TheSkySearchers

I was thinking of making a series about what goes into making a quality telescope and what better place to start than looking at some specifications.

Using only the information provided (No linking to other topics) pick which refractor you think is the best using the information below and why you think it is better than the other, we will go over peoples choices and in the process perhaps point out some things to help others make some better choices.

Price of both telescopes is the same.


#1 - 150mm triplet f/6.7 refractor
Description:
* 3-element objective lens system minimizes colour aberration
* Fully multi-coated optics ensures superior light transmission
* Finely CNC machining aluminium body and 1:10 focuser
* Deluxe high-glossy anodizing finish
* Retractable dewshield
* Accept all 1.25” universal telescope eyepieces
 
Specification:
Optical design: Refractor
Aperture: 150mm
Focal Length: 1000mm
Focal Ratio : f/6.7
Lens: Triplet , ED APO S-FPL51 Fully Multi-Coated
Attachments: 2.7“ Rack & Pinion Focuser, Steel lens cell, 2"-1.25" Brass Compressing Adapter, 1 set (2pcs) Tube Rings, Aluminium Carry Case 

#2 - 150mm f/6.7 refractor
Description:
* 2-element objective lens system minimizes colour aberration
* Fully multi-coated optics ensures excellent light transmission
* Finely CNC machining aluminium body, lens cell and focuser
* Deluxe high-glossy anodizing finish
* Retractable dewshield
* Accept all 1.25” universal telescope eyepieces
 
Specification:
Optical design: Refractor
Aperture: 150mm
Focal Length: 1000mm
Focal Ratio : f/6.7
Lens: Doublet , ED APO S-FPL53 Fully Multi-Coated
Attachments: 2.7“ Rack & Pinion Focuser, 2"-1.25" Brass Compressing Adapter, 1 set (2pcs) Tube Rings, 8” Vixen style extruded dovetail 

I think for my use I would go with #2.

The triplet of #1 is nice, but I am not interested in astrophotography and the ED doublet of #2 would suit quite well for my visual interests. #1 would be heavier in the objective, 150 is a lot of glass, and balance considerations would be on my mind. I suppose I could weight the focuser to counter balance. If not counterbalanced I think I would have a lot of tube behind my mount to put the triplet in trim and this would put me in the ground for views near zenith.

The steel lens cell on #1 is also attractive, but not a deal maker.

The triplet would maybe be my choice if I were doing astrophotography, but I would need a heavier mount to go with the heavy tube.

The glass in these two choices is another consideration.
The triplet has fpl51, sounds like an ES essential, the doublet has one fpl53 and maybe a schott glass front end, maybe a Skywatcher 150 ED.

The fpl53 ED doublet would do quite well for me.

The weight and balance considerations are the scale tipper in my choice. For my practice I like the lighter scope.

Well, that is my 2 pence.
Cheers !

Thank you for posting John, hopefully we will have some more responses so we can start discussing things

Nice game!

I'll assume both reach a Strehl of 0,95- 0,96- 0,97 or so, still machined finishing touches.
They are almost the same.


There are some differences though.

Triplet vs Doublet
FPL-51 vs FPL53
Steel lens cell vs Alu lens cell
R&P 1:10 vs R&P
No dove tail vs Vixen dovetail
Unknown weigth (heavier) vs Unknown weight ( lighter)
Unknown price (pricier) vs Unknown price ( cheaper)

Being a visual observer I would be more interested in the doublet. Weight, balancing and observer position. My own GPDX mount can handle it.( only just) I can handle it. The Triplet might need another mount. Costs is a point too; the Triplet will be more expensive than the Doublet, maybe $1000? Maybe the costs are even higher if my mount can't handle it properly. CA is no point of my considerations.
I can do some of my own corrections on some aberrations ( should it occur) on a doublet , a triplet needs a professional optician. Furthermore my considerations are the same as John Donne.

Are you up to something?

John Baars wrote: ↑Mon Oct 05, 2020 9:19 pm Are you up to something?

Of course I am John, I am trying to get people to read basic marketing and make a choice based on that.
Afterwards I will add some more questions, at the end beginners should be able to see what is and is not important.

Is there anyone else interested in placing a opinion on the telescopes in posting #1?

