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6" f/5 or 4.75" f8.3. Which Achro would you get and why?

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#1 Ghost332

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Posted 18 February 2021 - 05:21 PM

Say that you live on the third floor and the only way to use the scope, is to go down(No lift), get in your car and drive around 10 minutes to get to a nice bortle 4 sky. Which of the 2 achro would you get for all-around use, if large Apo-s are out of budget?


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#2 sg6

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Posted 18 February 2021 - 05:30 PM

4.75" f/8.3

Rather simply for the reduced CA.

Have a 102 f/6 and the CA "spoils" it. Fortunately not one I really bought for my own usage. An Outreach scope.

 

A perpetually poor view gets to be no fun.


Edited by sg6, 18 February 2021 - 05:31 PM.

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#3 MartinPond

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Posted 18 February 2021 - 05:33 PM

Say that you live on the third floor and the only way to use the scope, is to go down(No lift), get in your car and drive around 10 minutes to get to a nice bortle 4 sky. Which of the 2 achro would you get for all-around use, if large Apo-s are out of budget?

If it's a plain achro, I would shy away from the 6" f/5.

It's got too much chromatic aberration, and at the low powers that maximize field size,

  a 102mm F/6 would do for that.  If you want more light in a wide view,

   best to go Dobsonian  reflector.

 

I think a   4.75" f8.3    with an 80mm//40mm(F5) as a finder would be a blast 2 ways.

 

Interesting note on the 102 F/6  :   I was worried about that.

 

 

I quickly run out to the office park with

an 80x400(F5) and an 80x720(F9) with a minus violet filter (super-sharp).

I keep wanting to piggyback them, but my wife almost goes out with me

and it's fun to have two views.   

 

There are so many 102mm F6.5s....nah.

 

I did see a Celestron  and an Explore Scientific  at 102mm by 1000mm ..F9.8.

  (hey, and how about an 80x400 finder/widefield with a 2" eyepiece!) 

  That would be very nice, and could be sharpened with the minus violet, like

   my 80x720....  There is no way to filter yourself into more sharpness

   with an 80x400 or 102x660, though.


Edited by MartinPond, 18 February 2021 - 05:50 PM.


#4 t.r.

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Posted 18 February 2021 - 05:33 PM

4.7” no contest. The 6” f5 is a specialized instrument for low power wide field views....certainly not an all around performer. It actually exceeds what is called a filterable level of CA for visual use ( in other words filters can’t even tame the CA) so truly low power only.

Edited by t.r., 18 February 2021 - 05:36 PM.

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#5 Ghost332

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Posted 18 February 2021 - 05:39 PM

If it's a plain achro, I would shy away from the 6" f/5.
It's got too much chromatic aberration, and at the low powers that maximize field size,
a 102mm F/6 would do for that. If you want more light in a wide view,
best to go Dobsonian reflector.

I think a 4.75" f8.3 with an 80mm//40mm(F5) as a finder would be a blast 2 ways.


I personally ruled out dobs, carrying the 8" tube back and forth to the third floor feels exhausting.

That 80mm f/5 finder is actually a really great idea!

Edited by Ghost332, 18 February 2021 - 05:41 PM.

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#6 PNW

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Posted 18 February 2021 - 05:40 PM

I, too, have lusted after big refractors. Even APO's. Then I came to my senses when I got to the weight part. If you're going to be humping all your gear up and down 3 flights of stairs, I'd consider weight first, then length. I ended up with an AR 127, f/6,5, Achro. At 15 pounds the weight isn't bad although it's long. The scope itself is great. Then there's the mount. My AVX is 3 parts each weighing 17 pounds. That's a lot of trips up and down. So, I would consider the mount first. Heck, my Infinity 102 weighs 12 pounds for the whole setup and gives pretty good views.



#7 Supernova74

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Posted 18 February 2021 - 05:53 PM

Personally if you got access to sky bortale class 4 sky’s I would ditch the Achromatic idea altogether and get a mirror based scope instead food for thought.


