whjy such a price jump between 5 inch and 6 inch reflectors?

[kniclander]

But to be fair, calling anything under 6" (in his opinion) a jumped up toy is pretty imflamatory don't you think?

Having 4 scopes all 6" and under (and about to buy a fifth), I can safely say that optically, they're all pretty good and whilst they don't apparently qualify as scientific instruments, I have a lot of fun with them and that's what is important .

Tony..

it was pretty inflammatory - I think that was kind of the point

[FLO]

Wasn't the Messier list put together using a 4-inch telescope?

[steve_bham]

I think small telscope 6" or below are great, im a fan as they are portable light weight and the views you get from them are stunning.

Steve *aka toy telescope man*

[coffee_prince]

yeh, this cut off point to say 6 inch is good, but 5 inch isn't.. is kinda annoying. I mean a mere 20mm separates a cheap 130mm dobsonian with a £200 6inch reflector, but surely the 130 isn't that bad. Anyway...

I kinda want to save up for a 6-8 inch skywatcher explorer, I have a digital slr

[jgs001]

As you mention you have a dslr I presume this means you want to getinto imaging, in which case the mount becomes more important than the rest of the kif. Without a suitable mount imaging becomes much harderand more frustrating (it can be plenty of that with a decent mount anyway). Probably a good start point check out the c80edr offer at flo. It's for an 80mm fracon a cg5 mount.

[brianb]

As you mention you have a dslr I presume this means you want to getinto imaging, in which case the mount becomes more important than the rest of the kif.

I agree entirely. 80mm ED / apo frac on a decent mount is seriously useful kit for "small scale" deep sky imaging. Not that great for visual use though.

As for Messier using a 4" scope ... sure, but things have moved on a bit since then. Messier's 4" scope probably cost about a whole year's salary for one of the very few reasonably well paid professionals there were in those days ... maybe the equivalent of £30,000 now.

[acey]

80mm is more than enough aperture to see all the Messier objects - if the sky is dark enough.

In speaking of reflectors we've got to take into account the reduction in light gathering due to the central obstruction (secondary mount), so a 4" reflector is not in the same class as a 4" refractor. Because of this, and also the resulting poorer contrast, it was traditional to consider a 4" refractor equivalent to a 6" reflector.

The scopes of Messier, Herschel etc had metal mirrors that tarnished quickly and were of low reflectivity. The spiral arms of M51 were first identified using a 72-inch mirror, but can nowadays be seen with an 8-inch from a dark sky. Observers with twice as much aperture but light-polluted skies can find themselves struggling even to find the galaxy.

[brianb]

Because of this, and also the resulting poorer contrast, it was traditional to consider a 4" refractor equivalent to a 6" reflector.

I'm glad you used the word "traditional", with modern dielectric mirror coatings this is no longer true - in fact, with ordinary aluminimum coatings, a reflector is more than the equal in light transmission of a refractor the same aperture in sizes over about 6", despite the central obstruction, because of light absorbtion in the glass.

But then, people these days falsely accuse reflectors of having poor light transmission, then cripple their scopes (by at least 0.2 magnitudes) by using exotic multi-element eyepieces containing huge amounts of glass.

[Takahashi]

But then, people these days falsely accuse reflectors of having poor light transmission, then cripple their scopes (by at least 0.2 magnitudes) by using exotic multi-element eyepieces containing huge amounts of glass.

How much of an effect does this really have, in terms of light absorption? I mean, aren't some of the most-sought-after eyepieces of the multi-element variety? Is it not preferable to have a triplet-based apochromat over a doublet, sacrificing (I assume) minimal absorption for the sake of chromatic fidelity? Aren't you over-stating it somewhat, to say that people are "crippling" their scopes by using these eyepieces?

[ollypenrice]

To get back to what I guess may underly the question, the cost of figuring is proportional to the area to to be figured, not to the diameter. In terms of what you can see - well - bigger is better and as has been said, 6 inch is a lot bigger that five inch - but that doesn't make 5 inch a waste of time. Clearly, though, a five inch refractor is what we would call a large-ish instrument and a five inch reflector is what we would call small-ish. Wouldn't you agree?

Olly

[brianb]

How much of an effect does this really have, in terms of light absorption?

I'm an experienced variable star observer. I know I can get 0.2 mag deeper with a 20mm multicoated Plossl than with a 20mm type 5 Nagler. That's 20% light loss. Very significant in my book.

Sure the Nag gives a much wider field, and the star images are better towards the edge. It's a great rich field EP for that reason ... but, if you're concerned about light transmission, it's very far from the best.

Is it not preferable to have a triplet-based apochromat over a doublet, sacrificing (I assume) minimal absorption for the sake of chromatic fidelity?

