You can also use it for getting some nice calibration spectra with a Star Analyser SA-100 grating. Use a variety of light sources (incandescent bulb, halogen bulb, compact fluoro, "white" LED, etc), and in each case, the pinpoint "star" will generate a beautiful test spectrum.
Use a white light source such as a halogen bulb, and fit your various filters in front of the grating, and you can instantly see the absorption characteristics of each filter.
how far away do you place the ball bearing from the scope??????anyone do a diagram?
I use a 10 mm ball bearing at a distance of about 20 metres, as that is as close as I can get my 8" LX-90 to focus. I attach the ball bearing to a piece of black cardboard, to give a nice dark background to work against. At that distance, the Sun's image on the ball bearing is a brilliant pin-point of light - by my calculations, about 0.25 arc-seconds across, so easily fine enough to do collimation etc.
My 90 mm Mak will focus down to about 8 metres, which makes it easy to use indoors, by setting up down a hallway or in a large room, and using a halogen light pointed at the ball bearing as an "artificial Sun".
I find collimating my C8 to be very difficult using the doughnut technique spelled out in my manual.
Instead I use a 5mm eyepiece to give me 400X on a bright star, and look at how concentric the diffraction rings are, and adjust if they are not. Only problem is I might have to wait for one or two weeks before I get a night when the tiny diffraction rings stand out well rather than being a mushy blur around the star.
For cleaning the front corrector plate I use distilled water mixed with the proper isopropal alcohol (not the foot rubbing kind), together with lots of little lens cleaning tissue or DSLR CCD cleaning tissues, gently dragging the tissues out from the centre - with the plate being nearly perpendicular to the ground.
Regards,
Renato
I use a 10 mm ball bearing at a distance of about 20 metres, as that is as close as I can get my 8" LX-90 to focus. I attach the ball bearing to a piece of black cardboard, to give a nice dark background to work against. At that distance, the Sun's image on the ball bearing is a brilliant pin-point of light - by my calculations, about 0.25 arc-seconds across, so easily fine enough to do collimation etc.
My 90 mm Mak will focus down to about 8 metres, which makes it easy to use indoors, by setting up down a hallway or in a large room, and using a halogen light pointed at the ball bearing as an "artificial Sun".
Cheers
That may be an issue for my c9.25 though...
Cheers
That may be an issue for my c9.25 though...
Most SCT's will focus down to about 20 metres or so - if you can reach focus shooting across your backyard (or across the street), you can use a ball bearing for a daylight star test.
Preferably, set up with the sun behind you, fully illuminating a fence post or similar that you can focus on. (It's not essential to have the Sun at your back, as you will see a "glint" image of the Sun on the ball bearing from almost any position, as long as the ball bearing is not shaded. (If you set-up with the telescope facing South, you can actually view the Sun's image in the ball bearing from sunrise to sunset without ever needing to move anything, and without any risk of direct sunlight entering the OTA.)
Mount a piece of black card (for contrast) with a 10 mm - 20 mm chrome ball bearing on the post, and then focus on it in full sunlight.
Focus on the ball bearing, and you will see a minuscule "glint", which is the image of the Sun. Because the ball bearing acts as a reducing mirror, the image of the Sun's reflection as viewed in the telescope is MUCH smaller than the Sun itself. When you do the maths, the glint is approximately 25 microns across for a 10 mm ball bearing, and at a distance of 20 metres, this translates into a pinpoint that is about 0.25 arc-seconds across, which is plenty good enough to do your collimation tests etc.
The method works just as well inside or at night, using an artificial light source, such as a halogen light, as long as you can focus on the ball bearing.
Point the light at the ball bearing, but try to keep the light source some distance away from the ball bearing (to keep the image as small as possible). (If you imagine a light source that is right up close to the ball bearing, the reflected image of the light could be almost as big as the ball bearing.) If the light source is up near the telescope, the image of the light source will be minuscule, as desired. I find a halogen desk light works well, as the source itself is small and brilliant, and it can be placed near the telescope, directed at the ball bearing, without too much distracting light spillage coming back to you at the telescope.
Most SCT's will focus down to about 20 metres or so - if you can reach focus shooting across your backyard (or across the street), you can use a ball bearing for a daylight star test.
