25 replies to this topic

[gdavis]

I recently picked up a used Orion SkyView Pro 8 EQ and decided to check the collimation.  I've been searching and searching but can't find any explanation for what I'm finding.

 

I started by checking the secondary mirror which was way close to the primary, no where near centered in the focuser.

 

I tried centering the secondary, I've cranked it all the way back until it's bottomed out in the spider and it's still not quite centered.

 

Doing my best to look straight down the focuser:

 

If I center the secondary in the focuser, it's clear I'm not looking straight down the focuser:

 

I don't have any reason to suspect the previous owner has futzed with it and don't see any other signs of anything being amiss.  I don't think the focuser alignment is off, seems to me it would be pretty obvious if it was this far out of whack.

 

Shot of the spider:

 

Anyone ever seen anything like this?  Have any idea what's going on?   Worth worrying about?

[Couder]

I could easily be wrong, but it looks like someone glued a mirror on top of the original one, which is why it is too close. If I am correct, one way to correct it is to move the spiders up. Maybe use the top existing hole for the bottom hole on the spider, drill 4 new holes. But wait and see what others say.

[KerryR]

If the secondary support is butted tight to the spider hub, there would be 3 solutions that I can think of: 1) drill new holes for the spider, to allow you to remount it forward in the tube. Least elegant solution. 2) File off 1/8"+ off the secondary holder, the side opposite the secondary, to buy you the necessary room, possibly requiring a shorter center screw to ensure the screw doesn't poke through and hit the back of the secondary. Scary because you'd want to leave the secondary attached to the holder while you worked, lots of room for an accident... 3) Get or make a different spider with a thinner hub. Easier said than done.

 

But, you can probably just leave it alone.

 

The impact on the image should be minimal. I believe the only effect of this placement will be to tilt the optical axis a little further off the axis of the tube, which may cause vignetting at the front of the tube a little. This would manifest as image brightness falling off a little more on one edge if the full field of your lowest power eyepiece(s). This is usually pretty difficult to detect visually. Also, If the secondary is a little on the small side, as is often the case on mass-market Newts, some of the converging light cone from the primary may miss some of the secondary, at the focuser side. (This is the opposite of what we're trying to achieve with visually centering the secondary under the focuser.) Again, this would only cause a little dimming on one side of the image of your lowest power ep's, and would normally be difficult to see visually.

[Asbytec]

One thing that catches my eye is when you look down the focuser at an angle, the secondary looks circular. This means it is tilted properly for that angle and centered under the focuser. But, in the top image it is elliptical in appearance along the major axis suggesting it is tilted with respect to the focuser.

 

I wonder if you tilted the secondary a little so that it appears rounded in the direct view, if it will not look more centered. From your current position, remove the paper and align the focuser axis with the primary center and see where you are.

 

I understand you are bottomed out. One thing you might consider is shimming the focuser so it points a little toward the primary. Not much. This will cause you to move the secondary toward the primary and tilt it back to look directly up the focuser. The focuser does not have to be mechanically square to the tube, it needs only to capture the rounded secondary position. 

Edited by Asbytec, 04 December 2020 - 05:16 AM.

[KerryR]

One thing that catches my eye is when you look down the focuser at an angle, the secondary looks circular. This means it is tilted properly for that angle and centered under the focuser. But, in the top image it is elliptical in appearance along the major axis suggesting it is tilted with respect to the focuser.

 

I wonder if you tilted the secondary a little so that it appears rounded in the direct view, if it will not look more centered. From your current position, remove the paper and align the focuser axis with the primary center and see where you are.

 

I understand you are bottomed out. One thing you might consider is shimming the focuser so it points a little toward the primary. Not much. This will cause you to move the secondary toward the primary and tilt it back to look directly up the focuser. The focuser does not have to be mechanically square to the tube, it needs only to capture the rounded secondary position. 

I like this solution. Simple, easy, elegant.

