Bahtinov problem

[Tommohawk]

Hi all. I've been using a Bahtinov (standard FLO issue)  for some time with my 200P and it works a treat.

I've tried using it with my 300mm Tamron lens and it kind of works but the image is tiny, even when magnified on the laptop - usable though. However, I want to try some portable stuff and if I try and use just the camera LCD screen the image really is tiny even with x10.

So the question is, if I got a different Bahtinov would I get a better image on the screen? And if so, any ideas which would be best?

thanks in advance

[swag72]

I always thought that you needed a mask specific to focal length and aperture.....Perhaps ask FLO if they can get one specific to your requirements or go to Keith directly at Sharpmask and ask him

The image at 300mm will be very much smaller than the 200p (I assume that you are using the same camera?)

[Tommohawk]

Yes same camera. TBH the image i get loooks pretty good - its just very small

Do you think a specific mask would give a bigger image or just a clearer image?

[MARS1960]

You could alter the number of slots and change the dimensions by making your own, here is an interesting article about using a bespoke mask with a DSLR.

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

[cjdawson]

The batinov masks should be paired with the diameter of the apeture.  I use an 8" mask on my 8" LX-90.  If I wanted to use a mask on my ST-80, I'd get an 80mm mask for that.  Camera lenses also have different diameters, so again try to match up that diameter.

Using the wrong size mask will mean that you end up with either huge crosses or tiny ones.  Also the contrast may be lower than you'd get with a matched mask.

 

I'd have a look on flo and other sites to see if there's a better fit for each lens, telescope etc.

[Tommohawk]

OK thanks.

One issue with the camera lens is the F is variable of course, so not clear how to order - I'm using F2.8 and the mask generator shows tiny slots which I couldnt generate and it appears quite different to any of the commercial mask.

 

[MARS1960]

14 minutes ago, Tommohawk said:

OK thanks.

One issue with the camera lens is the F is variable of course, so not clear how to order - I'm using F2.8 and the mask generator shows tiny slots which I couldnt generate and it appears quite different to any of the commercial mask.

 

Have you tried using bahtinov grabber?

[Tommohawk]

Hmmm - my reply disappeared. Try again....

Yes have tried B grabber and works fine but my problem is with portable rig - so no laptop.

B generator give this which is pretty different to any of the offerings I've seen online, probably cos is F2.8.

 

[MARS1960]

That looks like it should work, you obviously need thinner slits and that has them.

 

[Tommohawk]

27 minutes ago, MARS1960 said:

That looks like it should work, you obviously need thinner slits and that has them.

 

sure, but I cant fabricate that and commercial offerings dont look at all like that. Maybe I'll speak to Keith Morris at Starsharp as suggested above.

[Helite]

Came across this page a while ago, seem to do camera sized bahtinov's that fit in regular camera filter holders. I have had no dealings with the company and know nothing about them.  http://www.lonelyspeck.com/sharpstar/

[Tommohawk]

3 minutes ago, Helite said:

Came across this page a while ago, seem to do camera sized bahtinov's that fit in regular camera filter holders. I have had no dealings with the company and know nothing about them.  http://www.lonelyspeck.com/sharpstar/

Thanks that looks neat - problem is my lens is ~130mm dia and I dont think I can get filter holder that big. Its probably just a question of getting a bespoke one cut I guess. 

[StuartJPP]

If you are making your own try a 'Y' mask...much easier to cut out or fabricate...have a read here:

https://stargazerslounge.com/topic/155800-details-for-a-lord-mask/

[Tommohawk]

Now that looks interesting.

Looking at the theory, for 300mm lens I need bar thickness 400/300 mm or 3 x that for third order mask. So that 1.33mm, or 4mm... so 4mm to be practical. And my existing Bahtinov is... 4mm.

Worth trying the Y mask though, so will give it a go. The theory is apparently a bit arbitrary, and bit of old stick or sundry outdoor detritus will work equally well according to some. Just need some clear sky to give it a go.