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#8 ngc7319_20

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Posted 18 February 2021 - 05:54 PM

Might be a situation for a 6" SCT or Mak-Cass?


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#9 kmparsons

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Posted 18 February 2021 - 06:23 PM

I used to own the Orion 120mm f/8.3 and found it to be a very satisfactory scope with little CA and pinpoint stars. I sold it and have since gotten fancier scopes, but I wish I had kept that one and outfitted it with a better focuser. I particularly remember the view it gave of Sigma Orionis, which clearly showed the dim C component that I had not detected in smaller scopes. 


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#10 turtle86

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Posted 18 February 2021 - 06:40 PM

Might be a situation for a 6" SCT or Mak-Cass?

 

If I were living on the third floor, that would be my choice too, as you'd get decent aperture with portability, but without CA.


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#11 Echolight

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Posted 18 February 2021 - 07:27 PM

Third floor? C6 sct with a .5 reducer.


Edited by Echolight, 18 February 2021 - 07:30 PM.

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#12 N-1

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Posted 18 February 2021 - 08:48 PM

If it had to be one of these two, I'd get the 6". Its tube is quite a bit shorter than the 120's so probably won't be harder to handle overall depite being fatter. More reach on DSO, and stopped down an inch or two will probably be more similar on planets than different, to the 120.


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#13 Wildetelescope

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Posted 18 February 2021 - 09:52 PM

I used to own the Orion 120mm f/8.3 and found it to be a very satisfactory scope with little CA and pinpoint stars. I sold it and have since gotten fancier scopes, but I wish I had kept that one and outfitted it with a better focuser. I particularly remember the view it gave of Sigma Orionis, which clearly showed the dim C component that I had not detected in smaller scopes. 

Agree.  the Synta 120 mm F8.3 is a venerable scope that in my experience has excellent optics.  It is an achromat and will show color, but for me it only REALLY shows on bright objects.   You will enjoy it a lot.  

 

JMD



#14 N-1

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Posted 18 February 2021 - 10:01 PM

I used to own the Orion 120mm f/8.3 and found it to be a very satisfactory scope with little CA and pinpoint stars. I sold it and have since gotten fancier scopes, but I wish I had kept that one and outfitted it with a better focuser. I particularly remember the view it gave of Sigma Orionis, which clearly showed the dim C component that I had not detected in smaller scopes. 

An Orion scope had better do well on Orion laugh.gif


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#15 Mitrovarr

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Posted 18 February 2021 - 10:22 PM

What do you want it for? These scopes do completely different things. The 4.5" is a generalist scope that's good at everything and the 6" is a super specialized very wide field DSO scope.
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#16 BarrySimon615

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Posted 18 February 2021 - 11:35 PM

I think in your first post you gave enough detail to answer your own question - "all-around scope and something to get down from the 3rd floor to drive 10 minutes to get to Bortle 4 skies."  So based upon that the 120 mm f/8.3 does this best for you.

 

Several other things to consider - you do not say what mount you would have under either scope.  That is an important consideration, the mount would likely weight more than the scope, as much as double if not triple the weight of the scope.  Not fun negotiating the stairs.

 

Another consideration is the skies, what are the skies like where you live?  If a typical urban or suburban sky, 10 minutes will maybe get you Bortle 4 overhead at the zenith but the sky looking back in the direction from which you came will not be Bortle 4 and as you get dark adapted the sky in that direction will look pretty bright.

 

I am attaching a chart that many have seen before, it may help you decide.  I think the chart is pretty much self explanatory.  As you can see the 6" f/5 as a fast achromat will have plenty of chromatic aberration that cannot be effectively filtered.  It is truly a low power specialized scope, I know, I have had one for the past 40 plus years.  It is not however an all around performer.  Carefully assess what celestial targets interest you and then make your decision.

 

Barry Simon 

 

Attached Thumbnails

  • 4662850-CA-ratio-chart-achro.jpg


#17 BillP

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Posted 19 February 2021 - 12:41 AM

4.7” no contest. The 6” f5 is a specialized instrument for low power wide field views....certainly not an all around performer. It actually exceeds what is called a filterable level of CA for visual use ( in other words filters can’t even tame the CA) so truly low power only.

