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Istar Phoenix WFT 204 “Comet HUNTER “…

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#1 Charlie Hein

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Posted 16 February 2014 - 12:17 PM

The Istar Phoenix WFT 204 “Comet HUNTER “…

By James Edwards

#2 stevew

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Posted 16 February 2014 - 04:12 PM

47 telescopes since 2001 ?
Wow.. :bigshock:
I hope you have finally found what you were looking for.

#3 galaxyman

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Posted 17 February 2014 - 09:20 AM

Amen brother of an 8" refractor :cool: :waytogo:

Congrats

Karl
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#4 RGM

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Posted 17 February 2014 - 09:30 AM

I have the 127mm f12 R30 Istar. I am looking at the 204 as my next scope. I really enjoy refractor views. In fact, my 10" Dob has not seen starlight in 2 years. The Dob can not put up the quality of views that my refractors can.

#5 Starman1

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Posted 17 February 2014 - 01:44 PM

Seriously, an 8" f/5.9 achromat?
That makes the chromatic aberration of this scope worse than a 6" f/5 refractor, which has some of the worst CA I've ever seen in a scope.
The 6" f/5, at least, is a short focal length, low power, wide field scope for comet hunting and wide views.

So do tell: how did you tame the CA in the scope? Which minus violet filter did you use? That might be an excellent one to use in the 6" f/5.

#6 galaxyman

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Posted 17 February 2014 - 02:44 PM

What's interesting Don, is his review seems to be in line what a CAS member's APM 7.1" F/6 does, particularly with DSO's and using up to 300x.


Karl
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#7 Starman1

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Posted 17 February 2014 - 02:56 PM

What's interesting Don, is his review seems to be in line what a CAS member's APM 7.1" F/6 does, particularly with DSO's and using up to 300x.


Karl
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Karl,
Yeah, a good lens is a good lens. My question was about taming the CA. I wouldn't believe a claim that there was none, but sometimes having a little (i.e. using a gentle filter) is just fine.
What does the CAS 7.1" owner use?

#8 galaxyman

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Posted 17 February 2014 - 03:12 PM

Here's a short review he did.

FYI he owns some other very serious scopes like a 22" f/3.6 dob and a AP 5" APO to name a few. Also as with most of CAS (yes thanks to me), he's more of a dedicated DSO observer, and most of the time the large refractor is used for that.

APM 180 mm F/6 Achromat Deluxe :


I couldn't stand it any longer finally broke down and set up the 180 on
my deck last night on the G11 in manual mode. Using the scope's rings as a
1x reflex finder I went hunting! It was like pointing a cannon at the sky!

Without a minus violet/Fringe Killer, the Moon was awesome (36 ortho, 24
Pan, 0-18 SMC ortho, 0-12 SMC ortho and the 3-6 TV Zoom set at 3mm (360x)!)
Obvious chromatic aberration at the bright limbs either side, but tolerable.

With the Fringe Killer, it's APO like!!
I have never seen so much surface texture and detail on the Mare, subtle
contrast was amazingly detailed.

Bands of clouds kept me away from the DC and UMa galaxies and Orion was
obscured by the house (I was set up on my deck blocking the southern
horizon)! But, over head, I got M37 and M36 in Aur and M35 in Gem. Pinpoint
stars, just like diamonds on black velvet! And the COLOR of the different
stars was extremely obvious, reds and blues and orange tints.

And then my heart stopped and I had a religious experience: Saturn!

Only about 1.25 degrees below the Moon, high clouds and lousy seeing, but at
180-360X with the TV zoom, planetary banding obvious and the Cassini
division winked in and out with the seeing. Five moons easy even in the
moonglow and skyglow!




