Is 5" the new 4" ?

[brianb]

maybe I should ask is there ANY telescope free from all types of abberation / distortions?

It's a physical impossibility to obey the sin criterion & the tan criterion at the same time. This means that you will always have at least one of distortion, coma and field curvature.

If you put sufficient surfaces into the system & compute very carefully you can make a sufficiently aberration free system for any practical purpose. But no system with lenses in it can be absolutely free of chromatic aberration, whilst if you compute a system to be free of spherical aberration it will be absolutely so for a fixed object distance, unless you allow the elements to change their seperation as part of the focusing mechanism, which really messes up the computations for the reduction of other aberrations....

The optical system which comes closest to the elimination of distortion and aberrations is the simplest of all - the pinhole - but unfortunately its "focal length" is very very large indeed in proportion to its aperture, making its resolving power and light grasp somewhat limited (to say the least).

[omneferuss]

:D

ahhh the path to enlightenment is often lost in the ruts of many bandwagons and barrows pushed.

the populous parade of opinions having passed, a chicken can now cross the road safe in the knowledge that 5" is the new 4".

but is as yet perplexed... so few eggs, so many baskets!

a quandry to sit on, and see what hatches.

[dweller25]

I've forgotten the original question

[coliea]

Well thanks for putting all of us straight on that !.

John

Sorry - didn't mean to rattle anyone's cage. In future I'll keep my opinions to myself lest I offend.

Col

[coliea]

Unfortunately there is no industry standard definition for what is, or is not, an apochromatic telescope. Those definitions that do exist are mostly from manufactures and others with a vested interest.

That's the point I was trying to make... and would have made if I were a little more eloquent. Anyhow, forget about it. I didn't want to instil ill feeling, so apologies to all.

Having spent two weeks watching smug, grinning politicians spout their ridiculous excuses, I foolishly, in a moment of temporary idiocy, took my ire out on something that doesn't really matter that much – think that excuse will fly

Lesson learned.

[John]

Sorry - didn't mean to rattle anyone's cage. In future I'll keep my opinions to myself lest I offend.

Col

Hey Col,

I value your input but I just thought this one post was a little too sweeping, although no doubt technically correct.

Sorry I sniped back - darn politicians have got us all bickering !.

Cheers and clear skies

John

[coliea]

Hey Col,

I value your input but I just thought this one post was a little too sweeping, although no doubt technically correct.

Sorry I sniped back - darn politicians have got us all bickering !.

Cheers and clear skies

John

No worries John - I was indeed a tad "sweeping"

Steve's post comparing the thread direction to "a cross between Cloudy Nights and DPreview" was quite sobering, as I'm familiar with how juvenile both those forums can be. It was an instant case of "OMG...what have I done :eek:"

All the best buddy.

C

[Ibbo!]

Well at least in this observatory the new 4" will be a 5 and a bit" as its on its way

I will let you know if I'm happy with it when I get over first light and imaging

I must compare the old 4" f13 against the newer 4" f8 now I have the mount capable to hold it for imaging.

When compared visually the f13 had slightly different colour on objects compared to the f8 and a newt

I put it down to the older Mgfl coatings

but personally preferred it to the 8" SCT on the planets when conditions allowed, due to physical length as the merest breeze really played havoc

but that could have been down to seeing conditions at the time

Steve

[John]

Well at least in this observatory the new 4" will be a 5 and a bit" as its on its way

I will let you know if I'm happy with it when I get over first light and imaging.....

Hmmm.... so that's a 130mm by my reckoning - what's on it's way Steve - c'mon, give us a clue

John

[adamsp123]

If 5" is the new 4" what do I do with my tape measure? :D

or if you, like me, have a 120ED (a superb bit of kit) should we not ask is 4.72441" not the new 4"

It is all down to affordability in the late 70's a 4" refractor cost at least a couple of thousand pounds in todays' money much more than a Takahashi now.

[Ibbo!]

Hmmm.... so that's a 130mm by my reckoning - what's on it's way Steve - c'mon, give us a clue

John

WO FLT 132mm and flattener

plus slap on head etc from other half

Steve

[John]

....plus slap on head etc from other half

A price worth paying IMHO

John

[Macavity]

I must admit, I'd come to dread the "Shoot Out" thread, too. LOL. But, at this level, such free-flowing(?) discussions interest me. As a (random!) example, I find these things (qv) quite instructive too: Courses in Astrophotography - As indeed the general conclusions.

I think I have also "realised my objective" (aagh!) in understanding e.g. diffraction patterns, with my (part deliberate) choice of the extremes in my ST102 + MAK127. Having the two scopes side-by-side on my Giro III, the difference in image, contrast, colour even, can be quite dramatic. And not always to the subjective detriment of the less expensive scope. Of course, I'm not sure my "liveliness" criterion is a very scientific measurement though...

<wibble>