Very true.  The 152 f/5 is going to be a limited use instrument and not good as a generalist for all objects.  The 120 f/8.3 however is going to show good views of everything, and best of all when you mask it to 100mm it becomes f/10 so will even show planetary with minimum CA!  I live in a Bortle 3/4 location and have 10", 6", 4" and 3" instruments and frankly, the 4" is the most used as the skies are dark enough that all the popular faint fuzzies are just fine, so a 120 will be even better!  The 60% light gain of the 152 over the 120 is not going to be all that impressive, and when you consider its limited usefulness due to its very fast focal ratio makes the 120 easily a best choice IMO.


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#18 Bomber Bob

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Posted 19 February 2021 - 04:57 PM

If you have to have a refractor, the 4.7" F8.3 for all the reasons already posted.

 

If I had your location, I'd use Stubby, my 6" F4 Newt RFT.  Ultra-light (rides well on an old Orion VersaGo 2), no CA, and good on lunar / planetary to 250x (center of field).  Fold up the tripod, and it's an easy carry up / down stairs.


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#19 DeanD

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Posted 19 February 2021 - 07:04 PM

I agree with the comments about the 120 f8.3 being a good "all-rounder". I think it would be better at very high power on Jupiter, Mars and Venus; and maybe the Moon.

 

However, I am not so sure that the 150 f5 is just a specialised low-power scope. (I have a TSA102, so I am aware of what a top Apo can do...) 

Mine (I have the Celestron version) does a great job on all sorts of objects. It is not my preferred scope for the planets, but it filters well, and it still gives very nice, sharp, unfiltered views at higher power (150-200x) of Saturn and even Jupiter (not so good on Mars or Venus...). The Moon obviously shows some edge colour, but it can still take a lot of power.  

I can see the "E" and "F" stars in the Trapezium in the 150 f5 quite easily, the surrounding nebulosity of M42 is wonderful, and my first views of Eta Carina and the Homunculus nebula (at around 150x)  blew me away: in fact this was the first scope in which I noticed the colour of the Homunculus (later confirmed in a nearby 24" dob!).

 

I have seen Barry Simon's chart before, but I don't agree with it wrt my scope. I suspect the appreciation (or rather the annoyance) of CA may be a more subjective thing than the chart implies. I don't find the 150 f5 is anywhere near "unacceptable" in terms of CA, except perhaps as I mentioned above on Mars or Venus. When I first got the 150 I was apprehensive because of this chart, but then I was pleasantly surprised at the quality of the view. In fact I often use it as my preferred DSO scope (even before my 12" truss-tube dob: it is so much more convenient to set up quickly, and has a much quicker cool-down time). I have also run it directly against both the Skywatcher 150 Esprit and Evostar Apo models. It can't match them for unfiltered planetary viewing because of the CA, but it is hard to pick any difference on the majority of other objects. I was thinking of buying the Evostar at the time, but when I did the comparison I decided it wasn't worth trying to convince the "accounts department"...

 

For general viewing and lugging around it is fairly light, and it sits quite happily on the SV M2 mount, and the iOptron Mini Tower.

 

All the best,

 

Dean


Edited by DeanD, 19 February 2021 - 07:06 PM.

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#20 daquad

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Posted 19 February 2021 - 08:04 PM

I agree with DeanD.  While the 120 f/8.3 is a good all-rounder, but the 6" fast Achro is not as specialized as one would think.  Mine is an Altair 6" f/5.9; Ok not f/5, but still in the same category.  Yes it excels at low power rich field views, but that is not its only forte.  It is an excellent double star scope.  Splits 52 Orionis (sep 1.0"), for example, much easier than my 4" f/15 refractor.  The E and F stars of the Trap are easy in good seeing.  The brightest globulars are easily resolved in my mag 5.5 skies.  The moon shows some color on the limb and the lunar shadows, but the detail is astounding.  I have seen 3 craterlets on the floor of Plato in good seeing.    Jupiter shows considerable detail in the the belts and the Galilean moons appear as discs.  The Cassini division. belts and the polar hood of Saturn are easily in view as well as 4 moon in addition to Titan.  If stopped to 80 mm the Ca is just about gone, but the view is better at full aperture.  Just because its intended use is RFT, does not mean it cannot be used successfully for other observing.