Karl
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#9 Dakota1

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Posted 17 February 2014 - 06:59 PM

In his review at the bottom he stated the weight was at 24 lbs. In Sept 2013 I E-Mailed Mike at Istar on how much the scope weighs he said the scope weighs 34 lbs not including the tube rings. This is an error and a pretty big one if someone is serious about this scope and to make sure they can handle that weight especially when mounting it on a tripod or any other support with a pier extension. I am possibly looking at one next year but have help available to mount it with out damaging it or myself. Thanks

#10 stevew

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Posted 17 February 2014 - 10:34 PM

Seriously, an 8" f/5.9 achromat?
That makes the chromatic aberration of this scope worse than a 6" f/5 refractor, which has some of the worst CA I've ever seen in a scope.
The 6" f/5, at least, is a short focal length, low power, wide field scope for comet hunting and wide views.

So do tell: how did you tame the CA in the scope? Which minus violet filter did you use? That might be an excellent one to use in the 6" f/5.


Don, you must have missed this part of the "review"... :scratchhead:

"The notion that “fringing” or “false” color hinders or hampers what you see in a “doublet” is phony baloney nonsense"

#11 Darren Bly

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Posted 17 February 2014 - 11:26 PM

"The notion that “fringing” or “false” color hinders or hampers what you see in a “doublet” is phony baloney nonsense"


This could be a very nice deep sky telescope but it would be a niche telescope. ANYTHING bright would have horrendous false color.

#12 Starman1

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Posted 18 February 2014 - 01:56 AM

Seriously, an 8" f/5.9 achromat?
That makes the chromatic aberration of this scope worse than a 6" f/5 refractor, which has some of the worst CA I've ever seen in a scope.
The 6" f/5, at least, is a short focal length, low power, wide field scope for comet hunting and wide views.

So do tell: how did you tame the CA in the scope? Which minus violet filter did you use? That might be an excellent one to use in the 6" f/5.


Don, you must have missed this part of the "review"... :scratchhead:

"The notion that “fringing” or “false” color hinders or hampers what you see in a “doublet” is phony baloney nonsense"

Having owned a 6" f/5 and a 4" f/15, and, now, a 4" f/5.4 apo, I can only say, "to each his own". Chromatic aberration definitely spoils the view of Moon, planets, and bright double stars. Deep sky? You're right.
Anything brighter than about 3rd or 4th magnitude? Give me an apo or a reflector. The 6" f/5 even still had too much false purple for me with the "atom bomb" of minus violet filters, the Baader Contrast Booster. I eventually sold it.

#13 David Knisely

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Posted 18 February 2014 - 03:02 AM

"The notion that “fringing” or “false” color hinders or hampers what you see in a “doublet” is phony baloney nonsense"


This could be a very nice deep sky telescope but it would be a niche telescope. ANYTHING bright would have horrendous false color.


Even some of the fainter doubles might have some haziness around them which some people might object to. Even my 100mm f/6 doublet has faint almost colorless halos around many doubles that really impacts the view at higher power, especially when trying for very faint companion stars that might get buried in that haze. Indeed, some tighter open clusters or globulars could get a little dose of extra "haze" in them with 8 inches of f/5.9 achromatic doublet to play with, although the aperture would certainly resolve them well. I suspect that my old 8 inch f/7 Newtonian or my friend's 8 inch f/5 RFT probably could probably do a little better planet-wise, although the 8 inch f/5.9 refractor could get true fields of view of up to 2.2 degrees or so with somewhat better edge illumination. Oh well, whatever trips your trigger (especially for around $3000). Clear skies to you.

#14 galaxyman

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Posted 18 February 2014 - 09:17 AM

Interesting that we seemingly are trying to dimminish what the owner of this scope saw through the scope. He seems to be quite positive on the views and scope.

I do understand why he seems so positive on the scope, not so much because of my longer focal length 8", but what has been seen in the APM 7.1" f/6. Yes, globs at high power in that scope also "rocks" as being well resolved to crisp points.

I'm sure if you send a PM to Deep Sky moderator Mr Mitsky, who has seen through this 7.1" :cool:

I am anxious to read more observing notes down the road from the owner of this 8" refractor.



Karl
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#15 David Knisely

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Posted 18 February 2014 - 03:53 PM

galaxyman wrote:

Interesting that we seemingly are trying to dimminish what the owner of this scope saw through the scope. He seems to be quite positive on the views and scope.

