Do I need a 5” ED doublet?

[Deadlake]

7 hours ago, DirkSteele said:

I fear for the prospects of the company here as even idiots like me who spend way too much money on scopes won't spend 20k on a 4" scope.

This is exactly the expression from the AP owners when they saw the price list.

At the moment the only think that could make me spend that sort of money is a weather machine to control the clouds. 
 

I suspect the amount of traffic on Astro sites is down due to the weather…

Edited December 8, 2023 by Deadlake

[mikeDnight]

I think the difference in performance between the big Apo names and the newer names that are now on the scene have become so close that it's really not worth losing sleep over, especially from a visual only perspective. I have a 5" because I was able to get it at a very good price, however I don't need a 5", butl wouldn't want to be without my 4". I know the OP is considering a 5" doublet, but looking at the Askar range of triplets, I have to admit they look very tasty.

[IB20]

Tbh, I hadn’t even thought about the Askar 120 triplets. They look very nice though and only 840mm and 6.5kg… 

I’m not even looking at the 140s. 😅😅

[Stu]

They do look nice scopes, although I think I would still prefer a doublet for visual. Faster cool down and arguably better transmission and contrast, although I’m not sure I’ve seen objective data to back the latter two up.

[F15Rules]

8 hours ago, Stu said:

They do look nice scopes, although I think I would still prefer a doublet for visual.

I'm probably showing my age here, but aesthetically I'm not a fan of short, stubby refractors and just prefer the look and (visual) performance of longer doublets, which to my eye look elegant, exciting and appealing..and ideally in white of course!😋.

Purely based on looks and visual performance I think an F10 Fluorite or FPL53 type Doublet would be my idea of observing Nirvana☺️.

Optically, I like the depth of focus offered by a slower optical system, and also the fact that scopes of c F10 or slower can use more modestly priced eyepieces with great results.

Of course, practically speaking, pretty much every scope has its compromises, which is why I think that modern 4" F7/F8 ED doublets are so popular: they seem to offer the most appealing compromise of performance, price and weight/practicality to the largest group of potential buyers.

We are certainly not short of plenty of choice these days!

Dave

 

[Stu]

13 minutes ago, F15Rules said:

I'm probably showing my age here, but aesthetically I'm not a fan of short, stubby refractors and just prefer the look and (visual) performance of longer doublets, which to my eye look elegant, exciting and appealing..and ideally in white of course!😋.

Purely based on looks and visual performance I think an F10 Fluorite or FPL53 type Doublet would be my idea of observing Nirvana☺️.

Optically, I like the depth of focus offered by a slower optical system, and also the fact that scopes of c F10 or slower can use more modestly priced eyepieces with great results.

Of course, practically speaking, pretty much every scope has its compromises, which is why I think that modern 4" F7/F8 ED doublets are so popular: they seem to offer the most appealing compromise of performance, price and weight/practicality to the largest group of potential buyers.

We are certainly not short of plenty of choice these days!

Dave

 

Agreed! Yes, f7 to f8 4” does seem to be a sweet spot and with fpl-53 they give excellent performance at great prices!

The larger you go, the longer the focal length needs to be to control the CA properly, particularly in a doublet so they do get big, fast! My Vixen Atlux was enough of a beast at 150mm f9 and 20kg, and I know @John has ‘been there’ with his 150 f12 cannon (Istar?? I think that’s right). So equally 130mm seems a good next step up giving a meaningful performance increase in a package that remains manageable. The 125mm f7.8s being a fine example. If I hadn’t found the Tak, I would likely have grabbed on of the SM Carbon Fibre ones from FLO 👍

[Deadlake]

52 minutes ago, Stu said:

Agreed! Yes, f7 to f8 4” does seem to be a sweet spot and with fpl-53 they give excellent performance at great prices!

The larger you go, the longer the focal length needs to be to control the CA properly, particularly in a doublet so they do get big, fast! My Vixen Atlux was enough of a beast at 150mm f9 and 20kg, and I know @John has ‘been there’ with his 150 f12 cannon (Istar?? I think that’s right). So equally 130mm seems a good next step up giving a meaningful performance increase in a package that remains manageable. The 125mm f7.8s being a fine example. If I hadn’t found the Tak, I would likely have grabbed on of the SM Carbon Fibre ones from FLO 👍

SM125 or TSA120?

