Astronomy.CJDawson.com

Focusing Aid


This device is specifically designed for use in AstroPhotography. It's purpose is to split an unfocused image, then by focusing the scope the images are bought together to create a single sharp image, removing the device should then reveal and very well focused object.

Parts List
MDF (16 mm Thick, but it doesn't matter too much).

Tools used
Saw
Router
WorkBench
Pencil
Pair of Compasses
Ruler

Assembly Instructions
As you can imagine, most of the work invloved in this project in getting the measurements laid out correctly. If you get even one measurement wrong, then you will need the throw away the entire project and start again. Fortunatly, a piece of MDF can be used to make several of these focusing aids.

So to the measurements.

It's worth taking the time to carefully mark out all the measurements prior to making the first cut, this way you'll be able to see the what you will need to do.

As you can see from the image, I marked out not only the places where I needed to cut, but also the locations of features of the scope, just in case I needed to make any modifications during the cutting phase.

Here are a list of the key measurements that you will need to mark out.

Concentric Circles. (all these have the same center point)

Diameter Radius description
240mm 120mm Overlap to allow for slippage
230mm 115mm Outer edge of telescope casing
226mm 113mm Inner edge of telescope casing
202mm 101mm Outer Mirror outer edge
73mm 36.5mm Seconadary mirror recess

Once you've marked those circles, draw a line from the center to the edge, marking out the radius. Next measure 120°, and draw a line. Then repeat the process, to mark out a third line. You should now have devided up all the concentric circles into thirds, This will allow you to accuractly place the centers of the three smaller holes. In order to be useful the three focusing holes will need to be placed so that they allow light through to the scope. This is accomplished by placing the hole between the 36.5mm and 101mm circles. This means that the maximum size of the hole is about 64.5mm. The larger you make the hole that more light will be let through to the scope, but at the same time the more difficult the final (and critical) focus will be. The smaller the hole the dimmer the image, but the final sharp image may be easier to pick out. The ideal solution here would be to make a couple of these aids, each having different sized holes.

For this aid, I've chosen to make 60mm diameter holes. These are comparible to some small scopes so light gathering shouldn't be a problem, also it will prove if this device is really worth the effort making. If I get used to using it I will consider making others of different sizes.

The picture on the right shows the board after making cutting out the slot around the edge of the scope, this was done using a plunge router, with a straight cutting bit and a circle cutting arm. This slot will be used to stop the device from slipping off the end of the scope.

The image to the left shows the device after the cut out for the secondary mirror has been made. I found that there is not enought clearance to use a flat board and since I didn't want to loose the other markings, this was the simplest solution to getting the focuser to lye flat.

Next image on the right shows the holes cut out from the board. All that is left to do now is cut the board loose from the rest of the MDF, the trim up the outer edges.

Having used this aid on several occasions now, I'm finding that it is still quite hard to get the final focus on the image, this aid does help, but I think that a smaller holed version would be more useful. Also the focusing screen on my camera does not seem to be helping much, as the image is very dull and the cental aiming point gets in the way.