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Bahtinov focusing questions...

NIGHTBOY

By NIGHTBOY
in Getting Started With Imaging

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NIGHTBOY

NIGHTBOY

Posted
Posted

Just ordered a 200p F5 mask for my skyliner 200p.

Couple of questions...

Could someone please show me a pic of what it looks like when focused?

Was star is needed to focus on? Using a dslr its going to difficult (but not impossible) to get the far away small/dim ones, maybe one like Altair???

If I set the focus just with my scope and dslr then add a barlow will I need to re-focus???

Think that's it. :grin:

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ronin

ronin

Posted
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What you see if a symetrical 6 pronged star, 3 up and 3 down, don't have a picture.

Key is to get it symetric.

Pick a bright star and use that one, so pick one that sits fairly much alone, Arcturus, Deneb, Altair, Vega, Aldebaren not sure what others. Rigel and Sirius are possoble if they are high enough.

Ignore the smaller fainter ones, just work on the bright one.

If you add the barloew then yes you need to refocus the set up with the barlow in place.

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RikM

RikM

Posted
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Here you go.

I use Bahtinov Grabber as well, works a treat. I either use a bright star and short exposure or a fainter star and longer exposure. Any brightish star should work as long as it's not a double.

post-5915-0-99275600-1386526881_thumb.jp

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majapala

majapala

Posted
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I think you will be pleasantly surprised at how unbelievably simple and straightforward using one of theses masks to achieve pinpoint accurate focus is, even visually without the use of a computer. Just make sure the middle diffraction spikes are spot on centre and there you have it.

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WaveSoarer

WaveSoarer

Posted
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I have one for a very similar setup. It's very easy to use. After a couple of minutes you should be focused up and ready to go. It's very easy to see when you're not in focus and it's just a matter of tweaking until all the spikes radiate from one point.

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ollypenrice

ollypenrice

Posted
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I use Bahtinovs on a variety of scopes but only use them as the definitve focus guide on the biggest one, or on nights of bad seeing. Having said that, they do give a good focus. They are great tools.

They throw up three cat's wiskers. The middle whisker needs to be in between the vee of the the other two as they cross. Move focus and the middle whisker will move up and down either side of the vee.

Easy answer, get it in the middle.

More serious answer, you can't. The middle whisker is always slightly offset either side of the focus star so, when you are bang on, the middle whisker is a bit low on one side and a bit high on the other. I always look out for this effect.

Olly

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ollypenrice

ollypenrice

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It's very easy with Liveview, just a tad off on this one. :grin:

bah1.jpg

Yes, the middle whisker is a tad high and I'd say that the left was a gnat's crotchet higher than the right but it's hard to tell till you have it nearly perfect.

Good post.

Olly

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sidelight

sidelight

Posted
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Just  quick thanks for reminding me (when I'm setting up) that I need to order a mask for the 130PDS, with the mask focusing is so much easier.  I tend to use short exposures in a loop as the dslr is often not sensitive to show the spikes nicely.  It also sounds like I need to check out the latest APT.  I got the email the other day that my subscription was nearly due.  If that is in there then I must renew.

Thanks all, for reminding me.

Cheers

Ross

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NIGHTBOY

NIGHTBOY

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Sounds like a sarcastic 'no' that steve!!! I was at Jupiter the other night and it takes about 15 seconds to go theough the viewfinder. With my slr set to video I can capture quite a few frames by moving the scope everytime the object goes through it

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Uranium235

Uranium235

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One thing tho, does the star need to be dead centre of the view finder when the masks on???

Depends on your optics.

If its a reflector: Focus using a centrally placed star (as that will be the centre of your corrected field)

If its a refractor: Focus on a star which is two thirds away from the centre (more important as your chip gets bigger)

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Yoddha

Yoddha

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Just to mention that the very same Bahtinov Grabber technology is incorporated into APT (even in the free version). It works both with LiveView and images (the Focusing Type Plans will not fill the disk with useless files ;))

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