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6" f/5 Achro Refractors

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#26 Phillysoc

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Posted 14 March 2019 - 06:46 PM

I've owned two of the 150mm f5 Omni's since 2015, the first was used and the second new.

 

I sold the first due to a grossly overcorrected lens, I did respace the lens and improved its correction somewhat. The second was better corrected, and I still own it but I'm struggling with the focuser. I've been through several focuser change outs on this scope, however the images are actually usable up to about 120x with filtering. In general you'll use less than magnification anyway for most deep-sky observing. One feature I do like: the tube is fairly well balanced and easy to mount on CG5-class mounts, it's very compact given 6" aperture.

 

I've seen more color in nebula and stars using these scopes than others I own (including larger reflectors) of course you might suspect false color is the cause. M42 revealed a beautiful red-green coloration several winters ago. Last summer I also experimented with planetary imaging, using dense color filters, see attached photo of Jupiter. While seeing in this case was worse than average, you can see some potential.

 

Loren

I could not agree more.  I owned a reflector....1100 Edge HD and since getting an ES 102 mm APO, I don't regret selling my reflector.


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#27 Phillysoc

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Posted 14 March 2019 - 06:47 PM

I could not agree more.  I owned a reflector....1100 Edge HD and since getting an ES 102 mm APO, I don't regret selling my reflector.

and my back and spine thank me too.


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#28 Vondragonnoggin

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Posted 14 March 2019 - 06:52 PM

and my back and spine thank me too.

This is why I want to switch up my AT152 at 25lbs for the more manageable 150 F/5 at 16lbs


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#29 kksmith

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Posted 15 March 2019 - 05:06 AM

This is a dangerous thread. Dangerous to my financial well being. I have an ST80 and a Sky Watcher 102/F5. And now I see this. I take it these are no longer made. That Celestron looks sweet.

 

Ken


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#30 rcg

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Posted 15 March 2019 - 07:36 PM

has anyone made comparison to IStar's 150 F/5?


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#31 ShortLobster

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Posted 16 March 2019 - 07:25 AM

has anyone made comparison to IStar's 150 F/5

Not against the iStar 150 F/5, but I had my 150 F/8 R35-S out next to the Skywatcher version of this scope last Fall. Thought not apples-to-apples, the iStar had much less CA, tighter stars and better contrast. IStar isn't building OTAs at the moment but they are building the lenses and I'm considering the R35 version. 


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#32 rcg

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Posted 16 March 2019 - 12:32 PM

There are a few reasonable Optical Tube assemblers out there, Seibert Optics looks like they are reasonable ...



#33 Exnihilo

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Posted 16 March 2019 - 08:43 PM

I have an Omni 150 and really like it. I bought it specifically because it’s F5 and gives great wide field views. It’s also very lightweight for its size, which I like. I just don’t look at Jupiter with it, but the CA on most objects doesn’t bother me.
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#34 Phillysoc

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Posted 23 March 2019 - 06:43 AM

This is why I want to switch up my AT152 at 25lbs for the more manageable 150 F/5 at 16lbs

Its just too much weight.  I dont have an issue with reflectors although managing the mirror is a pain...and its not that I cannot lift the mount, but I had a CGEMDX mount and it was just too heavy.  I was not in a situation where I could fix the mount so I had to set up every night I wanted to view or image and its just too much weight to carry around.  Now with my ES Carbon Fiber APO Triplet 102mm its like I an carrying around a small briefcase....and of course being a light scope means I dont need a huge mount.  Its a win win all the way around....and I astro image and quite honestly, the images I see from ES 102mm are just as sharp as from my, now sold, 1100 Edge HD.



#35 kfr

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Posted 20 September 2020 - 01:04 AM

well, lots of good stuff written about these kind of scopes. i have both the celestron xlt 150 f5 and a starfire 152 f5.9 v2 (the kunming factory product, in red of course). the xlt is pretty good, including moon & saturn (in a stopped-down mode). i replaced the focuser on the xlt with a gso 2 speed. much better & smoother. the red f5.9, however, is easily a couple notches better both in craftsmanship and view through the eyepiece. the f5.9 is 5 lbs heavier, but manageable on my cg5 mount. if you are trying to decide between the two, spend a few bucks more for the f5.9. if you can keep an objective attitude (and remember that neither of these are a classic alvan clark f15) the differences are easy to see. the xlt is a keeper, and the f5.9 is better. my dad had a large tom cave reflector for deep space and a small refractor for planetary views, but i live in the pacific northwest and am settling for a decent all around scope. i'm keeping the red kunming f5.9. i think it will do nicely. (40 years ago i got to see saturn through a 10" alvan clark - i'll never forget it!)


