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false color

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#1 Guest_**DONOTDELETE**_*

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Posted 24 November 2003 - 09:53 PM

hey everyone

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 :crazy:. anyway, the refractors i've been seriously considering is the stellarvue 102mm achromat, and the tal100. either would be purchased as an OTA. i know there is a pretty significant difference in price, but from what i've heard about stellarvue i'm hoping the scopes performance would be worth it, though the tal is attractive because of the lower price. and i've think i've read all the reviews i can find on the tal, as well. also, i don't want to go any smaller than a 4" aperture. but i guess the point of this post is that the subject of chromatic abberations is starting to deter me from gettin a refractor, and i would really like to think that it shouldn't. (ya know, it's easier to research and buy an automobile. then again, the "test drive" sure helps). if anyone could offer up some viewpoints, i would be much obliged :grin:

best,
Chris

#2 Guest_**DONOTDELETE**_*

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Posted 24 November 2003 - 11:20 PM

Chris;
As the local CSN (Certified Stellarvue Nut) I am highly opinionated, unfairly biased, and WAY over qualified to provide an objective opinion. (humor is such an ugly thing when being typed) :lol:

However, let me give you my beginners perspective. I am an older, new astronomer. I like quality things, but don't have a lot of money. (My wife says "Champagne tastes, Koolaid budget) I wanted a scope that was easy to move around, travels well, and sets up quickly, didn't like the Dob nudge, nudge thing, and didn't want to have to worry about cool down time. (rains a lot here in WA state, then clears) I was also worried about the "dreaded purple fringing." After visiting 2 astronomy shops, looking at a bunch of different scopes, I decided a refractor was what I wanted. I went to one shop during the day and looked through a Chinese short tube 80, a Stellarvue Nighthawk and a Magrez 80, at terrestial objects. All had purple fringing around the edge of objects at powers over 100x+. There was clearly a differance in the build quality of the Stellarvue over the Chinese import. The Magrez was as good quality as the SV, but cost more.

The SV was a little over my budget, then the company put them on sale.:jump: All the research and reading I did on the net showed me a couple of things.
1-The vast majority of people buying SV are very happy and seem to keep their stuff. Vic Maris, the owner of SV, does a great job of QC with his stuff, I think the optics in each scope are assembled and tested to the best they can be. Many of the Chinese scopes can be made very good if you know how to tinker. Vic uses quality components and does a good job assemblying them.
2-Most every one either has or wants a small Grab and Go scope. So even if I Upgrade this should be a keeper.
3-the scope that is the best for us is the one we will use.

So far I am very happy with my purchase. If I had the money SV's new 102DN would be on order. I think the bigger aperature would be worth the trade in weight and size. However the OTA alone is more than my setup. :money:(Koolaid again)

Finally: Purple fringing. There is a little at high power on the moon, at the edge of the view, if I go looking for it. From what I can tell some of the less expensive imports have a lot more of it, but I don't have the experience to know for sure. What is visible does not bother me at all. One caveat: I don't do photography so I don't know the implications for that.

One option, the Orion ED80, not 4" opening, but great reviews.

Stellarvue has great customer service, makes a good product, and has a 30 day money back guarantee. Order one up. The folks on the SV forum say "seeing is believing" :bigshock:

Forum URL below:
http://groups.yahoo....oup/Stellarvue/

Blessings

Keith CSN

#3 Guest_**DONOTDELETE**_*

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Posted 24 November 2003 - 11:40 PM

I can't wait to put the 80/9D and the AT1010 up against each other!

#4 Guest_**DONOTDELETE**_*

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Posted 24 November 2003 - 11:42 PM

Draw Pardner!

#5 Guest_**DONOTDELETE**_*

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Posted 24 November 2003 - 11:59 PM

It'll be a fun evening...wonder when??? I have rain forcasted from here until next July 5th! When I pull the Dob out, we'll see what we are supposed to see!!!

#6 rboe

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Posted 25 November 2003 - 12:09 AM

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.

#7 Guest_**DONOTDELETE**_*

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Posted 25 November 2003 - 12:30 AM

rboe;

You've been reading way too many posts.

Put down the mouse and back slowly away from the monitor.

Keith :lol:

#8 Ron B[ee]

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Posted 25 November 2003 - 01:52 AM

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 :crazy:. 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 :bawling:)? 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]


#9 Guest_**DONOTDELETE**_*

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Posted 25 November 2003 - 08:48 AM

Chris,
There are of course safer routs as well you know....You could opt for a Maksutov of some size and then not worry ever about color or a reflector...........Personally having owned and looked thru a lot of different sizes of achros my opinion is get the longest fl you can manage an f9 has a lot less color than a f6 and is also a better moon/planetary also as it takes hi-mags better. Dave

#10 Guest_**DONOTDELETE**_*

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Posted 25 November 2003 - 09:29 AM

What Dave said. Get the longest focal length you can afford if you are worried about the color.