I'm not convinced that a doublet will cool down that much quicker then a doublet of similar size and the TSA120 is similar to a 4" scope in handling.

[Stu]

11 minutes ago, Deadlake said:

SM125 or TSA120?

I'm not convinced that a doublet will cool down that much quicker then a doublet of similar size and the TSA120 is similar to a 4" scope in handling.

It’s a fairly commonly held view that doublets cool quicker than triplets, and the TSA 120 is 5.7kg vs say 2.8kg for my FC100. Lovely scope I know, but not the same weight class. The SM 125 which I was referring to is 4.7kg in comparison.

[Sunshine]

Can’t wait to see the difference in light gathering capability at the eyepiece, going from my 102 to an FS128, I look forward to my first view of Jupiter under good seeing.

[Xilman]

A 120mm refractor makes a very good guider. 😉

That's what mine is used for mostly. It is also far far better for solar work than the 400mm for which it acts as a guider most of the time. Solar heating would destroy that one.

 

[Mr Spock]

I keep looking at these 120 - 125s class of scopes and fancying one. Then reason hits and I remember why I have a 100. It's light, small, fits an EQ5 without struggling and in terms of performance only a shade behind a 120 - only 0.19" less resolution and 0.4 mag less faint. I conclude each time the extra size and weight isn't worth the difference.

See, I just talked myself out of one again 

[MartianHill]

1 hour ago, Mr Spock said:

I keep looking at these 120 - 125s class of scopes and fancying one. Then reason hits and I remember why I have a 100. It's light, small, fits an EQ5 without struggling and in terms of performance only a shade behind a 120 - only 0.19" less resolution and 0.4 mag less faint. I conclude each time the extra size and weight isn't worth the difference.

See, I just talked myself out of one again 

Interesting numbers but the eyepiece views are noticeably better in a 5” over a 4” as long as the optical quality is the same.

[Mr Spock]

15 minutes ago, MartianHill said:

Interesting numbers but the eyepiece views are noticeably better in a 5” over a 4” as long as the optical quality is the same.

 

1 hour ago, Mr Spock said:

I conclude each time the extra size and weight isn't worth the difference.

I think you missed the point 

[MartianHill]

47 minutes ago, Mr Spock said:

 

I think you missed the point 

Soooo, why do you keep looking at 120/125mm scopes ? 🙂

[Mr Spock]

27 minutes ago, MartianHill said:

Soooo, why do you keep looking at 120/125mm scopes ?

None of your business.

[Naughty Neal]

53 minutes ago, MartianHill said:

Soooo, why do you keep looking at 120/125mm scopes ? 🙂

One expects like a lot of us, the allure to look and want is all that is needed.

Edited December 10, 2023 by Naughty Neal

[Ags]

On 05/12/2023 at 13:57, JeremyS said:

The TSA 120 is a step up from the FC 100 and not that much bulkier.

A lot depends on your mount. The FC 100 just about works on a ScopeTech Zero.

Want this NOW!!!!!!

[JeremyS]

1 hour ago, MartianHill said:

Soooo, why do you keep looking at 120/125mm scopes ? 🙂

Nothing wrong with window shopping 🤣

[johnturley]

19 minutes ago, JeremyS said:

Nothing wrong with window shopping 🤣

I spend ages looking at things I've no intention of buying. 

John 

[Ags]

With all this cloud, window shopping is the only way to enjoy this hobby

[JeremyS]

Well, one can always arrange and rearrange eyepieces 😊

[MartianHill]

2 hours ago, Mr Spock said:

None of your business.

Oh come on, it’s a hobby !

[Naughty Neal]

I window shop a lot esp at Delitte's & XW's.

[Xilman]

1 hour ago, johnturley said:

I spend ages looking at things I've no intention of buying. 

John 

My favourite telescope porn can be found at https://planewave.com/product/pw1000-observatory-system-rc-optical-design/

If I had a megabuck to spare I would buy one like a shot.

 

[John]

Over the years I've been in the astronomy hobby I've found the best cure for an "itch" is to scratch it. I read a lot of different view points from a lot of different sources but there comes a time when only "seeing for yourself" will do. 

Occasionally I have regretted that, and the example of the 6 inch F/12 achromat refractor that I acquired, as mentioned by @Stu earlier in the thread is one of those. But I learned from it and it's quite possible that had I not actually tried it, I would still have that itch of curiosity for a really long, large aperture refractor. They do look so alluring 🤩