I do understand why he seems so positive on the scope, not so much because of my longer focal length 8", but what has been seen in the APM 7.1" f/6. Yes, globs at high power in that scope also "rocks" as being well resolved to crisp points.

I'm sure if you send a PM to Deep Sky moderator Mr Mitsky, who has seen through this 7.1"

I am anxious to read more observing notes down the road from the owner of this 8" refractor.


I don't think anyone is diminishing what the owner saw in the scope at all. From his almost gushing report, he seems quite happy with it (and more power to him if he likes what he got). However, as a true review, this article lacks some of the vital critical analysis which really needed to be there to make the article somewhat more useful. We need to know if this objective is some kind of "miracle" and tames chromatic aberration enough to work well, rather than being just another simple achromat (which I suspect the 8 inch f/5.9 actually is). The author hardly mentions how much false color there was in the views, and does not fully explore this in terms of the magnification when it became visible and how it varied between bright and fainter targets. Were there faint halos around double stars and how big were they? Did a minus-violet filter need to be employed at higher magnifications? How much scattered light appeared around brighter objects? Questions like this weren't really answered properly. All he says is the somewhat questionable response of:

"False color is prevalent on brighter objects (planetary, moon and larger stars) but guess what? The notion that “fringing” or “false” color hinders or hampers what you see in a “doublet” is phony baloney nonsense, "

For many observers, this latter statement is definitely not true. Without some really exotic (and expensive) optics, a two-element eight inch achromat at f/5.9 will show some really significant residual chromatic aberration, which those of us who have used such scopes of significant aperture know quite well. This color excess reduces the contrast of finer planetary detail and impacts the view notably at high magnifications much more than in reflectors or refractors of significantly longer f/ratios. It is nice to report on some of the good qualities that an instrument might have and how much fun one can have with it, but as a "review", this report left me with a few more questions than answers. Clear skies to you.

#16 Jim Curry

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Posted 18 February 2014 - 07:47 PM

You guys are so mean. Do you just like to hear yourself type?

The reviewer "gushed" about the deep sky views. Planetary views getting a 3.5/5 rating (70/100) is not gushing. In fact, I would call that panning. “False color is prevalent on brighter objects”. Prevalent=dominant. It doesn’t sound like he was mincing his words or glossing over the deficiencies. “WHAT the 204 gives you is breath taking views that more than compensate for any color that shows”. That must be what set you guys off!! Way too much joy there.

The man is experienced, has owned some high(er) end refractors and was relating his joy of deep sky viewing with an 8" doublet. 8" is where deep sky views start to really get fun, noticeably brighter and more extensive. Some objects can start to take on a 3D look. He didn't write the review for you guys or perhaps you should have sent him your checklist so his review would have met your standard. What's with the straw man “We need to know if this objective is some kind of "miracle" and tames chromatic aberration enough to work well, rather than being just another simple achromat”. I could suggest you lower your proboscis enough to look over it and google the Istar website. But I won’t suggest that because I’ll bet a chocolate chip cookie you know darned well it is a simple achromat. And congratulations, you beat down two straw men in one paragraph, double stars (objects he didn’t even discuss).

Oh, I don’t want to neglect the red herring: “So do tell: how did you tame the CA in the scope?” Prevalent=dominant. There’s a lot of attitude in that sentence, Don. (Me too in this missive) :>)

"If you aren't having fun in this hobby, you aren't doing it right." But apparently the fun needs to be pre-approved.

Big tough guys here.
Give it a rest.
Oh, yeah. To keep it all pleasant:
Clear Skies

#17 galaxyman

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Posted 18 February 2014 - 07:58 PM

Hi David

Certainly and I respect your comments, and one reason why I want to read a more detailed observing report.

What I find interesting is that James (the owner) seems to be experienced, and has written reviews of other quality scopes here on CN. So I may take it from his experience that the scope is quite good, for that’s all I can go on short of actually viewing through the scope.

