M45 The Pleiades 30-10-2017 Manche, France

[The Thing]

A short exposure experiment with a misty sky and first quarter plus Moon on the other side of the sky. I'm pleased that the short exposures have kept the stars and spikes under control. BTW can anyone tell me what mirror adjustment will fix the double converging spikes? I have tried re-collimating several times with no apparent improvement.

124x 10s,20s,40s @iso800, bias, darks, flats.

Image date, time and location:   2017-10-30 21:00 Manche, France
Telescope aperture and focal ratio:   Teleskop Service TS1506UNC 6" f4, TS KomakorrF4
Camera and filters used:   Canon1100D defiltered, Astronomik UV/IR Cut Clip Filter
Processing applied:   DSS, Gimp 2.9.6dev

[Carole]

Very nice Duncan, would love to see this on a Moonless night.

Can't help with the spikes I am afraid, I have read about the problem somewhere, might be worth googling it.

Carole

[MarkS]

Looking good.  The double diffraction spike is almost certainly caused by the spider vanes.  One pair is not in an exact straight line but there's a very slight angle between them.  The angle between the offending diffraction spikes is the same angle by which that pair of vanes is wrong.

Hope that makes sense.

Mark

[Carole]

Ah, that might be it, I was thinking vanes but could not reason out the explanation.

Carole

[MarkS]

The other possibility is a twisted vane.  Either possibility is equally likely.  The way to think about it is that wherever you see a diffraction spike in your image then there is a straight edge perpendicular to that spike interrupting the optical path somewhere.

Mark

[ApophisAstros]

Looking at max res those spikes are like bhatinov spikes ..multicoloured.
Roger

[The Thing]

Looking good.  The double diffraction spike is almost certainly caused by the spider vanes.  One pair is not in an exact straight line but there's a very slight angle between them.  The angle between the offending diffraction spikes is the same angle by which that pair of vanes is wrong.

Hope that makes sense.

Mark

Makes perfect sense Mark, I was thinking of a vane as a unit creating a spike across the star but each creates a spike on each side of the star and so a pair of spike needs to be in a straight line. I hope Teleskop Service drilled the holes in the right place! Actually I found one of the vane adjusters loose yesterday when I was doing flats - may be the cause. I will check them all out and check lengths and tension next time I'm down the observatory.

[MarkS]

Looking at max res those spikes are like bhatinov spikes ..multicoloured.
Roger

That's right, true diffraction spikes will be multi-coloured.  Fake ones, added to an image for effect, are generally not multi-coloured unless the faking software is good.

Mark

[Carole]

Talking about multi coloured diffraction spikes and coloured Bahtinov spikes. 

Well with a mono camera you just get a mono Bahtinov star pattern, but when I use my Newtonian scope, once I have combined all the coloured filters, I get those rainbow diffraction spikes on the largest stars.  Still don't understand it, but it looks pretty so am happy with it.

Carole

[MarkS]

The diffraction spikes for a narrowband frequency have regular gaps in them - it's because of the pattern of constructive and destructive interference of light at that wavelength.  It's easy seen by looking at the image for a single narrowband wavelength. The longer the wavelength, the more widely spaced the gaps.  With a broad spectrum of wavelengths the gradual migration of the gaps according to wavelength is what generates changing colours.

Mark

[Carole]

Interesting.

Carole