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#36 DeanD

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Posted 20 September 2020 - 02:13 AM

Just revisiting this thread.

 

As with my earlier post (#15) I am still very happy with the Celestron Omni version. Since then though I have replaced the focuser with a GSO two-speed crayford, and I am now able to use my binoviewer with it: very cool. I have also been able to run it against both a 6" Skywatcher Evostar and a 6" Esprit. It shows classic achromat CA (although nowhere near as bad as I had feared based on some theoretical tables which give this configuration an "unusable" tag), so they both obviously killed it for planetary viewing and bright stars: but I found it very hard to notice any difference in dso/galaxy viewing (which was what I got it for): plus it gives a wider FOV at the same magnification. One of the reasons I did the comparison was that the guy was selling the Evostar for a good price, and I was very tempted; but after the comparison I decided to save my money for other things...

 

Oh: and "kfr": I am not at all jealous of your viewing Saturn through the 10" Clark...  whistling.gif  waytogo.gif

 

Happy viewing folks.

 

- Dean


Edited by DeanD, 20 September 2020 - 02:13 AM.

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#37 beanerds

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Posted 20 September 2020 - 02:31 AM

I had a SW 150 f5 and its optics were very , very good , I was suprised as to the views it gave of Jupiter at 200 - 250x , a little CA but extremely SHARP !!! with great natural browns and yellows seen on the disc , impressive but stupidly sold it , this scope and my Zeiss Telementor are the 2 scopes I regretted selling .

DSC00054 (2).JPG
DSC00054 (2).JPG
DSC00054 (2).JPG

They are an impressive deep sky scope , easily as good but with typical refractor like stars as a mates 8 inch Meade SCT .

 

Beanerds.

 

OOPS ! I somehow added 3 photos HMMM ? 


Edited by beanerds, 20 September 2020 - 02:32 AM.

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#38 j.gardavsky

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Posted 21 September 2020 - 02:54 PM

My Sky-Watcher 6" F/5 achro is the same as in the above post #37.

Purchased at Teleskop Service, as one of the first sold in Germany back in the day.

Mounted on a Cullmann tripod with the central geared column for a comfortable height adjustment, it is a comfortable to move and to use grab-and-go refractor.

 

As it is an achromatic refractor, it has its CA deviation C-e-F (0.35mm), and being fast (F/5) it has its spherochromatism. The spherochromatism has been corrected to some extent with the 2" BK7 prism.

 

Mostly used on the diffuse emission and reflection nabulae at the low to medium magnifications, where the CA does not matter much.

The globular clusters are spectacular through the Pentax XO 2.5mm

Moon and the planets can be viewed through the filters in the range of magnifications 150x - 300x.

 

Another grab-and-go telescope I have at the moment, is the Leica 82mm APO Televid, so I know the difference between a true APO (3 air-spaced lenses in the objective), and an achro doublet.

 

Best,

JG


Edited by j.gardavsky, 21 September 2020 - 02:54 PM.

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#39 gnowellsct

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Posted 21 September 2020 - 03:40 PM

I could not agree more.  I owned a reflector....1100 Edge HD and since getting an ES 102 mm APO, I don't regret selling my reflector.

Yeah but he don't want no apo.   Don't want no c8.  He wants an achro.

 

My feeling about this is that after you have been around scopes a while, which OP has, you either know you are an achro-man, or you know y'aint.  If you're an achro-man and you have the funds and a mount you shouldn't waste time talking to us.  You should go and get one.

 

Greg N


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#40 Manu#1

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Posted 22 September 2020 - 03:17 PM

I had a Celestron CR6, the whole package with the GoTo equatorial mount. It was a very nice scope. I even added a 12” pier to get it higher for viewing. I was able to find quite a few DSOs with that. The only problem I had with it was the fact that I always had to set up and tear down after viewing. It is the type of scope to set up in one area and leave up, preferably in an observatory or a runoff shed. I always had 4 or 5 trips in and out to set up and view. After awhile it got very tiring.
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#41 Echolight

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Posted 22 September 2020 - 08:55 PM

I had a Celestron CR6, the whole package with the GoTo equatorial mount. It was a very nice scope. I even added a 12” pier to get it higher for viewing. I was able to find quite a few DSOs with that. The only problem I had with it was the fact that I always had to set up and tear down after viewing. It is the type of scope to set up in one area and leave up, preferably in an observatory or a runoff shed. I always had 4 or 5 trips in and out to set up and view. After awhile it got very tiring.

