434 replies to this topic

[Jason D]

XLK offset pupil

Attached Thumbnails

• 

[Jason D]

XLK central pupil

Attached Thumbnails

• 

[Jason D]

Below are photos for a slightly miscollimated scope.

Attached Thumbnails

• 

[bsim]

What is so complicated about iterating between aligning the primary mirror with the Blackcat and aligning the secondary mirror with the XLK offset pupil?

Alternating between the Blackcat and the XLK offset pupil is simplicity. I have to thank you for those "simple" instructions earlier in the thread. I do the initial collimation with my Glatter laser and TuBlug and then fine tune using your method.

[cdndob]

FYI, for using the hotspot in the field I mounted a switch and 12v car brake light LED bulb to a paper clamp. I just coonect the power to my battery box and attach the paper clamp to the outside of the tube then flip the switch to illuminate the hotspot when using the XLK, works great indoors or outside.

Steve

[CatseyeMan]

Jim, you might want to consider placing the white HotSpot on a darker paper.

I've got no choice on this one. The reflective media the spots are cut from comes with the "white" backing paper.

[Jason D]

Jim, you might want to consider placing the white HotSpot on a darker paper.

I've got no choice on this one. The reflective media the spots are cut from comes with the "white" backing paper.

No problem. It would have been nice to have the darker paper but I was still able to place the white HotSpot with high precision on the template and mirror.

Jason

[Jason D]

Alternating between the Blackcat and the XLK offset pupil is simplicity. I have to thank you for those "simple" instructions earlier in the thread. I do the initial collimation with my Glatter laser and TuBlug and then fine tune using your method.

Glad to hear the steps are working for you.
By the way, how easy is it to fine tune adjusting the secondary mirror to stack P+2 from the offset pupil on your Teeter? Do you have to deal with backlash movement when using the secondary mirror tilt set screws?
Jason

[Jason D]

FYI, for using the hotspot in the field I mounted a switch and 12v car brake light LED bulb to a paper clamp. I just coonect the power to my battery box and attach the paper clamp to the outside of the tube then flip the switch to illuminate the hotspot when using the XLK, works great indoors or outside.

Steve

An interesting solution.
I use a clip-on as shown here

[Starman1]

Alternating between the Blackcat and the XLK offset pupil is simplicity. I have to thank you for those "simple" instructions earlier in the thread. I do the initial collimation with my Glatter laser and TuBlug and then fine tune using your method.

Glad to hear the steps are working for you.
By the way, how easy is it to fine tune adjusting the secondary mirror to stack P+2 from the offset pupil on your Teeter? Do you have to deal with backlash movement when using the secondary mirror tilt set screws?
Jason

I would welcome another way to adjust the tilt of a secondary mirror other than the screws we currently use.

Though my spider is as taut as piano wires, when the screws are tight, the spider flexes when pressure is applied to the collimation screws and it appears to move too much, then springs back. When the images in the lateral hole in the XLK are nearly perfectly stacked and only require a tiny tweak to make the stack perfect, it is hard to apply just enough force to the collimation screw.

I always wonder if the twisting of the vanes is completely released, i.e. if the vanes truly return to exactly the starting position before the tiny tightening of the collimation screw that imparts a temporary twist to the vanes.

It may be that my spider flexes that way because all four of the vanes join in the center. Perhaps if they were offset I'd get less torsional twist during collimation. But then I'd be afraid they would no longer be in a straight line and cause even more diffraction.

Has anyone tried having a primary collimation bolt/spring/nut arrangement on a secondary? Perhaps that would allow adjustments without torque being applied to the spider vanes.

[Jason D]

Don, give the Milk jug washers idea a test drive. You might be happy with the improvement. When I fine tweak the secondary mirror, I do not have a problem with stressing the spider vanes. The only downside to the Milk jug washers is the backlash which takes little time to get used it. The backlash is caused by the elasticity of the Milk jug washers. When you subconsciously account for the backlash then the secondary mirror adjustment becomes a breeze.
Jason

[bsim]

By the way, how easy is it to fine tune adjusting the secondary mirror to stack P+2 from the offset pupil on your Teeter? Do you have to deal with backlash movement when using the secondary mirror tilt set screws?
Jason

My dob has an AstroSystems heavy-duty spider and 3.1" secondary holder. The secondary holder has four thumb screws. With practice, I can stack P+2 with slight tweaks of one thumb screw. Still, I regard these very fine adjustments to be more difficult than my previous setup which was a Protostar secondary. The Protostar didn't have tools free knobs, but had grub screws. With a long t-handle hex wrench, I found very fine adjustments to be very easy. I'm thinking of replacing the thumb screws with longer socket head screws to make fine adjustments easier.

