Focuser Project

It's been some time since I last posted and even today the topic is an on going focuser update project. The purpose is to make a crayford style focuser to replace the rack and pinion focuser of the AstroMaster. The reason for this focuser update is that the stock focuser does not have enough backfocus to allow for DSLR primefocus photography as it is. In order to take the photos I've posted in my blog, I had to do a quick and dirty modification of the stock focuser and it has left me with some slack between the drawtube and the focuser frame.

In order for prime focus photography to work, the telescope needs to have enough back focus to allow for the DSLR imaging sensor to move to the focal plane. The amount of back focus needed depends on your setup (camera+adapter+acessories). Quite often with the "budget" newtonians this is not possible without doing some modifications to the telescope. There are a few possibilities to get the focal plane to hit the sensor of the camera with a newtonian:
  1. Move the main mirror inwards in the tube. This often means shortening the telescope tube and also there is the possibility that the secondary does not catch all the light coming from the main mirror, depending on the size of the secondary.
  2. Get the focuser to move inward the necessary amount. This can be achieved by getting a lower profile focuser or modifying the original.
  3. Move the focal plane with the use of a barlow lens or other optical means. This however means that there are more optical elements in the light path, amounting to additional light loss and possibly more coma due to extended focal length.
In my case I started off with modifying the original focuser. The DSLR I'm using (EOS 1000D) has the sensor 44mm from the front mounting flange + additional 15mm for the T-adapter and the adapter for the 1 1/4 inch eyepiece holder, for a total of 59mm backfocus. After doing some measurements I needed some 30mm of additional backfocus for the camera to achieve focus. What I did was first of all remove the top portion of the drawtube frame. The top of the drawtube frame is what keeps the drawtube from wobbling, so when I removedit, I had to improvise with some small screws and teflon strips. That got me some 15 mm and the other 15mm comes from attaching the camera directly in to the drawtube, without the eyepiece holder, using a self made adaptor.

This setup has gotten me this far, but I started thinking about a rugged re-usable focuser. Meaning that if I switch scopes I can use the same focuser  with small changes. The basic idea is to have a straight base and then have an adapter plate to fit to the shape of the telescope tube. Also I want to have the possibility to adjust the tilt of the whole focuser.

Rummaging through the metal scrap at work, I found a nice piece of aluminium to act as the starting point for the focuser frame. A 40mm diameter hole was drilled in the center (to be finetuned later).

I machined the rough dimensions at work with a larger milling machine, the fine tuning and smaller work will be performed with a smaller milling machine.

After some hours (and lots of aluminium chips) later the main components were finished. From left to right the components are;
  • The drawtube
  • Main frame of the focuser (one piece)
  • Focuser axel
  • Holder for the focuser axel
  • Eyepiece holder

The pros at work were nice enough to make me the draw tube and eyepiece holder with the lathe there. The height of the focuser frame is 50 mm and together with the drawtube+eyepiece holder is a total of 70mm. Height is 10mm less than the current, quick and dirty, modified stock focuser on my AstroMaster.

Currently there are no bearings in the main frame of the focuser for the drawtube. The mechanism seems to work fine since the sides polished to some degree. The white part in the focuser axel holder is teflon, which is used as a bearing for the focuser axel. The screws (one visible in the photo) protruding from the backside are used for setting the correct pressure to the axel-drawtube contact.

The drawtube axel is made from acidproof steel, which makes for a "sticky" contact between it and the drawtube. No additional friction providing material is needed.

The four large hex screws are used for adjusting the tilt of the focuser frame (they are at a staright agnle to the base even though in the photo they don't seem to be). The smaller screw on the right side of the frame is used for locking the drawtube if needed. The frame also has some spare to acommodate for a larger drawtube, for 2 inch accessories, if necessary.

All in all the focuser turned out fine. Now what is needed is some black coating on the inside of the drawtube and possibly on the outside (it's a bit too shiny) and the actual knobs for the focuser axel.

Clear Skies


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