Mine lives outside in a deck box 50 feet from my viewing area. I have an AVX, but usually set it up on the Unistar.

 

Tripod stays assembled in the shed. Mount head stays attached to the C6R. I carry out my diagonal and a couple of eyepieces. Whole thing goes up in about 2 minutes and takes down just as quick. Although I do leave it up for days on end as long as no rain in the forecast.

And always acclimated.

20200809_091013.jpg

It could use a heavier tripod. But I manage.


Edited by Echolight, 22 September 2020 - 08:57 PM.

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#42 Echolight

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Posted 22 September 2020 - 09:10 PM

Yeah but he don't want no apo.   Don't want no c8.  He wants an achro.

 

My feeling about this is that after you have been around scopes a while, which OP has, you either know you are an achro-man, or you know y'aint.  If you're an achro-man and you have the funds and a mount you shouldn't waste time talking to us.  You should go and get one.

 

Greg N

I like my big achro. And will keep it maybe forever. It's a pleasure to use and look through.

But I wouldn't pigeon hole myself with a label like achro-man.

 

I also have a C8, that I don't use as often.

Maybe because I don't have it set up with some nice finders for an easy alt/az mount.

 

And I have designs for a little apo. And may eventually get a dob some day.

 

Variety is the spice of life.



#43 KBHornblower

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Posted 22 September 2020 - 09:36 PM

Just revisiting this thread.

 

As with my earlier post (#15) I am still very happy with the Celestron Omni version. Since then though I have replaced the focuser with a GSO two-speed crayford, and I am now able to use my binoviewer with it: very cool. I have also been able to run it against both a 6" Skywatcher Evostar and a 6" Esprit. It shows classic achromat CA (although nowhere near as bad as I had feared based on some theoretical tables which give this configuration an "unusable" tag), so they both obviously killed it for planetary viewing and bright stars: but I found it very hard to notice any difference in dso/galaxy viewing (which was what I got it for): plus it gives a wider FOV at the same magnification. One of the reasons I did the comparison was that the guy was selling the Evostar for a good price, and I was very tempted; but after the comparison I decided to save my money for other things...

 

Oh: and "kfr": I am not at all jealous of your viewing Saturn through the 10" Clark...  whistling.gif  waytogo.gif

 

Happy viewing folks.

 

- Dean

My bold.  If that rating is based on the diameter of the color fringe over the whole visible spectrum, it may be exaggerating the fault as perceived on most objects.  Much of that diameter is at the red and violet extremities of the spectrum, where our eyes are less sensitive.



#44 KBHornblower

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Posted 22 September 2020 - 09:46 PM

I've owned two of the 150mm f5 Omni's since 2015, the first was used and the second new.

 

I sold the first due to a grossly overcorrected lens, I did respace the lens and improved its correction somewhat. The second was better corrected, and I still own it but I'm struggling with the focuser. I've been through several focuser change outs on this scope, however the images are actually usable up to about 120x with filtering. In general you'll use less than magnification anyway for most deep-sky observing. One feature I do like: the tube is fairly well balanced and easy to mount on CG5-class mounts, it's very compact given 6" aperture.

 

I've seen more color in nebula and stars using these scopes than others I own (including larger reflectors) of course you might suspect false color is the cause. M42 revealed a beautiful red-green coloration several winters ago. Last summer I also experimented with planetary imaging, using dense color filters, see attached photo of Jupiter. While seeing in this case was worse than average, you can see some potential.

 

Loren

My bold.  I would not expect a reflector of equal light gathering to suppress the true colors of these objects.  I would attribute your sensation, beyond a reasonable doubt, to the CA and our eye-brain combination's complex processing of it.



#45 Echolight

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Posted 22 September 2020 - 09:55 PM

My bold.  I would not expect a reflector of equal light gathering to suppress the true colors of these objects.  I would attribute your sensation, beyond a reasonable doubt, to the CA and our eye-brain combination's complex processing of it.

Maybe the obstruction suppressed it.



#46 KBHornblower

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Posted 22 September 2020 - 10:38 PM

Maybe the obstruction suppressed it.

No way.  The obstruction merely dims the light slightly, by the same percentage at all wavelengths.  We need only to increase the aperture slightly to equalize the through-put and eliminate that variable.



