Multiple diffraction spikes

[badgerchap]

Hi guys,

Obviously the focus is teeeeeerrible in this cropped image, but what is more worrying is the appearance of two extra sets of diffraction spikes. The most obvious doppelganger is parallel to the one expected set, but then more faintly there is another diagonal set.

Is this something to do with misaligned spider vanes? I have now measured and equalised the lengths of the spider vanes, so can I expect an improvement?

I would just test for myself but I've had one clear night this week, and don't expect another any time soon!

If it helps it was a 1 min sub at iso 800 through a 6" f/4 Newt with a 6 MP DSLR (EOS 10D) Uniguided.

Thanks,

Guy

[dugpatrick]

You just need to work on the focus. The diffractions spikes will come togther as you come to focus. Also the donut stars will become pinpoint stars.

Doug

[Josh Wilson]

+1 on that. ^^

[steppenwolf]

You can actually use those diffraction spikes as a focusing aid - adjust the focus unti you get just one set of diffraction spikes in a cross formation and bingo, you will be in focus!

[ollypenrice]

You can actually use those diffraction spikes as a focusing aid - adjust the focus unti you get just one set of diffraction spikes in a cross formation and bingo, you will be in focus!

...which is how we focus the ODK14. We can't get much sense out of FWHM at that focal length.

Olly

[Tim]

Which corner of the image was that taken from?

Your collimation looks suspect, and that won't help with the additional flaring around bight stars, but it's almost impossible to decipher an image so far from focus, those flares might disappear.

There could be many reasons for the extra flaring, including bright or reflective surfaces anywhere in between the secondary mirror and the camera, or even the sides of the secondary itself.

But you need to start by getting the focus tight, getting good collimation, and take it from there. If you're still worried, post up another piccy

The most common issue with diffraction spikes in newts is that they diverge as they move out from the central star, and this is as a result of the vanes not being square to one another.

HTH

Tim

[Daniel-K]

i have just had a similar problem with my quattro , its mainly down the the spider vanes not being at equal distances to each other and being slightly twisted easy fix.

[badgerchap]

Which corner of the image was that taken from?

Your collimation looks suspect, and that won't help with the additional flaring around bight stars, but it's almost impossible to decipher an image so far from focus, those flares might disappear.

There could be many reasons for the extra flaring, including bright or reflective surfaces anywhere in between the secondary mirror and the camera, or even the sides of the secondary itself.

But you need to start by getting the focus tight, getting good collimation, and take it from there. If you're still worried, post up another piccy

The most common issue with diffraction spikes in newts is that they diverge as they move out from the central star, and this is as a result of the vanes not being square to one another.

HTH

Tim

Here is the whole pic. The star in the OP is from the right hand side, about 2/3 down

While I have your attention, has anyone got any suggestions for improving my focus? I have no liveview, and the focus for this was with a mask (not a bhatinov - the one with the two round apertures) and was still clearly waaaaay off. I have no idea how I'm supposed to make it more accurate :/

i have just had a similar problem with my quattro , its mainly down the the spider vanes not being at equal distances to each other and being slightly twisted easy fix.

Yes, I have a slight twist on a couple of the vanes, but I think I have that sorted now.

[JamesF]

Here is the whole pic. The star in the OP is from the right hand side, about 2/3 down

Eeek. I don't know what it is about the way that picture is out of focus, but it does really unpleasant things to my head.

Regarding better focusing I can think of two possible solutions. The first is just to take, say, 15 second exposures whilst you're focusing and keep going until the stars are nice and tight. The other is to use a flip mirror and fix an eyepiece so it's parfocal with the camera.

James

[badgerchap]

Ah, that's what a flip mirror is for! Always wondered haha. Excellent suggestion, thanks James. So what about a Bhatinov mask, will that not help?

[JamesF]

Ah, that's what a flip mirror is for! Always wondered haha. Excellent suggestion, thanks James. So what about a Bhatinov mask, will that not help?

You'd still need to see the image created by the mask. In combination with the flip mirror or taking repeated exposures it might well help though.

James

[badgerchap]

Thanks James, much appreciated. I'll try out the repeated exposures effort next time the clouds clear. 2014 it is then!