Diagnosis of my SW188p

[Captain Scarlet]

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

[jetstream]

7 minutes ago, Captain Magenta said:

I checked the primary mirror for lateral freedom to see whether it was sliding around, and it was secure.

Magnus, the primary does need a bit of room to move, all my reflectors are like this- my truss dobs can move at least 1/8" laterally in any direction.

Have you checked out secondary sizing vs illumination with Bartels excellent calculator? It gives insight with respect to central obstruction size vs illumination.

http://www2.arnes.si/~gljsentvid10/diagonal.htm

[jetstream]

Here is a guestimate of your scope with the stock 58mm minor axis secondary

Remember that illumination may drop to 70 % for visual use.

---

Off-Axis	Illum.	Light Loss
0.00 mm	100.0%	0.00 mag
2.00 mm	100.0%	0.00 mag
4.00 mm	100.0%	0.00 mag
6.00 mm	100.0%	0.00 mag
8.00 mm	100.0%	0.00 mag
10.0 mm	100.0%	0.00 mag
12.0 mm	100.0%	0.00 mag
14.0 mm	99.58%	0.00 mag
16.0 mm	96.38%	0.03 mag
18.0 mm	91.96%	0.09 mag
20.0 mm	86.91%	0.15 mag
22.0 mm	81.50%	0.22 mag
24.0 mm	75.87%	0.29 mag
26.0 mm	70.12%	0.38 mag

---

Max field for visual use with 68 deg eyepiece and 7mm exit pupil = 20.76 mm radius.

offset = -2.43 mm parallel to focal plane

[Captain Scarlet]

2 minutes ago, jetstream said:

Magnus, the primary does need a bit of room to move, all my reflectors are like this- my truss dobs can move at least 1/8" laterally in any direction....

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...

[jetstream]

What is the measured distance to the focal plane? I guessed 180mm.

[Captain Scarlet]

5 minutes ago, jetstream said:

Here is a guestimate of your scope with the stock 58mm minor axis secondary...

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...

[jetstream]

what is the actual measured distance from the centre of the sec to the focal plane? I ask because I've done these numbers many times same as you.

One thing that the calcul;ator thankfully shows is the relationship between aperture, f ratio and central obstruction vs illumination. ie my 24" runs 19% CO at F4.1.

[Captain Scarlet]

7 minutes ago, jetstream said:

what is the actual measured distance from the centre of the sec to the focal plane

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.

[jetstream]

Is this an imaging scope? sometimes they push the focus out using shorter tubes etc. OOUK does this on their VX's unfortunately. 10.5 inches is excessive IMHO. It should be 1/2 dia of tube + focuser height +.5" roughly

[Captain Scarlet]

It may well have been an imaging scope, but I bought it just to break down and build up and generally practice on. In that it's doing its job, and I've learned a lot.

Personally, for now, I'm visual only. But I can't think why an undersized secondary would be useful for any sort of user. I can only imagine it's a very early model and the Synta designer hadn't thought it through properly. Especially as a knock-on effect of the small secondary is the large distance-to-focus amplifying the structural weaknesses of the tube to boot.

[Captain Scarlet]

 

12 minutes ago, jetstream said:

10.5 inches is excessive IMHO.

yes I agree...

[jetstream]

12 minutes ago, Captain Magenta said:

 

yes I agree...

well, maybe a nice 63mm sec?

[jetstream]

15 minutes ago, Captain Magenta said:

I can only imagine it's a very early model and the Synta designer hadn't thought it through properly.

maybe they just put it together with what they had laying around... maybe an H130 sec

[John]

The spec of the same scope marketed under Orion (USA) branding gives the secondary mirror minor axis as 58mm. Granted that would be bevelled as well so the aluminised surface will be more like 55mm or so.

I wonder if the secondary mirror on your example has been changed by a previous owner ?

I also wonder if the primary has been moved up the tube at some point - the Synta (the manufacturer) newtonians generally have the focal plane of the scope not far outside the scope tube wall.

Interesting stuff all the same - good experience.

FWIW my Orion Optics 12 inch F/5.3 has an aluminum tube and a 63mm MA secondary which is the OO stock size for this scope.

 

[Peter Drew]

My guess is that someone along the line has set this up for planetary observation. A 50mm minor axis is plenty large enough for full aperture resolution for a planet, full field illumination is unimportant in this instance.     🙂