Preferably, set up with the sun behind you, fully illuminating a fence post or similar that you can focus on. (It's not essential to have the Sun at your back, as you will see a "glint" image of the Sun on the ball bearing from almost any position, as long as the ball bearing is not shaded. (If you set-up with the telescope facing South, you can actually view the Sun's image in the ball bearing from sunrise to sunset without ever needing to move anything, and without any risk of direct sunlight entering the OTA.)
Mount a piece of black card (for contrast) with a 10 mm - 20 mm chrome ball bearing on the post, and then focus on it in full sunlight.
Focus on the ball bearing, and you will see a minuscule "glint", which is the image of the Sun. Because the ball bearing acts as a reducing mirror, the image of the Sun's reflection as viewed in the telescope is MUCH smaller than the Sun itself. When you do the maths, the glint is approximately 25 microns across for a 10 mm ball bearing, and at a distance of 20 metres, this translates into a pinpoint that is about 0.25 arc-seconds across, which is plenty good enough to do your collimation tests etc.
The method works just as well inside or at night, using an artificial light source, such as a halogen light, as long as you can focus on the ball bearing.
Point the light at the ball bearing, but try to keep the light source some distance away from the ball bearing (to keep the image as small as possible). (If you imagine a light source that is right up close to the ball bearing, the reflected image of the light could be almost as big as the ball bearing.) If the light source is up near the telescope, the image of the light source will be minuscule, as desired. I find a halogen desk light works well, as the source itself is small and brilliant, and it can be placed near the telescope, directed at the ball bearing, without too much distracting light spillage coming back to you at the telescope.
[QUOTE=multiweb;1233462
You can use an artificial star no problem to collimate during the day. You'll need at least 20m or 40m line of sight on a grassy field to minimise seeing. It will get you ball park.[/QUOTE]
I believe collimation using an artificial star can be used to precisely collimate a SCT if it is done correctly. The distance of the artificial star is of importance as the diameter of the star in arc minutes must be smaller than the resolution of your scope, after all we can't resolve a star as a disk. For this to happen the physical diameter of the artificial star must be unresolvable at the distance it is observed. For an 8" scope using a common diameter star this would be in excess of 40 metres. Seeing and convenience are the best reasons for collimating this way. It is rare for seeing to be good enough to observe a star at high power for collimation .
Philip
The distance of the artificial star is of importance as the diameter of the star in arc minutes must be smaller than the resolution of your scope, after all we can't resolve a star as a disk. For this to happen the physical diameter of the artificial star must be unresolvable at the distance it is observed. For an 8" scope using a common diameter star this would be in excess of 40 metres.
It depends on the diameter of the artificial star - and that is what makes the image of the Sun on a small ball bearing such a good artificial star.
The Sun's image on a 10 mm ball bearing at a distance of 20 metres is about 22 microns diameter (about half the size of the hole in commercial "artificial star" devices), which translates to a subtended angle of about 0.25 arc-seconds, which is below the theoretical resolution of even a 12" SCT. It is a brilliant pin-point of light, which is easily observed when the ball bearing is placed against a black card or similar.
I believe collimation using an artificial star can be used to precisely collimate a SCT if it is done correctly. The distance of the artificial star is of importance as the diameter of the star in arc minutes must be smaller than the resolution of your scope, after all we can't resolve a star as a disk. For this to happen the physical diameter of the artificial star must be unresolvable at the distance it is observed. For an 8" scope using a common diameter star this would be in excess of 40 metres. Seeing and convenience are the best reasons for collimating this way. It is rare for seeing to be good enough to observe a star at high power for collimation .
Philip
Sorry, got to disagree with this one. When you have an SCT in excess of 10" looking horizontally at an artificial star there are a couple of things to look at. First the distance between the mirrors will be less than ideal when you're in focus at 30m or so. If you use extension tubes at the back it will only make things more difficult. Second, the scope is not pointing up and the weight repartition is all wrong. If you think you're collimated accurately when you do a star test at the zenith I can guarantee you'll be off by a fair bit. So it gets you close enough to do a star test. I see people tend to over complicate things as well. Ball bearings, diameter of the reflection dot, etc... Truth is that it doesn't matter. Just get a small length of discarded fibre optic or even a small fishing line, stick it through a piece of black cardboard and shine a led torch at the back. That's all you need to get you close. Do a star test to finish off and pick a star right up.
