Is 5" the new 4" ?

[John]

Sorry for the rather cryptic thread title

When I started out in astronomy (mid-1980's) I used to dream of owning a 4" refractor. Not an apo either - a long tube achro was what mere mortals yearned for. I actually thought Tele Vue and Astro Physics sounded rather naff names for scope companies back then - shows how much I knew

A 4" refractor was considered a "serious" instrument then, probably not really portable (achros = long focal length) but capable of "opening up the universe before your eyes" according to Celestron's 1980's refractor brochure.

Fast forward 20 plus years and 4" refractors are 2 a penny, apos less expensive than achros used to be, Chinese sourced optics occupying the lens cells of reputable names like Vixen and William Optics, etc, etc.

In fact I'm now wondering if the 4" refractor has now been supplanted by the 5" (I include 120mm in this category). The current 5" class refractors are lightweight, in plentiful supply and affordable - even the apo versions !!!. You do not need to have the build of Geoff Capes to operate one either - I saw an ED120 at SGL4 and honestly thought it was an ED100 !.

In the 1980's the only 5" refractor I knew was the Cooke that Sir Patrick Moore owned. Today we can all have one which is great, but where does that leave the 4" - does it still have a strong niche of it's own or has it now been relegated to just a "stepping stone" on the path towards a "big gun" ?.

Please excuse the ramble - I'm just going off to talk softly to my Vixen ED102 in case I've hurt it's feelings ........

John

[Jamie]

John

I think that there is still room in our equipment line up for a 4" refractor.

Its a great grab and go size of scope for visual use,yet it is one of the prefered sizes when looking for an imaging refractor too.

The thing with all the scope manufactures making refractors in incremennts of between 5,7 & 10mm intervals is that they can then use a slightly oversized tube for the first in the line of scopes, then they use the same size tube for the larger of the series. For example the WO 105 triplet APO was identical in tube size to the WO 110 triplet APO....

I think that is the reason why we can look at a scope these days and just not be quite sure what size we are looking at till we give it a closer examination...

I still think the 4" frac is the way to go IMHO...

[acey]

The classic 4" refractors were very long - f/15 was typical. I wonder how the views through one would compare with modern scopes? I suspect they'd stand up pretty well.

Design has been driven by portability and imaging, hence shorter tubes, wider fields, and exotic glass to deal with all the problems of false colour, coma etc, arising from those changes.

It was always said that for planetary viewing, 4" was as much aperture as you needed, as there are rarely times when the air is steady enough for the higher resolving power of larger aperture to be of benefit. Well, that's what everyone said, though maybe because 4" was as big as they could reasonably expect to get.

And when light pollution wasn't an issue, 4" was enough for decent views of deep-sky objects; a 4" scope in the mag 6.5 skies of yester-year is equivalent to a 10" in the typical mag 4.5 suburban sky of today.

Newtonians have undergone the same inflation: the standard for most of the twentieth century was 6". By ten years ago that had gone up to 8", then it became 10", and now 12" seems to be taking over as "entry level" aperture, as people try and see through the ever-brightening suburban skyglow.

That's progress!

Andrew

[moondog]

When I started to be interested in astronomy a 3ins refractor was considered to be ' a powerful instrument, capable of showing much lunar and planetery detail..' ! (Quote from 'Guide to Astronomy' by James Muirden 1972)

MD

[dweller25]

I think the short answer is yes.

The price of a good 5" scope (Equinox) has tumbled so a lot more people can afford them - I have an old Astro Systems brochure - a 4" Vixen FL refractor on an undriven Super Polaris mount cost £1500 - 25 years ago !!!!

In 1985 I bought a 8.5" F5 newtonian from Astro Systems on a driven Super Polaris mount, the price was just £1100 - the same price as a 10" Skywatcher on an EQ6 now.

These are good times for astronomers.

Just wish Synta would bring out a 140mm ED for around £2500.............

[gliderpilot]

I guess we are all benefiting from the economic expanision of China. It is also amazing how much binoculars have reduced in price while retaining a high standard.

I am really enjoying my 102 Skywatcher, enough aperture for some decent deep sky objects and wide angle viewing as well. Pushing the magnification though reveals crisp details on moon and planets. Plus it is much lighter and easier to use than my 8.5" newt - no need to worry about collimation, cool down etc.

[Mark at Beaufort]

I thought long and hard before deciding which refractor to buy. I looked at various scopes in the 4" to 5" bracket and eventually felt that a 4" APO would make the best grab and go together astro photo needs in the future.

I was pleased with the comments that I received at SGL 4 with members saying that it was the cleanest view of Saturn that they had had that evening.

