Bahtinov mask: This is about right isn't it?

[irtuk]

 

This is the masked view of Alkaid for tonight's session. From what I can find online this looks to be pretty good focus

 

[steppenwolf]

Yup, almost perfect. It could be argued that the bisecting line is a FRACTION high but to be honest, this really is close enough 👍

[Freddie]

Have you tried Bahtinov grabber? If not, give it a go, it takes the guesswork out.

[irtuk]

Great thanks. I was going to measure it in Photoshop and count pixels but on a basic mark 1 eyeball that looks pretty close. I'll take a look at that software too.

I have dialled in another hundred subs, currently picking up some light pollution that I am pretty sure is my neighbours getting ready for bed, it happened last night between about 9:30 and about 10:30

Ed.

[AlexK]

2 hours ago, irtuk said:

 

This is the masked view of Alkaid for tonight's session. From what I can find online this looks to be pretty good focus

 

Nope. It's a fraction low  You should look at farthest from the center regions. In fact the most accurate position is the center beam being vertical. That's how eyes perception works. Adding a perpendicular to the center beam narrow slit would also help. With the Photoshop the best way is to copy the thing, invert the color, mirror it over the center beam, and overlap with the original with color subtraction mixer.

 

Edited April 6, 2021 by AlexK

[DaveS]

But not if you're looping exposures and adjusting focus between each. all that PS palaver is just wasting time.

[irtuk]

Tomorrow I will try to make some sense of this then! right  now I can't be in and out of the house to re-fit and remove the mask and tweak the focus.

At least the temperature here in the southeast seems to be set to "bloody cold" I started at 2c and its now 1c so, we will see. Its only the second time I have used a mask for focus so, plenty to learn.

Ed.

 

[Freddie]

As said, just use the grabber and it will take all this subjective guesswork out of it!!

[ollypenrice]

On 06/04/2021 at 22:19, steppenwolf said:

Yup, almost perfect. It could be argued that the bisecting line is a FRACTION high but to be honest, this really is close enough 👍

Heh heh, and I thought it was a fraction low (but, like you, good enough for jazz!) Then I looked again and now I don't know. Freddie may have a point but I like the B mask because it is software free!

I don't know about anyone else but I always find the bisecting line slightly offset from one side to the other. In this case I see the left as a tad lower than the right.

But when it looks as above, just get on with it!

Olly

[steppenwolf]

5 hours ago, ollypenrice said:

Heh heh, and I thought it was a fraction low (but, like you, good enough for jazz!) Then I looked again and now I don't know. Freddie may have a point but I like the B mask because it is software free!

High or low, it is certainly good enough for government work and I reckon an old amateur like me would be happy with it too! 🤣

[tooth_dr]

5 hours ago, ollypenrice said:

I don't know about anyone else but I always find the bisecting line slightly offset from one side to the other. In this case I see the left as a tad lower than the right.

I've always noticed this too, end up going with what looks 'average'

[iapa]

22 hours ago, ollypenrice said:

 

I don't know about anyone else but I always find the bisecting line slightly offset from one side to the other. In this case I see the left as a tad lower than the right.

Time for the straight edge test as I “see” the offset too.

and the results are now in dum dum dum - damn that’s so annoying - dum dum

it’s a straight line

[BrendanC]

If you're doing this with a laptop then I highly recommend Bahtinov Grabber. It basically tells you when you're in focus by analysing the Bahtinov mask image. The Bahtinov Tool in APT is based on it, but odddddddly, Bahtinov Grabber is better. I struggled with APT's implementation until I read about this, now I use it all the time: http://www.njnoordhoek.com/?p=660

[tomato]

I started off doing Bahtinov Mask focusing by eye but moved on to Bahtinov Grabber, after I found out about it. 
 

I found it easier to fine tune to a number rather than by eye.

[tooth_dr]

What sort of exposure are you using?  I can see my line moving between exposures presumably due to seeing. I would use 3-5s exposures with region of interest selected and just select a small area around the star in question.  

[michael8554]

+6 for Bahtinov Grabber

You can adjust focus so that the "wobbling" due to Seeing can be eyeballed to be either side of perfect focus.

Michael