Thursday, November 07, 2013

Astro-Tech 8" Imaging Newtonian, Part V

I am still seeing comatic stars in the AT8IN in spite of my best efforts.  Before adjusting the laser collimator, I have decided to stiffen up the Newtonian tube some more, using a much larger aluminum plate.  Actually I wanted to use a steel plate that went a full 360 degrees on the inside of the AT8IN tube (it would also match the coefficient of expansion of the existing rolled steel tube) but I don't have such steel and I don't have the roller.  But I did have a good supply of aluminum sheet.

So I decided to use an entire pre-cut sheet. Here's the before (small reinforcing plate):

And after (much larger plate, bolted with the four M4 focuser base bolts, four additional M3 bolts, and two M4 bolts of the finder base).

The previous reinforcing plate was rather small, and held in place by the four M4 bolts of the focuser base itself. I had made it small to avoid interference with the spider vane bolts and the two bolts that hold the finder bracket.  The new plate is much larger - I used an entire pre-cut sheet, didn't bother cutting it.  Due to its size, it interferes with one of the spider vane bolts and with the finder bracket bolts, so I added holes for those.

I removed the spider, secondary, and holder, then hand-bent the aluminum sheet and fit it inside the tube. The rear end of the sheet is right up against the first baffle.  I then marked the four holes for the focuser base, and the circular hole for the focuser itself, then removed the sheet and drilled the four M4 holes.  I then put the sheet back in and bolted it in place.

With the aluminum sheet held in place by the four bolts of the focuser base, I drilled additional holes for the finder bracket.  I also added four holes at the corners of the aluminum sheet, by marking the exterior of the Newtonian tube and drilling through the tube and through the sheet behind it.  This is the only destructive and non-reversible part of this modification.

Because I was too lazy to remove the aluminum sheet after it had been bolted down (and I didn't want to use the "hundreds of holes" technique to cut the large hole for the focuser drawtube) I simply took a 76mm bimetal hole saw to the aluminum sheet.  The hole saw cut through the aluminum in less than a minute, and also gouged the internal diameter of the focuser base. Ugly, but not visible once the focuser was mounted.  I painted the gouge with some automotive touch-up paint even so.

So now the focuser base should be much more secure, there should not be any shift in collimation when pointing to different areas of the sky.  Of course, the Taiwanese focuser itself may be the next suspect.  I am somewhat peeved because between the cost of the AT8IN, the Keller reducer, and a Feathertouch (if I add one), this scope would cost the same as a used Takahashi Epsilon 160 being sold on Astromart right now (7 November).  Of course this one would be faster (f/3 versus f/3.3) and have a larger aperture, but it's not as solid and the optics are almost certainly inferior. And it's not a Takahashi.