Mark

[acey]

When I started to be interested in astronomy a 3ins refractor was considered to be ' a powerful instrument, capable of showing much lunar and planetery detail..' ! (Quote from 'Guide to Astronomy' by James Muirden 1972)

MD

My first scope, which I got in 1971, was a 2" refractor with a fixed magnification of x30. I recently dug it out, aimed it through the kitchen window at Jupiter, and could clearly see the main cloud belts. I still recall my first childhood view of Albireo through it - the colours seemed extraordinary. And from my back garden in a town in central Scotland, I got my first view M31, easily found in the dark sky.

Changed times.

[oceanheadted]

I bought a 90mm refractor for my daughter to look through, when I mentioned it to my dad he said "ooh 90mm - that had to be expensive!".The market has changed enormously in the last few years, plus electronic gizmos have got a lot cheaper.

[adamsp123]

Just wish Synta would bring out a 140mm ED for around £2500.............

150ED for £1750 !!!!!

[dweller25]

150ED for £1750 !!!!!

I wish

[Astro_Baby]

Its true enough - I was looking at a web site recently which features old telescope manual and brochures and say my 'dream' scope back then - a 6" Tasco Reflector.

I had the 4.5" on a wooden tripod complete with a wind-up clock drive. It actually looked like an old Smiths alarm clock - seriously Heath Robinson complete with a small bell that went 'ding' when the spring was running down - though mine never did after the firts few windings. It just died usually after about 10 minutes.

It was amazing looking at the old ads for 6" refelectors which were all long tube stuff on pretty primitive mounts. When you calculated the costs these things were about £450 in 1960s money. That was when a new mini cost about £500.

Today you'd be talking about a scope of vastly improved better optics on a computerised GoTo mount of a quality undreamed of outside a proifessional observatory back then - and the cost would be less than a decent pushbike - let alone a car.

There was the same inflation for reflectors. Back in 79(ish) the 4.5 reflector was the kick off for reflector owners. 6" would be considered 'serious' and an 8" would have been only for people who had paid off the mortgage or could make their own.

These days an 8" on a GoTo is almost the starting point for a lot of people.

[Macavity]

I seem to remember Sir.P saying (1960's Observers Book!) something about the 3-4" size being optimal for solar projection(!). Or rather a 5" was deprecated for collecting too much light & heat? But a lot of those "size comparisons" & "inch-for-inch" etc. aphorisms are rather permanently etched on my mind. Yet probably motived my purchase of an ST102 on "return" to a (never really stated?) hobby. But a 4" refractor feels "right-sized" and even a 5" MAK seems BIG to me!

[brianb]

I seem to remember Sir.P saying (1960's Observers Book!) something about the 3-4" size being optimal for solar projection(!). Or rather a 5" was deprecated for collecting too much light & heat?

Let's face it, with modern eyepieces with cemented components held together with plastic spacers etc., 2" is way too big for solar projection.

Solar work is unusual because the Sun itself tends to create awful seeing, some people claim that unless you have a specially selected "good seeing" site (probably on a desert mountaintop) there's not much point in having more than about 6" of aperture for solar work because the gains in resolution get offset by losses to poor seeing. Light grasp is of course a non issue.... 4" scopes collect sufficient light even when used with ultra narrow waveband hydrogen alpha filters.

But, for all other work, the more aperture, the better.

Personally I find it hard to understand why achromatic refractors, especially short focus ones, remain so popular. They don't collect enough light (except for solar work), the small aperture limits resolution and the false colour inherent in the short focus design is a serious handicap. They're not even compact compared with Maks or SCTs. Small aperture apos make good imaging tools but that's mostly not what people buy them for.

Is 5" the new 4"? Well, I'd much rather have a cheap 8" Dob!

[John]

A very interesting selection of views folks - thanks for posting them

On the Cloudynights site there is a thriving section on vintage scopes with some fabulous looking instruments on there. Even some of the older Tasco's are considered "classics" including my 1960's Tasco 12TE-5 60mm refractor which I have a big soft spot for as it gave me my 1st views of Saturn:

[Beulah]

Some great opinions here, the light pollution opinion is certainly a consideration.

I think the shifting standards of telescope size are also commercially driven. It's the same with whatever specialist interest....

My 8" Dobbo does a jolly grand job of revealing the skies, however after two weeks of owning this formidable piece of equipment, and because of the Internet, I am already thinking about a 12 incher!!! Somehow, one has to find contentment with what one has........

For example, I have seen the goalposts for entry level guitars shift exponentially since the birth of the web. As with PCs, bikes, mobile phones...web advertising is an extremely powerful and persuasive tool, IMO, more powerful than mags and TV adverts.

[John]

My 8" Dobbo does a jolly grand job of revealing the skies, however after two weeks of owning this formidable piece of equipment, and because of the Internet, I am already thinking about a 12 incher!!! Somehow, one has to find contentment with what one has........

Yes it's all the fault of the internet - I had owned just a couple of scopes until I got my internet connection - I've since been through at least 20 different scopes - very undisciplined

John

[The Warthog]

What you are saying about telescopes is true of almost any field - Cameras, watches, computers, calculators, even bicycles. In 1963 I received my first watch, which cost $15, the equivalent in todays money of about $100. Today you can buy a very good watch for less than $100, and you can get a nice looking windup watch for about $20.

