How to focus manually a lens for which it was not designed.

[xilman]

Just discovered http://nova.astrometry.net/ and tried it out with the 512x512 image. It identified the field nicely but has the declination axis running in the wrong orientation for some reason I don't understand.

Nonetheless it appears to be an extremely useful resource.

[Spherical Cow]

Originally Posted by xilman

Just discovered http://nova.astrometry.net/ and tried it out with the 512x512 image. It identified the field nicely but has the declination axis running in the wrong orientation for some reason I don't understand.

Nonetheless it appears to be an extremely useful resource.

Thanks for pointing out that site- looks like it could be very useful, though I admit I don't completely understand its operation yet. Should be fun to explore.

Norm

[xilman]

Originally Posted by fivemack

The mount I was using for the smaller lens ( http://www.ioptron.com/product-p/3302b.htm ) has the lens on a ball-head, meaning that it will be at some arbitrary rotation with respect to the sky.

For the larger lens, and the telescope, I use a proper equatorial mount ( http://www.orionoptics.co.uk/MOUNTS/...hereq5syn.html ) on which up-down on the image corresponds to north-south on the sky, and left-right to east-west, though there tend to be a few degrees of deviation because I can't leave the telescope aligned and polar alignment is fiddly.

In particular, the instructions for aligning the mount assume that you have something mounted on it with a reticle eyepiece that you can look through, and I haven't found a good way to get the DSLR to display an indicator for the centre of the frame that I can line up stars against.

This isn't so much a problem with 6-arcsec-per-pixel or 3-arcsec-per-pixel lenses, where I have enough of a field of view that I can see the object despite a few degrees of misalignment, but with the 0.75-arcsec-per-pixel telescope a few degrees of misalignment places the object invisibly outside the FoV.

I think the answer is to align very assiduously using a shorter lens and then switch to the telescope without adjusting the mount at all, but that's not something I can do during a brief break in the clouds. I got not a single usable image from a couple of hours with the telescope on 26 December, and now the moon is up.

First light through my telescope for several years. Saw blue sky, white clouds and green leaves! Perhaps something more interesting soon, though not likely to be imaging yet.

Although the scope has a GOTO system that will take some fiddling to get it set up as I have absolutely no experience with it at all. A rummage around the web came across a PUSHTO system (wonderful name) which runs on an Android phone strapped to the telescope tube. Semi-automated finding even for Dobsonians! You might find it useful too as it claims to have pointing accuracy good to about 5 arcmin with care and decent calibration.

[Spherical Cow]

Originally Posted by xilman

First light through my telescope for several years. Saw blue sky, white clouds and green leaves! Perhaps something more interesting soon, though not likely to be imaging yet.

Although the scope has a GOTO system that will take some fiddling to get it set up as I have absolutely no experience with it at all. A rummage around the web came across a PUSHTO system (wonderful name) which runs on an Android phone strapped to the telescope tube. Semi-automated finding even for Dobsonians! You might find it useful too as it claims to have pointing accuracy good to about 5 arcmin with care and decent calibration.

Wow- thanks for that link. There are 3 Dobsonians in my family; this could be very useful.

And Congrats on getting some photons on your optics. Hope that soon you'll be gathering photons that have traveled a few million years.

Norm

[xilman]

Originally Posted by fivemack

One of my other hobbies is astrophotography.

I have an optically-decent autofocus 70-200 f/2.8 lens, which I expect ought to be able to take pretty good pictures of star clusters.

The problem is that autofocus doesn't work on point sources; the manual focus ring is very loose and the difference between fifty-pixel-diameter blobs one side of focus and fifty-pixel-diameter blobs the other side of focus is no more than five degrees of motion. I can use live-view and zoom in as far as it goes, but that is not one-screen-pixel-per-sensor-pixel, still less the ten screen pixels per sensor pixel that I would like.

Is there some sort of clever contraption that could be made (I'm thinking of some kind of worm-and-wheel) to turn the focus ring in hundredths-of-a-degree increments, and if so is it an item of commerce? Is there some kind of optical contraption which could make accurate-infinity-focus discernible on something other than a star? There is no focus-lock mechanism that I can see.

