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FOCUSING - by John Drummond

Extract from the Astrophotography Camp (Taupo, NZ - May 2005)

Coupled with the challenges of guiding are the challenges of positioning the camera’s film or chip exactly where the incoming cone of light from a star forms a stellar point…Camera lenses are sometimes okay because they can be set at ‘infinity’ and should be in focus (unless it’s a zoom lens). A telescope has no ‘infinity’ setting – so has to be focused in or out until the ‘sweet spot’ is found (and it’s easier said than done!). The shorter the f-ratio of a scope the harder it is to focus – e.g. an f 10 is easier to focus because it roughly has a 0.3mm depth of field - as opposed to an f 4 which has a 0.05mm DOF. (Unless stated, all images are by John Drummond and are copyrighted)

NGC 4755 (the Jewell Box) taken through my 41cm (16") f4.5 - in focus
NGC 4755 - 1 mm out of focus

 

Some Methods of Focusing (see below for more info)

1. Eyeballing it through viewfinder (plus magnifier)

2. Par-focal eyepiece

3. Hartmann Mask (plus star-test images)

4. Diffraction spikes

5. DSLR focus and other software programs

6. Trial and error with a number of test shots

Please note that these techniques are aimed at the digital SLR camera, but can be used with film. Because of this knife-edge and Ronchi focusing are not mentioned...

 

One absolute necessity is fine focus increment readouts. I use a JMI Motor Focus Digital readout on the 16" and scales from a photocopied ruler (reduced) with a needle. See images below...

JMI Digital Read Out (with Motor Focus) - allows 1/100mm increments
A photocopied ruler with a needle on my Sigma 70-200mm lens

 

1. Eyeballing it through viewfinder (plus magnifier)

Right angle viewfinder - allows 2 x magnification when looking through viewfinder
Focusing scope – 200mm FL with a 20mm EP (focused to 6 ft (2 m))

 

2. Par-focal eyepiece

Basically this is a 12.4mm eyepiece and a 3x Barlow which are set to be focused at the same spot where my Canon 10D DSLR will be focused (hence the term 'par-focal'). It takes a little tweaking until it matches the camera's focal point but is a beaut to use once set (and screwed securely with the thumb screws so it won't move!).

To use I place the diffraction spikes (no. 4 below) over the end of the scope, insert the par-focal eyepiece into the focuser tube, select a magnitude 1-2 star, watch the star's spikes until they merge (described below), then insert the camera and complete critical focus with DSLR software (or test shots if you don't have DSLR).

 

 

3. Hartmann Mask

This handy focusing aid (16" on left, 8" on right) is cheap and easy to make and works fairly well. The mask is inserted over the telescope end and the twin-star image visible through the SLR camera view finder is focused until they form one star - with spikes (if the star is fairly bright). If a 2x magnifier is used to look through the camera FOV it accentuates this process... I initially tried four circles, but after research found that two triangles (one pointing at the middle of the other) worked better. I have also found that using the par-focal eyepiece (mentioned in 2 above) instead of the camera body yields acceptable results which places the focus in the ballpark field. Example photos can be seen below...

Harmann mask for the 16" - two triangles
Harmann mask on the 8"

 

A mag 1-2 star out of focus creates two triangles
As the focus gets better the triangles move together
In focus - note the spikes and faint stars visible

Another method is to take one exposure without the drive running. Start the exposure let the stars drift for 10 seconds, cover the scope with a 'black hat' for 5 seconds, adjust the focus by a set amount (e.g. 0.1mm), take the hat off, expose for another 10 seconds, cover, adjust focus (always in the same direction), and so on. See the image below. Note that the fainter stars are possibly the best 'ambassadors'.

 

4. Diffraction spikes

Placing some 'spikes' across the face of the scope can create diffraction spikes when viewing brighter (>mag 3) stars. To utilize this method I either image a star with the spikes on and examine each image until the spikes merge as one after adjusting the focus each image (see images below). The most common method I use however is to place the par-focal eyepiece (with 3x barlow) in the focuser and put the diffraction spikes on and physically watch until the spikes merge. When merged the scope will be pretty close to focus. Then I do step 5 (DSLR Focus) or step 6 (test shots). I have heard of people placing branches, tape, string etc across the scope to make these spikes. A friend places them across his SCT (has no secondary mirror holder - and thus no spikes) to create spikes on the brighter stars to aesthetically enhance his stars.

 

 

Inside of focus - each spike consists of several spikes
In focus - the spikes have merged into one
Outside of focus - each spike consists of several spikes

 

5. DSLR focus and other software programs

This is an excellent program for focusing and one I use most often. It costs about US$50. For details go to: http://www.dslrfocus.com/

 

6. Trial and error with a number of test shots

As it suggests, I sometimes just takes a series of seven images. First I set the focuser at '0' after getting it as close to focus using the par-focal eyepiece and diffraction spikes method. Then I set the DRO focuser to 0.60mm inside focus, take an image, set it to 0.40mm, take another image and so on through 0.00mm and then out to 0.60mm outside of focus. I then download the images, look at them closely, move focus to the best image's focus position, reset the DRO to '0' again and take a fresh set of seven images - this time adjusting the focus by 0.10mm, check them, adjust the focus position to the best image's focus position and viola, I'm ready to image...

 

 

For other extracts from the camp click on: piggyback astrophotography, guiding, focusing, deconvolution

 

For details regarding the 2005 Taupo Astrohotographers Camp see -

astrocamp - photographic journal

http://www.star-mate.com/astrophoto2005/