Bahtinov masks - again, sorry

[dawson]

I know some people think these are great and others don't, but when using one, and this might sound like a daft question, does one use it at the end of the dew shield or does the dew shield have to come off?

 

I ask as I see they have specific slit patterns for specific telescopes, and I wondered if the additional distance the mask was from the telescope might interfere with the focus.

 

Thanks for any replies.

 

James

 

[T A WOW]

I would have thought with a Mak, Dew shield off my old bean.

Edited March 9, 2013 by T A WOW

[Doc]

I'm not an imager but I would say without dewshield.

[Sheila]

only time i have seen one used was at end of dew shield on a frac

 

sheila

[Doc]

My thoughts on this are as James said, they produce these in certain focal ratios for actual scopes. So surely if one adds say 8" to your scopes length due to a dewshield then this alters your focal ratio.

[andyboy1970]

It doesn't matter. I use a C8 mask on my 200p mainly cause i'm cheap.

Use bahtinov grabber for spot on focus.

Edited March 9, 2013 by andyboy1970

[dawson]

Andy

I downloaded the grabber programme but i just couldn't figure out how it worked and there were no instructions anywhere to be found?

James

[Brantuk]

Here you go James - you need to get this pattern when scope's in focus - it can be done with a webcam (or live view on a dslr) or I think by eye:

 

http://www.nightskyimages.co.uk/bahtinov_mask.htm

 

http://www.daltonskygazer.com/using-a-bahtinov-mask-to-achieve-focus-for-astrophotography.html

 

or http://www.youtube.com/watch?v=KnKOynYBnlI

[Perkil8r]

As far as I am aware, what ever your scope is manufactured with is how it should be used. Ie, on my old st80 it had a dew sheild as supplied from the manufacturer, so it was used with this in place, same with the ED80. The 200p though has nothing in it's manufacturers supplied state in the way of a dew shield, therefore focus without then pop the dew shield on after.

[andyboy1970]

Andy

I downloaded the grabber programme but i just couldn't figure out how it worked and there were no instructions anywhere to be found?

James

Its easy, 1) Open your capture program and Grabber.  2) take a short sub (5 second) of a bright star near your target.  3) On grabber you can input your scope apature and focal length then click set capture area. 4) click on a star and drag out a square that covers the diffraction spikes. 5) Grabber will give an instant Yes or No and figure for how close your focus is. 6) adjust your focus and keep taken snaps until you get the Yes for focus. 

[dawson]

Right. Here are my fool proof instructions on how to use the Bahtinov Grabber program for anyone else who is thick like me

 

[Disclaimer: I've only ever tested this with images off Google and not yet on a real star.]

 

There are various versions of the Bahtinov Grabber software, I've downloaded the version without autofocus, and the 64 bit one for my laptop. It's just one file which is zipped and can then be extracted and dumped straight onto the desktop and run from there; no 'installation' necessary.

 

The Grabber will work with 'live view', either a live feed image from the telescope via a DSLR (and appropriate software) or from a web cam, however I suspect the constant changing atmosphere would continually alter the focus, so I suspect, as Andy said above, it would be best to grab an image of the star in question with the mask over the end of the scope and use that.

 

Once the image has been captured it will hopefully have a bright centre and then six lines shooting out of it, three in either direction. Ideal focus is when the middle lines (which shoot through the star in either a north-south or east-west direction) are equidistant from the other lines, like the image below on the right:

 

http://img820.imageshack.us/img820/8111/bahtinovdiffractionspik.png

 

So open up the captured image you've taken, you can open it in any viewing software, the Grabber won't care as long as you can see it, it can see it. Open up the Bahtinov Grabber software too. Make sure the Grabber window isn't hiding the important bit of image; you need to be able to see the bright centre and the radiant arms.

 

The Grabber software wants to know some information about your set up first, focal length of the telescope in meters, diameter (aperture) in meters, and the pixelsize of the camera capturing the image (this website allows you to input your camera (on the right hand side) and reveals the pixel size [thansk to Kim for the link]: http://www.12dstring.me.uk/fov.htm); I'm not sure how important inputting all this information is to be honest.