The one thing that may need to be acknowledged, is though I’m sure it’s a standard doublet achro, the optical quality (including using high quality glass) may be far better (as mine is), then the standard mass-produced Celestron (Synta) achromats.

I can tell you with all honesty that his rave or shall we say positive review did not surprise me. Partly yes because what I’ve seen with my own 8” refractor albeit longer focal length, but again as I mention the APM 7.1” f/6, which of course is close to the instrument in this review as in aperture and focal ratio.

The owner here has pushed this scope, as you can read in the short review I posted above.

I’ve seen this scope many times in action, and the views even at 300x plus using high quality eyepieces are quite good. Whether it is a small planetary nebula or a tight glob.

We’ve seen 14th mag plus galaxies in this scope from good (not great dark sites).

Anyway, maybe the owner of this Istar 8” scope will post some more comments here or answer any questions, and hopefully a detailed observing report.




Karl
E.O.H.


Chesmont Astronomical Society - www.chesmontastro.org
Galaxy Log - http://www.youtube.c...r/GalaxyLog4565
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#18 David Knisely

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Posted 18 February 2014 - 11:50 PM

Jim Curry posted:

You guys are so mean. Do you just like to hear yourself type?


No, I just wanted more information. My questions were not answered quite completely enough by this review for the reasons I stated earlier, that's all.

The reviewer "gushed" about the deep sky views. Planetary views getting a 3.5/5 rating (70/100) is not gushing. In fact, I would call that panning. “False color is prevalent on brighter objects”. Prevalent=dominant. It doesn’t sound like he was mincing his words or glossing over the deficiencies. “WHAT the 204 gives you is breath taking views that more than compensate for any color that shows”. That must be what set you guys off!! Way too much joy there.


No, you have definitely overreacted to what I have said. When someone says "refractor", many observers think, PLANETS, as planetary viewing is what they tend to be most known for. I know some refractor "lovers" who put up with the fairly minor chromatic aberration in their D&G f/12 to f/15 large refractors in the six to eight inch aperture range and yet still complain about it (at least a little). They would very probably be a lot more critical of an f/5.9 8 inch doublet unless some more detailed information was presented about how it performed as far as color excess is concerned. Instead of just the quick "on a planetary scale the jury is still out, but Mars and Jupiter looked good (3.5 out of 5)" statement, it would have been nice if the author of the article had been somewhat more descriptive about what the scope exhibited in that residual color on the moon, planets, and double stars. That way, one could truly judge if the statement of "breath taking views that more than compensate for any color that shows", is fairly accurate. We just need a little more information and not blanket emotional statements like "The notion that “fringing” or “false” color hinders or hampers what you see in a “doublet” is phony baloney nonsense". Like the author, I enjoy what my simple f/6 doublet does at low to moderate power on deep-sky objects, but for the planets or double stars, well, it is functional but isn't exactly overwhelming. It would have been nice to put a little more in the description of the views at high power in that 8 inch f/5.9 refractor, and on objects other than the deep-sky ones which the author obviously feels are the forte of that instrument.

What's with the straw man “We need to know if this objective is some kind of "miracle" and tames chromatic aberration enough to work well, rather than being just another simple achromat”


It is not a "straw man" argument. There are recent (and sometimes fairly frequent) changes in optical materials and design that have made a few 2-element achromats perform notably better than earlier designs (in some cases, almost as well as some of their more expensive 3-element bretheren). I would have been interested to see if this objective was better than what I have seen previously with 2-element objectives (especially those with f/ratios below f/10). In the past, I have been pleasantly surprised occasionally by something like the Vixen 103mm f/7.7 doublet which gave sharp almost color-free images of Jupiter one night at the Nebraska Star Party (wish I would have bought that one). I can't really effectively judge how good the eight inch performed based on a simple "3.5/5" rating. I needed just a little more. It's that simple.

And congratulations, you beat down two straw men in one paragraph, double stars (objects he didn’t even discuss).


