Optical defects gallery - an idea.

[Flying Eye]

Here's another for the gallery taken with a WO 80mm Megrez II triplet, seems appropriate really given the three sided star shapes.

Which defect does this image illustrate?

[rocketandroll]

This is an awesome idea Olly... I'll add my pics from my Newt to the pot once it's confirmed what the issue is... 90% its down to the focuser being bent.

Ben

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[ollypenrice]

OK I'll drop a line to Admin to see if they like the idea.

Olly

[Mike Hawtin]

Which defect does this image illustrate?

Triangular stars are usually ascribed to pinched optics.

[Gina]

Here's another for the gallery taken with a WO 80mm Megrez II triplet, seems appropriate really given the three sided star shapes.

That image looks fine to me. What am I looking for and what aberration is it called?

[michael.h.f.wilkinson]

That image looks fine to me. What am I looking for and what aberration is it called?

The deviation is small, but the stars are slightly triangular in shape. It is all relative. One man's cracking image is another man's demonstration of the inadequacy of his current setup or fine-tuning . These experienced DSO imagers set the standard very high indeed (and it shows in the results ).

[Mike Hawtin]

For clarification this is a pixel for pixel crop showing the defect at full size.

[Gina]

Ah yes, I can see it now. Thanks

[Flying Eye]

I may have misunderstood, but I thought the idea was to show and say what the defect is. I can see it, but I don't know how come it's there.

As a newb I'd find it most useful if it said what the defect is called as well as showing what it looks like. Otherwise it could simply become a collection of poor images, albeit ones of the highest quality in all other respects. Which is probably not quite so useful.

Perhaps I'm missing the point?

[ollypenrice]

I may have misunderstood, but I thought the idea was to show and say what the defect is. I can see it, but I don't know how come it's there.

As a newb I'd find it most useful if it said what the defect is called as well as showing what it looks like. Otherwise it could simply become a collection of poor images, albeit ones of the highest quality in all other respects. Which is probably not quite so useful.

Perhaps I'm missing the point?

No, you have hit the point smack on the head. My suggestion to Admin is that the gallery should be just for images showing identified defects. A room full of sick patients and no doctor in the house wouldn't be at all useful!

For the record, the triangular stars above are 99% certainly due to pinched optics, the lens cell squeezing the glass. This often arises and disappears with temperature changes.

Olly

[Gina]

Ah, thank you That is what I wanted to know

[barkis]

I Imagine there will be images, and or sketches of optical defects, together with an explanation of what the probable cause is.

These will possibly in the main, be refractor anomalies, and in particular, the Expensive Apochromatic refractors. Pinched optics in those instruments, would probably require a return to the supplier, and then on to the manufacturer. Simply because the fix is unlikely to be possible by the end user. Also, it would be wrong to tamper with it, as it's warranty would probably be rendered invalid.

The same could be said of Achromatic refractors I guess.

Reflectors with a pinched mirror can be a user fix, with help should it be necessary, available via this forum.

This proposed diagnosis list will be very useful, and it will draw attention to existence of the flaws that may exist in some instrument. In some cases the user may be ignorant of these flaws.

For no other reason than they don't expect them to be there.

It goes without saying though, that there has to be no doubt whatsoever over any conclusions drawn. We certainly do not want to see instruments returned recklessly.

So whilst the idea is a good one, it will need to be managed in a very responsible manner.

Ron.

[michael.h.f.wilkinson]

Collimation errors in refractors and catadioptrics could also be useful, all the more because they can be corrected by the user. We could even have three categories: refractors with identified aberrations, reflectors and catadioptrics with identified errors, and unknown (i.e. input from others desired). When the latter are identified, they can be moved to the appropriate section.

[Gina]

That sounds a good idea Michael

[ollypenrice]

Also some optical defects don't arise from the optics. There are several ongoing sagas involving focusers (I never know whether to spell that with a double s or not!!) which are not orthogonal.

I take Ron's point. We don't want to turn this into a specific scope-bashing thread (and I promise not to bang on about Crayfords!!!). It should be a strictly objective look at aberrations which have been pretty solidly identified. Being 100% certain is not the way of the world but being very confident is often possible.

Olly

[Gina]

The spell checker seems to think "focuser" is spelt with one S while "focussing" has two - seems weird to me! But I guess our language is weird

[Macavity]

I think the idea is potentially and hugely useful. Easier perhaps to show "imaging" (telescope primary?) defects than visual ones. But, for the latter, the odd non-image (diagram, description etc.) could have merit. As a beginner, interpreting "reviews": One wo/man's definition of "sharp" is different to anothers. That said, as I get older (further away from "bloomin' science") I'm more of a convert to subjectivity.

[michael.h.f.wilkinson]

Some errors are simply tracking errors, mount backlash, etc. These are not scope specific (I feel another category coming )

[Peter Drew]

Another + vote, should be most interesting. Astigmatism is going to be a good one, it can be introduced by tight optics, poor collimation, cool down time, users eye or worst of all, inherent in the production of the optics.

[barkis]

A catalogue of traceable optical anomalies and their proven causes, could become a very useful resource, and perhaps a team of experts can be recruited from the wealth of talent SGL has in it's membership.

No doubt Admin. will undertake to put that into effect as this proposal proceeds.

Ron.

[ollypenrice]

I think that mount-generated issues belong here as well since the primary aim is to be helpful rather than intellectually tidy-minded (never my strong point at the best of times!)

So, for example, the dumb-bell double stars that arise from classic backlash, or the need to specify the orientation of RA and Dec when looking at trailed stars would all help beginners.

Olly

(PS, note my crafty hyphen in dumb-bell which gets me out of deciding between b and bb.... Groan.)

[earth titan]

Sounds good to me. Tracking down faults with reflectors is always a challenge as there are so many user 'customisable' components. Would really help at them mo with my f4 reflector.. So I'll have some good photos to show once I've worked out whats causing it..

[Gina]

If you post some of the problem images here then others may have the answer to the probable cause.

[barkis]

I think that mount-generated issues belong here as well since the primary aim is to be helpful rather than intellectually tidy-minded (never my strong point at the best of times!)

So, for example, the dumb-bell double stars that arise from classic backlash, or the need to specify the orientation of RA and Dec when looking at trailed stars would all help beginners.

Olly

(PS, note my crafty hyphen in dumb-bell which gets me out of deciding between b and bb.... Groan.)

Olly, and you a teacher too

[Karen]

Optical defects can go all the way back to the eye. I had a problem with glare on bright objects which turned out to be my contact lens, put a new one in and glare had gone!