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Apochromatic or Achromatic does it really matter?

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#76 Maurice

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Posted 13 September 2013 - 03:20 AM

Thanks for all the good advise, really appreciated. But I'm a bit more confused now :o
Although the 152 achro seems a very good deal to me, I'm concerned about how good it will be as an allround scope, especially in light poluted skies and how easy or hard it is to handle.

Furthermore, the achromat is way havier than the 120 ED, so part of the money I'll save on the achromat needs to be spend on a heavier mount. I was considering an iOptron MiniTower (II or pro? That will be the next discussion) but I asume that's out of the question for the 152 achromat.

Hmmmmm.... Tough decision, luckely I have some time to think it over since I have to take care of my bank account first :crazy:

Likely I will settle for the 120 ED, for now it seems to me the most practical and user-friendly scope of the two and coming from a 80mm I believe it will still be an noticeable step up. At least that is what I hope to achieve...

Boy o boy, I don't know anymore. Our hobby is all about making tough decisions :grin:

Btw, unfortunately I am unable to look through any of these scopes prior to purchase because they are not present on the star parties I visit and the shop that sells them is too far away from where I live to visit them.

#77 Kunama

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Posted 13 September 2013 - 04:29 AM

If you're considering AP in the future I would suggest you get a 120mm ED scope rather than the 150 achromat. The biggest difference you will see will be on bright targets like Saturn, Jupiter, Luna, etc. You will also have a slightly wider view for clusters etc. The mount requirement for the 120 will be EQ5 equivalent or bigger, the 150mm will need EQ6 or bigger.

The 120ED will be fine on the iOptron MiniTower Pro but your AP options will be limited to short exposures due to field rotation when using an AltAz mount.

I am currently doing visual only and use a 120mm F7.5 (900mm fl) on a manual T-Rex mount and find that to be a fantastic setup for me.
My previous scope was a 150mm F10 Istar which is an excellent scope but does require an EQ6 mount as a minimum. The smaller diameter of my 120 is more than compensated for by the reduced extraneous colour of the better optical design (mine is a Tak TSA120 Super Apochromat)

Good luck with your choice.

#78 beanerds

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Posted 13 September 2013 - 04:31 AM

Yes Maurice , its a tough decision , I know but mate its your money :cool: .

Just to put into some sort of perspective down here is Australia , our 'Ice in Space' classifieds , our local astro web site and it has 2 x 120mm refractors currently for sale ,

#1 is an Orion 120mm f8.3 achro , ( great scope ! ) for $100 aud .

#2 is a no brand ( probably a North Group and another great scope ) 127mm f7.5 triplet for $1800 aud .

Take your pick ? , if I was just starting out the $100 achro would be awesome , that's what you originally asked ? $200 vrs $2000 ? , stick it on an Astro Tech Voyager alt/az for another $279 aud ( Brand new ) and you are away with a great scope .

The triplet will fit on the AT voyager ,,, just ! but it will be a pain to use because the triplet's are 2x the weight of the achro's and it will wobble horribly . ( I have owned both , see above ), so you would need a HEQ5 ? type mount or HD alt/az mount for an already $1800 scope ??? ,,, its your money mate , but for starting out I would personally ( If I needed another 127mm achro ) grab the $100 scope and AT mount for a total of $379 , and be very happy with that set up too.

Look 2nd hand , there are some awesome bargains out there , like these two I have just mentioned .

Brian.

#79 Niklo

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Posted 13 September 2013 - 05:23 AM

Hi Maurice,
the 6" 900 mm is very short so I think that the EQ6 is not a must. An Celestron ADM mount could be sufficient (haven't tested it) the EQ5 maybe a little bit too weak.
A 6" f/10 has a much longer lever arm and will require a more powerful mount than a 6" 900 achromat.

For the EDs there are cheaper versions of Skywatcher Equinox or Evostar ED 120/900. That could be an alternative that works well even for very high magnifications with an EQ5.

Cheers,
Roland

#80 Jon_Doh

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Posted 13 September 2013 - 08:41 AM

Maurice, I have a 152 achro and it does very well for nebulae. The CA is not bad at all on the moon. I see a very thin yellow line that disappears when I shift my eyes. The detail on the craters is excellent and free of CA. CA is worse on Jupiter but it can mostly be removed with a Baader Fringe Killer filter. This filter also helps focus the stars tighter and improves contrast on the planets. You should get one and screw it onto your diagonal.