Again, not a straw man, but missing information in that the reviewer didn't discuss doubles at moderate to high powers. Refractors are a mainstay of many double star observers, so again, it would have been nice if the reviewer had included at least a few in his tests of the instrument. With a short f/ratio achromatic doublet, there would undoubtedly be at least some issues with the performance on some close brighter doubles, but it would have been nice to know exactly how good (or bad) the instrument performed on them.

But apparently the fun needs to be pre-approved.


Nope, it doesn't have to have any "approval". I would have just liked the reviewer to have covered a little more of the bases when it comes to something I might want to someday purchase. I guess the only way to judge this instrument is to see it at a star party and get a look for myself. Clear skies to you.

#19 PGW Steve

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Posted 19 February 2014 - 12:28 AM

It's a golf club.


I have a 6" f5.9 and I know better than to point it at a planet or the Moon. I do love the crisp stars delivered to my eye in a huge FOV. Knowing the limitations of this optic, and its strengths, I employ it as a putter.

If I need to look at colourless views of the Moon or planets, I'll grab my 7" LX200 Mak, my driver.

Thank you for the review, been drooling over this OTA for a while, you confirmed my suspicions.

What I would like to know from the reviewer, it looks like you said the rings and dovetail were included. However, on the Istar site, it looks like the scope is a-la-carte. What was the total cost of the scope as reviewed, did the focuser come from Istar installed?

#20 Starman1

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Posted 19 February 2014 - 02:16 AM

The reviewer "gushed" about the deep sky views. Planetary views getting a 3.5/5 rating (70/100) is not gushing. In fact, I would call that panning. “False color is prevalent on brighter objects”. Prevalent=dominant.

Oh, I don’t want to neglect the red herring: “So do tell: how did you tame the CA in the scope?” Prevalent=dominant. There’s a lot of attitude in that sentence, Don. (Me too in this missive) :>)

Actually, you read into my words a meaning they didn't have. Since he had said what he did, I actually was curious about how he compensated for it.
I know a short achromat will work fine on deep sky--I enjoyed my 6" f/5--but there are a lot of objects up there where the false color would be obtrusive.
The reviewer of the 7.1" mentioned he used a Baader Fringe Killer filter, whose characteristics are well known. Had this reviewer mentioned his solution for violet fringing, it would speak volumes on just how tamable the violet was.
I used a Baader Contrast Booster on the 6" f/5, and it was barely strong enough. In one sentence, you know exactly how my scope was on bright objects.
Perhaps the author didn't buy the scope for any purpose requiring a minus violet filter, and if so, that's just fine. I was merely asking.

#21 Kevin Barker

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Posted 19 February 2014 - 11:13 AM

I have been reading this thread with some interest. Mostly because I have recently constructed a 180 mm 7.1 inch f-8 achromat myself. It was sold as a R30 improved achromat etc and it does appear to control CA a fair bit better than i would expect for a standard achromat.

I am well aware of the limitations of this type of fast large aperture refractor(e.g the 204 f-6). A cruise at low to medium powers across star fields with a big fast achromat and modern well corrected wide field eyepieces is pretty darn good. As has been stated above they can illuminate a wide field well and provide very sharp pin point stars. Views of deep sky wonders are also quite impressive. They can also do surprisingly well on double stars.

In my own experience I have found the scope I have to be pretty darn impressive with respect to CA as seen at the eyepiece. Planetary performance is not up to a long focus achromat but heh I knew that. For example to some extent CA washes out some belt detail on Jupiter compared to a newtonian of slightly larger aperture. This is tightened up a bit using a fringe killer.

I have enjoyed reading Singaporean astronomer James Ling's reports using an Istar 8 inch f-8.8 achromat over recent years have been interesting. His scope has performed well in most respects. On You tune I found a video of a lunar view through this scope. It shows quite well the type of CA a big achromat produced on bright white objects. My 180 has a heck of a lot less CA than this.

CA is an issue but realistically I think it is often overstated as much as it is understated. For visual observing I tune it out and appreciate the sharp diffraction spike free views unobstructed apertures can deliver.