I am reasonably confident that the odd-shaped stars are a result of optical aberrations rather than of a wobbly mounting, because they are very consistent in shape over several exposures of the same field.

As it doesn't get dark at night this far north and this close to the summer equinox I'm reading up and preparing equipment ready for the season starting in September. In particular, I've been re-browsing Sidgwick's Amateur Astronomer's Handbook and came across something which may be appropriate in this thread.

On page 426 of the 4th Edition:

A triangular objective diaphragm allows a fine adjustment of focus to be made. The diffraction pattern of a stellar image with such a diaphragm is 6-rayed; the extrafocal images are triangluar, with the angles of the inside-focus image occupying the position of the sides of the outside-focus image. A precise focus is easily obtained by adjusting the drawtube so as to make the 6 rays of the focused image equal.

That advice is for visual observers but I don't see why it shouldn't apply to photographers. For the price of a piece of scrap cardboard and some fiddling around it seems worth giving it a try.

[xilman]

Originally Posted by fivemack

s for aligning the mount assume that you have something mounted on it with a reticle eyepiece that you can look through, and I haven't found a good way to get the DSLR to display an indicator for the centre of the frame that I can line up stars against.
...
I think the answer is to align very assiduously using a shorter lens and then switch to the telescope without adjusting the mount at all, but that's not something I can do during a brief break in the clouds. I got not a single usable image from a couple of hours with the telescope on 26 December, and now the moon is up.

Having played with distroastro and a Canon EOS 10D at the end of a USB lead, I may have found a useable solution, not only for this but also for camera shake, focussing, etc. Distroastro comes with ghoto2, ImageMagick and astrometry.net as standard.

The approach goes as follows. Plug in the camera to a USB port and unmount its filesystem if it automounts because gphoto2 requires exclusive access to the camera. For alignment and focussing, set the camera focus and direction manually as best you can. Set the image quality to give as small a data transfer as you can, unless you've particularly fast USB at the camera. Pick a plausible exposure time. Then run

gphoto2 --capture-image-and-download

The resulting image can be checked for focus and exposure to ensure that you have at least a dozen or two sharp stars visible. When a reasonable image is acquired convert it to FITS with ImageMagick's "convert" utility; this will take a few seconds. Then use astrometry.net (the local version, not the web facility unless you are patient and have good net access at the scope) to determine the image center. If you're pointing in the right direction, good. If not, move the scope a known number of degrees in a known direction^*, take an image there and determined the new center. You should be able to get within half a degree of so after one or two such steps.

Finally, switch the camera to RAW mode, set the desired exposure times and snap away with

gphoto2 --capture-image

to leave images on the camera until you're ready to download them all in bulk.

If you prefer, you can run the gphoto2 shell but I've not found it to be any great advantage over running each command through the CLI.

* Old-time astronomers will know how to star-hop ...

[xilman]

Anyone have any experience with Bahtinov Masks?

For little over a tenner I'm tempted to give one a try.

[xilman]

Originally Posted by fivemack

The problem is that autofocus doesn't work on point sources; the manual focus ring is very loose and the difference between fifty-pixel-diameter blobs one side of focus and fifty-pixel-diameter blobs the other side of focus is no more than five degrees of motion. I can use live-view and zoom in as far as it goes, but that is not one-screen-pixel-per-sensor-pixel, still less the ten screen pixels per sensor pixel that I would like.

Returning to the original post as I have almost the same problem, though not with an autofocus lens.

This evening I took a series of 20" shots with the focus ring first set to "hard infinity", i.e. as far round as it would go, and subsequent shots with the ring rotated a small amount, always backwards from the stop to try to counter backlash. A photo with my phone was taken at each setting to have a record.

Turns out that I was too cautious and only the final image of 200mm had a decent focus in that the star trails were 6 pixels wide. For photometry this is close to ideal! At 80mm the same setting yielded images which were 60 pixels across and a further set of images will be needed.

I'm hoping that once calibrated the focus setting will be sufficiently reproducible that per-session calibration won't be needed. If it is, knowing roughly where to set the focus ring should be helpful.

Tonight off-line processing was used. In the field I wouldn't bother with the phone photos and I'd evaluate the focusing images from a laptop connected to the camera.

Perhaps this approach may suit Tom, after I return his mount to him!