You'll be happy with anything above f/9. The smaller apertures tend to come in the longer focal lengths because they are smaller overall. As the objective diameter gets larger, focal lengths tend to shrink because of the physical size of the OTA.

I have a 90mm at f/11 that is physically shorter than my 152mm at f/8.

I wish Meade would have squeezed one more f/ stop out of the 6".

#11 Guest_**DONOTDELETE**_*

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Posted 25 November 2003 - 12:13 PM

Question:

How much difference does 1 f/ stop make in color correction? Is it discernable by the eye?

Keith

#12 Guest_**DONOTDELETE**_*

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Posted 25 November 2003 - 03:58 PM

Keith, are you bored and just trolling?
I think you wont notice a great deal of difference between 1f as im sure,........................ you already know. The most noticable diff would probably be a slight larger/smaller FOV between them unless the refractors are from different manufactures then anything could be different wildly obviously. The major differences are between scopes of several fls. All flint/crown glass refractors in the f6-f9 show color at mid/high mags. The f11-up flint/crown glass lenses take hi mags a lot better and produce higher magnifications on similar eyepieces allowing use of widefield eyepieces for high mags. APOs on the other hand using rare special lenses can be produced at about any fl with minimal color if any at all, of course there is a price paid for low fl and high mag with no color. Quality of lenses and focusers is more importaint than any 1 fl/ stop between any similar scopes ever period, no matter who made em. Dave

#13 Guest_**DONOTDELETE**_*

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Posted 25 November 2003 - 04:18 PM

If your question is in relation to your wanting to build your own finder scope, I think I would want the shorter f-ratio for the wider field of view. I like to envision the f-ratio as a cone of light that is bounded by the eyepiece, the lens and infinity. It defines what you will see at infinity. the shorter the cone the more sky you will see. Probably didn't help since you know this but you never know!

#14 Guest_**DONOTDELETE**_*

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Posted 25 November 2003 - 05:43 PM

Dave;
No, not trolling, a serious question.

For example, in the audio or RF world, power is measured in decibles. (db) To produce a 3db change in volume or Rf power requires a doubling of the power used. 3db is the minimum change a normal person can detect as being louder.

If you are listening to music and the amplifier is a 1 watt amplifier and you hook up a 100 watt amplifier the signal is not 100 times as loud it is about 20 times as loud.

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, ?

I hope this made some sense.

Keith



#15 jrcrilly

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Posted 25 November 2003 - 06:09 PM

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.

#16 Guest_**DONOTDELETE**_*

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Posted 25 November 2003 - 07:05 PM

thanks for the input, everybody. there's an awful lot of experience speaking, on these forums, leading to some great advice and insight. i really hope that other future aspiring astronomers and veterans to this hobby find their way to this wonderful community. from what i have gathered, i could get a long FL 4" achromat, a filter to suppress the CA, and be probably be pretty happy with it. i'm pretty set on a refractor, i've heard a lot of good about the views of both achros and, of course, apos. also, living outside of buffalo, ny, i really feel i should get a good cold weather scope :grin:(there's a great deal of truth to that, but, hey it doesn't hurt to have an excuse, right?) btw, speaking of filters that cut back on levls of false color, are these typically pretty useful? do they do at least a decent job? thanks again, everyone :waytogo:

best,
Chris

#17 Guest_**DONOTDELETE**_*

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Posted 26 November 2003 - 12:26 AM

Thanks all, next question.

A 3" refractor has approx 7 sq in of surface, a 4" has approx 12.5 sq in, and a 5 " has approx 19.5 sq in of surface.

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?

I would expect the step from a 3" to a 5" would be greatly noticable.

Comments?

Thanks
Keith

#18 Guest_**DONOTDELETE**_*

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Posted 26 November 2003 - 12:31 AM

I would imagine that the image will be a little bit brighter in the 4" vs. the 3" and be able to go a little bit deeper into the sky. The 5" would be noticable and able to go much deeper. Thinking about getting or making something bigger?

#19 Ron B[ee]

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Posted 26 November 2003 - 12:47 AM

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]


#20 Guest_**DONOTDELETE**_*

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Posted 26 November 2003 - 01:34 AM

Tom;

Just thinking at this point. I would like to be able to magnify the planetary views and make the dim fuzzies less fuzzy. (I know there will always be dim fuzzies) I am beginning to understand that darker skies will help a lot more than I first understood.