I have a 105mm refractor which you can have when you can pry it from my cold, dead hands. Scopes don't deteriorate in optical quality, and if cared for even tings like focusers go on and on for years. It was a good scope when it was first bought, and it's still a good scope today. It's not a 6" APO, but nothing is, except a 6" APO. I get very good views through it.

[FLO]

I checked FLO's statistics to compile a top-ten list of bestselling telescopes. Without being specific, there are 5x Newtonians, 3x short-tube APO refractors and 2x SCT telescopes. There are no four or five-inch refractors and no achromats.

It is a shame they aren't getting much of a look-in as they offer excellent views and there are some very good deals out there, like the Celestron Omni XLT 120 for example.

Ah well

[brianb]

Without being specific, there are 5x Newtonians, 3x short-tube APO refractors and 2x SCT telescopes. There are no four or five-inch refractors and no achromats.

Eh? I'm amazed .... or are you using the term "apo" to cover refractors with two elements, one of which is an exotic glass?

"Apochromatic" implies coincidence of focus at 3 wavelengths (in the blue, green and orange/red), that simply can't be done with only two elements.

[FLO]

are you using the term "apo" to cover refractors with two elements, one of which is an exotic glass?

Yes, most astronomers consider a doublet with an FPL-51 or an FPL-53 element to be an APO. Telescopes like the Zenithstar 66, Skywatcher ED80 and Megrez 90 are all good examples.

HTH

[Macavity]

Let's face it, with modern eyepieces with cemented components held together with plastic spacers etc., 2" is way too big for solar projection.

I must admit, I've always "jibbed" at the prospect. I was reminded by the recent pic of the space shuttle in front of the sun was a 'frac plus Herschel Wedge? Despite believing myself to be infinitely careful, I did once ALMOST look through my scope without the objective solar filter in place.

But the 'frac + wedge might seem a safer way for the (overly!) casual white light solar observer.

Personally I find it hard to understand why achromatic refractors, especially short focus ones, remain so popular.

Perhaps an F5 achromat for "sky sweeping" or Open Clusters? Something I enjoy. Certainly in the 3-5" range, and on a small budget, I cannot see why one wouldn't buy a MAK102 or MAK127. Cooldown, perhaps (not). The potential for wide fields is not much below that of (standard f~48") reflectors. But indeed, if you're young and fit, a straight forward DOB.

From a theoretical(!) POV, I did start to wonder recently about e.g. the Skywatcher 190MN MAK-Newt. Aside from weight and cooldown, if one wanted an F5 wide field (visual) view what's not to like? But I suppose we get back to my "Observers Book" and the notion of inch for inch, a reflector is "less efficient" than a refractor. I guess those "old astronomers" were implicitly alluding to lesser CONTRAST due to the central obstruction... I sense the gap has closed with the advent of the MN. I guess (for DOB + 'frac fans etc.) the MN is still "twice as heavy" as need be!

Probably should have started a new thread on the latter, but the "general wisdom" here has intrigued me...

[Izar]

The classic 4" refractors were very long - f/15 was typical. I wonder how the views through one would compare with modern scopes? I suspect they'd stand up pretty well.

Andrew

Very, very well indeed Andrew. I own 6 refractors, and it is always interesting seeing how they compare.

My 35 year old 80mm f/15 Achromat, beats the pants off my modern 80mm APO for visual use.

[brianb]

But the 'frac + wedge might seem a safer way for the (overly!) casual white light solar observer.

Well the wedge only reduces light & heat by 90%, what's left is still very dangerous without an objective film filter ... so why not use a density 5.0 film filter instead of a density 3.8 film filter plus an expensive wedge. Used to be that people with small refractors used a wedge plus the dreaded eyepiece solar cap ... that way the thing didn't crack (when used with a 3" refractor), but I know of one astronomer who set fire to his tie by getting it in the way of the "exhaust" from the wedge.

I make a habit of passing my hand through the exit beam before putting my eye to the EP. If I'm foolish enough to leave off the objective filter, my hand will get scorched. Better that than my retina. But IMO the solar film filter is the safest way of viewing the Sun in white light ... provided it's inspected before each use and that it's taped firmly in place so that it can't fall or blow off whilst in use.

[brianb]

But I suppose we get back to my "Observers Book" and the notion of inch for inch, a reflector is "less efficient" than a refractor.

That's another myth that modern coatings have laid to rest. Modern reflectors with UHTC/Hilux/XLT type coatings almost certainly have a higher light gathering capacity than a refractor with the same nominal aperture, despite the central obstruction - going through glass scatters & absorbs light. The CO may reduce contrast slightly but the effect is subtle & likely to be swamped by other factors. In practice the quality of seeing, proper collimation & cooling and optical quality will have much more effect on the quality of images than the optical design of the scope.