Fidelity is hardly the issue; light which goes the wrong way i.e. doesn't come to focus at the right place is wasted, & contributes to contrast loss (as well as horrendous "blue bloat" if used for imaging with a camera with good sensitivity in the violet). Nevertheless a triplet apo refractor has 6 air/glass surfaces and, in 15cm aperture, contains ~ 3cm of glass for the light to get through. Losses are certain. If you're a solar or lunar observer, or a bright planet observer, the light loss may well not be an issue at all, nevertheless it's real enough.

The Yerkes 40" refractor loses over 2 magnitudes in light grasp compared with a reflector of the same aperture. Or, to put it another way, for observing faint objects, a 16" reflector will beat it. If they made a 'frac OG much bigger than 40" , it would have to be so much thicker to support itself without sag that the light grasp would actually start to decrease!

[coffee_prince]

do i need to get an apochromatic telescope, they seem pretty small, I mean will an 8 inch reflector produce nice pictures, I am not looking for professional quality, just nice pictures of the moon, or star clusters etc.. Thanks

[brianb]

will an 8 inch reflector produce nice pictures,

Yes ... provided it's on a good steady mount, properly collimated & allowed to cool to ambient temp (will take 1 - 2 hours) a 8" Newtonian with reasonable optical quality will beat the very best 4" or 5" apo into the dust, even though it costs a lot less (and probably weighs about the same).

The only thing it will not be so good at are wide views of e.g. M31 & M42 - you need a short focal length for those, a small apo with a short focal ratio, a field flattener and a good quality 2" focuser is a better tool for "wide angle" DSO imaging.

[ollypenrice]

''a 8" Newtonian with reasonable optical quality will beat the very best 4" or 5" apo into the dust, even though it costs a lot less (and probably weighs about the same).''

An intemperate claim, I think. Here are some (suitably dusty!) images taken with small apos (3 to 5 inch) and all I do is ask Brian to link to images taken with 8 inch Newtonians which do blow them into the dust.

Here are M42 and the Iris by Tom O'Donoghue with a four inch Tak apo. I'll also throw in my own effort on the Cocoon (3 and 5 inch apos) as well so as to stand up and be counted! As far as I am aware the only way to beat an apo in the sub one metre focal length area is with a highly corrected Newtonian like the Tak Epsilon or Orion Astrograph and for me these give a result which is more 'diferent' than 'better.'

Olly

Flickr Photo Download: M42 RGB improvement

Flickr Photo Download: Iris Nebula NGC7023

[ollypenrice]

PS Perhaps I should have chosen some images taken with longer focal length apos, since you do concede, Brian, that the Newtonian has its limitations in widefield. Since most cmmercial 8 inch Newts have a focal length of around, or just over, a metre images from 5 inch apos with focal lengths of around a metre would be a better comparison. My apologies. So here's one taken with an inexpensive Meade 5 inch triplet. (F=950mm)

If the original poster is aiming to use a DSLR, though, it would be a terrible shame not to make the most of that large chip, surely? The cheapish Meade 127 will give pinpoint stars out to about a 15mm square, after which it will need a flatenner. The excellent Skywatcher 120 doublet will do likewise, I gather. I've no first hand experience of them.

However, most imagers would urge newcomers to stay short on focal length to start with. The ED80 has been the way in for many and really takes a bit of beating in my view. The FLO offer on the Celestron is again available to those of you with UK addresses so I'd grab while you can!

Olly

[acey]

But then, people these days falsely accuse reflectors of having poor light transmission, then cripple their scopes (by at least 0.2 magnitudes) by using exotic multi-element eyepieces containing huge amounts of glass.

I'm with you on that - in eyepieces I generally want light transmission more than field of view which is why I stick with Plossls.

But regarding the reflector/refractor comparo, the real issue is contrast, with the reflector design inherently being more prone to light scattering and contrast loss.

Hence I agree that on planetary views it's quite possible to have superior performance from a refractor of smaller aperture, compared with a reflector. But the larger aperture will always win in terms of limiting magnitude, hence wins for deep-sky.

All of which is getting a bit off-thread. To someone looking to get a scope and wondering whether to get a 5-inch or 6-inch reflector, I'd say go for 6 because if you get 5 you'll only wish you'd gone the extra yard (sorry, inch).

With 6 inches of aperture you can theoretically see 0.4 mag fainter than with 5 inches. An experienced observer would see a significant difference on deep-sky objects, a less experienced observer would see very little. But depending on the scope design, you'd possibly find that the central obstruction of the 6-inch is proportionately lower, and this could potentially give it better contrast than the 5-inch. The larger aperture also has inherently better resolution (if both mirrors are properly figured) though again the difference is not huge.

As to the original question about price difference, I don't know.

[Macavity]

POTENTIALLY, smaller mirrors might be spherically ground, rather than parabolic? (Worth checking the small print?)

In the PAST, maybe 4" reflector could be deemed "satisfactory" as the former. Or does that simply show my age?