Still I'd love to spend a night under a dark sky with his big 204 f-6 refractor.

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#22 galaxyman

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Posted 19 February 2014 - 11:15 AM

I think we all agree that a bit more info would be good.

The owner of the APM 7.1" added it to his ever and still growing line-up of quality scopes after some observing sessions that I was using the APM 8".

His initial thought was that it would make a wonderful wide-field instrument (which it is of course), but soon realized that the scope could do much more. I believe James here in the review of the Istar 8" saw the same, hence his very positive review. Both of these scopes are of course more for DSO observing to those who enjoy refractors.

Each fine scope has it's strengths and yes some negatives, but I find it wonderful that with the owner of the 7.1" and this new 8" gives their respected owners a bit more than expected.



Karl
E.O.H.


Chesmont Astronomical Society - www.chesmontastro.org
Galaxy Log - http://www.youtube.c...r/GalaxyLog4565
Galaxy Log Blog - http://galaxylog.blogspot.com/
HASB - http://www.haveastellarbirthday.com
Telekit (Swayze optics) 22" F/4.5 Dob
Homemade (Parks Optics) 12.5" F/4.8 Dob
Vixen 5.1" f/5 reflector
TMB/APM 8" f/9 Refractor”The Beast”. One great DEEP SKY achro
ES 6" f/6.5 achro. Good one
Celestron Omni XLT 102 refractor.
Celestron 10x60mm Binos

#23 Starman1

Starman1

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Posted 19 February 2014 - 05:31 PM

For those planning to build their own refractors using commercially-available doublet achromatic lenses, the standards for "no appreciable chromatic aberration" are:
Conrady's Standard (f/ratio = aperture in inches times 5)
Sidgwick's Standard (f/ratio = aperture in inches times 3).
Correctable chromatic aberration, using filters, normally considered to be:
f/ratio = aperture in inches times 2.

Special purpose scopes, of course, violate these rules all the time. And they don't apply to ED doublets, either, where FR = 2A is considered to be similar to Sidgwick's standard for normal achromats.

With a plethora of really good minus violet filters out there, though, it's tough to apply these old standards.

But, having owned a 4" f/15 instrument, I can tell you Conrady's Standard probably applies if you want to see NO chromatic aberration at all, ever.
With all the different glass types available today, though, I wonder if some of the modern lenses may bend the rules some.

And, they certainly don't apply to special uses for refractors, like comet hunting, deep sky viewing (how many of us have experienced that much aperture in a refractor, eh?). An 8" refractor sure sounds like a fun instrument to use.

#24 jrbarnett

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Posted 19 February 2014 - 10:48 PM

Very simple, Don. Permanently install an OIII filter. :lol:

- Jim

#25 jrbarnett

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Posted 19 February 2014 - 11:04 PM

"I do love the crisp stars delivered to my eye in a huge FOV."

A scope with an 900mm focal length isn't capable of a particularly HUGE FOV. Any scope with a shorter focal length will produce a considerably larger maximum FOV. Case in point: 78mm f/8 fluorite doublet with a 630mm focal length will produce a larger TFOV with any eyepiece than the 6" f/5.9. A scope with a slower focal ratio will produce a flatter field as well, and a scope with less chromatic aberration will produce crisper stars if the stellar magnitudes are 3 or brighter. If a big TFOV and crispy stars is really what lights your rockets, for $900 a used Takahashi FS-78 would be a better scope than the 6" achromat.

I have one of the Kunming 6" f/5.9 achromats as well. What I like about it is moderate (rather than small, as is usual with refractors) unobstructed aperture with the inherently superior contrast that comes with a well-baffled, unobstructed optical system. While it's far from my favorite scope, the fact that I haven't dumped it tells me that id does some things I like that my alternatives don't do.

Still, fast achromats have plenty of faults. While I'm certainly not "against" them or being "mean", whether a reviewer likes or dislikes a scope is the least interesting and least useful part of a review. The meat of a good review is the description of what the scope does well and what it could do better. :shrug:

Mi dos centavos.

- Jim






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