Friday, April 10, 2015

Astro-Tech 8" Imaging Newtonian, Part VIII

Final refinement on the Astro-Tech Imaging Newtonian: rather than paint, powder-coat, or anodize the tube, I simply wrapped it with fake carbon fiber wrapper (used to make regular cars look more ricey).

A 1m x 1.7m cut cost me S$25 (about US $20) and I only ended up using half of it.


A useful tip: putting the holes in the wrap for the screw holes is much easier with a hot soldering iron!

The large hole for the focuser must be cut out with an X-Acto knife, box cutter, or similar implement.


The finished telescope actually looks pretty good, and no one's the wiser that it isn't real carbon fiber.
One caveat though, the wrap is rather soft. I dinged the telescope tube against an aluminum door frame, which cut into the wrap and exposed the metal underneath. I was able to push back the wrap over the exposed metal, but the finish is no longer pristine.  I would expect that for larger dings, it might be necessary to overcoat the damaged portion with clear varnish or similar to prevent the wrap from peeling.

Sunday, March 22, 2015

Berlebach Dovetail Clamp with Shoe for Takahashi EM-11

One problem with the Takahashi EM-11 (and EM-200) is that they are designed for Takahashi tube clamps, and don't come with a Vixen or Losmandy dovetail saddle.

ADM Accessories is probably the only company left making saddles (since Robin Casady no longer does).  I was not interested in a Losmandy saddle because it is large and requires large Losmandy dovetails which drives up the weight.

Vixen makes a compatible clamp, but frankly, it's ugly, and is of the wrong diameter. The photo below shows the (white) Vixen dovetail and how it's obviously of the wrong diameter and finish. It also uses two set screws which ding your dovetail and aren't terribly secure.  This was what I was using when I got the EM-11 because it is what came with the mount.
Luckily I found this Berlebach dovetail clamp with shoe.  It is not specifically made for Takahashi mounts (it's probably designed for Vixen Porta type mounts) but it has the 35mm hole spacing for the two central bolts.

Unfortunately, I discovered that the holes are 6mm diameter, not the 8mm needed for Takahashi mounts.  So I had to drill them out.  The Berlebach clamp uses countersunk machine screws, not the counterbored ones that the Vixen uses. Hence the Berlebach cannot use M8 SHCS machine screws, but rather M8 x 25mm countersunk ones.

A nice bonus of the Berlebach clamp is that its diameter is only slightly larger than the Takahashi mount top (at least for the EM-11) so it doesn't look too out of place.  The clamping shoe is also a nice touch, avoids marring your Vixen dovetails.

In my case, I used M8 countersunk machine screws with socket heads. The socket heads made the machine screw tops a bit too large, as is obvious in the first picture. I will have to find replacement machine screws (probably Philips) with slightly smaller heads that will fit exactly in the dovetail clamp's countersunk holes.  On the other hand maybe M8 bolts will never fit - the clamp was designed for M6 bolts.

Sunday, March 15, 2015

CCD Inspector Results

Here are CCD Inspector results from several of the telescopes and flatteners/reducers which I've owned or had the chance to use.

Astro-Tech AT90EDT FPL-53 triplet, 600mm focal length, with William-Optics 0.8X Reducer/Flattener II, APS-C chip (QHY8). Curvature is 22% which is not too bad, there is a hot spot in the center.
The same AT90EDT APO triplet, with the Orion 1.0X non-reducing flattener (which is very similar to the AT2FF). Curvature is actually slightly worse than the William-Optics reducer.
William-Optics ZenithStar 70ED, 420mm focal length, with William-Optics 0.8X flattener/reducer II.  Curvature is rather high at 34% on the QHY8 APS-C chip, this seems to indicate that the WO Flat II is optimized for higher focal length (and thus smaller field curvature).
My current imaging refractor, TMB design Lomo 80mm f/6 OK-4 APO triplet, with Televue TRF2008 0.8X flattener/reducer (400mm - 600mm focal length) and Canon EOS 40D.  The Lomo 80mm has a native focal length of 480mm, so within the specification of the Televue flattener. Curvature is 23% which is about the same as the AT90EDT with the WO Flat II. Stars are smaller, although that could be due to better guiding. I believe these results are comparable to the above two, since the Canon DSLR has an APS-C chip, same size as the QHY8 (although smaller pixels).
The same Lomo 80mm f/6 but with the much smaller Kodak KAF8300 chip (4/3 chip size, half the area of APS-C and one-fourth the area of 35mm full frame). Curvature is much less but minimum FWHM is only slightly better than the same scope and reducer with the Canon APS-C chip.
I've seen CCD Inspector results for the Televue NP-101, and frankly they don't look much better (and some reports are considerably worse) than my Lomo 80mm  with the Televue flattener. So I should probably not consider the Televue going forward.
Now here's a slightly different setup: Astro-Tech 8" f/4 imaging newtonian, with a Televue Paracorr Type 1.  It actually has better numbers than the much more expensive Lomo 80mm which is quite embarrassing.
Now for the absolute best case, I dug out some raw FITS from my 2012 telescope rental at GRAS/iTelescopes in New Mexico.  This is the curvature data for a Takahashi FSQ-106ED, KAF8300 sensor.  It looks bad because there's a major difference in the FWHM in the center and at the edge, but the FWHM's are very, very small. I'm not sure if I'm measuring it correctly.
If the measurements are correct, then all of my current results are basically invalid due to poor seeing and atmospheric conditions.