 

Here is a screen shot of the autofocussing version of the Grabber:

 

http://www.njnoordhoek.com/wordpress/wp-content/uploads/2011/06/screenshot.jpg

 

Next, with the image open, and the Grabber open, click the "Set capture area" button on the Grabber screen, and the whole screen will go misty. With the mouse, click and hold on the centre of the bright area of your image, and drag the box out so that you've selected the central bright area and most of the lines coming out of it, probably two to four times the width of the bright central area in each direction (so five to ten times the width of the bright central area in total). Release the mouse button.

 

The software will show the captured area on the Grabber screen now and overlay some of its own lines (red, yellow and green in my case). It will give you an answer immediately to "within critical focus:" as either YES or NO. If YES, then you are good to go and have good focus, if NO, you need to change the focus, re-capture an image and repeat the process again. Someone clever will be able to say what all the other values and numbers that the Grabber generates mean, as they probably will tell you which way to focus and by how much. I'd just go for trial and error!

 

Trouble shooting:

 

Make sure the three boxes at the bottom of the Grabber (Red, Green and Blue) are all ticked, else it won't work.

It doesn't seem to matter that when you first click on your image to make the selection, whether you are bang in the centre of the bright area or not, the software seems to not take your click as the centre, it works its own centre out subsequently.

I have no idea what the option "Rotating focuser" means!

 

I tried it out by just running it on various images on Google images (just search for "Bahtinov focus" on Google Images).

 

Good luck and I'll let you know what happens when the sky it clear, with or without the home made dew shield.

 

James

 

Grabber software from here:

 

http://www.njnoordhoek.com/?cat=10

 

 

[Guest Ely Ellis]

I have used the grabber program in APT and it works great.

Having a refractor, I put the mask on the end of the dew shield. I am not convinced whether it matters where it is placed providing it is the same diameter as the orifice it covers. (I have used it in both positions and focus is the same even though the dew-shield adds about 6 or 8" to the length).

 

APT works the same, take an image of a bright star, drag the box over it and it tells you if your close, or far away and with a positive or negative number.

It will work on live view too, but I find it too unstable, the figures just jump all over the place.

 

One thing to note for specy people like myself, is that when you visually focus using a mask, if you do it without glasses, then you must observe without glasses and if you focus wearing your glasses then you must view wearing your glasses as the diffraction pattern is focused onto YOUR eyeball and will change if you remove / add glasses after!

 

Cheers

 

Martin

[dawson]

Used the Bahtinov mask for the first time tonight. I like it. I'm still transferring files so not processed anything yet, but it's easy to use and I'm pretty sure it made me sharpen up my focus. Here is a screen shot of it in action in live view mode from BackYard EOS, you'll see how it struggles in live view, so I think it is better to do as described above and take some stills for it to analyse.

 

James

 

[Guest Kheldar]

Bahtinov grabber and masks should be used in live view, that's why it has a 15s average reading

 

Atmospherics don't change the diffraction pattern either, unlike FWHM and other measured focusing aids.

Edited March 13, 2013 by Kheldar

[dawson]

Well that's useful to know, thank you.

 

My grabber kept flicking YES then NO when in live view. I'm not sure I left it alone for 15 seconds though.

 

James

[Brantuk]

Great info there Stephen - I never knew that thanks

[Guest Ely Ellis]

Is that the same (15s etc) in APT too Stephen ?

 

I have tried it in live view and it just jumps all over the place.

 

Cheers

 

Martin

Edited March 14, 2013 by Ely Ellis

[Guest Kheldar]

I've not used the functionality in APT yet Martin so it's bit hard to say - but given it was developed with the original developer, I see no reason why not.

 

The biggest reason you will have problems (flickering YES/NO, strange patterns on zoom) with a DSLR is the fact the stars are sooooo small - sometimes the patterns aren't even visible (Ask Sheila, we never got Bahtinov grabber working properly for hers ) You may need to consider binning the exposures for focus if you are able to do that with live view?

