Saturday, October 11, 2014

Skywatcher Star Adventurer Tripod

I wanted to find a more suitable tripod than the flimsy Benro A350EX, and less clumsy than the Vixen Polaris tripod.  I initially purchased a Sirui N2204X, carbon fibre, with detachable leg, etc. It was not terribly stable (about 4-5 seconds damping time) in spite of its alleged 33lb rating.

But, due to a 20% discount promotion at KEH, I was able to purchase a beat-up Gitzo G1340 Pro Studex tripod, without centre column.  That last part is important, because the centre column induces instability. Coupled with the rubber bungs on the Gitzo's feet, damping time is now 2-3 seconds. The Gitzo is actually more stable than the larger aluminium tripod from my Vixen Polaris. This is a tripod that's rated 20lb.  The total weight of this setup (Stellarvue SV80ED, diagonal, eyepiece, mount, and tripod) is just a hair under 20lb, and can easily be lifted and carried around.

Of course, Gitzo tripods are eye-wateringly expensive new; a major drawback.  A new 2-series Gitzo carbon tripod and the Star Adventurer would cost very close to an Atlas or EQ6, so definitely not cost-effective anymore.

I guess we all can derate Chinese tripod weight ratings by 50% now..

Sunday, September 21, 2014

Sirui R2004 Tripod for Astronomy

I was looking for a lightweight tripod that would fit in carry-on baggage and be useful for my Star Adventurer.  The Sirui R2004 tripod seemed like the perfect fit: it is 520mm collapsed (will fit diagonally into a carry-on suitcase), weighs about 1.8kg, and is rated for 15kg.

It also only costs S$85 ($68 US) which is mind-bogglingly cheap.

All of the Sirui 2-series tripods (R2xxx, N2xxx, T2xxx) have 28mm thick legs, only differing in the number of leg segments, whether one leg can be detached to form a monopod, and the material of the legs - aluminium or carbon fibre.  The R2004 is the most basic model, and hence the cheapest.

As payload I used a Sky Watcher Star Adventurer, APM Lomo 80mm apochromatic triplet in a William Optics tube, 2" Televue diagonal, and a 4mm Vixen LV eyepiece giving 120X magnification.  The total weight of all this is 8 kg, well below the alleged 15 kg capacity of the Sirui tripod.

The long and the short of it: at 120X, it takes a mind-boggling 8 seconds for vibrations to damp out after touching the focuser. Hence this tripod is wholly incapable of carrying the mentioned load.

As a control, I used the short aluminium tripod from a Vixen Polaris; this has the benefit of much larger legs, is much shorter, and has a tray to reinforce the legs.  With this tripod and the same mount and telescope, damping time is reduced to 4 seconds, which in my opinion is still excessive.

In short, the Star Adventurer is overloaded with the Lomo 80mm, and the Sirui tripod is also overloaded with an 8 kg load. Of course 120X is equivalent to 6000mm focal length.  So the Sirui tripod and Star Adventurer may still prove useful for astrophotography.  But as a visual setup for planetary, this combination is unusable.

Friday, July 25, 2014

Sky Watcher Star Adventurer, Part II

I had an old counterweight shaft and counterweight for an EQ1 mount lying around (the EQ1 had already broken some years back and I had thrown it away).  The shaft has a large-pitch thread on one side, and an M6 tapped hole for the toe saver on the other side.

I drilled out the M6 tapped hole with an 8mm bit, then epoxied an M8 bolt into the hole.  I then cut off the head of the bolt leaving the M8 threaded portion exposed.  A more robust solution would have been to drill out the M6 hole with a 6.5mm or 7mm bit, then use an M8 tap to cut threads into the hole.  The M8 bolt would then screw into the threaded hole, a more secure fastening than just epoxy.  But I did not have an M8 tap handy.   The epoxied bolt seems secure enough (I used Araldite brand epoxy) and hasn't wobbled through a 2-hour plus testing session.

With the stock EQ1 counterweight, the Stellarvue SV80ED still would not balance; however the result is much more in balance than having no counterweight at all.

However, the counterweight did not really improve the guided performance, or reduce the periodic error to in any meaningful way (and I did not expect it to).  Guided performance is still around 2" RMS, which I believe is the best this mount is capable of.  Hence it should be used at a pixel scale of 4" to 6" per pixel, or 200mm to 300mm focal length.

A longer sampling period has confirmed that the worm fundamental is 10 minutes, hence the worm wheel has 144 teeth (like the Vixen Polaris, Great Polaris, and CG-5).  This was confirmed by a poster on Stargazers Lounge who tore down the Star Adventurer.  The fundamental is about 30" peak-to-peak.

I got a better polar alignment this time (as shown in the minimal DEC drift in the graph above) and in the process noticed that the polar scope is pretty well-aligned - better than my PASILL3.  It's not perfect, but it's good enough that I'm not going to mess with it.

