Hi

I have been trying to take a few snaps with my old newtonian telescope with an SLR using a prime focus configuration, however when I look through the viewfinder the image is very sharp but when I look at the images from the camera they are not in focus. Does anyone have any idea why this might be?

Cheers

Franky

Does the viewfinder have a diopter knob? It may be misadjusted--try getting pictures of objects a few hundred yards away in the daytime to adjust this. Also--if you wear glasses for distance viewing--you'll need to wear them when looking through the viewfinder too or you will focus to correct your vision rather than to true focus.

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Todd (Bowie, MD, US, North America, Earth, Sol System, Vega region, Local Bubble, Orion arm, Milky Way Galaxy, Local Group, Virgo A Cluster, Virgo supercluster, the universe in which spock is clean shaven)

Quidquid latine dictum sit, altum sonatur.

personal page: http://blog.astrosketches.info

hi Franky,

if You wanna go for astrofotografy with this setup (EOs...?) You might find this here useful:


here is the link to Wolfgang Grzybowski`s websitre and e-mail address.
http://astro.uni-tuebingen.de/~grzy/

he has helped me a lot - testing my optics and providing good advises.

best Dietmar

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best regards,
Dietmar
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www.stargazer-observatory.com

I cant focus my DSLR without a barlow on my reflector because the focuser cannot go in far enough for prime focus

Three ideas spring to mind.

1 The dioptre adjustment of your viewfinder is not right for your eye. Just because the image you see through the viewfinder is focusing on your retina does not necessarily mean that the image coming through the telescope is focusing on your film/digital sensor. There is some good software available for focusing digital SLR's called DSLRFocus. Send me a PM if you're interested. The software gives you information on how well focused you are on a star, and you can adjust the focus to get the optimum. In my experience, even with the dioptre adjustment of the viewfinder correct, you still can't focus well enough by eye.

2 Your mount is vibrating and blurring the image. Maybe it's windy, maybe it's just a cheap mount. Maybe shutter slap is causing the vibration. When an SLR camera flicks the mirror up there is enough of an impact to make the telescope wobble slightly. I have a heavy duty mount but it still suffers from bad shutter slap when photographing at prime focus. Some cameras allow you set a delay, whereby the shutter flicks up, then the camera waits (mine is set to 5 seconds) then it opens the shutter. My camera, a Canon 300D, doesn't have this in the factory software, but there are a couple of hacks you can download and install on the camera to allow you to do this. More expensive cameras (like the Canon 10D I believe) have this function built in.

3 You're photographing something too close to the horizon and atmospheric refraction is making everything wobble around (stars twinkle). I had major problems when I started in astrophotography, always getting blurred stars etc., because I was photographing things low in the sky. Once you aim at something more than 30 degrees up the problems go away.

Of course, it could be all of these things in combination.

Hope this helps,

clop

Boy, ain't that the truth?! Prime focus astrophotography is probably one of the hardest things you can do, and Murphy's Law will work against you 100% of the time

clop, those are good ideas!

Clear skies!

Paul

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