How good is this stuff in optical terms?

[russ.will]

Just wondering out loud but, how good is say a 127MAK or 200P?

Back in the day when I worked my college holidays in photography shops, I used to own Contax cameras with Zeiss West lenses and the build and optical quality was beyond reproach. There were also some serious optical bargains to be had in the way of lenses like the Sigma 400mm/f:5.6 APO that frankly, was embarrassingly good value at £189 (circa 1990) in its manual focus variant.

Bung one of these on an OM to 4/3s adaptor and you have an 800mm (effective) f:5.6 solution. How would this compare for DSO imaging to the current crop of budget scopes?

I'm asking because I'm going to buy a SW 200P/HEQ5 with a long term view to imaging DSOs (once I've learned the considerable ropes of the basics), but wonder if piggy-backing such a camera/lens combo may offer a serious alternative.

Russell

[Carbon Brush]

H

i Russell.

Interesting thought. I don't own any long camera lenses so can't say 'this is what I found'.

I would though add this comment.

Could we buy old lenses 2nd hand, that are much more handleable than huge scope tubes and get comparable performance? If the answer is yes, then why are so few astronomers doing this, and so many of us sticking with big scopes?

Over to those with 'long camera lens' experience.....

[Ad Astra]

Hi Russ,

mirror optics are generally defined by the maximum error (deviation from a perfect parabolic curve) in wavelengths of light from the middle part of the visual spectrum (~5500 angstroms = 1 wavelength).

It is generally considered that if you have a mirror that is 1/10th wave (the Airy limit, I believe) that the mirror will perform essentially perfectly for visual work. Many manufacturers make claims that they have "1/10th wave optics", but each mirror needs to be tested by the Ronchi method (or similar) to determine its 'figure'.

1/10th wave optics also assumes that this is the maximum deviation over the surface. Some mirrors can be essentially perfect over most of the surface but may have a flaw in one area (zone problem) or the edge or middle may deviate from the perfect curve. As strange as it seems, glass is also flexible, and if you put too much tension or compression on a mirror during collimation, you may warp the mirror out of shape and create astigmatic error. Fortunately, this corrects itself when you back off the pressure on the collimation screws!

Camera lenses are different. Like a refractor, they pass light through their volume in stead of bouncing off a precisely curved surface. This means that lenses must also correct for color error (chromatic aberration). Lenses require multiple elements of different types of glass (different index of rerfraction) to correct color error. Two element lenses correct color fairly well and are called achromats (this includes most mid-range refractors). Lenses with 3 or more elements go farther and correct color error across a broader range of the visible spectrum (some correct beyond into the infra red and ultra violet ranges). These beauties are calle Apochromats or 'Apo's'.

I hope that helps.

Dan

[brianb]

The big difference between camera lenses & scopes is that (with a few exceptions) scopes are designed to be ultra sharp on axis without much care for what goes on away from the centre of the field, whereas camera lenses have to be reasonably good over a wide field.

In olden days, "long tom" tele lenses were essentially just doublet or triplet scope objectives. Nowadays we have sophisticated multi-element refractors with field flatteners, more or less equivalent to camera lenses & just as good over a fairly wide field. So the distinction is being lost.

However, it still follows that a camera lens does not have to be as sharp on axis as a scope objective does ... the film / sensor just does not have the resolution to cope with the difraction pattern made by a "perfect" objective, unless the working focal ratio is in excess of f/20. For this reason, camera lenses fitted with an eyepiece usually perform quite badly.

[russ.will]

<Edit> BrianB replied whilst I was responding to Dan, so my response should be considered in that light.

Thanks for the detailed reply. You clearly have a greater understanding of the limitations of optics than I.

I understand that mirrors are free of chromatic aberation, although they suffer other unique issues that impact image quality such as physical distortion, so they're not a total answer. I guess the users attention to detail is as much a limit as anything.

I also understand the difference between achromat and apochromatic lenses, but my point was how do the modern telescopes compare? I realize that in photography the pixel pitch of modern sensors has really placed a spotlight on optical quality that legacy lenses meet to a greater or lesser degree.

I realize this is a bit of an apples and oranges question, but the Sigma in question still stands up in optical terms when subject to the resolution of 12mp+ camera sensor; It's a solid set of optics and when attached to a 4/3 sensor camera, even less questions are asked of it as the sensor is half frame thus it is less demanding of resolution/aberations at the edges of the lens field of view. This of course places double the demand on centre of the field of view....

