Bahtinov Mask and Planetary Imaging

[Altomar718]

Hi All

Can anyone advise me on Bahtinov Masks and Planetary Imaging? - I know that the mask cannot be used for planets as they are supposed to be used for focusing on stars, but I wondered if the scope was focused on a nearby star and then the scope slewed (with Synscan) would that work and improve focusing on the planet?

Thanks

[Freddie]

Yes, that's exactly what I do.

[JamesF]

There are SGL members who have said that using the mask on the target itself will work so that might be worth a go. I've never tried it myself.

James

[Freddie]

It will only work on the planet if it is very small. If you have much of a disc at all (which you should have for planetary imaging) it will not work and you will have to focus on a local bright star.

[Spikey]

As Freddie says Bahtinovs tend to work better with a 'bright point' source of light such as a star.

I've tried using a Bahtinov to focus on Jupiter and Saturn with no success at all.

You've already hit upon the best solution of using a star with the mask which should get you an excellent focus.

Personally I prefer to locate, centre and sync the planet and get it in visual focus then it's off to a nearby bright star, pop the mask on, refine the focus, take the mask off and slew back to planet.

[Astrosurf]

I image with a webcam and, even with gain and brightness right up on Sharpcap, I can't see a bright star. Mind you, I'm using a 4x Barlow! I have to use my awful eyes as best I can!

[michael.h.f.wilkinson]

I have a Bahtinov mask for my C8, but do not use it for planetary imaging. I can see the diffraction spikes of bright stars (Aldebaran is positioned nicely for Jupiter now), but I found moving the scope around is a hassle. I prefer to tweak a while to get focus right. I might try the mask again at some point.

[Altomar718]

Interesting stuff and thanks for the replies ........ it may sound dumb - but - if you focus on a star as suggested, and then slew to Jupiter which is much, much closer, would the focus be the same as the star may be 1,000's of light years away ???

[Astrosurf]

Not dumb at all. But no, they're all so far away it won't make any difference.

[michael.h.f.wilkinson]

As I like to put it, the depth of field at infinity is infinite

Put mathematically, the distance d between optical centre of the scope and the image plane, for a given focal length f and distance to target D is

d = f D /(D+f)

At huge distances D this simplifies to d = f

For example, if D is a million kilometres (close by by astronomical standards), and f is one metre, the change in focus needed is about one nanometre. I doubt the average focuser would manage that.

[Spikey]

Interesting stuff and thanks for the replies ........ it may sound dumb - but - if you focus on a star as suggested, and then slew to Jupiter which is much, much closer, would the focus be the same as the star may be 1,000's of light years away ???

It's a question I have asked before now but as Alex says they are both far enough away for it to make no noticeable difference to the focus and Michael has a more cerebral explanation

[Altomar718]

As I like to put it, the depth of field at infinity is infinite

Put mathematically, the distance d between optical centre of the scope and the image plane, for a given focal length f and distance to target D is

d = f D /(D+f)

At huge distances D this simplifies to d = f

For example, if D is a million kilometres (close by by astronomical standards), and f is one metre, the change in focus needed is about one nanometre. I doubt the average focuser would manage that.

Eh? - I have only just got into this astronomy lark and failed my 'O' level in maths Ha! Ha!

Thanks anyway

[Astrosurf]

Wow Michael! Impressive. Don't unnerstan tho!

[Altomar718]

I went for the first time to the meeting of local astronomy club here and they had a guest speaker who was talking for an hour about radio astronomy - I didn't understand ANY of it until he mentioned he had got a shed to keep his gear in and I have got a shed as well !!!

He was talking about baseline inferfeometry and put formulas up like P=I(p d²/4)(1/r²)(p D²/4)

Some clever people out there or I am really thick !!!

[knobby]

The best astronomy club is this one. ... Star gazers lounge

[Astrosurf]

Some clever people out there or I am really thick !!!

We're all good at some things and not at others. Maths HATES me! But I consider myself good at art, music and (when I was younger and fitter) dance. It all balances out.

Edited March 7, 2013 by Astrosurf

[tingting44]

i have the mask and tbh i get better results focusing by eye, many people swear by the mask's, i guess its what ever suits you really, planetary focusing is not too bad, i can see the mask being very helpful for DSO imaging as you can not see the DSO's to grab focus and have to use a tiny star (thats where the mask comes into play) but i just find with planets, there so big compared to stars that its just easier for me to focus by eye for planets (i also enlarge the planet via live view on the laptop either via my webcam or dslr and that works nicely for me

Martin

[badgerchap]

I'm doing algebra and calculus as part of my degree - the formulae are the most evil things you have ever seen. Or in physics we have y(x)=w^2A^2sin(kx-wt+p)-w^2A^2cos(kx-wt+p). Yuckyuckyuck!

[tingting44]

how do u guys even write these formula's!! i cant even see them characters on my laptop! loool let alone understand them :D

[leelee970]

I image with a webcam and, even with gain and brightness right up on Sharpcap, I can't see a bright star. Mind you, I'm using a 4x Barlow! I have to use my awful eyes as best I can!

I'm surprised you don't see any bright star Astrosurf- are you using the SPC900?

I usually set mine to "auto" to see the star, then pop on the mask, focus, take off mask (sometimes I forget) and slew to the planet. I'll use anything from x3 barlow to x5 barlow.

then it's just a case of taking "auto" setting off and away I go.

Admit I don't use Sharpcap, but I shouldn't think it makes any difference.

Lee

[Astrosurf]

I'm afraid I tried it and failed. I'll have another go. What's this auto setting?

[michael.h.f.wilkinson]

how do u guys even write these formula's!! i cant even see them characters on my laptop! loool let alone understand them :D

Writing equations? Easy, use LaTeX. You can always resort to proof by intimidation:

http://iwi.eldoc.ub....esWilkinson.pdf

For more serious stuff see with loads of math (and a picture of M81 thrown in)

http://www.cs.rug.nl...sonPAMI2011.pdf :evil:

[leelee970]

I'm afraid I tried it and failed. I'll have another go. What's this auto setting?

I’ve just had a look at Sharpcap (lucky I’ve my webcam here today at work).

Go to Options, then Video capture filter which brings up your camera properties. Now click picture and you will see a box labelled “full auto control”. This will give the PC full control and will pick out a bright star. Remember to deselect it once you’ve focused with the Bahtinov mask.

I use Wxastrocapture which I find easier – on there you go to video device properties and click the full auto box – simple and it’s with the other control needed so you’ll not forget the auto setting.

[Astrosurf]

That's great. Thanks for your advice.

[Jerzy_K]

I use a 10" RC GSO ( 2000mm  focal length) and a slightly modded Logitech Quickam c905 ( svbony 1.1/4 adapter + uvir filter) and I find that unless using an eyepiece, through the cam Bathinov mask does not give clear diffraction spikes i can see on my capture laptop display ( and that's  with gain and exposure on auto or not). It looks very nice in the eypiece when focused on a bright star, but cam focus is off the eyepiece focus  by some turns.  There has to be abetter way, or a better cam anyway .