[Vic Menard]

...I understand you are bottomed out. One thing you might consider is shimming the focuser so it points a little toward the primary. Not much. This will cause you to move the secondary toward the primary and tilt it back to look directly up the focuser. The focuser does not have to be mechanically square to the tube, it needs only to capture the rounded secondary position. 

I would suggest getting a simple peep sight/collimation cap (like this https://agenaastro.c...t-eyepiece.html  ) to ensure your view is centered above the focuser--pictures through an empty focuser are woefully inadequate when assessing and correcting the secondary mirror placement. I would also recommend leaving the primary mirror uncovered--this adds the edge of the primary mirror and the primary mirror center marker for reference.

 

I would also suggest looking closely at your spider vanes--if they're tapered with one side perpendicular to the spider hub, your vanes may be mounted backwards.

[Asbytec]

Good point. Points.

Edited by Asbytec, 04 December 2020 - 09:18 AM.

[Vic Menard]

Good point. Points.

I'm pretty sure the spider vanes are mounted backwards...

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[KerryR]

OP: Be sure and let us know what your final solution ends up being. I really like the focuser tilt and vane-flipping possibilities. (If you don't just leave it as is.)

Inquiring minds want to know.

[gdavis]

I won't be able to mess with it until tonight, but the spider vanes do indeed have the taper towards the primary.  Didn't know if that was frontwards or backwards or if it even made a difference, but if you're saying it's backwards it sounds like that's the first thing I should try.

[KBHornblower]

If I were doing this I would not have a piece of paper concealing the primary mirror.  I would put it on the inside of the tube across from the focuser to give a bright background, so the edges of the secondary mirror and the focuser drawtube are clearly visible.  I need to see the reflection of the primary in the secondary for reasons I will address.

 

When centering the secondary under the drawtube visually, the position of my eye or camera along the focuser axis is critical.  That needs to be at what we call the apex, from which the apparent diameters of the primary and secondary are equal.  From there a correctly positioned secondary will appear exactly centered under the focuser.  From too close to or too far from the secondary, perspective effects throw the apparent position off.

 

I do not bother with the apparent circularity of the secondary, because in the rotation adjustment it is not sensitive enough.  I rotate it until the reflection of the primary is centered in the crossways dimension, and then move the secondary toward or away from the primary to center it in the dimension along the tube.  The details of how to do this depend on the construction of the components.  My one-off homemade assemblies are different from each other, not to mention typical commercially made scopes.

[KerryR]

I won't be able to mess with it until tonight, but the spider vanes do indeed have the taper towards the primary.  Didn't know if that was frontwards or backwards or if it even made a difference, but if you're saying it's backwards it sounds like that's the first thing I should try.

My old GSO 8" f8 has the tapered side of the vanes toward the primary. That might suggest that that's normal, but I wonder what other folks see in their mass market Newts.

It looks, both in your photo and on my scope, like the holes in the hub are offset in a specific direction to accommodate the tabs on the vanes, so I don't think the vanes would be centered on the hub if they were flipped. If not, could the spider/hub be flipped and still work while buying you that last 1/8" or so of clearance?

[Asbytec]

I agree with KBHornBlower. That's one method, and the method I prefer, too, for the same reasons. One can cover the reflections from the primary, but finish secondary rotation using those reflections.

I have the same spider, seems pretty common these days. When my neice wakes up, I'll check the taper of my vanes. Im not sure which way mine taper, but may make a difference.

[Tank]

Make sure you do a collimation first since if the tilt of secondary is wrong it may give the illusion of it not being centered!!!

[Vic Menard]

I won't be able to mess with it until tonight, but the spider vanes do indeed have the taper towards the primary.  Didn't know if that was frontwards or backwards or if it even made a difference, but if you're saying it's backwards it sounds like that's the first thing I should try.

From the image you provided in your first post, it looks like you should be able to switch top for bottom and left for right to get the taper facing the opposite direction. This should give you a little more room between the secondary mirror stalk and the spider hub. Do not try to flip the hub, you need to flip the vanes. You're not the first to report this problem...

