Ultra-portable 8" f/4 travel scope

[janoskiss]

For sale is my 8" (200mm) f/4 ultra-portable custom-made Dobsonian. This is an unusually large aperture telescope for something that you can take on an international flight (economy class, regular luggage) or in your backpack for hiking/cycling trips.

I've travelled with it between Australia and Europe and within Europe. The base packs up into flat panels and goes in the check-in luggage along with the top half of the tube and the truss beams. The primary mirror box comes with me in the carry-on luggage. Great for exploring DSOs in parts of the sky not observable from your usual observing locations.

I've also gotten around with this scope on a push bike. Without disassembling the base, a small trailer is required. Partially disassembled (nowhere near to the extent shown in attached photo #3), the scope fits in a modest sized backpack. I've taken it places that way as well. Assembly & collimation then takes about 10 minutes.

As I've worked long and hard creating this scope (starting with a Synta/Skywatcher solid tube OTA), I am reluctant to let it go. But I have not used it in over 4 years so better it go to someone who will use it more often.

Comes with motorised focusser (Orion AccuFocus), 6x30 finder, primary mirror fan on removable plywood mounting plate, 12V deep-cycle lead-acid battery, teflon + ebony-star-like laminated azimuth base (upgraded from the vinyl+furniture sliders shown in photos).
Rigel quickfinder and 25" Obsession seen in the photos are not included, sorry.


$500 complete
$450 without AccuFocus and fan (for up to ~100x power you don't need them)
Free pickup from Preston, Victoria (~7km north of Melbourne CBD).
Postage Australia wide at buyer's expense.
Rough estimates of postage cost to selected destinations (using Aust. Post parcel post calculator and my guess at size & weight of package): Melbourne $20, Sydney $50, Hobart $60, Perth $110, Townsville $130.

Price is ONO negotiable. Also, if you're buying for a school or astronomy club and/or have educational purposes in mind, I'd be more inclined to drop the price, possibly even donate it to a worthy cause (but not to individuals). Note that this is a specialised scope best suited for long distance travel, or perhaps for the cyclist/motorcyclist/backpacker deep-sky astronomer who wants portable aperture. It is not the ideal scope for home/permanent-location use. For that you're better off with e.g. a GSO 8" f/6 Dob.

I've uploaded some photos but you can see more in this thread (http://www.iceinspace.com.au/forum/showthread.php?t=11358). To see earlier photos and read more about the painstaking development see this thread (http://www.iceinspace.com.au/forum/showthread.php?t=10887). Note: The furniture sliders and vinyl record azimuth bearing have been replaced with teflon on dimpled ebony-star-style laminated surface. See next post for more details.

Detailed description follows in the post below.

[janoskiss]

Mechanical properties and static stability

The dob base was made with special care and holes in panels and joining brackets have been drilled with precision. All the brackets and panels fit together easily but snugly. This makes for a very sturdy and solid structure. All bolts use wing nuts. You only need a Phillips-head screwdriver for assembly/disassembly.

The truss tube structure is very rigid with no flex or twist: as good as a solid tube. It requires no tools for assembly (wingnuts tighten by hand). The ends of the truss beams are correctly offset by ~22.5 degrees, so the 8 of them (16 ends top and bottom) make the full circle (360 deg) without undue stress. The ends of all truss beams are labelled as to exactly where they should go and how they need to overlap (which goes over, which under). Collimation does not change upon disassembly and reassembly. Inner surfaces are painted matte black.

Collimation

Collimation is super easy because the tube is short enough so that you can look through the focusser (with collimating tool, Cheshire, in place) while turning the collimation bolts with your fingers, which are nice and beefy with knurling for grip. The locking bolts also work great without any tools and the scope tends to hold its collimation very well (even after transport, disassembly, reassembly; but be good and always check it anyway, hey). The mirror is centre spotted. Short collimating tools are a bit easier to use than long ones but either will get the job done. Star test refinement is also feasible in decent seeing (but it's unnecessary, overkill).