Saturday, March 14, 2015

Putting the Go-To Back in the AP600E

Howard of OnStep fame clued me in to these motors from 2Engineers. They are cheap Chinese steppers (these are not Vexta's - look at the prices) with 50:1 spur gearheads, 48 pulses per revolution, and 0.4 amperes per phase.

The value proposition of these cheap motors is that they are exactly the same form factor as the stock Nippon Pulse motors in the AP600E QMD.

So I fabricated another mounting plate and attached two of these cheap motors to the plate.  Here we can see the 50:1 motors and the stock 150:1 motors that came with the AP600 mount:



As we can see they are pretty darn identical. I reused the old cover plate from my 2012 Go-To conversion using the Littlefoot. The new motor plate was much easier to fabricate because I didn't need to use the step bit. Also, because the replacement motors have a circular cross section, they don't interfere with the insides of the AP600 mount anymore. This is the biggest benefit of the new motors' being physically identical to the old ones - they are completely drop-in replacements.  In fact I could have used the original motor plate, but decided not to.

The new motors can slew at 160X sidereal (about 0.7 degrees per second, glacial but actually slightly faster than my EM-11 Temma 2 Jr). This is with 0.4 A per phase configured on the Littlefoot. So at 48ppr, 50:1 gearbox, and the 192-tooth worm on the AP600, it's 2.81" per full step, and if doing 1/16 micro-stepping, 0.18" per micro-step. I'm fairly certain the inherent periodic error in the cheap gearheads themselves would be in the 2" to 4" range, peak-to-peak.
Since the AP600 is in Manila now, I haven't had the chance to test the new motors "for real" - I have so far just been limited to slewing the RA and DEC via the direction buttons.

Tuesday, March 10, 2015

Astro-Tech 8" Imaging Newtonian, Part VII

I've blogged extensively about the modifications and improvements I've made to my Astro-Tech AT8IN Imaging Newtonian.

Part 1 - spacing the Keller reducer, and first light

Part 2 - oblong stars, and thinking about reinforcing the tube under the focuser

Part 3 - reinforcing the tube under the focuser, and relocating the collimation locking bolts

Part 4 - replacement (stiffer) collimation springs

Part 5 - an even larger and thicker reinforcing plate under the focuser (at some point I also replaced the stock GSO linear bearing focuser with a Feathertouch)

Part 6 - gave up on the Keller reducer, switched to a Paracorr, and extended the tube to reduce focuser racking out



Now for what is hopefully the last modification to the AT8IN.  After all the improvements made above, the newtonian was usable - it held collimation, and the stars were round across an 8300 sensor. Unfortunately, one side effect of all the modifications (as well as the tube falling a couple feet) were a lot of dings, dents, holes, and rivets on the tube.  In other words, the AT8IN became very ugly.

A new carbon fiber tube from Klaus Helmerichs would cost about $500, which I was not really willing to spend.  Instead, I commissioned a local machine shop to roll an aluminum tube out of 2mm thick sheet.  The tube ID is 232mm and the OD is 234mm.  The tube is 840mm long and has the correct spacing for the lengthened original tube.

The tube weight is about 3kg - about the same as the original steel tube, which had 0.8mm wall thickness.  So there is no weight savings.  The new tube is (I calculate) about three times stiffer than the original steel tube.


The only challenge at this time is that the tube is unpainted (I asked the machine shop not to paint, powder-coat, or anodize it, because I had to drill holes in it for the hardware and focuser).  I could remove all the hardware and have the tube powder-coated white; that's the best and most aesthetically pleasing solution.  However the tube cost $200 and a powder coat will cost another $100 plus, bringing the total cost very close to that of a carbon fiber tube from Klaus!

Teleskop-Service sells replacement carbon tubes for the GSO newtonians (actually made by Klaus Helmerichs), the weight is about 1kg - 1.9kg.  So there really is not that much weight savings in going to a carbon tube (right now the telescope weighs 12kg, it would be reduced to 10kg with a carbon tube).

So I am probably just going to wrap it with 3M vinyl wrap with a carbon fiber weave pattern.  So the tube will look like carbon fiber but will be heavy and metal.  A 1-meter length of the vinyl wrap is only $20 so the price is definitely right.