As for backlash, there's a tiny bit but I compensate for it when I make my slight tweaks.

The issue I'm having and trying to fix is that my P+2 is moving slightly when I move the scope in altitude. The errors are worst at the zenith and horizon. I have a Glatter cable sling and 18-point mirror cell. This question is for Don: Is it reasonable to expect a truss dob to keep perfect collimation from horizon to zenith using the more stringent autocollimator?

[Starman1]

This question is for Don: Is it reasonable to expect a truss dob to keep perfect collimation from horizon to zenith using the more stringent autocollimator?

This is the Holy Grail of collimation. There are so many reasons why it is difficult to achieve:
--focuser flexure
--focuser board flexure
--spider flexure
--pole flexure
--UTA inherent sag
--mirror box sag
--movement at pole attachments
--mirror movement on the cell
--cell flexure
All can be mitigated to the point of disappearance in 10"-15" scopes. I'm not so sure about larger scopes unless the larger scopes are heavily overbuilt.
But, use a good focuser, mount it on at least a 3/4" thick board, use a spider that tightens to a very high tension without breaking, construct the UTA to be stiff, use larger diameter poles (1.25" for shorter lengths, and 1.5" or larger for longer poles), make the pole pairs non-parallel, keep the mirror box short for stiffness, have strong and stiff pole attachments, keep collimation springs strongly compressed or use firm attachment points, and over-build the cell, do not use a nylon sling, etc.
The most important factor for collimation movement over the 90 degrees is spider flexure--the secondary hangs below the spider at zenith, but beside it at the horizon. A secondary counterweight is not a bad idea when the secondary is larger than 3.5", and it helps to mount the spider low, near the focuser so the extension of the secondary weight away from the spider is held to a minimum.

As to whether you can achieve no visible movement in the AC from zenith to horizon, yes, it's possible. Is it easy? No. And, depending on the scope, it may not be possible without substantial structural changes.

[Parkit]

Don, to compensate for all the different points of movement I assume most people just re-tweak the collimation when it's thrown off by the factors you listed. Which seems to be the shortest and simplest route, especially if you're targeting and tracking one object for an extended period. At least that's what I do. My scope is too big and too bulky to use as a "find all the objects I possibly can in one sitting" tool, it's MUCH easier to use it for extended viewing of a few interesting objects. I can collimate to the approximate position of my target, recheck once I'm on track, and I usually only check it again if the image degrades and I don't suspect thermal or other issues. I agree that it's a lot to ask of a big dob to hold collimation at any and all altitudes, I find it easier to just adjust as needed with the collimation tools at hand.

Steve

[bsim]

Thanks, Don. As always, your answers are detailed, complete, and succinct.

[Vic Menard]

...The issue I'm having and trying to fix is that my P+2 is moving slightly when I move the scope in altitude.

The problem with watching P-2 change is that it only tells you that the focuser axis and primary mirror axis are changing their relative parallelism--it doesn't tell you specifically how each axis is changing.

The errors are worst at the zenith and horizon.

That's pretty typical.

I have a Glatter cable sling and 18-point mirror cell.

The Glatter cable sling maintains the primary mirror edge support at the CG of the mirror mass, and progressively supports the primary mirror as the scope is moved from zenith to horizon. The 18-point flotation does it's best work when the scope is pointed toward the zenith. If the sling is mounted to the flotation cell, there should be very little change in loading and cell flexure will occur at the collimation bolts. If the sling is mounted to the mirror box, loading changes are more complex and are distributed throughout the flotation/alignment systems.

The question is--what is the impact to axial alignment, specifically, the focuser and primary mirror axes?

[Starman1]

Don, to compensate for all the different points of movement I assume most people just re-tweak the collimation when it's thrown off by the factors you listed. Which seems to be the shortest and simplest route, especially if you're targeting and tracking one object for an extended period. At least that's what I do. My scope is too big and too bulky to use as a "find all the objects I possibly can in one sitting" tool, it's MUCH easier to use it for extended viewing of a few interesting objects. I can collimate to the approximate position of my target, recheck once I'm on track, and I usually only check it again if the image degrades and I don't suspect thermal or other issues. I agree that it's a lot to ask of a big dob to hold collimation at any and all altitudes, I find it easier to just adjust as needed with the collimation tools at hand.

