Monday, September 30, 2013

Astro-Tech 8" Imaging Newtonian, Part III

After last night's funky stars, I decided to make some modifications to the Astro-Tech 8" Imaging Newtonian in an attempt to improve the optical performance.

From this very good blog post I was able to get some ideas on how bad the focuser sag is on this telescope (check out the video in the link).  I had a look at my focuser and it's also the same CPC MR7W linear bearing, a not-bad focuser (I contrast-enhanced the area of the linear bearing where the part number is printed). Maybe I can hold off on buying a Moonlite, as my gadget budget has been reduced to zero.

First item of business was to remove the focuser.  To do this, you must first remove the two grub screws that hold the focuser body to the curved plate (one of the grub screws is visible in the above picture).  Then remove the four screws that hold the curved plate to the telescope tube.  These are M4 bolts with a nut on the other side; ensure that the nuts don't fall into the innards of the telescope.

Find a sheet of cardboard (I used some spam postcard from the local Omega dealership, which was extolling the America's Cup Omega Team New Zealand). Put the cardboard inside the tube and up against the focuser hole and bolt holes; and trace the outline onto the cardboard.

Transfer the pattern to an appropriate sheet of aluminum.. Make sure that the aluminum sheet is small enough so that it clears the bolts for the finder bracket, and the baffles behind the focuser. Approximately a 5" x 5" aluminum sheet is the correct size.

Then cut out the focuser hole and drill the bolt holes, by whatever means necessary.  I ended up drilling dozens of tiny holes along the focuser hole line, then whacked the remaining aluminum bits with a chisel until the center fell out. A very laborious task.  If you have an appropriately sized bimetal hole saw, you'll produce a much nicer hole, much faster.

It's also necessary to fit the aluminum to the curve of the tube.  Resist the temptation to use the telescope tube as a form, you might end up denting the tube.  I found a round piece of wood and hammered the sheet against it until a rough curve was formed.

The aluminum plate actually goes up inside the tube;  then re-affix the focuser base plate.  The original bolts won't do because they are too short. You will need M4 x 20mm bolts.  The original nuts can be used.

The focuser assembly feels much more rigid with the aluminum plate inside. Note that I didn't bother to paint the plate; although it is shiny, I'm gambling that the baffles would block any stray reflections from the plate.

The second step is to relocate the collimation locking bolts. As can be seen in this photo, the collimation bolts (black) are spaced 120 degrees apart, and the locking bolts (white) are also 120 degrees apart equidistant between the collimation bolts.

I quickly discovered that this arrangement is no good; after achieving a good primary collimation with the black bolts, when you torque down the white bolts, the collimation shifts.

The solution is to drill new holes as close as possible to the black collimation bolts. Use a small drill bit (like a 3mm) and go slowly.  I did not remove the mirror from the cell so had to proceed very slowly. If the drill bit punches through the metal, it should hit the metal of the mirror holder; but you could slip and ding the mirror. So better safe than sorry. If you drill the holes with the mirror in place, make sure to vacuum up all the swarf and metal shavings afterward; you don't want that stuff getting on the mirror coating.

After the three holes have been drilled, put in a 4mm, then 5mm drill bit and enlarge the holes.  Once the holes are 5mm in diameter, you can tap them with an M6 tap.  The locking bolts are M6 thread. This is the result.  Now collimation only shifts slightly when the locking bolts are torqued down. More importantly, because the locking bolts are right next to the collimation bolts, the collimation shift is very controllable.

I also replaced the white collimation locking bolts with some M6 leg levelers with round plastic feet that cost $0.50 apiece.  This isn't strictly necessary, but it's useful to be able to distinguish the collimation and locking bolts by feel while peering through the focuser.

And here's the result: seven, 5-minute sub-exposures, unguided, of M27.  Depth of the 5-minute sub-exposures is about the same as 20-minute subs with the AT90EDT.  However stars aren't nearly as round. Hopefully better collimation with rectify that..

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