The downside to the achro is that they are huge, not so heavy as bulky. Because of their length when viewing something like M57 which is right at zenith now you have to sit on the ground. The 120 may be somewhat better in terms of viewing angles, but basically that's the cost of a refractor over a dob or sct.

#81 gdd

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Posted 13 September 2013 - 11:27 AM

If cost is a stumbling block and you wish to take black and white photos (not necessarily mono) you can use a light yellow or violet minus filter on the achro and get some nice shots. Doing this you will be taking black and white photos in red and green light and will not have blue/violet bloat around the brighter stars.

Using such a filter for visual just depends on your tolerance for a slightly dimmed yellow cast on all views.

Gale

#82 Jon_Doh

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Posted 13 September 2013 - 01:01 PM

For astrophotography the best results with an achro are obtained by stacking a Baader Fringe filter with their Skyglow filter. You'll get white stars with no yellow cast or purple halos. This combo really works nicely on the moon and Jupiter too for visual.

#83 PeterR280

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Posted 13 September 2013 - 01:08 PM

I think you still get a greenish tint with both filters. You can always correct the color in photoshop if there is a tint.

#84 jrbarnett

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Posted 13 September 2013 - 01:36 PM

Here's why it matters. No, it's not about chromatic aberration, or field of view, or anything else achromat and apochromat fans tend to argue about. Apochromats are better because of *economics*. Wha?

Yeah, that's right. The economics of making and selling apochromats favor superior quality. While it's true that low dispersion glasses cost more than traditional optical glasses, but this is greatly over-emphasized. Increased glass costs are only a small component of the increased relative cost of an apochromat over a similar achromat. Instead, because apochromats (fairly or not) are in higher demand. This means that manufacturers and dealers of such designs enjoy much larger per unit margins. With a larger margin to play with, there's more money available to spend on materials and workmanship quality. No one can afford to put $800 worth of glass working labor into an achromat that sells for $600. A similar aperture apochromat that sells instead for $1700, could indeed - economically - receive $800 in materials and workmanship investment.

As a result, apochromats today tend to be better quality than like aperture achromats.

Quality is king. Optical quality helps you beat the seeing. Helps reveal fine detail. It's well established that the difference between a 1/8 wave optic and 1/4 wave optic is quite detectable visually at the eyepiece.

:grin:

- Jim

#85 PeterR280

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Posted 13 September 2013 - 03:07 PM

From what I am reading, FPL-53 glass raw material cost could be over $400 for a 4" blank.

#86 Bonco

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Posted 13 September 2013 - 03:37 PM

I'm going to add my two cents by saying that a long achro might well be corrected as well as an apo but that the nice compact package of a modern apo makes such scope truly portable...you can take it to whatever dark skies you wish with very little transport worries and get phenomenal optical performance. Or, if permanently mounted, you can put it in a smaller facility. Yes, an apchromat definitely matters for more than one reason. Can you attain the same performance from an achromat...probably...but with the same functionality? I don't think so.


Here's a picture that illustrates your point. Televue 4 inch f/5 next to Antares 4 inch f/15. Both great telescopes and offer the best of both worlds. Wide field views in the f/5 are stunning. High power double star views in the f/15 are amazing. Take your pick, or buy both. Bill :grin:

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#87 Kunama

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Posted 13 September 2013 - 04:12 PM

Very nice comparison Bonco !
Here is my version:
Istar Perseus AT150mm F10 on c1865 Gaunt Mount versus Takahashi TSA120 F7.5 on Vixen GPDII

Posted Image

oh by the way Jim, I think you got the info the wrong way around:

"Instead, because apochromats (fairly or not) are in higher demand. This means that manufacturers and dealers of such designs enjoy much larger per unit margins. With a larger margin to play with, there's more money available to spend on materials and workmanship quality."

I think the demand was a result of the high quality, not the cause of it ! :cool:

In all seriousness, when it comes to visual on nebulae and other faint objects, APERTURE RULES !!! Go for an Istar Perseus AT150, one of the nicest modern achromats I have seen.

#88 Mark Costello

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Posted 13 September 2013 - 04:28 PM

Based on what I seen it can do, I'm of the opinion that my achromat can perform to its design limits as well as any aprochromat can perform to its design limit. This may be true of a lot of achromats out there.

#89 VNA

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Posted 13 September 2013 - 04:36 PM

Apochromatic or Achromatic does it really matter?

Yes it matters a great deal in the pocket book!

No one has really answered it other than proclaiming virtues of one or the other!

