Which Canon DSLR?

[DRT]

...and now Polaris and some very faint friends...

ISO-400, 60 second exposure.

Does anyone know what the radiating lines from the bright star are caused by?

[nigelbeaumont]

Diffraction pattern from the leaves of the aperture diaphragm in your lense

[DRT]

Diffraction pattern from the leaves of the aperture diaphragm in your lense

Thanks. Is there anything I can do to reduce that effect?

[Alien 13]

Thanks. Is there anything I can do to reduce that effect?

The best way is to make an aperature mask for your lens rather than using the internal iris the way I do it is to use a very cheap filter off e-bay remove the glass and add in the mask made with the plastic from a DVD case.

Alan

[DRT]

The best way is to make an aperature mask for your lens rather than using the internal iris the way I do it is to use a very cheap filter off e-bay remove the glass and add in the mask made with the plastic from a DVD case.

Alan

Sorry, Alan, I don't understand that. Is this a mask on the front of the objective of the telescope or on the camera?

What is the "internal iris"? Is this getting into "astro modding"? Which I don't want to do as I want the camera for normal use as well as astro.

Sorry for the dumb questions - I'm very new to this stuff.

[Knight of Clear Skies]

What is the "internal iris"?

Hi, this wiki article should help. This is what the camera iris looks like:

It's not circular so the edges cause diffraction spikes on stars. 

[Alien 13]

An apperature mask can be fitted to the front of a scope or a camera lens it helps with the control of star bloat and coma issues its basically a ring with a circular hole in it.

Alan

P.S thanks Knight for posting that link for the description of an iris.

[Tinker1947]

You can put a filter reducer on the front of the lens i have one for my 100mm its a 58mm filter size and the reducer takes it down to 52mm this should remove the spikes from imagines, as it only arrived yesterday not had a chance to try it....i normally stop the lens down a bit to get some Depth of Field so focus on a bright Star, then use F2.8 this gives the focus point some front and back focus, it does help to have a fast lens when doing this.....

http://www.ebay.co.uk/itm/271759512920?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

[DRT]

I think I might have confused things here.

The advice above (gratefully received!) seems to relate to taking astro pictures with a camera and a camera lens. The pictures above were taken with the camera stuck in the back of a SW ED100 Pro with no camera lens involved.

If I put the lens cap on the front of the ED100 and take the centre cap off would this have a similar effect? The aperture in the end cap is 52mm.

[Alien 13]

If I put the lens cap on the front of the ED100 and take the centre cap off would this have a similar effect? The aperture in the end cap is 52mm.

The scope should not need one I was assuming that the 2nd image was done with the camera lens.

Alan

[DRT]

I just updated my previous post as you typed yours, Alan. I just realised we were talking about different things :-)

So what is the cause of this effect when using a telescope?

[Alien 13]

The diffraction effects are strange for an ED refractor I can only think of pinched optics if the scope temperature is not in equilibrium with the surrounding air or something in the lightpath that is causing internal reflections.

It would be worth looking down the tube for any shiny bits and/or none of the internal baffles (if fitted) have shifted.

Alan

[DRT]

Thanks, Alan, will take a look. It's the older gold tube version and not in the best nick so it could well be the problem. I'm not overly worried as my objective is to take quick snaps rather than serious imaging so just looking for things I might be (will be!) doing wrong rather than thinking about building an ideal imaging set-up.

Thanks for your help.