DSLR focusing

[Kaptain Klevtsov]

Right so I'm going to have a bash at the bane of my life, here are some of the options:-

Hartman mask

This is a piece of cardboard or similar that fits over the end of the telescope and has two or more holes in it. The idea is that, unless the camera is focused correctly, each star will give an image for each hole in the mask. Focusing consists of adjusting the focus until each star only gives one dot on the image.

Ronchi screen

A much more expensive way of doing it and one that I have had very little sucess with. This method uses a device which has the same bayonet fitting as your camera (so you need to buy the correct one) and you fit it to the telescope in the same way. Using it involves looking down an eyepiece at a pattern of stripes and adjusting the focus until the stripes are gone. More stripes means further off being in focus so you go through fewer and fewer until they start coming back. At this point you have gone too far so you have to guess where the middle is. This probably works better with a Crayford type focusser as there is no backlash and the middle position with the knob would be the in focus position.

Preview screen

Just what it says on the tin. Centre a bright star and reduce the shutter speed to give a dimish image. Keep snapping away whilst adjusting the focus. Best focus is when the image is pin sharp. Don't forget to use the zoom facility on maximum zoom and use the highest resolution image setting as this usually gives you more zoomability. Next step is to increase the shutter time until you get more stars. Adjusting finely will give you fewer stars each side of the in-focus position so aim for the largest number of visible stars. Also check the histogram. Brighter stars and more peaky histogram outputs mean better focus.

TV output

Many DSLRs have the ability to output the preview to a TV screen. This gives you a bigger image to work with so that the focus is easier to spot. Use as above. (not tried this one as I haven't got a TV in the shed yet).

PC control software

DSLRfocus and proprietry software allows you to control the camera from a laptop and automatically gives full size previews on the laptop. Probably the best method though only really works well with Canon cameras. Nikon users can get a demo of camera control off the Nikon USA website but you can't buy it anywhere that I've found in the UK, nor off the US website as you have to have a US zipcode to match your plastic.

Card reader

Poor man's way of PC control. Slip the card out of the camera and into the card reader fitted into the laptop to zoom in really big and check the focus.

Must have missed something but this might help somebody starting out.

Captain Chaos

[geppetto]

Good info there CC.

If they all had "live" image review, that and a monitor would makes life very much easier.

Dont laugh but whilst I was babysitting a Canon 300D some time back, I had a play at using

my toucam bracketed to the tripod thread of the camera and set to view the 300D's viewfinder.

Got a certain amount of success with it and could see what the camera was seeing at 640*480

on the laptop making focusing a lot easier.

Never followed it up though cause the camera went back to its owner

[Albireo380]

Interesting idea Gepetto.

I use a Hartman mask and also a Flip Mirror (parfocaled with a mid power EP). As the EP has greater magnification than the camera image, then when parfocalised, a crisp image in the EP means I am pretty bang on focus with the camera image. Seems to work.

Tom

[moondog]

Since I don't have a laptop or a handy TV, I can only use the Hartman mask and the preview screen.

On the moon I don't bother to focus on a bright star first using the Hartman mask, just the preview screen with a moon test shot. (You cannot use the mask on the moon directly)

Up to press I've not had a problem with focus using these methods.

Question ? I know that none of us want to lose aperture, but presumably if you got the best focus you could and then stopped down the scope from say F8 to F11 you should get more depth of field. Perhaps this would cover up any slight out of focus issues? It can help with normal camera lenses, or would the longer focal length of the scope prohibit this? I might try this on the moon on those nights when focussing is a bit of a struggle.

I'll report back if it makes a difference.

MD

[Kaptain Klevtsov]

It can MD, stopping down will always help but you would obviously lose apertureby having a higher f/ ratio. Whilst not a bad thing per-se on the moon and planets, it's a real no-go on DSOs. The guy with more aperture generally has a better chance with faint things which is why we buy / make / borrow the biggest aperture that we can get our hands on for imaging DSOs.

You might notice some 'scopes are described as "planetary", this reflects (no pun intended) the long focal length to aperture ratio (high f/ number). You can stop down larger newts to improve the depth of field by putting the cover on and removing the smaller cover provided. You have to make sure that the light coming in misses the spider, but that's no big deal.

Focussing on the moon with a DSLR can be a struggle because of the seeing. This causes the moon to move in and out of focus because of atmospheric disturbances rather than nything to do with the focus mechanism on the 'scope. Using higher shutter speeds helps "freeze" the seeing, so more aperture might help if a wobbly moon is causing you problems.

HTH

Captain Chaos

[moondog]

Thought that might be the case. Thanks.

If I try for DSO's I always focus on a bright nearby star first.

I've been imaging the moon pretty extensively these past few months and not found a lot of difference by varying the DSLR shutter speed - provided it doesn't go below 1/20 sec. I just put the Canon on continuous shoot RAW mode 200 ISO between 1/30 sec and 1/125 sec, depending on the moon's phase. This seems to give me the best results.

Even at a higher ISO level and a faster shutter speed there seems to be just as much chance of capturing a blurred image when there's a wobbly moon. In this situation stopping the scope down wouldn't help. Another great idea bites the dust !

MD