Focus of DSLR

[kirkster501]

Hi all, quick question since never done widefield with DSLR.  I have a modded 450D from my pre-CCD days.  It works great still and I want to get into wide field a bit since I love it. I can leave it gong on the scope whilst it is imaging other stuff.  

Do you guys focus the DSLR with a Bhat mask? 

Thanks.

 

 

[tooth_dr]

3 minutes ago, kirkster501 said:

Hi all, quick question since never done widefield with DSLR.  I have a modded 450D from my pre-CCD days.  It works great still and I want to get into wide field a bit since I love it. I can leave it gong on the scope whilst it is imaging other stuff.  

Do you guys focus the DSLR with a Bhat mask? 

Thanks.

 

 

For widefield with a lens I would tend to use live view at 10x, and focus on a star until it is at it's smallest.  Also you might notice faint stars coming in and out view as the focus is just exactly right.  If I'm using a telescope I use a bhatinov mask.

[Scooot]

Yes I use an unmodded 450D, and with my 135mm lens live view at 10x seems best. Rather than only concentrate on getting the star as small as possible I also try to make sure no red is showing around the star’s fringe. Doing this has helped a lot.  

[knobby]

Or you could use a Y shaped piece of card across lens (similar effect to Bahtinov)

Take a 10 second exposure and adjust.

[Jkulin]

I bought one of these, still not had a chance to use it, but it looks very promising: -

https://www.lonelyspeck.com/sharpstar/

[StuartJPP]

Are you talking about LiveView focusing or focusing, taking an image and then re-focusing?

Although I didn't buy a SharpStar mask, I did make some various similar ones on my engraver. The longer the focal length the better for LiveView, at 135mm for my Samyang Lens, a bright star is usable in LiveView. When you get to say 50mm and wider, you really struggle in LiveView as the stars are so small.

Have a look on this thread...

 

[Guest]

I am focusing with my DSLR + BAHTINOV MASK or DSLR + BAHTONIV MASK + APT SOFTWARE or BACKYARDEOS, depends how easy  & quick  I can find a star tro focus on, the software supports me to find & focus on faint stars when the CAMERA DISPLAY fails..

[clarkpm4242]

I have no problem using LiveView at x10, even with wide angle lenses.  Looking for the colour fringing is a great aid.  Never err on the red side!

Am usually out 'in the field' so only have camera.

Paul

[MarsG76]

Yes, I use my 80mm frac bahnitov mask. First I find the brightest star and focus on it before framing the area I want to image wide field. I use either live view or take 5 second exposures, what ever shows the best diffraction lines on the night.

 

[kirkster501]

Great stuff guys, many thanks !!!

Maybe daft question though...  Why not just focus on infinity and be done with it?  Why doesn't that work? 

[StuartJPP]

32 minutes ago, kirkster501 said:

Great stuff guys, many thanks !!!

Maybe daft question though...  Why not just focus on infinity and be done with it?  Why doesn't that work? 

Because most lenses focus past infinity (no Buzz Lightyear jokes please!). If there was a hard stop at infinity then you wouldn't be able to achieve infinity focus when the temperature changed dramatically.

[geordie85]

I use the bahtinov mask I bought for my star 71 with my Samyang 135mm and it works a treat, especially at f2 where focus is such a tiny point.

[Josh40996]

On 18/12/2017 at 19:36, kirkster501 said:

Great stuff guys, many thanks !!!

Maybe daft question though...  Why not just focus on infinity and be done with it?  Why doesn't that work? 

I have a lens that has a hard stop at infinity and perfectly focuses, but another of my lenses does have the hard stop and focuses past infinity. 

 

 

I zoom in as far as I can using live view, focus until the star is smallest and then it's done. I purchased a bahtinov mask, used it once and never again. Every second counts for imaging in the UK, live view is simple, quick and effective.