How accurate is a Bahtinov mask?

[toryglen-boy]

i need to get in focus, for taking flats with the QHY8, no mean feat when the scope is pointing skywards and has nothing to focus on.

One possible solutin for this is a mask, TBH some of the images i have seen taken with people who have focused with such a mask, have been a bit off, although that could come down to mask accuracy, collimation etc.

So how good are they? and is it a good solution for getting focused, when there is nothing to focus on?

thanks

;)

[TrevorW]

I use a program called DSLR Focus my preference over a mask

more accurate IMO for those with imperfect eyesight

similar programs are availabel for CCD cameras

[toryglen-boy]

Thanks for your post Trev, but it didnt answer my quesiton at all !!

:lol:

[TrevorW]

Sorry what i was implying although good for visual IMO not 100% accurate for astrophotography even moreso if you have imperfect eyesight

[toryglen-boy]

Gotcha Trev, thanks for that. i am find the process of focusing with a CCD to be a challenge, take a shot, wait several seconds for it to download, make a slight adjustment, take a shot ...

:P

[Robbie]

I have a bahtinov for both the ed80 and the 10" sct (kendrick )
deadly accurate!!! visual CCD or DSLR . it makes a huge difference over winging it with the eyeballs.:thumbsup:

of course its useless unless you have a decent star to focus on..... so in response to the overhead flats with nothing in fov focus first then point!!!

[toryglen-boy]

Thanks Robbie, but you have to take flats while focused, right? so how do you get to focus when pointing at the sky with a white t-shirt pulled across the scopes aperture?

:shrug:

[JohnG]

You leave your camera attached to your scope, in the same locked focus position and orientation as when you took your lights, also leave any filters in place as well, you then take your T-Shirt Flats.

Cheers

[Octane]

Shoot object first, and then take flats, so you'll be in focus.

I don't think there's any point in taking flats before you begin, unless you have an observatory and nothing ever moves. Consider that you may need to rotate your camera (yuck) to frame your object (more yuck).

+1 for being a stickler with keeping camera always perfectly oriented east-west or north-south.

Regards,
Humayun

[toryglen-boy]

Thanks H, so basically, if you dont have an observatory, then a lightbox is pretty essential ?

:doh:


EDIT : anyway fella, i think i have a handle on flats, i was just really looking for a way of getting really sharp focus

[Octane]

Duncan,

Yeah, I think that's a reasonable assumption to make.

It all depends on how critical you want to be. I'm sure you could just leave markings on the focuser drawtube for both focus travel, and where a marking on the camera lines up with a marking on the drawtube when it's in the focuser. So, you can always go back and orient the camera roughly. This is a big no-no. Whilst you may solve vignetting problems, you'll be left to deal with holes in your light frames from where the camera didn't line up to the pixel.

Regards,
Humayun

[multiweb]

Flats need to be taken at the exact focus position and camera orientation you were imaging at in order to work 100%. (i.e. remove dust donuts and vignetting).

[Robbie]

what John said;)