Diffraction spikes

matt · #1 01-12-2005, 05:20 PM

G'day lads and ladettes

Not sure if this is the right place to be asking this?

But further to my recent posts about the drama I had with my primary.

I noticed last night, during an hour or two of relatively clear skies, that the diffraction spikes when looking at Venus and Sirius seemed particularly long.

I was using a 6mm Vixen LV eyepiece at the time, and switched it with a 6mm Skywatcher SW6 EP. Same result.

The spikes extended all the way out to the edge of the FOV. Is this normal, or could it be an aberration created by my recent probs with wiping the primary clean of faint bathroom silicon smears???? Could there still be residual material on the mirror surface exaggerating the spikes?

I'm thinking maybe it's normal, given the small FOV created by a 6mm eyepiece in my 8" f5???? But as always your opinions very welcome

cheers

asimov (John) · #2 01-12-2005, 05:38 PM

Good question Matt! I'd be tempted to say this is normal. But I can totally understand you being slightly (is paronoid the right word here?) after the bathroom sealer incident.

I don't reckon there will be any visual consequences from what happened..

matt · #3 01-12-2005, 05:48 PM

Thanks asi

I think paranoid is veeeeeeery much the right word!

I don't think there's too much damage done either. I turned the scope on Rigel last night and it's fainter companion was very easy seen. The trap in M42 I'm sure I saw a fainter 5th star. Tuc47 with pin-points all the way to the edge of the FOV. Loads of detail.

Mars was not bad. Albedo surface features popping in and out of view, and had my first view of Saturn with my scope this morn. Wow!

I'm sure the Cassini division was visible, but it was so bright I couldn't make much else out. What sort of filter do you reckon would help for visual observation of Saturn and Jupiter?

Loved your Saturn shot by the way. Great work with the refractor.

janoskiss (Steve H) · #4 01-12-2005, 05:49 PM

The brighter the object the longer the diffraction spikes. You will not get diffraction spikes from normal dirt on the mirror; you only get them from sharp edges in the light path. One is the ring around the Airy disk, from the sharp edge of the mirror itself, then there is the bright spikes from the spider vanes, more larger (but fainter) rings from the edge of the secondary, and there are still more spikes from the edges of the mirror clamps. I know it's a fair bit of self-pollution.

matt · #5 01-12-2005, 05:54 PM

thanks steve

Just hadn't noticed the spikes so long before. Was concerned I'd created a thin film of silicon crud on my primary which could be causing diffraction issues or scattering light in some way etc????

venus was so bright last night. very pretty too, in its current phase

asimov (John) · #6 01-12-2005, 06:58 PM

Hi again Matt. I'm no filter expert....but on saturn I don't usually use anything. On jupiter I've gotten some fantastic contrasty images from my reflector by using a 3 holed aperture mask (the 3 holes are round rather than triangle on a hartman mask) sometimes combined with a contrast booster, sometimes not. The aperture mask also eliminates those diffraction spikes caused by your spider! Oh...If you have a 4 vaned spider you would use a 4 holed aperture mask.

avandonk · #7 01-12-2005, 07:05 PM

Any contamination of your optics if random will only lower contrast.Janoskiss said it already.The brighter the object the more noticeable the diffraction spikes.

Have a look at some Hubble images the length of the diffraction spikes is proportional to the stars brightness.

This is why I started playing with expensive large aperture mid telephoto lenses,the major problem you have if at all is chromatic aberration.No diffraction spikes!Unless the lens has a small number of aperture leaves.

No optic is perfect,some are just better at some things than others.
If you are wondering if a lens has no diffraction,it does due to the aperture.It is distributed circularly.The light intensity that should be in the Airy disk ends up in the diffraction rings (Bessel functions) and so lowers contrast.
Thats why the Dob enthusiasts are correct,the bigger the aperture the better the contrast and resolution.So if you make three or four holes make them as big as you can without exposing your spider.

Bert

janoskiss (Steve H) · #8 01-12-2005, 07:27 PM

Filters that reduce the brightness can help a lot without sacrificing resolution. Darren (WombatInSpace) showed me some in his 12" Dob and they worked great on Mars. The filter got rid of the diffraction spikes and glare and I could see detail on the surface even better than without it.

matt · #9 01-12-2005, 07:35 PM

Thanks guys.

Like I said, brief observations last night suggested all was well, but it's reassuring to get a 2nd...3rd...4th etc opinion from friends

Still not sure about the filter/s though.

Am in need of advice on a few filters which really aid in visual work on planets, particularly saturn and ol' joops

ballaratdragons (Ken) · #10 01-12-2005, 07:40 PM

Matt,

Here is a site that will help you with what filters help with what objects.

http://sciastro.net/portia/advice/filters.htm

asimov (John) · #11 01-12-2005, 07:44 PM

Theres obviously differences of opinions between aperture masks & filters. I look at it this way: I'd rather spend 2 hours making an aperture mask out of card board (which doesn't interfere with the planets natural colour & free) as apposed to buying some filters....but that's just me.

janoskiss (Steve H) · #12 01-12-2005, 07:47 PM

I don't know what filters were used at Snake Valley star camp. Try PM-ing Wombat if he doesn't reply to this thread. I'm sure he would be happy to tell you all about it.

Otherwise try contacting a competent dealer. I found that AOE are very good at explaining things and very patient with persistent questioning. I just looked at their filters page. I think what you might be after are the Neutral Density and/or the Polarising filter.

ballaratdragons (Ken) · #13 01-12-2005, 07:52 PM

Quote:

Originally Posted by asimov

Theres obviously differences of opinions between aperture masks & filters. I look at it this way: I'd rather spend 2 hours making an aperture mask out of card board (which doesn't interfere with the planets natural colour & free) as apposed to buying some filters....but that's just me.

Yeah, I'm not a filter person either John. I have the Moon filter but I keep forgetting it's in the EP after using it on the Moon and have trouble seeing anything until much later when I remember it's still in there LOL!

asimov (John) · #14 01-12-2005, 07:53 PM

I was about to ask which filter you had a look through Steve. I've found a contrast booster works good on mars. Knocks back the glare without interfereing with the colour too much visually speaking. I've also taken images of mars with the same filter on but they came out yuck.

janoskiss (Steve H) · #15 01-12-2005, 07:53 PM

A simple aperture mask with one opening will reduce resolution. You can retain resolution by using an aperture mask with multiple openings that dodge the spider while still covering the full size of the mirror. But this will reduce contrast because of more edges per unit area (more edges = more 'scattered light' via diffraction). But the end result might still be better than the unmasked view. Like Asimov said, it costs next-to nothing, so why not give it a go.

matt · #16 01-12-2005, 08:00 PM

Steve

Excuse ignorance, but which retailer are you referring to with the acronym "AOE"???

janoskiss (Steve H) · #17 01-12-2005, 08:06 PM

www.aoe.com.au

asimov (John) · #18 01-12-2005, 08:08 PM

Aquila Optical & electronics Matt..... http://www.aoe.com.au/index.html

asimov (John) · #19 01-12-2005, 08:10 PM

Ooops! Sorry Steve, thought you had gone.

RAJAH235 · #20 02-12-2005, 06:52 PM

fwiw, I find the use of good quality coloured filters works well on bringing out diff details on diff objects. Light blue for both Saturn/Jupiter & Mars, ocassionally use green/yellow.
It's really personal pref again, as well as how old your eyes are & what colours they see best. Try before buy, if poss. Do not skimp on quality.

L.