 

Does APT have an option to increase the live view frame time or are you fixed to what the camera delivers? If you can try it on a really big bright star it will be much easier than a fainter one.

 

Don't worry too much about it jumping around, so long as that focus error comes down to sub pixel, and the 15s average holds steady at sub pixel, you're about at critical focus

Edited March 14, 2013 by Kheldar

[dawson]

What is FWMH?

If the atmosphere doesn't influence the spikes, why run it for 15 seconds in live view?

James

[Guest Kheldar]

What is FWMH?

If the atmosphere doesn't influence the spikes, why run it for 15 seconds in live view?

James

 

Full Width Half Maximum - APT and other software provides a numeric reading that indicates the "focus" of the image, you're normally aiming for as low a value as possible (sub 2.0) but that value will be higher with bad atmospherics (some nights you may not be able to get it below 3.0)

 

Because general noise and stuff on the image can throw Bahtinov grabber a little bit, you will notice that each image "looks" different and the calculated pattern on the screen moves a bit, though I assure you the spikes are in the same place each time - by giving it the 15s you let it average out any confusion introduced by non perfect images coming through

 

If you get to a dark site when we have a meet I will be happy to run through what I do with anyone who wants to see

Edited March 14, 2013 by Kheldar

[dawson]

Thanks. That's useful again.

With the DSLR i just clicked the 5x button because as you say, the spikes were pretty small otherwise. I could see by eye i was pretty close, but when i changed the focus a little the grabber was happy.

My focus knob is awful. There is no option to fine focus, it's all or nothing. My next OTA is going to have a fine focus knob, it's on my list of requirements.

James

[Guest Kheldar]

AH ... that's not going to help - fine focus is crucial for using a tool like this, the difference between 1px out of focus one way and the other is probably something like 0.5mm focuser travel ... I'm not saying you can't do it, but it will be harder.

[dawson]

No, I have no fine focus. The focus knob is quite stiff anyway, so making small adjustments is very hard plus the mount shakes for 5 seconds afterwards too! I've got a remote electronic focusser but it isn't any finer than me, it just means it shakes the scope less and I can do it remotely from the conservatory

[Guest Ely Ellis]

Stephen: Not sure about this "You may need to consider binning the exposures for focus".

What is binning? or do you just mean don't use.

 

What I tend to do is take longer exposures to get brighter and longer diffraction patterns, this seems to give me more accurate and stable numbers. I always have it zoomed in anyway. If the star is super bright, then obviously I use a shorter exposure but it then limits me in numbers of bright stars.

 

James: As for focus's, you should try mine!

I have a refractor with just the bog standard rack and big wheel, no slow mo for me. Compared to mine, I know at least on my brothers 127mm SCT (very similar to yours) that his focuser is a dream to use. I cannot imaging how a 10:1 reduction would be to use, must be so easy!

 

As mentioned, I take an image, changing the exposure level to get a good bright solid diffraction pattern, then drag the box over it and centre it. Do the calculation, then do a tiny tiny tiny move on the focuser know, and take a shot again. Normally I can get down to around 0.0n of a decimal place. Just to be certain, I will take a few more shots and compare to make sure it wasn't a freak low number.

 

Another thing to note, with mine anyway, is that if your off centre with you box, it does change the value. You don't have to be spot on centre, but don't have it near the edge of the analysing box.

 

I also find that if I try live view, not only do I get a jumpy rough and mostly dim image, when of course you touch the focuser, the image jumps so much in the frame that it makes it so hard to use. Could all be down to the magnitude of the star though, the brighter the better.

 

Although I am not that up on these things as I have only used it on about 8 or 10 occasions, I recommend taking longer exposure like I explained and don't bother with live view.

 

Using stills of course, I normally end up with a directory full of diffraction images, unfortunately I can't show you any as I normally delete them at the end of the evening.

 

Cheers

 

Martin

[dawson]

Thanks Martin, I'd like to see your screen shots next time.

 

James