I have been looking at a couple of good quality Achro refractors that are less than an APO. D&G optical being one, homebrew being another option. But I didn't know whether the f12 or f15 would be resonably color free or if I should just decide an APO is the way. I think if I ever want to experiment with Astrophotography using a refractor, an APO will be needed.

Ron;

I think you might be right about the 8" Newtonian. I will just need to mount it on an EQ mount. Pretty big mount setup. However it is probably equal to what a 5" refractor would need. I have been looking at EQ platforms, yoke mounts, etc for reflectors. Lots of options. I am not planning on doing anything for a while, would like to attend a few star parties and examine and look through all kinds of scopes first. The thing about the Newt is I can build it myself. I would probably buy the mirror, so finding a good one would be a priority.

For now; I would just settle for the rain to quit so I can take my Nighthawk and binos outside and use them.:(

Blessings
Keith

#21 Guest_**DONOTDELETE**_*

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Posted 26 November 2003 - 08:07 AM

Hi Keith,
Like John says,
a high fl is necessary, unless you want to use a V-block filter on it for high mags on Moon and planetary. Actually a Meade 5" or better yet 6" dosent even need a V-blocker filter for deep sky viewing just planets, and the price complete with EQ mount with go-to is pretty darn hard to beat. Ive thought about the D&G and other manufactures myself too as APOs are so highly priced. I think id be wary to even use a scope that cost me $5,000 alone just for the OTA! I had the Meade 6" and a APO 3.7" and finally went to a Meade 5" on the LXD55 combined with a V-block filter and im happier with it then my 3.7 APO because the images are brighter and it mags higher. Its not as portable as the APO but I dont go out to dark skys much except in summer anyway.
As far as comparing lenses to music volume I dont have a musical ear or 20/20 vision but from all the different scope sizes ive owned (over 10), I must agree with all people above that stated brighter images, slightly larger eyepiece image size too but not startling unless you jump from 3"-7" or 8" then there is a pretty good difference, to see that much difference again would probably take a monster sized scope like a 14" and up......To be honest I have a 5" refractor and a C8 and a 7" Meade Mak and the one that gets all the use usually is the Mak. Reason is the aperture and resolution on planetary, the other 2 cant compete with it on Saturn and Jupiter. Ive gone thru an awful lot of scopes to finally settle down happily with the ol Mak. Its f15 by the way so its as close to an APO as a lens'mirror can get at its pricetag range, and im not afraid to use it because of such. Something to consider. Dave

#22 rboe

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Posted 26 November 2003 - 09:16 AM

One this one point (going from 3" to 4") I am going to disagree with Ron B[ee](just this once :D ). 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.

#23 Ron B[ee]

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Posted 26 November 2003 - 10:03 AM

One this one point (going from 3" to 4") I am going to disagree with Ron B[ee](just this once :D ). 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 :thewave:. 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]



#24 Guest_**DONOTDELETE**_*

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Posted 26 November 2003 - 02:02 PM

Keith, I finally got my choices down to the Stellarvue and the Meade LXD-55 AR-5. It was a tough call for me put the SV won. I was concerned about the LXD-55 mount to begin with and I also wanted it portable enough to take out at any time. The 5" would probably do that ok but the 80 is just going to be that much simpler. I hate to say it, but finding the 114GT mount was the final tipping factor. Should fit the SV perfectly and price was under $500 for the scope/mount vs. $700 for the AR-5...just enough to matter (to me).

I think any f-ratio over 9 will greatly reduce the color on bright objects (hence the 80/9D vs. the Nighthawk). I also have a contrast filter that I hope will help even more on the moon and planets.

I would like the chance to see Stacy's 8" reflector on the EQ mount and try it out sometime to see if it is a viable option for me in the future. If I find an HEQ5 mount at a reasonable price (or an EQ6), I'll snatch it and convert my Dob.

#25 Guest_**DONOTDELETE**_*

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Posted 26 November 2003 - 03:59 PM

Here here Rboe!
I whole heartedly agree with you about the subtile differences between all but the hi end APOs within several inches anyway. I will say however that for DSO observing yes a few inches makes a BIG diff. For planetary tho quality is king mostly hence, APOs & Maks and the larger sized scope over 8" seeing permitted. To me high fls is what I want verses a wide, low power sky view. I get my wide angle views thru my binoculars using both eyes anyway. Dave


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