1st Clever Question: Star 'Cross'.

[BiG JiLM]

I noticed early this Am that on a V bright star, Betelgeuse I think it was, or Venus or maybe both.. I got this rather nice star '4 cross' effect from the object a bit like one sees in a hubble pic for eg (or from fillum lights iircc, fresnels? anyway I know the reason cos Im clevererer now, that being light diffracting around the scope's 4 spider vanes, but I wonder: as nice an effect it is, can it be seen in any way as an indication/ a barometer if you wlll of the scope being well collimated? And/ or an indication of good seeing conditions? or just a flare so not much more than just a nice effect.

Just a thought- noticed for 1st time only after my 1st efforts at collimating you see.

thanks JiLm.

[John]

It's an artifact being added by the design of the scope. Inevitable where the secondary mirror is supported by vanes but, in my opinion, it would be nicer if it was not there. You need to use either an unobstructed scope (eg: refractor) or one where the secondary is supported by an optical glass lens (eg: maksutov, schmidt) to be rid of it though and newtonian scopes have many other positive advantages to balance this issue.

I'm not sure that it indicates good collimation, or bad, though.

I'm not an imager but I've heard that some add these diffraction spikes during image processing, where the scope has not added them. Can't understand that, myself but thats probably just me

[knobby]

personally, i like them, but i have them, so i have to like them !

[brantuk]

You can do a star test to check collimation but far as I know it doesn't involve diffraction spikes. What you do is focus on a bright star as accurately as possible then defocus slightly in both directions (inward and outward focus). When you do this you will see bright airy rings in the eyepiece which should be concentric. If they're skewed or not concentric then it's a good indication the collimation needs checking.

With a bhatinov focus mask you can check focus using diffraction spikes (also using software on your laptop/pc). See here for an explanation and the pattern you'll see: http://en.wikipedia.org/wiki/Bahtinov_mask HTH

[Rustmonkey]

It's all about opinion. I quite like the images to have the odd Star Cross on them. Adds to the overall effect.

[BiG JiLM]

I still wonder why I only saw it once, does it not only occur when conditions allow? or do you expect to see it on Newts most times you observe with this design of scope? Im not asking how to be away with it at all- I rather like it, but only seen once: as said my Q is why.

[Moonshane]

it's only obvious when observing bright (and small) objects. e.g. the moon does not generally show the effect but very bright stars like Vega will as well Jupiter and Mars but not usually Saturn or the outer planets.

in a way it can show you that your vanes are lined up correctly as if you see four they are and if you see more, they are not. not something to worry about though. another way to 'remove' them is to make an off axis aperture mask - in fact you will have one built into the end cap most likely. this works OK but will seriously reduce aperture, brightness and resolution, albeit increase contrast and reduce glare. it's better with larger apertures e.g. my 16" dob.

[BiG JiLM]

Aha thanks Moonshane- yes I did notice it when I switched to Venus the smallest object I was viewing and Betelguese. Quite like it tbh. Btw Ive yet to trawl your 16" dob build.. but help me cut to the chase- how goes it? has it blown your noggin/ as expected etc?

[ollypenrice]

Although it would be hard to detect at the eyepiece, a doubling or slight separation of the spikes can be used by imagers to see that they are not in focus. In fact I use the diff spikes to focus the 14 inch because I've found it's the best method. Other defects can be revealed in images by strange effects on the spike pattern but I doubt they would ever show visually. It is possible to used a curved secondary support instead of a spider. This produces a smeared diffraction effect which is supposedly invisible.

Olly

[Moonshane]

Aha thanks Moonshane- yes I did notice it when I switched to Venus the smallest object I was viewing and Betelguese. Quite like it tbh. Btw Ive yet to trawl your 16" dob build.. but help me cut to the chase- how goes it? has it blown your noggin/ as expected etc?

it's the best thing I ever did in terms of astronomy. everything looks better even in bad LP and at dark sites the views are just amazing. the Orion Nebula looks virtually photographic (no major colours though like in pics obviously)

[BiG JiLM]

Although it would be hard to detect at the eyepiece, a doubling or slight separation of the spikes can be used by imagers to see that they are not in focus. In fact I use the diff spikes to focus the 14 inch because I've found it's the best method. Other defects can be revealed in images by strange effects on the spike pattern but I doubt they would ever show visually. It is possible to used a curved secondary support instead of a spider. This produces a smeared diffraction effect which is supposedly invisible.

Olly

Thats interesting I'll test that idea when I focus- may be a useful trick to save some faffing/ time. And 'Diff spikes' is a far more experienced expression than 'star cross'. Great- It will make me sound more impressive.

Cheers Moonshane- glad the MonsterDob is doing the biz, must've been hugely satisfying to focus your 1st object- I can only equate to s'thing like getting the 1st notes from a guitar valve amp build, thats pretty special.

[Laowei Tony]

Hi BiG JiLM,

This youtube video explains it perfectly -

:-)

[paul schofield]

I can only equate to s'thing like getting the 1st notes from a guitar valve amp build, thats pretty special.

Aah the sound from my first Vox valve amp....wow. Anyway just my 2 pennies worth diffraction spikes are par for the course with newts and they can add a certain charm to certain images, but given the choice, I could live without them. Don't think stars should be crosses.

[Skuber]

I can attest to the fact that when your scope is not perfectly collimated, without a coma corrector, that you still get the diffraction spikes.

They are not always in the precise centre of the star when it's out of collimation.

I wouldn't recommend using them as a test of collimation accuracy

[BiG JiLM]

Ah ok- was just a hunch. I will go with the focus into and out of object to check as a basic (primary mirror) collimation test.

Interesting that if one is nerdy enough one can tell precisely which of the world's big-gun scope the image has been taken through, IE the Keck's 6 vanes! that's proper astro-nerdy-nuts.

[66mikeg]

Only costs about £2k to get rid of them, I like the simile with the Vox tube amp though ( Marshall valve driven bass amp sound pretty special as well)