I note that there are occasional bargains like the Sigma and yet Apo scopes of similar focal length/aperture seem very expensive, whilst Newtonian/Maks seem cheap in comaprison, even if we factor in inflation with the passing of time.

In other words; Why do people buy Apo refractors?

Russell

[russ.will]

...However, it still follows that a camera lens does not have to be as sharp on axis as a scope objective does ... the film / sensor just does not have the resolution to cope with the difraction pattern made by a "perfect" objective, unless the working focal ratio is in excess of f/20. For this reason, camera lenses fitted with an eyepiece usually perform quite badly.

Okay... and yet DSLRs are offering up 12mp+ resolution which ask serious multi-megapixel questions of lens performance even if we just consider performance in the centre of the field.

Given the comparatively low resolution of sensors applied to astro photography, what are astro scopes doing different? Come to think of it, I was stunned to see the Kodak sensor used in astro imagers. The KAF-8300 wasn't even liked as a sensor in my ancient Olympus E-300 which I dumped as soon as I could - It's low light noise was hideous and yet here it is in the ultimate low light arena when it's about four generations out of date...

Russelll

[Ad Astra]

<Edit> BrianB replied whilst I was responding to Dan, so my response should be considered in that light.

In other words; Why do people buy Apo refractors?

Russell

Hi Russell,

Every optical design is a compromise of some sort or other. Modern optical design has run leaps and bounds over what we had even 10 years ago. The best astrographs today Richey/Cretain designs, Dall Kirkheim (forgive my poor spelling from memory here!) even the corrected Schmidt Cassegrain designs like the Celestron Edge HD and Meade ACF scopes have tremendous flat fields with sharp, pinpoint stars out to the edge of a full size sensor field.

That said, refractors have no obstructions, and when the Apo is well designed, excellent glass (absolute witchcraft there, mate) and properly baffled, the contrast and resolution is beyond anything you can get with an obstructed design like most reflectors offer. The little diffraction spikes that are common on many photos may be charming, but they are optical errors. Stars should be round or pinpoints! There are off-axis, unobstructed designs like Schiefspeiglers and stuff - but nothing you can buy commercially.

My big refractor is a 133mm f/12 - and it is a one-of-a-kind instrument. (One of 23, actually... ) I can get as many moons of Saturn with my Apomax as I can with the school's C-11, and on the moon - the Apomax will show you detail that the C-11 never will simply because of the superb contrast and optical accuracy. Put them both on a nebula or a galaxy though, and the C-11 remains completely dominant.

I love my Big Frac!

Dan

[ollypenrice]

I'd hesitate to say why people buy apos but I can tell you why I have bought some...

Imaging:

- They are absolutely hassle free. No collimation, best resistance to dew, snap focus, easy on windy nights, high mechanical integrity (orthogonality etc).

-Excellent optics, flat field (the expensive ones!) even illumination, no diffraction spikes. (I don't like them aesthetically, especially tiny square stars in Milky Way fields and I like to combine images from different focal length scopes.)

Visual

-They can't compete on light grasp but nothing can compete with them in terms of giving a beautiful image. I am reviewing a mid priced refractor at the moment which gave a perfectly circular diffraction pattern at the very edge of a Nagler widefield EP. Amazing. You get a gorgeous, contrasty, pinpoint starry view of the night sky. I just love it. This is a science-free observation but totally factual. (I do love it!)

I fully accept that to get F4 at a metre or or more of FL refractors are out. Pity!

Optical quality of modern cheapish scopes from China? Extraordinarily good. You have to dig deep into your pockets to beat it.

Camera lenses? I don't know about earlier ones but I have had lots of fun using a Canon EF200L on a CCD camera. The lens is fast, sharp, flat and evenly illuminated. At the exotic f ratios involved focus is hyper-critical so I bought a micro-focus do-dah to get more control. Haven't tried it yet.

Couple of lens shots.

http://ollypenrice.smugmug.com/Photography/Widefield-images-including/CONE-ROSETTE-HARGB/1178011112_8HpnY-X3.jpg

http://ollypenrice.smugmug.com/Photography/Widefield-images-including/HA-COMPARATOR/1182344610_T6hNK-X3.jpg

Olly

PS Dan, it was love at first sight when I saw your refractor... Gorgeous!

[russ.will]

Thanks for the replies chaps - Massively informative and interesting as ever. I may hunt down the Sigma in the future just for the fun of it, after all it's not like you cam loose money on something that's already 15-20 years old!

Russell