Captain Scarlet

It's 264mm. The focal plane is 146mm outside the main tube, and the tube outside radius is 118mm, from a measured circumference of 741mm. Offset is 2-3mm.

this is the problem ... my secondary is 50mm across. Perfect interception requires a 53.3mm, using the formula I derived: mirror short axis = s/(2f+1) + s/(2f-1) where s is distance from focal plane to OTA axis, and f is the scope's f-number In this case, "s" is 264mm and it's an f/5 scope...

Yes it's not actually wedged, it does have an infinitesimal freedom to move, and when I get around to actually looking at stars with it I'll free it up a bit more. But for these purposes I'm more interested in keeping it as a stable reference position for the secondary collimation. As for the illumination calculator, yes I have perused his and various other resources. When I put the specs of this scope into that link, it simply returns this, which is what my own calcs are telling me...

I recently got a Skywatcher 200p 8” f/5 Newtonian, mainly to use as an inexpensive “practise-horse”. I paid £145 for it, including rings and dovetail. The idea was to use the lessons learned on my more precious 12”. Several episodes now of disassembling, reassembling, collimating and theorizing about this scope have revealed some interesting things. Secondary Undersized: Actually a Skywatcher 188p. The first thing I noticed was that no matter what I did, I couldn’t position the secondary to see all 3 mirror-clips simultaneously from my viewing position at the focal point, suggesting an undersized secondary, missing out on the outer part of the light-cone coming from the primary. So I measured everything, effectively performing a “survey” of the scope. The secondary mirror turned out to be 50mm across, for example. The blank was 53mm, but the edges had been bevelled to reduce the flat surface to 50mm. A bit of geometry-and-trig showed that for this scope the secondary needed to be 53.3mm across its short axis to perfectly intercept the main ray’s whole light cone. So at 50mm, my scope was only behaving as a 188mm scope. Which may not sound like much but it amounts to nearly 12% of the primary’s collecting area not being used! Main Tube Not Rigid Using my Concenter (marvellous tool), I was easily and quickly able to place and orientate the secondary correctly. Putting my high-quality (and heavy!) Glatter laser collimator into the focuser, I adjusted until the laser-spot fell right into the doughnut on the primary. Perfect, I thought. Next, of course, I needed to tilt the primary to get its axis to coincide with the focuser’s. To get a better view of the laser’s returned dot at the laser-source, I changed the scope’s elevation on its mount (a SkyTee2). Uh-Oh! Problem! Simply changing the alt angle was enough to move the laser spot away from the doughnut! Perhaps the secondary’s support structure was not, after all, “rigid and stable”? I grasped the mounting and spider and applied a bit of torque to check for flex. No, the secondary and spider were all solid, and there was negligible movement in the laser-dot. I checked the primary mirror for lateral freedom to see whether it was sliding around, and it was secure. I gently touched the laser unit, and Lo and Behold! Just by applying light finger-tip pressure to the laser-acting-as-eyepiece, I could easily move the spot around! I checked that the laser was securely clamped into the eyepiece holder, and it was (Baader Click-lock). Which meant that either there was slack in the focuser or the whole focuser unit wasn’t rigid on the main telescope tube. A little further experimentation showed that I could also move the laser spot around just by depressing the main OTA with my finger anywhere near the focuser-block. This was not good. I checked whether the housing was securely bolted to the OTA shell, and it was. The guilty party: OTA flexure. Conclusion: the main tube of my 200p seems to be not rigid enough to properly support the focuser unit – another design flaw. ***** I have read that some versions of the 200p has or had an undersized secondary, and so it is. But I’ve never read of this tube-flexibility problem. I don’t think my copy of this scope is damaged or unusual in any way, and if that’s true then this tube-flexure must be an endemic problem. So why has nobody else mentioned it? On reflection, I can think of a few reasons. The conventional wisdom is that once you set the secondary, you don’t need to do it again for a while. Supposedly it’s rigidly enough held that it stays in place, and it seems that this is true: it’s not the secondary that’s moving around, it’s the focuser. Also, the advice is to make the scope horizontal or nearly so when setting up the secondary, to avoid dropping tools down onto the primary. So the whole initial collimation process often gets done without moving the scope. Obviously the laser spot will remain stable. Subsequent collimations are for the primary only and often “in the field”, with the scope in a more elevated angle: people may not notice the laser (if that’s what they’re using) spot has moved, they’re just looking for its return-spot. And the most common lasers and Cheshires are quite lightweight, perhaps not heavy enough to noticeably pull the focuser around. Put in a heavy and tall eyepiece, though, and it’s a different story. I was “lucky” to have a monster Glatter 2-size laser which, when using its 1.25” barrel, is both heavy and tall and so I was able to produce the effect. Remedies The remedy for the undersized secondary was obvious: buy a new one in the correct size! This I plan to do. The OTA-flexure problem was more difficult and still open. I got a large-diameter (236mm) stainless steel bolt-closing hose-clip, as used to affix air-conditioning pipes and placed it just alongside the focuser to put the OTA under compression and hopefully stiffen it up (without buckling the tube). I think it has improved things, the spot movement is less but it’s still there. And by the way this is in no way intended to be a criticism of the person on here I bought it from. This scope has already fulfilled its purpose: learnings. It’s provided me with much fascination and pleasure. Ongoing… Cheers, Magnus

try this: https://www.astroshop.eu/carrying-bags/oklop-padded-bag-for-small-telescopes/p,56169 I have 2 Oklop bags and they're very good... Magnus

A read of Harold Suiter's "Star Testing Astronomical Telescopes" gives a very good guide through all of this, in some detail but not too text-book. And much much more besides, including a great section on Newtonian Collimation.

