30 replies to this topic

[rboe]

So which is better? Astro-Physics or Takahashi? I mean the Orion ED80 or the AT1010? Or would you prefer to talk about cashmere sweaters?

Chris, any chance a local astronomy club; with fewer wacko's than you see here, having a star party you could attend to get a better feel for some scopes? Nothing like some real world experience to set some benchmarks. Then again, there is a lot to be said for only having a $300 scope in the corner while it rains for weeks on end instead of a $4000 scope.

I vote for Genie! In a ...well, let's just say proper observing attire.

[Ron B[ee]]

so i'm really lookin into gettin a refractor and i've been doin a lot of research. but this whole issue of false color has actually got me kinda frightened. i mean, there's some much emphasis on whether or not or how much false color a scope shows, that i'm starting to think i'm goin to be dissapointed in any refractor, and an apo, i just can't afford, at least not for a good many years . anyway, the refractors i've been seriously considering is the stellarvue 102mm achromat, and the tal100. either would be purchased as an OTA.

Hello there Chris,

Well, it all depends on you. Some people are more sensitive to the false color than other. There's nothing wrong with an achromat, but caring people are trying to make you aware of the false color. Personally, having own a 4" f/9.8 achromat and a 4" f/8.6 APO (and side-by-side for about a month), I much prefer the APO. I would not want a short focal ratio achromat however if it's gonna be my general purpose scope. But don't let me or anyone steer you from what you want so long as you want it for the right reason and understand what you're getting .

I think the bottom line is: if you have not look through an achromat and you're this fearful, you should arrange to look through one at the Moon and Jupiter later when it's up higher (hopefully under good seeing). You can then decide for yourself. Alternatively, you can buy a used chinese made C102-HD fairly cheaply on Astromart and find out for yourself and you could probably turn around and sell it for about the same price or less (assuming you got a good deal to begin with of course :o). Also, you can't look at an 80mm achromat (or 78mm) and extrapolate because of the difference in light gathering; the larger the aperture, the easier the false color would be seen (i.e. brighter).

Last but not least, ask yourself do you need a 4" achromat at this very moment (Mars is practically gone )? Or do you want compromise now or do you want to save up for a year (maybe more) to get really what you want? Or perhaps you don't care about the better color correction of the APO anyway!

Ron the 4-inch Tall Evangelist B[ee]

[jrcrilly]

I am trying to understand focal ratio in terms of color problems in an achro scope. On a practical Amateur level can I get a 5 or 6 inch scope that will have minimum color problems in the 300x power range and what focal length will it take. f/10, f15, ?

Hi, Keith the only guidelines with which I'm familiar for minimum focal ratio for good CA performance vary between three and five times the aperture in inches. For a 5" achromat this would lead to focal ratios in the neighborhood of 15 to 25. I suspect this is pretty conservative and I'd rather live with a little color than pay for a 5" APO or put up with a 75" optical tube; either way is too much trouble if all I'm gonna have is 5" of aperture anyway.

[Ron B[ee]]

Does the step up from a 3" to a 4" provide a large improvement in light gathering, or is it more like, it's noticable but not spectacular?
Keith

It would be very noticeable, but not spectacular. Heck, going from my 4-inch APO to 8-inch Dob does not result in a spectacular light gathering increase either and made everything looked maybe twice as bright! So, the math has been misleading to me. The exit pupil thingy is much more important consideration than the sq. math as you can go to a lot higher magnification with aperture (providing the seeing cooperates) and because of the thingy difficult faint fuzzies become easier, and the brighter faint fuzzies more detailed.

In my opinion, if you have a 3" scope now and would like to see substantially more, much more, I would go to at least an 8-inch Dob f/6 (or 6-inch if your space is tight) with good quality mirror.

Ron B[ee]

[rboe]

One this one point (going from 3" to 4") I am going to disagree with Ron B[ee](just this once ). I've looked thru the Pronto and the TV 101 and do not see that great a difference - we're talking light gathering here. Did not see that big a difference between my 16" dob and the 11" SCT (now you know one reason I want to recoat it - I expected a visible difference).

Difference between a C9.25 and the C11 was very minimal too.

This could be because I EXPECTED big differences but have learned that small incremental differences in eye pieces or telescopes, noted by many experienced observers are simply not seen by plebes such as myself. Our scales are different.

When you say there is a huge difference I expect to go from a flashlight to those spotlights you see on trailers at movie openners. Not the 2 D cells to 3 D cells difference.

Many times the difference is subtle and unless you do a comparison like EdZ has done with the binoculars you will not notice it.

This is one HUGE reason to go try some of these scopes at a star party. It will ground you to the correct expectations. You will also see that a 20" dob, in some cases will knock the socks off a 4" APO, in other cases it's a toss up. I was not expecting that at all.

[Ron B[ee]]

One this one point (going from 3" to 4") I am going to disagree with Ron B[ee](just this once ). I've looked thru the Pronto and the TV 101 and do not see that great a difference - we're talking light gathering here. Did not see that big a difference between my 16" dob and the 11" SCT (now you know one reason I want to recoat it - I expected a visible difference).

Touche, Brother Ron . You're quite right if you're casually comparing 70mm to 4". However, if you're carefully scrutinizing for features and details, the difference will be quite noticeable. Here's a comparison I made between 70mm and 102mm aperture using aperture mask.
http://members.cox.n..._TW-70_moon.htm

Ron B[ee]