Quote:
Originally Posted by GOTO
varying staff focus is perhaps a bigger more problematic issue. If anyone can suggest a fast reliable way we can standardise visual focus, again please let me know.
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Geoff,
There are several ways to accurately focus a scope for infinity, which I'll post here as I am sure others may be interested. My own experience with public nights indicates quite a few people are a bit short/long sighted but have been ignoring it - when they try to look through a scope their focus is significantly off, and as a result they need to refocus a telescope to see anything properly.
Of these:
- I often use method 1 and method 3.
- Method 1 is not so accurate,
- Method 2 assumes your eyepieces are parfocal;
- Method 3 is quite accurate, and does not rely on your eyepieces being par-focal;
- Method 4 works in daylight only;
- Method 5 is obvious to camera buffs, and requires a specially modified eyepiece;
- Method 6 works for a point source (stars) only and requires a specially modified eyepiece;
- Method 7... requires a camera and makes some assumptions about your eyepieces. I have used this a few times but is fiddly.
1. The first method is simple - put your glasses on and focus while wearing them ! This relies on the simple fact that over 45 people lose their ability to accommodate different focal distances (I'm 56) and if you have some glasses that have been accurately selected for long-distance by a good optician (as mine are), then your eyes should have perfect focus for infinity when wearing them. I have separate reading glasses. It's even a good reason to go see your optician regularly when you are a bit older
If your glasses are chosen for a range of 1-2 metres (rather than infinity) this may not be a bad thing either, as most can manage to focus on that.
2. If you have parfocal eyepieces (mine are, Vixen LVW's) switch to the shortest focal length eyepiece and focus with that (on anything) then leave the focus alone and switch back to whatever eyepiece you wish. For low power eyepieces this will give an accurate focus - if the eyepieces are parfocal.
3. The next method works for small objects with a well-defined edge, particularly the brighter planets (Venus/Mars/Jupiter/Saturn). Having got the object reasonably well centred and more-or-less focussed, slowly pull your head back a few cm from the eyepiece while continuing to look at the object.
If it stays in focus - and it's apparent diameter STAYS THE SAME (the latter is the most important part) the 'scope is focussed. If it's apparent diameter increases/decreases, the eyepiece is not accurately focussed. This method is incredibly sensitive at high magnifications on the planets or moon, and very accurate.
4. This is a variation on 3, easy to do in daylight as you need to be able to see the exit pupil of the light cone coming out of the eyepiece. Point the telescope at blue sky so you can see the blue disk of light (the exit pupil) emerging from the eyepiece. Using a dynameter (a tapering slit with a numbered scale), hold the dynameter at the eyelens and measure the apparent width of the exit pupil at the eye lens. Now pull the dynameter back away from the eyepiece a few cm and measure the apparent width again. Should be the same diameter if focussed. If not, adjust the focus so that it is the same width.
This is easy to do in daylight. if you are tempted to set it then wait till dark, remember your telescope OTA may shrink as it cools and the focus will shift.
5. The next also assumes you have parfocal eyepieces, and one of them has a piece of ground glass in its focal plane. The ground side must be facing the telescope. Put this eyepiece in the scope and focus on anything bright - moon, planets or bright stars. I was able to cannibalise an old eyepiece which had a filter thread cut well into the barrel, and an old filter which I ground with some spare abrasive (wet-and-dry carbide paper, grade 1600 will do this nicely). There are 2 tricks with this one
a) to set up the eyepiece with the ground glass accurately in the focal plane of the eyepiece;
b) to add a ring around the eyepiece so it is parfocal with your other eyepieces.
To do these accurately I used an optical bench.
Alternatively, if you pick up an old film SLR body and dismantle it, you can cannibalise the focussing screen. Many old SLR's had a central split-focus or micro-prism surface that provides a much brighter image than ground glass, notably any of the Pentax MX, ME or K series bodies which had a plastic screen easily reworked to fit an eyepiece with a Dremel and some sandpaper. Put masking tape over both surfaces to prevent scratches while you're working on it.
6. Basically this method is a star-test and it is VERY accurate but also quite fiddly and time-consuming. You need a 12mm eyepiece modified so that:
a) it has a knife-edge, graticle or very fine wire in the barrel, but this must be somewhat out-of-focus in the eyepiece, ie located in front or behind the field-stop by a couple of mm.
b) You need a collar fitted so that once the knife-edge or graticle is at the focus of the telescope, your other eyepieces will be par-focal with the knife-edge (and not par focal with the lenses in this eyepiece).
You will need an optical bench of some sort to make this modified eyepiece.
With a star in the field of the telescope, look into the eyepiece, it should be a out-of-focus so you can see a disk of light instead of a point. Allowing the star to drift across so the knife-edge or graticle cuts into the light-cone near the focus, you will see the disk go dim then blank. If it goes dim evenly all over, the knife edge is precisely at the focus. if it goes dim on one side first, the knife edge is in front or behind the focus, so re-focus and try again. This takes a bit of practice to get used to.
Way back in the 1970's I used this method to set the focus for a cold-camera plug on an 8" Newtonian. It is incredibly sensitive, and you are seeing a Schlieren view of your objective which will reveal any tube-currents and atmospheric turbulence. This may be so bad that you can't accurately focus.
7. This last method works especially well at high magnification. It relies on eyepiece projection into a good quality compact camera with manual focus (i have a Panasonic LX5), you can use this on the moon or bright planets but its impossible on anything else. To work well, the eyepieces should have enough eye relief to put the exit pupil at the iris inside the camera lens so that it will fill the field of view of the camera without vignetting (again, I'm using Vixen LV and LVW's that have a pretty consistent 20mm ER.)
Basically, I manually set the camera aperture to f/2 (iris wide open), choose a rough ISO setting (ISO 100 for the moon, or 1600 for Mars/Jupiter/Saturn) and manually set the camera lens focus for infinity.
Holding the camera at the eyepiece, focus the image in the camera using the telescope focusser (the camera's autofocus must be disabled). Once focussed, remove the camera.
In my case the rubber eyecups of the Vixen LV and LVW eyepieces are a neat fit and will hold the LX5 in place without rings.
