Aaaarrrggg, help needed in focusing DSLR

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Aaaarrrggg, help needed in focusing DSLR

Postby bmzjastro » Sun Apr 23, 2006 1:20 pm

Sorry for the title, buti,m having kittens trying to focus my Nikon dslr through a skywatcher 80 ed, have to use cls filter, have tried hartman mask but very hard to see, cant afford laptop, would be gratefull for any hints, i am disabled and i think my brain is a bit fried + i,m new to this, just have a love of deep sky objects, many thanks for any help, and just a gripe why is astronomical equipment so expensive in UK and so cheap in US? Just lie to say huge amount of thanks to all on forum, as i cant get out much it's like having access to all of britains experts, there is a lot of clever people on this forum, and ive learned more reading it than all the web site put together, many thanks
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RE: Aaaarrrggg, help needed in focusing DSLR

Postby Chanctonbury » Sun Apr 23, 2006 1:53 pm

Bmzjastro,

This subject causes much pain and I have tried many solutions:-

1. Hartmann Masks. Didn't really do it for me as just as the 2/3 images (depending on mask design) start to merge as you approach focus you are back where you started - not being absolutely sure where the exact pont of perfect focus is! In addition, the masks - of necessity - also cut down the light in the viewfinder making a bad situation worse ...

2. A small black and white television (£15.00) to view captured test images. Thought this had the problem licked but I could artificially make the focus appear to be good by changing the brightness and contrast of the image at will.

3. Checking test images zoomed in using the camera's LCD monitor - this tended to indicate better focus than was actually being achieved!

I didn't bother with a parfocal eyepiece as this would not guarantee that the focus would smack on as there would be too many potential variables.

I am afraid that the only way to be [b]sure[/b] of focus on the dark objects that we are interested in is to check what is happening on the camera's sensor and for me the PC is the only way to accurately see what has been captured in test shots.

This does introduce extra cost and complexity and I appreciate that it is not the answer you wanted to hear but from my own experience ( but then what do I know? ) this is the only system that consistently works for me.

You also have private mail from me.
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RE: Aaaarrrggg, help needed in focusing DSLR

Postby dazza1639 » Sun Apr 23, 2006 4:23 pm

Hi bmzjastro,

You could try using a flip mirror. This is basically a small tube with a mirror in it. When the mirror is up, the light path is stright through on to your DSLR focal plane (ie just an adaptor to mate the DSLR to the 'scope). When the mirror is flipped dpown, it acts as a diagonal and directs the light to an eyepiece.

If you set it up in daylight, focus the image of a distant terrestrial object onto your DSLR focal plane (run off a few shots and put on computer to ensure they are in focus) then flip the lirror and parfocalise into your eyepeice. then next time you use, you should be in good focus for the camera focal plane when you are in focus in the eyepiece.

If use a flip mirror for mant prime focus astro images and it has improved the focussing markedly. I can now be reasonably sure that I can take (in focus) images of many DSOs and the moon (at prime focus).

However, if you want to move to negative or positive projection photography, you may not be able to reach focus with a flip mirror in place - not enough back or forward focus, depending on the set up.

Hope this is helpful


Tom
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RE: Aaaarrrggg, help needed in focusing DSLR

Postby sky geek » Sat May 13, 2006 12:13 pm

Bmzjastro

I had the same problem with focusing my Rebel XT (Canon 350D).
I ended up buying at STI Stiletto Focuser (series IV), I have no problems now, it provides spot on focusing everytime.

They are reasonably expensive though around £200.00, however you can find them second hand occassionally on Astromart [url=http://www.astromart.com/classifieds/]http://www.astromart.com/classifieds/[/url]

The link for the Stiletto (Stellar Technologies) is [url=http://www.stellar-international.com/]http://www.stellar-international.com/[/url]

Good luck & clear skies..
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