For the packages as they stand I would go for #1. The Lady is giving hints! "marketing" All other things being equal there are differences: As for 3 vs 2 element, pick your poison but 3 element SEEMS more alluring. #1s 1:10 focuser, allows fine adjustment and specs ALWAYS impress. #2s focuser, at least it has one. Steel vs Aluminum cell? Being a novice I've no idea the best as they should both be machined to specs. Maybe expansion differential between OTA body and cell or weight factor, I dunno. #1 includes a aluminum case, no dovetail. #2 no case but you get a vixen dovetail included...nice but what if you have a Losmandy mount. So do you want to purchase a dovetail or ante up more money for a case?
Also in the coatings descriptions "Superior vs excellent light transmission". Superior does SOUND better.

Am I giving hints or hiding something?
Some good comments.

Pros for no. 1
I do more astrophotography than visual.

Pros for no. 2
The Vixen dovetail will fit on my CGX.
I don't travel much so carry case not important (I can make one, I can't make a dovetail)
If the doublet is the same price as the triplet then the quality is probably better (you get what you pay for!)

Overall, I would go for No. 2 and buy a coma corrector.

Intriguing thread!

Regards

Graeme

konza wrote: ↑Fri Oct 16, 2020 2:46 am (..)
Also in the coatings descriptions "Superior vs excellent light transmission". Superior does SOUND better.

Quite so.

Let's do some simple assumptions / calculations.
Let's say the doublet loses 2% on each glas-air-surface (4X) and 3% on transmission through the glass of two lenses. Result is a transmission of 89%.
The triplet has a superior transmission ; loses 1% on each surface (6X) and 4,5% on transmission through the glass of three lenses. Result 89%.
So, even the advertisement is correct, in reality there is no difference.

In practice transmission through lenssystems ( in this case eyepieces) can be seen here: http://www.amateurastronomie.com/Astron ... /tips3.htm
The trend is for multiple lens systems to have a lower throughput.

Moreover, no human being will be able to notice the difference between 90% and 95% transmission. With great difficulty I can establish the difference between my Jena and Swarovski binoculars with 83% vs 96% transmission: the color of the total image is a more decisive factor than the transmission. In the case of the Jena and the Swarovski: more amber vs. whitish with a hint of blue.

Very good points John but it is taking the conversation away from the main question

Well, I happen to disagree a bit. It just refutes one of the reasons people would choose one or the other. If people read "superior" they tend to choose it above "excellent". In our case it is just the same. As a result I eliminated one of the reasons. But...ok, maybe I am a bit too far off.

I'm not really a refractor guy, but, knowing nothing else but was originally posted, I would go with #1. I know that three lenses are *generally* worse than two. But to me the important thing is the 10:1 focuser. Fine focus is so important for me, whether for visual or AP. Getting a mounting system (ie dovetail) for MY mount would be a price I would pay (in addition to the OTA) to have the improved focuser, as a better focuser might be hard to retro fit on #2. So as a relatively naive person in the refractor world #1 is the one for me.

Well, I have a number 2 but one day would like to get a number 1. One can always chase after better optics, and a triplet certainly would be better for the future for things like AP, if my understanding is correct. Right now I am satisfied with the 2 though, even though there is a bit of a hint of color around the brighter objects - moon and planets.

Thank you for the reply Michael

that's a hard one ....

well ,i image in mono , so i would pick number 2 " the doublet" this will cost alot less and any small ammount of ca can be controlled with filters and focus . the extra cost savings can go elsewhere ,dew heating, filter upgade,a nice field corrector ectt .

(what am i kiding, mirrors for me please .. haha)

yobbo89 wrote: ↑Fri Oct 16, 2020 4:53 pm (what am i kiding, mirrors for me please .. haha)

Thank you for that blasphemous reply Yobbo

i have a question, oil spaced doublet ?? , are the above scopes air or oil spaced ?

yobbo89 wrote: ↑Fri Oct 16, 2020 5:04 pm i have a question, oil spaced doublet ?? , are the above scopes air or oil spaced ?

Both are air spaced, using oil spacing does not in itself make a lens set better.

I think I would go for #1 the triplet configuration outweighs FL 53 for me. The steel cell should also let me collimate the optics. The focuser is an important feature for me this all gives me more control.

I think the FL 53 is a classic ’buzz word’ marketing approach, covering some of the deficits of the second option.