Thursday, July 24, 2014

Sky Watcher Star Adventurer Portable EQ Mount

I purchased this portable mount in London from The Widescreen Centre in London (very close to Baker Street!) instead of lugging my old Vixen Polaris to the UK.  I got the "Astro-Photo Bundle" for £299.00 (about US$ 500) and am still waiting for the £40-odd VAT rebate, which would bring the price down to about US$ 400.  This mount is also available from Perseid in Malaysia for 1700 MYR, or about US$ 540.

The build quality is better than expected, with some nice touches such as the worm-and-sector drive for the altitude axis of the equatorial wedge, a design very reminiscent of the Astro-Physics Mach1 GTO, albeit with much slacker tolerances. There even is a ratchet on the altitude locking bolt (again, just like the AP). Here I have secured it via the 3/8" bolt to the aluminum tripod of the Vixen Polaris.  The mode dial and power switch is also visible here:

Right Ascension and Declination clutches are the large knurled plastic wheels.  On a less sturdy tripod, it is quite easy to knock off the polar alignment when tightening or loosening these clutches.

The declination slow-motion knob is visible in the photo below.  Note that the equatorial wedge, declination slow-motion, counterweight shaft, and counterweight, are all part of the "Astro-Photo bundle" and are separately priced if one purchases the base package.  On the other side of the Star Adventurer body are two electric RA slow motion switches (12X sidereal) to assist in centering objects, since tightening the RA clutch is a fiddly affair that tends to throw objects out of the field.

There is a fairly nice polar scope, although the reticle illuminator is attached to the far end of the polar scope bore, which is obstructed by the declination assembly.  Hence use of the illuminator requires removing the entire declination assembly (and any payload on top), which throws off the polar alignment.  The 4x AA batteries are under the top cover, and are supposed to last for up to 72 hours of tracking. Maybe that's with lithium batteries.

Below is a 100% crop of the area around Deneb, 2-minute exposures with a Canon 70-200mm f/2.8 lens at 200mm.  There is a bit of RA drift, and a much smaller DEC drift.  The RA drift is a combination of periodic error  and polar misalignment in altitude (as I only used the polar scope for alignment, this was in Lancashire in the UK).

On returning to Singapore, I decided to load up the Stellarvue SV80ED, and a Meade DSI.  Widescreen hasn't received their shipment of counterweight shafts and counterweights yet, so the mount was severely unbalanced (no counterweights).

Using PHD2 I was able to measure a bit less than 2" RMS error in RA, after guiding.  The Star Adventurer has a standard ST4 guide port, but only can guide in RA, since the DEC is not motorized.

Unguided performance shows about 23" peak-to-peak periodic error, and there is no PEC.  The worm period is about 15 minutes, although there is an almost-equal amplitude harmonic at 7.6 minutes.  This implies that peak periodic error over a 2-minute period would be approximately 6" - so unguided exposures at 200mm and of 2-minute duration should be possible. Guided performance should be acceptable at around 2" to 3" pixel scale, limited only by declination drift.  So a scope like the SV80ED would be a good choice if guided.  However, the mount is close to its limits with such a payload. A 200mm range camera lens or small refractor like a Takahashi FS60 is probably a better choice.

In summary, this mount is a far cry from my Mach1.  Although it looks better-built than other Chinese mounts, and some of the parts are CNC machined and not cast, the illusion of quality falls apart under close scrutiny.  That said, if you want something supremely portable for up to perhaps 300mm focal length, this mount will do very well.  It says something that the entire mount weighs not much more than a Mach1 Eagle half-pier (and also doesn't cost a lot more than said half-pier).

If I ever travel to Gran Canaria,  I will certainly not be able to bring the Mach1 along, but this mount will fit perfectly in check-in luggage.

I did manage to get about 30 minutes of the area around Deneb with the Star Adventurer from Lancashire, but I missed the Pelican Nebula by a small amount.  The wispy nebulosity next to Deneb that I thought was a DSO, turned out to be dirt on the DSLR sensor. D'oh!  image scale with the 200mm lens was about 6" per pixel.

Sunday, June 29, 2014

Motorizing a Vixen Polaris Mount, Part II

I finally got around to re-making the motor bracket, and made a tripod attachment to the Vixen ring mounting points (there is no saddle).  The Polaris carries the Stellarvue 80ED and DSLR quite well.  I also used an intervalometer from e-bay to allow repeated bulb exposures.  Power is (literally) provided through the USB port on the Arduino; because the stepper motor is only 4V, providing 12V through the barrel jack on the Arduino causes a lot of noise and motor heating.

I took some 10-minute long exposures with the DSLR (an exposure this long is normally not practical, as thermal noise in the DSLR becomes unmanageable).  The reason for these long exposures was to measure the periodic error.

Unfortunately, the total PE is a massive 52.5" peak-to-peak over the 10-minute worm period.  This means that the maximum error is 0.175" / second or about 0.012X the sidereal rate.  With my (old) Canon 40D and the SV80ED, the plate scale is 2.1" / pixel. Assuming star FWHM of 4" and a maximum eccentricity of 0.60, this means stars would be 4" x 7" at worst - a drift of 3".  This corresponds to a maximum exposure time of only 17 seconds, which is pretty bad.

This means that PEC is not optional, but mandatory.