Is it worth 10 times more than an achromatic?

Is the view 10 times better?

(Law of diminishing returns)

#90 PeterR280

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Posted 13 September 2013 - 04:42 PM

The same can be said about size. Looka at the price differential between 100, 130, 150 and 175 mm. The resolution improvement is linear. Light gatherin does increase with the square of aperture. Cost seems to be logarithmic.

#91 Jon Isaacs

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Posted 13 September 2013 - 04:47 PM

Apochromatic or Achromatic does it really matter?

Yes it matters a great deal in the pocket book!

No one has really answered it other than proclaiming virtues of one or the other!

Is it worth 10 times more than an achromatic?

Is the view 10 times better?

(Law of diminishing returns)


Your eyes are free. There is not telescope that can be justified in terms of how much better the view is. If one is looking for performance per dollar, a mirror is the way to go...

Jon

#92 Kunama

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Posted 13 September 2013 - 05:20 PM

Apochromatic or Achromatic does it really matter?
Yes it matters a great deal in the pocket book!
No one has really answered it other than proclaiming virtues of one or the other!
Is it worth 10 times more than an achromatic?
Is the view 10 times better?
(Law of diminishing returns)


Of course the view is not 10x better, in fact on some targets there is almost no difference in the view, but then you point the scope at a bright cluster, Saturn, Jupiter or the Moon and you see the difference.

If the difference is worth the extra $$$ depends on the individual.

Like anything else, to get a small improvement in performance you have to spend quite a bit more.

#93 jrbarnett

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Posted 13 September 2013 - 10:25 PM

I doubt it given that Synta was selling complete 4" FPL-53 doublets for $450 under several different brands (Sky Watcher and Celestron). Of course, Ohara blanks come in different quality levels too, at widely varied costs.

Regards,

Jim

#94 Mike Clemens

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Posted 14 September 2013 - 02:10 AM

When I first started, I owned a 6" achromat and then picked up a 4" APO. I almost never looked through the achromat again. The APO view was so insanely aesthetic in comparison.

#95 Maurice

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Posted 14 September 2013 - 03:40 AM

Thanks again for all the help gentlemen, this forum is really great!
Thanks to all your input I will be able to make a better choice.

To answer some of your questions: my current setup is a 80mm WO Zenithstar II ED mounted on a Voyager alt/az. I'm not into AP, but I do sketch the objects that I'm looking at, hence my wish for an alt/az with tracking ability. This little scope performs beautifully on the moon and star clusters, especially in combination with my UWAN16 eyepiece.

This shorttube however is not that useful on planets and nabulae due to the small aperture and short focal length. That's why I'm looking for a second scope, not to replace the shorttube but as an addition to it.

All your input makes me wondering, is the best addition to the shorttube not simply a "slow" refractor with an larger objective? So instead of the 152/900mm it looks to me I'm better off with for instance a 130/1200mm scope. The larger focal length also helps in reducing CA.

As an example my 80mm next to this one: http://www.teleskop-...ITE-127-Refr...

Makes sense or is the 120/900 ED a better option (regardless the price)?

#96 gdd

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Posted 14 September 2013 - 08:47 AM

All your input makes me wondering, is the best addition to the shorttube not simply a "slow" refractor with an larger objective? So instead of the 152/900mm it looks to me I'm better off with for instance a 130/1200mm scope. The larger focal length also helps in reducing CA.



Adding an AP CCDT67 reducer would convert the 130/1200mm to a 130/800mm.

Gale

#97 Jon_Doh

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Posted 14 September 2013 - 09:32 AM

There have been some folks who have tested an AR152 against the smaller aperture 127ED and reported that the smaller scope put up better views even on dimmer deep space subjects. I suspect this is due to the superior optics and the way the light is focused. This is something Jim may know about and can comment on.

I'm Just a former dumb football player and all this physics stuff goes over my head. But give me a receiver coming out of a crossing pattern on a single four wideout spread and I'll stuff him :p

#98 Mike4242

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Posted 14 September 2013 - 09:51 AM

I think this might be the review you had in mind:

http://www.cloudynig.../Number/5369924

#99 Jon_Doh

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Posted 14 September 2013 - 11:43 AM

Yes that is the review. DrWho posted it on another forum too. What is your take on this Mike? Have you had a chance to compare the achro with an apo? I'm wondering after your post about the difference the filter made on M13 whether the views between the 127ED and AR152 might not be closer.

#100 Jon_Doh

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Posted 14 September 2013 - 11:43 AM

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