#47 Marcsabb

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Posted 05 October 2020 - 07:25 AM

I'd really love to see an ED version of these large "skyweepers".

 

A 140-150mm doublet composed of a fpl-51/fk61 crown mated to a lanthanum flint. F-ratio between 5 and 6. 700-800mm long, around 6kg (13lb) in weight. The ultimate "hand cannon".  

 

It wouldn't be completely CA free (I expect it to match or slightly exceed an F/20 achro of similar diameter) and spherochromatism could be a problem as well, but it would be more versatile than classic big achros, while keeping the low weight and compactness of these.

 

Incidentally, APM does employ a similar design in their 150mm giant binoculars (they're F/5.5) but it's not available in scope version.


Edited by Marcsabb, 05 October 2020 - 07:28 AM.

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#48 daquad

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Posted 05 October 2020 - 10:57 AM

Yeah, I think if it were a 6" f/6 with FPL51 it would just meet the Sidgwick standard for suppression of CA.

 

Dom Q.



#49 Marcsabb

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Posted 05 October 2020 - 12:01 PM

Yeah, I think if it were a 6" f/6 with FPL51 it would just meet the Sidgwick standard for suppression of CA.

 

Dom Q.

As a 'rule of the thumb' a doublet with FPL51 reduces longitudinal CA by a factor of ~3.5; depending on the choice of the mating element it could be a bit more or a bit less. For example, the APM 6" F/8 is said to have CA comparable to a F/30 achro. That's a 3.75x improvement.

 

If I were to design a large, but portable and compact rich field refractor with ED glass I would probably choose a diameter of 140mm. There is already a wide choice of 140mm refractors, but all expensive triplets and all quite heavy and long, with f-ratios of 6.5-7. A larger diameter, like a 6", would be preferable for faint DSOs, but the resulting telescope would be too much front heavy (a 6" doublet lens cell weights 2.5-3kg, which would be 50% of the weight of the whole scope and all concentrated on one end of the tube)

 

As for F ratio, F/5 surely looks doable: the resulting scope would have FL= 700mm and would be also around 700mm long, lightshield included. Sidgwick ratio for such a scope would be around 3.2 if we use the mentioned 3.5x factor of improvement for CA over a standard achromat lens. However I would relax a bit the F-ratio increasing in to 5.5-5.6 to reduce CA a bit further and also to make lens design less critical.

 

The resulting scope, with a FL of ~ 780mm (F/5.6) would be still very compact and lightweight, being able to be mounted on any mount that can take a 100mm F/9 such as the 100ED or the slightly larger 120ED. The extra aperture would make a difference with faint DSOs and the decent CA, coupled with a generous 140mm aperture, could make the scope useful also for some occasional moon/planet gazing.


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#50 StarAlert

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Posted 05 October 2020 - 12:10 PM

As a 'rule of the thumb' a doublet with FPL51 reduces longitudinal CA by a factor of ~3.5; depending on the choice of the mating element it could be a bit more or a bit less. For example, the APM 6" F/8 is said to have CA comparable to a F/30 achro. That's a 3.75x improvement.

 

If I were to design a large, but portable and compact rich field refractor with ED glass I would probably choose a diameter of 140mm. There is already a wide choice of 140mm refractors, but all expensive triplets and all quite heavy and long, with f-ratios of 6.5-7. A larger diameter, like a 6", would be preferable for faint DSOs, but the resulting telescope would be too much front heavy (a 6" doublet lens cell weights 2.5-3kg, which would be 50% of the weight of the whole scope and all concentrated on one end of the tube)

 

As for F ratio, F/5 surely looks doable: the resulting scope would have FL= 700mm and would be also around 700mm long, lightshield included. Sidgwick ratio for such a scope would be around 3.2 if we use the mentioned 3.5x factor of improvement for CA over a standard achromat lens. However I would relax a bit the F-ratio increasing in to 5.5-5.6 to reduce CA a bit further and also to make lens design less critical.

 

The resulting scope, with a FL of ~ 780mm (F/5.6) would be still very compact and lightweight, being able to be mounted on any mount that can take a 100mm F/9 such as the 100ED or the slightly larger 120ED. The extra aperture would make a difference with faint DSOs and the decent CA, coupled with a generous 140mm aperture, could make the scope useful also for some occasional moon/planet gazing.

Sounds interesting but why not just buy an APM 140? 




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