[Starman1]

And, one other solution if you're handy:  use a Dremel tool to "slot" the tube holes where the spider attaches.

This would allow you to slide the entire assembly up tube a tad.

 

But I like the idea of reversing the vanes if that is easy to do.  It looks like the top pair in the picture could be reversed by flipping them to each other's position.

And ditto the bottom pair of vanes.  When you replace them, be sure to measure the center bolt to tube dimension as you tighten the vanes--the bolt should be dead center in the tube.

[Vic Menard]

...When centering the secondary under the drawtube visually, the position of my eye or camera along the focuser axis is critical.  That needs to be at what we call the apex, from which the apparent diameters of the primary and secondary are equal.  From there a correctly positioned secondary will appear exactly centered under the focuser.  From too close to or too far from the secondary, perspective effects throw the apparent position off.

While the view from the apex should ensure the best offset--an inch above or below will have no significant impact on the setting (perhaps a couple hundredths of an inch). I usually prefer to keep the primary mirror reflection a bit smaller than the reflective surface of the secondary mirror--and when I'm using my adjustable TeleCat, I like to keep the bottom edge just a bit larger than the actual edge of the secondary mirror. This ensures good visibility of the complete primary mirror reflection (including any mirror clips if your scope has them) and provides discrete "steps" between each circle. Using this method (containing the actual edge of the secondary mirror between two circles), I find it's much easier to resolve (and correct) combined tilt/rotation and tilt/offset errors.

 

...I do not bother with the apparent circularity of the secondary, because in the rotation adjustment it is not sensitive enough.

Using the method I described above, and given reasonable mechanicals, I can usually get the secondary mirror "rounded" pretty quickly. Of course, if the mechanicals are worse than what I normally expect with a mechanically centered secondary mirror and an offset alignment--the "optimal" secondary mirror placement may not deliver a "round" or "circular" presentation (in most of these scenarios, it will usually appear sightly elliptical--it's usually pretty obvious). It can be even more complicated when the secondary mirror is housed in a shell--especially if the fit is not good--but the OP's secondary mirror is simply mounted on a stalk (hopefully correctly!)

 

In the scheme of things, what matters most (for image performance) is the axial alignment. While the optimal secondary mirror placement delivers centered and balanced field illumination, for simple Newtonian configurations and typical visual applications (variable star work may be problematic), close is usually good enough.

[Asbytec]

From the image you provided in your first post, it looks like you should be able to switch top for bottom and left for right to get the taper facing the opposite direction. This should give you a little more room between the secondary mirror stalk and the spider hub. Do not try to flip the hub, you need to flip the vanes. You're not the first to report this problem...

I might add, when rotating the entire spider, be sure the single screw inline with the vane on the left remains inline with the focuser (either top or bottom of the spider). To my way of thinking, this configuration makes secondary tilt a little more intuitive. 

[Vic Menard]

I might add, when rotating the entire spider, be sure the single screw inline with the vane on the left remains inline with the focuser (either top or bottom of the spider). To my way of thinking, this configuration makes secondary tilt a little more intuitive. 

[gdavis]

Alright, it looks like flipping the vanes did the trick.  I feel like I can get it centered in the focuser much better now.

 

Thanks for all the info, there were several details I would have overlooked.

 

I do need to pick up some sort of collimation tool, the cap that normally comes with this scope wasn't included.  Any recommendations for something 2" that doesn't cost $140?  And preferably in stock?  Sight tube... cheshire... autocollimator... I'm rather confused.

[Asbytec]

Check out the Astrosystems light pipe. It's a Cheshire and site tube combo tool that collimates both axes. You can find one tuned to your focal ratio. Eyepeices etc most likely has them in stock. I think under $100 for 2". It fits snug in a 2" system. Also check out a 1.25" Far Point laser (for the focuser axis) and Cheshire (primary axis) combo on the same site. A good quality tool is worth the modest additional cost.