Azimuth and altitude bearings

The azimuth movement has been upgraded with dimpled laminate (closest thing to ebony star I could get) and teflon pads. (Gone are the vinyl record and furniture sliders you see in some of the photos. Teflon setup works 10x better.) The same type of laminate is used on the sides with the alt-bearings, which you can see in some of the close-up photos. The alt bearings are PVC plumbing hardware and they rest on plastic bumpers/sliders and are also supported by tensioning bolts (see Balance below). Perhaps the bumpers could be replaced with teflon but I found them plenty adequate and the tensioning bolts are precisely aligned for their height, so I think it's best to stick with them.

Balance

Counterweights are not required unless you mix very light and very heavy eyepieces (photos show a little speaker magnetically attached to the top of the tube; there is no need for that normally). There is a range of alt-bearing mount points to get the correct balance for your eyepieces, and the tensioning bolts on either side work well if you happen to end up somewhat off-balance and for adjusting the resistance in the alt-movement response.

Focusser with motorised controller

The optics and the mirror and focusser box were originally a Skywatcher/Synta solid tube. The focusser is the original 1.25" rack-and-pinion. It does have some backlash but I am including an Orion AccuFocus electronic focuser with the telescope (with a hands-off-the-scope cable-remote controller), which makes for a much nicer viewing experience at powers over ~100x. For wide field up to ~100x I prefer no fan and manual focus (removing the AccuFocus and fan) to keep the tube assembly lighter. You can see the AccuFocus unit fitted to the scope in the second photo above, where the 8" f/4 is pictured next to the ASV's 25" f/5 Obsession.

Mirror condition

The parabolic primary mirror is over 10 years old and has never been recoated. It is still quite capable, but definitely not as good as new. I haven't washed it for some years so it's collected a fair bit of dust. I'd be happy to wash it before sale and get it as good as it gets. The figuring is pretty good for an f/4, and with the right barlow+EP combination, it reveals good detail on planets at 200-300x. I'd say 300x is its practical limit, and even that's a bit much for hand-nudging the mount (see Usability ... below).

Finder scope

The photos show a Rigel quickfinder. This is not included (it got stolen :( along with another scope). The Rigel base is still on the scope and the original Synta 6x30 finder will be included.

Mirror fan and battery

I will include the primary mirror fan assembly and the 12V battery, but I cannot vouch for the battery's condition. It's nearly 10 years old and I haven't used it in 3-4 years, maybe more. You'll need your own deep-cycle 12V lead-acid battery charger.

Usability, eyepieces, barlows, observing tips

The Dob mount, while sturdy enough, is still a compromise between stability and portability and low weight. With manual focussing it is good up to about 150x, where it starts getting annoying. With the AccuFocus unit, 250x is not a problem. Nudge-nudge tracking gets tedious though at such high powers and there is some wobble. But really, this is a deep sky and not a planetary scope and 30x-150x is what it's best at. The 2mm exit pupil sweet spot is at 100x (8mm eyepiece). The Pentax XF 8.5mm works extremely well with this scope, as do the XW-10 and XW-7 but their weight makes them less ideal. (No eyepieces are included with sale.) Long barlows also let you use most EPs with minimal abberations at the edge of the FOV. Shorty barlows work, but you will lose sharpness near the edge of the FOV. Same goes for most unbarlowed EPs due to the short f/4 focal ratio (but not the Pentax XF-8.5; or the equally capable but less practical XW-10 and 7). Masuyama style Plossls fare better than regular 4-element ones, e.g. GSO or even Televue. The 25mm Antares Elite served me well as a finder/wide-AFOV EP.

Best barlow to use with this scope is the Orion 2x fully-baffled Japanese made barlow (http://www.thienvanviet.com/ong-kinh-orion-2x-barlow-lens-125-fully-b-p205.html). (No longer made afaik, but if you can get it, it's better than Televue regular barlow, Powermate, Orion Shorty-Plus, etc.) It makes just about any EP sharp edge to edge in this fast scope.

Misc

Cosmetically, there are some scuff marks on some of the panels, also some marks on the exterior of the focusser/spider tube section. There are bits of velcro hook tape here and there for AccuFocus hand controller resting spots, the battery, etc, and cable guide hooks for the fan wiring.