A Tale of Two Telescopes

As I mentioned before, I am building a 4.25" spotter / CCD imaging scope to attached to the main assembly. This is not to be confuser with the actual finder scope which with direct the telescope.

For this task, I chose to refit an old OTA I had laying around (my first telescope as a matter of fact).

For those of you interested in building a similar scope, here is the link to the plans I used back in 2001: http://www.lymax.com/cosmicone/4inchdob/4inch01.pdf

Anyway, I started by finding a 5.5 I.D. 6 foot length of 6061-T6 aluminum tubing at the local scrap yard (Frank Sahd Salvage Center in Columbia, PA) and convinced them to let me cut off a shorter 6-foot piece (it was 16 feet long originally, and I suspect it was some sort of street light pole). Then I trimmed it to 48"in length.

Next, I came up with a primary mirror holder and collimation design by scrounging the internet. I eventually adapted a design used by Rick Scott for his Lurie-Houghton Project and located here: http://members.cox.net/rmscott/lh_scope/scope_rear_view.html

After settling on the design and having the parts machined ($150), I shifted my focus to the tube itself. It was in pretty rough shape when I got it, so I sanded the outside down until it was shiny and touched up the surface with a bunch of hand sanding.

Next, I determined where all the holes should go, namely that the focuser hole should be located exactly 37" from the rear mirror mount bolt holes and the secondary spider close by that. I was able to find a local metal finishing shop to commit to polishing the tube with a high shine and anodize it blue, so after drilling all the appropriate holds, I will drop the tube off next Monday, so stay tuned for the results!

As for the focuser details, I settled on the Orion 2" Crayford style Focuser available here: http://www.telescope.com/control/product/~category_id=focusers/~pcategory=accessories/~product_id=13039/~sSearchSession=a0b27e12-903e-4088-861a-b3ff5ea7c714

I had contacted Orion about a month ago, but despite speaking with several top managers, I was turned down cold on a donation. Thankfully though, after a month of stewing on the problem, the thing went on sale for $65, and even with shipping, it was still $25 cheaper than it could have been.

I also purchased a cheap, but awesome 60mm neon blue LED case fan to mount on the back of the primary mirror assembly, both to act as a cooling device when setting up, and to make the thing look awesome, since the OTA will also be blue anodized.

Here's the link from Newegg: http://www.newegg.com/Product/Product.aspx?Item=N82E16835119021

Suffice it to say, it looks awesome, though I will take more pictures upon final assembly. I also centered the fan on the hole in the mirror plate and drilled and tapped 4mm metric bolts holes for some stainless steel hardware.

More Spider Stuff

Despite the snow, I continued the work on the spider today, to great results.

I cleaned the surface with Easy-Off over cleaner to rough it up, then I epoxied the 1" washer to the bottom of the mirror holder tube.

The next step is to paint the whole thing flat black and silicone the mirror in place. After that, I'm going to concentrate on getting the tube drilled and off to the the anodizing shop for polishing. With any luck, the next time you see the tube, it will be shiny and blue, ready to install the guts and get collimated up!

The Itsy Bitsy Spider

Well, folks, I admit that I have been very intimidated by the secondary mirror holder and spider assembly. So, I decided to experiment a bit on the finder scope. Oh, did I mention I'm building a 4.25" f/10 finder scope for this Beast?

Anyway, I got my first inkling of a decent design from Barislav Bracun and his website: http://www.scopemaking.net/dobson/ota.htm

Granted, it was a pretty rough example made from PVC and such, but I decided to upgrade the design a bit for a 3-vane 1.0" diagonal spider mount, and the result was pretty satisfactory. I've included some pictures and directions below.

If you have all the tools (taps, dies, drill bits, etc.) then this spider only costs about $15 in parts from ACE Hardware! That is a far cry from the $155 quote I quote from another company for this part, so I'd say I've saved money once more.

• First, find a suitable hexagonal barb coupling to drill. Mine was a 1/2" Barb x 3/4" FPT Female Adapter in Brass (Watts Water PEX Model # P-516) - $5.99

• Next, mark the middle of the hexagon faces on three side, each 120 degrees apart (every other face) and drill an appropriate hole for tapping. In this case, I used 10-24 threaded rod, so I drill a #25 drill hole.

• Then, tap the hole straight through and test the threaded rod fit. In this case, I used a 10-24NF tap in the #25 holes I already drilled.

 

• Next, mark, drill, and tap three holes on the top of the adapter, perpendicular to the faces you did not already drill for the spider vane rods. Make sure you use a small screw size that will clear the inside threads of the main adapter but be close enough to ensure clearance for the guts of the spider mount. (In this case, I used a #30 drill and tapped a 4mm metric bolt hole). Then, test fit the bolts for clearance.

• At this point, you have a brass adapter that has been drilled and tapped to accommodate three 10-24 threaded rod spider vanes and three collimating bolts. Next, you need to populate the assembly. In this case, I used a 6mm metric center bolt with two washers offset with a #140 compression spring and 1" flat washer and two locks nuts. Don't lose me here; it's not that complicated and anything that fits will do. Then test assemble everything. At some point I will put together some better instructions for all you ATM's out there.
• The assembly order should be: 6mm bolt, washer, adapter, washer, spring, flat washer, nut, lock nut.

• Then, cut the threaded rods to length for your optical tube assembly. Use lock washers and a lock nut against the brass adapter to ensure it won't unscrew in the future. You can use Locktite in the nut in the final assembly, but remember that you'll have to leave 1 vane off in order to fit it into the tube at installation.

• One point of notice is that the upper washer above the spring might not fit around your bolts in a smaller spider like mine, so I had to file 3 round indentations in the washer at 120 degrees apart. This shouldn't be necessary if you have a large assembly.

• Next, you will cut an aluminum tube of appropriate size at a 45 degree angle for later mounting of the secondary mirror. Once the cut is made, trim below the cut at a length sufficient to clear the bolt and nuts inside. It doesn't have to be long, so long as it clears the bolt and mirror. Then test fit for clearances. At this point, you have constructed your spider and vanes. The next step will be to complete the secondary mirror holder.

Secondary Cage Almost Done

Progress on the scope is advancing, albeit slowly.

But, the good news is that with the installation of the Moonlite Focuser, the secondary cage is nearly complete. I cannot say enough good things about Ron at MoonLite. He really helped out with his generous focuser donation, and I am excited to stump his products to other ATMers.

So, everybody BUY MOONLITE FOCUSERS! They're Grrrreat!

Next, I'm going to work on the secondary mirror holder and spider assembly, but I still might have to do some finagling to get ahold of the FPI Protostar assembly I need. I drilled the tubes to accommodate their 4-vane spider, but I might be able to get another type in there or make my own....

I've posted progress pictures below. Let me know what you guys think.