over corrected ??

[beamish]

Here's a pic, approx' 800x so 100x per inch.( way over what I would ever use-300x max !)

Is this an example of over corrected optics and if so is that it? or can something be done about it? is it a problem with collimating or production or am I just becoming a little obsessive ?

Thanks, Karlo

PS the interferences from the circles are known about , and the dark banding across the bottom is a telegraph wire !

[barkis]

Is this related to the (Glow) you mentioned in an earlier post Karlo?

That's a hell of a magnification for a star test:D

I would have liked to see a more defined edge to the disc.

Are you inside or outside focus?

Ron.

[beamish]

This was racking the focuser away from the scope and towards focal point. looking at Arcturus this am

Karlo

[barkis]

Karlo, have you always had this glow you described, irrespective of whether the seeing was good or bad. Or has the problem recently developed.?

[beamish]

No, it's recent ( it might still have been down to dew!) But of course now it's a case of getting the scope collimated again. I have learnt a great deal from this experience !! I also understand from the little I can find on 't web that sometimes mirrors are figured to be overcorrected/undercorrected.

Bearing in mind that primary is a spherical mirror, this may be the case. At least we're having a good run of clear night's to keep checking.

I was more concerned about this last night, today in the cold light of day I'm thinking that I have been very happy with the quality of views, so as long as the optics are aligned/collimated then I'll leave well alone.

Ron, in your experience would you say that the optics are happily aligned judging from what you see ? What would be your thoughts on seeing this result ?

[barkis]

The concentricity of the rings in the disc are almost perfect Karlo, and considering the very high power involved, very good indeed.

Collimating the Cape is something I know nothing about, except to say, as with any other reflecting scope, it has to be perfect, especially at low f ratios. The added lenses in the Cape, would dictate that accuracy in alignment is even more essential.

Good luck anyway mate.

Ron.

[beamish]

Thanks Ron. Collimating the Cape appears to be something hardly anyone but P Wise is entirely familiar with ! However I think I've done okay, I just can't see how I might improve on this .

Given the fact that I've had the front plate off and the mirror cell out, the mirror completely way off collimation too, it's frustrating ,yes, but doesn't scare me anymore !

Cheers Ron !

[Merlin66]

Did you run the earlier results through Win RoddierV2.2???

A similar image ,closer to focus with a 3rd mag star would tell a lot more.

[beamish]

Dya know I had completely forgotton about that ! Cheers Merlin will do !

[Vlad]

This looks poor to me. Take a comb from the house, fine teeth if possible. Focus the scope on a bright star. Remove the eyepiece. Put the comb up to touch the eyepiece opening, and look through the comb. You may have to rack in or out until you see some dark lines. These should be dead straight. Any deviation is big a problem. Let's know how you go on.

Phil

[MartinB]

This is very interesting Beamish. Sorry I haven't a clue about optics other than to say the scope looks beautifully collimated. Is over correction when the inner rings are fatter than the outer ones?

[beamish]

The reason I asked/was concerned Martin, is due to the lack of a ring at the outer edge at the same intensity as the inner most ring.

Although I'm now convinced the bmp shown above and my concerns has alot more to do with the settings of the cam and the extreme high power mag I was using-we'll see.

I am becoming A *al about this I think.

Vlad, with respect I don't have any real worries about the quality of the mirror, given it's previous owner and it's history and of the views I've had thru it (excuse the lack of detail) I have since re-collimated it !!

I made a sight-tube from an ally cap from an AE 4x mate and the body of a longish barlow. The underside of the AE cap was polished up to a mirror finish and fits nice and snug into the barlow, which in turn fits snugly into the focuser and if you remove the AE logo from the top you'll find a 2mm hole PERFECTLY centred hee hee !

Having got everything lined up text-book fashion ,I then went back to focus upon the 1/2" stainless steel bearing at the end of the garden. Initially I was a little diappointed with the de-focused image, but, with just a smidge of alteration with the primary collimating screws (and I do mean only a smidge !) the resulting airy rings were , well, very good indeed.

Interestingly when I then I had a wider look thru the focuser the generally accepted view of everything being perfectly aligned,was not the case-the secondary was slightly offset. There I've said it !! Offset-my scope was bound to have it wasn't it ??

All I need now is Arcturus to become available or some sun to fall on my ball bearing and I'll get the w/cam out.

I've also been looking into getting the avi's required to run thru winroddier2.2. This will be interesting.

I'm really really don't think this going to be improved upon.

Karlo

[Vlad]

The ball bearing is good, but it won't show you spherical aberration in daylight. If Arcturus has a feint halo of light around it, double check with the comb. Get about three lines. No harm in checking this.

Cheers,

Phil

[beamish]

Okay , seems fair enough,though if it is there it's not like there's owt that can be done. the combination of the two lenses either side of the secondary are designed to negate any spherical aberrations that may occur in the primary.

Here's one of Regulus in focus 2sec, though it's looking reasonably fair to have a go later tonight.

[Vlad]

Regulus looks pretty fair, but 2 sec is a bit short to show a feint halo I suppose.

The first photo showing the rings are not the true diffraction rings of intra and extra diffraction focal rings though. They look more like Newtons rings, or rings of interference when two surfaces that almsot match are placed together. I am confused by this. Can you give a full story of what you are doing? Whatever, please give it the comb test, rough and ready, but most informative.

Phil

[beamish]

In a nutshell Phil, I had been putting off opening up the scope 'cos I was petrified of not being able to collimate it again. ( I tried once before and failed resulting in a trip to Nth Wales to visit P Wise ). However there was a lot of crud sitting on the inside surface of the flat optical glass which was very irritating and very noticeable.

Anyway having registered the various screws that hold the front glass cell and the mirror cell, I set to. I thought that since I was opening it up I may as well place a centre mark on the primary (a reinforcing ring).

I put it all back together and then took some piccies of Saturn-

http://stargazerslounge.com/imaging-planetary/77771-saturn-b-w-colour-31-03-09-a.html

There was a slight area around some the planet that looked abit like glow. Could've been dew but I felt it more likely mis-collimation.

The image at the top of the thread is taken at high mag but also with little/no gain and longish exp in order to capture.

[Vlad]

I have mentioned this glow already. If the system has spherical aberration, planets are likely to have a halo of light around them The moon will show it too, as if there is high mist. The planetary/Lunar imnages will otherwise look great. There is one sure method to discover spherical aberration, and that's with the fine tooth comb. This is more than well worth a try. Collimation looks good, not collimated shows an elliptical shape, not circular as yours is. PLEASE, please let me know about the comb. I am aching to see the results. I sincerely hope the lines are dead straight right to the very edge without a hint of bending anywhere.

Phil. (Need puting out of my misery!)

[beamish]

Iwill I will, as soon as I can get back out to my pier. I've been extending the paved area around the pier today, just finished and reasonably happy with it.... anything would have been an improvement really, given I was trashing parts of the lawn and stepping onto flowers/plants etc previuosly Hey even the missus liked it !! can't be too bad then ??

Karlo,

PS latest planetary images show no halos

These are the latest test images and I'm pleased with them, the lines, just as a reminder, is a telegraph wire X-ing the path, the primary has since been tweaked a little more, so the rings are now far more concentric.

[Vlad]

Thanks, again and again, for putting me out of my misery!. Those diffraction rings look pretty good to me.. I noted you had them intra and extra focal. Great stuff! Poor old souls such as wot I is, live, nay, thrive, on this stuff.

Great news, you got a good scope there.

Phil:)

[beamish]

Vlad, I've done the comb test and everything is fine, parallel lines.

Karlo