It is! A handful of years ago I was sitting in the cafe after a rowing outing with some crew-mates. One of them, without a car, fairly urgently needed one for an upcoming trip to commentate a regatta in Europe. Another crewmate happened to want to sell his. "Just a minute I'll go to the cashpoint", and the deal was done. £400 as I recall...

It looks as though Jupiter and Venus will be 1.4 degrees apart this Sunday, I plan to try to see them during daylight and even get a shot. They'll be 11 degrees up and just East of South at 1pm, looking from SW of London. Cheers Magnus

The laser I’m using is quite heavy, the dual-size Glatter, and the Delos eyepieces I have are heavier still and taller. I’m going to play around further this weekend and try to isolate what’s moving. The vanes and spider all seem good, as the spot doesn’t shift at all when I grab the secondary and apply torque with my hand. My best bet is it’s draw-tube flex, but I need to pin it down before I go Moonlite-ing.

Roger Clark has recently added the below to his extensive site to try to help illuminate this topic, which I found interesting... https://clarkvision.com/new_articles.html

No, first thing I checked. And the spot, wherever it was, stayed stable if I rotated the laser. Laser is a Glatter, which I confirmed is itself collimated a couple of weeks ago.

... fiddling around with this again over the weekend I put the laser in the eyepiece-holder with the laser dot on the primary centre-spot. As I moved the scope around on its mount, the spot moved around On the primary too! Up to 5mm of movement at the primary, and definitely not the primary sliding around, I shimmed that in. The focuser is a newish SW dual-speed one as supplied with their Quattro series which a friend gave me. It FEELS secure enough, there’s no *feeling* of movement when I stress it manually, but the spot also moves when I apply finger pressure to the eyepiece holder. I’m not sure if it’s tube flexure or focuser flexure, ie whether it would be sorted by getting, say, a Moonlite...

Also think about what you mean by the word "spent". If you've purchased new, then sure you've "truly lost" about 25-30% of what you paid, and you won't get that back. That's what I would consider you've truly "spent". If you've purchased second hand, you'd get most of the value back if you wanted to sell, so arguably your "spend" is quite small, could even be zero or negative. It all has value, whether it's in machined glass/aluminium form, or foldable paper, or an electronic record in a server farm in Slough... Cheers, Magnus

Superb through 15x56 binocs, lots of passing gaps in clouds. Got neighbours in too including their 10-year old daughter. They were all amazed...

Terve Praseodyymi! I'm sure you’ll have no trouble working out the technicalities if you’re an aerosol physicist... M

Well I’m now ready with my home-made cloud-seeder..

Thanks. I’d be surprised if it were different from 70mm, would just be nice to have it confirmed by someone with the identical scope...

Thanks both. I’ve seen SW’s current Dob specs at 70mm, but there’s nothing “out there” that definitively states the dimensions for their solid tube any more. But the GSO also at 70mm strengthens the case...

I’ve got some baader solar film being delivered Sunday, I’ll make a makeshift pair of filters for my 15x56s and take them to work, and sneak out during the morning, weather permitting, hopefully with a colleague or two...

If anyone has one of these, the solid tube Sky-Watcher 305mm f/4.9 Newtonian as pictured below, could you perhaps kindly let me know what the size of the secondary is please? Minor axis. My 300p is in Ireland and I’m not there until Christmas so I can’t measure it myself... Thanks in advance... Magnus

It depends what gets moved to change the focus. If the objective is moved to achieve focus, then the eyepiece will remain focused on the reticule and there will be no parallax. However if you move the eyepiece to get focus on closer objects, you’ll be moving the image plane away from the reticule and you will get parallax. ... assuming all the while there is no parallax at infinity.

It was actually on your advice if you recall at a wag meet ... I’ve kept my eyes open ever since!

Courtesy of @parallaxerr my first ever filter of any description...

Glatter laser, but you have to swallow hard before paying up. I was lucky enough to find the once-in-decade 2nd hand one a few months ago... And another vote for the Concenter too

I shake my head in wonder that people can get away with selling something, often expensive, that simply doesn't do the only thing it's supposed to, and be not designed to be adjusted by the user without e.g. digging out sealant. I'm not sure who I blame more, the manufacturer or the retailer if it's a specialist retailer. With collimating lasers it seems you'd be sending back items time after time after time: perhaps those retailers themselves should ensure they're collimated before they go out. In similar vein I've had to throw away useless spirit levels that were more than a degree out... rant over M