 

Dom Q.


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#21 nirvanix

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Posted 19 February 2021 - 09:01 PM

If you're into deep sky objects the 6 f5 without question will deliver a profoundly satisfying experience.


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#22 RichA

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Posted 20 February 2021 - 11:03 AM

If you have to have a refractor, the 4.7" F8.3 for all the reasons already posted.

 

If I had your location, I'd use Stubby, my 6" F4 Newt RFT.  Ultra-light (rides well on an old Orion VersaGo 2), no CA, and good on lunar / planetary to 250x (center of field).  Fold up the tripod, and it's an easy carry up / down stairs.

But not very good at the field edge owing to coma, needs much more expensive, higher-correction eyepieces or a coma corrector. 



#23 Jon Isaacs

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Posted 20 February 2021 - 11:55 AM

But not very good at the field edge owing to coma, needs much more expensive, higher-correction eyepieces or a coma corrector. 

 

Still:

 

A 6 inch Newtonian should be in this discussion. Bob has a F/4 but an F/5 or F/6 makes a lot of sense. Both the 120 mm F/8.3 and the 150 mm F/5 have significant chromatic aberration, I've owned three 120 mm F/8.3s and could never get past the chromatic aberration. 

 

As an all-around scope, I think a 6 inch F/5 or F/6 Newtonian with a 2 inch focuser is better than either of these two refractors, zero CA, short focal length for wide fields.

 

Jon

 

 


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#24 balticsensor

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Posted 20 February 2021 - 02:23 PM

I own the 120 f/8.3 and am very happy with it.

For my widest views I use a 40mm William Optics 70° SWAN. This gives me ×25, 2.8° TFOV and 4.8mm pupil. Right about the lowest power I might want. Dark lane in Andromeda - check. The entire half of the Veil Nebula filtered - check. Comet F3 - big check! Now if I were to aim for the same field in the 150 f/5, I'd have more aberrations, especially off-axis due to struggling eyepiece, field curvature and chromatic aberration. I've observed the polar ice cap of Mars a half dozen times with my ortho eyepieces last summer, and have steadily resolved a 1.5 km craterlet in Theophilus. It's a cracking all-arounder.

dup.jpg

Having said that, and the whole reason I found this topic, I'm having a serious itch about trying that 150 f/5 refractor, especially since my mount could hold it well. Unfortunately, as others have said, such an instrument is gaining a few extra points here at the cost a dozen points in other areas. I think, eventually, I will get myself that 150 f/5, but only after I replaced my 120 f/8.3 by a bigger aperture high fidelity all-arounder. Like a C9.25. In other words, I don't want my largest aperture scope to have limitations. Smaller secondary scopes - no problem. That's why I cannot see a 150 f/5 achromat as my one primary scope, unfortunately. I say unfortunately because it's immensely aesthetic to me - scanning the galaxy with the eye of a giant squid - imagine that... No. I must resist. The 120 f/8.3 is a strong telescope. The 150 f/5 is more toy-like, in it's single-role, if you know what I mean...


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#25 j.gardavsky

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Posted 20 February 2021 - 03:05 PM

I agree like with the post #19, #20, #21, as above.

 

My big grab-and-go is the old Synta achro 6" F/5.

When I feel, there is too much CA, then I mask the aperture down to 110mm, or I will mount bandpass filters for the Moon, planets, doubles.

 

The Synta comes into a travel golf bag, the big Cullmann tripod (with the alt/az head), and with the geared central column comes into a Breuninger bag.

The total weight of the both bags, and including some accessories, is about 15 kg, still no problem at my age of 75.

 

Best,

JG


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