Edited by Asbytec, 05 December 2020 - 06:39 AM.

[Vic Menard]

Check out the Astrosystems light pipe.

Since I prefer discrete steps between the three alignment circles (bottom edge of the sight tube, actual edge of the secondary mirror, and reflected edge of the primary mirror), I would suggest the f/3-f/4 LightPipe for an f/5 scope.

 

(Edit: Eventually, gdavis will probably want to replace his primary mirror center marker/donut with a matched center marker for the LightPipe. I seem to recall Don Pensack saying the new LightPipes are calibrated to the CatsEye primary mirror center markers for higher precision primary mirror alignment. But save it for later.)

Edited by Vic Menard, 05 December 2020 - 05:25 PM.

[JKowtko]

I assume when you flipped the vanes, you flipped them in groups so that the mounting flanges of each vane position retained it's direction when mounted on the cylinder.  The screw holes in the secondary mirror holder are offset to account for the mounting flange of the vane being bent left or right.  Not all your flanges are pointing the same way.

 

Fyi below are pics of the Zhumell Z8 / Apertura AD8 ... (ignoring the clunky thumbscrews I added) notice that the vanes on this scope appear to be symmetrically tapered ... however the mounting flanges are bent two left, two right, and the holes are offset on the holder to accommodate the flange position ...

 

 

 

 

 

Now in your scope maybe it doesn't matter if the holder is rotated a bit ... but as you can see in the pics this scope has specific positions for the adjustment screws to seat in the mirror holder so you can't really rotate the holder.

 

-- John

Edited by JKowtko, 05 December 2020 - 09:18 AM.

[Eddgie]

I assume when you flipped the vanes, you flipped them in groups so that the mounting flanges of each vane position retained it's direction when mounted on the cylinder.  The screw holes in the secondary mirror holder are offset to account for the mounting flange of the vane being bent left or right.  Not all your flanges are pointing the same way.

 

Fyi below are pics of the Zhumell Z8 / Apertura AD8 ... (ignoring the clunky thumbscrews I added) notice that the vanes on this scope appear to be symmetrically tapered ... however the mounting flanges are bent two left, two right, and the holes are offset on the holder to accommodate the flange position ...

 

 

IMG_1498.jpeg

 

IMG_1496.jpeg

 

IMG_1497.jpeg

 

Now in your scope maybe it doesn't matter if the holder is rotated a bit ... but as you can see in the pics this scope has specific positions for the adjustment screws to seat in the mirror holder so you can't really rotate the holder.

 

-- John

I have two scope using this same type of secondary mirror, and while they cannot be rotated much, the holes are slightly oversize. I have a center spotted secondary on my f/2.8 and there is enough room that I can rotate the spot out of the primary spot. Likewise, my 4.7 also had a small amount of rotation ability. Not much, but enough that I can see it when collimating. 

 

Just as an aside, the f/2.8 came with a center spotted secondary (well, actually a little diamond shaped mark) and it makes checking collimation and focuser tilt a snap. I wish all secondary mirrors came center spotted. 

[Starman1]

I have two scope using this same type of secondary mirror, and while they cannot be rotated much, the holes are slightly oversize. I have a center spotted secondary on my f/2.8 and there is enough room that I can rotate the spot out of the primary spot. Likewise, my 4.7 also had a small amount of rotation ability. Not much, but enough that I can see it when collimating. 

 

Just as an aside, the f/2.8 came with a center spotted secondary (well, actually a little diamond shaped mark) and it makes checking collimation and focuser tilt a snap. I wish all secondary mirrors came center spotted. 

Just a note: if the spot on the secondary were actually in the center, it would be worthless.  It needs to be offset from center or paying attention to it would disable the ability to collimate.

If you remove the secondary and measure, I would bet the manufacturer offset the mark by several millimeters from the exact center.  It only appears to be centered when looking at the secondary from a 45° angle.