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bryand

Four inch to five inch: worth the upgrade?

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bryand

I have two scopes, a 9.25" SCT and a 102mm (4-inch) APO triplet refractor.  
After 18 month's use, I am coming round to the idea that I prefer the view from the refractor over the SCT, despite differences in scale etc.
But the four-inch refractor is a tad small and since I can't afford a six-inch, I am looking at a 130mm (5-inch) APO triplet refractor.
I am hoping for a useful increase in light-gathering power without it becoming so unwieldy that it discourages use.
I would keep my existing eyepieces: ES 92° 17 mm and 12 mm and Ethos 8 mm.  

Transportable observation rather than photography is my main interest.

 

Has anybody compared 4" and 5" scopes, and if so what did they find?

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tuckstar

There are a few discussion thread on cloudy night about this very subject. May be worth a browse.

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Ibbo

I have a Tak 102 (old doublet) and a WO 132 (triplet with issues)

 

I prefer the 102 as it gives the sharper view/image, though the 132 is brighter.

the 132 is heavy and did have pinched optics so is an early version, but having said that we seem to have cured that, Thanks Phil.

The wo is a short tube that is designed to be used with bino viewers so if you are doing cyclops viewing or imaging you need stupid amounts of extension tubes.

 

 

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philjay

You can see an awful lot with a 4" , you can see a bit morewith a 5". A 4" is lighter and more transportable, a 5" is bigger and heavier. I went from a 4" to a 5" refractor and I must say the 4" used to get used more than the5" does. So a question you must ask yourself is, is the extra small gain on fainter stuff worth the extra cost (especially with Tak prices) and will the bigger scope be used as much as a more manageable 4"

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bryand

Thanks for your contributions folks.
The Cloudy Nights consensus seems to be that (comparing like for like) the 5" is better, provided you can manage the size and weight.
My mount can already cope with the C9.25, so a 5" wouldn't be too massive.
I wouldn't intend keeping both refractors (no storage space) so for me it's a case of 4" OR 5".

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Bino-viewer

They say a 5" hits 'the sweetspot'

And for planetary observing, where you want that contrast, and nice unobstructed optics,

i'd certainly be opting for something 125mm+

(my new refractor, which should be with me soon will be 140mm)

 

 

My personal opinion is that as long as the focal length is below a metre, its manageable.

 

A 6" on the other hand is a big scope indeed ; certainly the 1200mm versions.

 

Another thing to consider is design. A big air spaced triplet takes its time to cool (those in the know suggest about 20mins / inch to reach ambient)

so with one of these sizes of scope you need to allow well over an hour if the scopes been stored in a warm house.

And make sure you use a Losmandy type rail with a big 120mm + refractor. They weigh  around 10 kg and if you are using an equatorial mount

they can end up in some precarious positions.

 

Keep us posted with what you intend to buy......i myself rather like refractors ?

 

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philjay

Just a little tip re apos and cool down.

 

All apos take longer to cool than their achromatic counterparts. Apo doublets less than triplets due to less glass. Even a 60mm apo will take longer than a 60mm achro. Why? The glass formula. Apo glass is not as hard as achro crown and flint and is more susceptible to temperature variations.

 

Therefore expect to wait longer for a larger apo to give its best when setting up. Not quite as long as a C9.25 but not far off. I dont use my 5" triplet for detailed stuff for at least an hour.

 

 

 

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