looking for low power views with some high power observing thrown in I would give a serious look at a used Vixen 102EDSS f6.5. I've had the Vixen f9 fluorite and a Tak FS102 as well. I still have the 102EDSS. I found that all three of these scopes have CA but in varying degrees, The f6.5 shows the most but it is barely noticeable when compared to every achro larger than 60mm that I have ever used up to 6" and f13. The 102EDSS is very light and compact but it is not in the same league optically or mechanically as the triplets or Petzvals that I've seen but it's much less expensive. I put a 2 speed Moonlite on mine and now it's a pleasure to use. A 4.25" mask for your Dob will give you some idea of what a 4" apo will show you at high power. My quick look planetary scope went from a 4" apo to an 8" f7 Newt and I've never been happier with my views.

Clear Skies,

Steve

Good idea,Inge.
I bought a little TV-60 to supplement my 14.5" Starmaster, and I really like it.
It's great for taking in wide field views of Milky Way star fields.
I can't see all of M-24 in the Starmaster, for example.
And it's exciting to see the entire Veil nebula at once,
even at greatly diminished brightness.
I'm actually considering trading up to a TV-85 or 102.
Enjoy whichever you pick!
Linton

Thank you all for feedback. There is absolutely no urgent need for me to acquire a second scope, but now I have an idea about why we see the enormous price differences of 4-inch scopes, and also have a fair chance of telling what is a good deal and what is not.
One of the reasons why I'm thinking about a second scope is that I sometimes could make good use of a grab-and-go scope. With my previous scope (an 8-inch Portaball) this wasn't an issue since I kept it fully assembled all the time, and could carry it out and start observing in a minute or two. With the 12.5-inch Portaball it isn't that easy. I have to disassemble it for car transport, and it is heavier to carry around than the 8-inch. For a 2 hour observing session, my choice would always be the 12.5-inch, but for a quick look at Saturn and some open clusters and double stars a smaller scope might do the job.
At my age ergonomics becomes more and more important. I don't like to strain my back, and I find it difficult to see optimum detail while viewing from an inconvenient position. Once again the Portaball is superb. With the 8-inch I always was sitting while observing, with the 12.5-inch I stand, but still always in a relaxed position. Therefore I figured out that if I'm going to use a refractor, it should be a short one where the eyepiece position doesn't vary much depending on the location of the target. Unfortunately, nobody seem to make Coude-refractors any more. Ergonomically this would be the perfect scope, the drawback being a heavy setup, and long focal ratio limiting the maximum field of view.
I once owned an 102mm Vixen achromat(f/9.8). Next to the Portaballs it was the best scope I ever owned. I was never annoyed by colour fringing, and I still can't forgive myself for trading it for a Monday product with higher aperture. But even the Vixen was a bit large. A shorter scope f/6.5 - f/8 seems as a very good choice.

Inge S

Going back to your original question, in addition to differences in the lens, there are differences in the overall construction; what some refer to as "fit and finish." For example, I understand that the higher end APO's are made of a heavy spun metal, whereas lower cost versions may be made with a welded tube. The focusers also tend to be different with the higher end versions now including fine focusing mechanisms as a standard feature. It's clear however, that there are now more choices in the marketplace for 4" APO's and the differences in viewing may be due as much to the observer's eyesight and nighttime viewing conditions as it is due to the scope.