Steve

Sure, and since the majority of movement of collimation is typically in the secondary, a simple single-beam laser can work. Star collimation is a tool to collimate primary mirrors, not secondaries. But what a pain--having to recollimate several times during the night. It's better to figure out where the majority of movement is occurring and then address that specific issue. Typically, the goal is elimination of collimation changes with altitude, but eliminating 90% of it with a couple changes is often viewed as good enough on a large scope.
If the scope is 15" and smaller, there is no reason not to be able to make a few mods, track down the causes of collimation change, and eliminate collimation changes entirely except for those caused by differential cooling of sub assemblies. If you enjoy the challenge, that is. When I was getting close to the goal it only made me desire it even more because it was a challenge. I had to address even the little things, like the fit of the secondary housing's bolt through the spider. And the use of brass screws to hold the spider in to the UTA because they can be tensioned higher (brass acts as if it were self-lubricating and as a result doesn't get as tight and hard to turn as a steel screw at the same tension). Etc. Along the way I learned a little about engineering, truss structure rigidity, torsional strengths of tubing, etc.

But you don't have to get that deep into it. Simply tightening the spider significantly can have a profound effect.

[Jason D]

There have been new recent developments for the CAM. Jim Fly sent me a new CAM made from a different reflective material with a smaller Inner-Diameter (ID) for the reflective donut. The new reflective material solved the poor reflectivity problem with the original CAM and the smaller ID improved the readability of the CAM.
See attachment.
Top photo: The CAM on the top is the new CAM and the other is the old one.
Bottom photo: The old, new, and home-made CAMs. Jim’s CAMs are precision laser cut unlike my home made. The photo also includes the XLK-C and XLK. You can’t use the CAM with the current XLK. The XLK-C barrel has a recessed inner edge to accommodate the CAM.

Attached Thumbnails

• 

[Jason D]

I have already included a good amount of details about the CAM functionality in this thread but I will have a quick review in this post about what it is. In a nutshell, the XLK+CAM (or XLK-C) will provide comparable accuracy to the combination of XLK+Blackcat_XL. That is, one tool (XLK-C) is comparable to two tools (XLK + Blackcat XL). Using the XLK alone will provide accuracy within 0.5mm for both FAE and PAE. The XLK plus Blackcat_XL improves accuracy to within 0.25mm for both FAE and PAE. The XLK-C alone provides comparable accuracy – better than 0.25mm for both FAE and PAE.
See attachment, every row of photos were taken off the same setup. First two columns are from the XLK-C central pupil. The next two columns are from the XLK-C offset pupil. The last column is from the Blackcat XL pupil. Starting from the top row, collimation error is reduced for each consecutive row until best collimation is achieved at the bottom row. Starting from the top, PAE/FAE ~ 2mm, ~1mm, ~0.5mm, ~0mm.
Note how the combination of stacked CAM and stacked P+2 represent excellent axial collimation setup without the need for Blackcat-XL.
The CAM can only be evaluated from the offset pupil when collimation is close to perfection. From the central XLK-C pupil, the CAM reflection will start to fade around the same time reflection2 starts to fade too. However, no refection will fade from the offset pupil.

Attached Thumbnails

• 

[Jason D]

In the attachment, the top row represents what you see through the XLK-C pupils during daytime pointing to the sky. The middle row represents what you see when the OTA is pointing close to a source light inside. The bottom row represents what you see at night when Catseye’s Red LED is used. The new CAM reflection is well suited for all conditions. With the old CAM, I had to use a second source light attached to the spider vane to illuminate the CAM. No need for a second source light with the new CAM.

Attached Thumbnails

• 

[Jason D]

When I use the Catseye’s Red LED, I clip it to the OTA edge at the far side then point it to an area on the primary mirror between the primary’s edge and the center – not at the center spot. I clip it at the far side to get it out of the way. I do not want to hit it accidentally when I move my hand at night to adjust the secondary mirror and end up dropping it on the primary mirror.

Attached Thumbnails

• 

[Jason D]

There are several procedures to follow with the XLK-C. The simplest procedure is to stack P+2 by only adjusting the secondary mirror followed by stacking the CAM by only adjusting the primary mirror. Iterate between the two until both are nicely stacked. Only the offset pupil is used in this procedure.
Another procedure is to start with the CDP from the central pupil before iterating between the CAM and P+2 stacks; however, the CAM does complicate CDP by limiting the area where P+3 can be stacked as shown in the attachment.

Attached Thumbnails

• 

[Jason D]

The following link is to a 3 months old. Though the post is unrelated to the CAM, it includes interesting information pertaining to the clarity of the center spot reflections as seen by different individuals.

http://www.cloudynig...sb/5/o/all/vc/1

[Starman1]

Jason,
Through the lateral pupil in the XLK autocollimator, I see one pair in focus and the other pair out of focus.
When I put my glasses on (they're reading glasses), the pair that was in focus goes out of focus, while the pair that was out of focus becomes focused.
After stacking one pair (without glasses), and iterating a cheshire/autocollimator adjustment sequence a few times, I found I can tweak the collimation even closer by moving from glasses to no glasses several times to make sure both pairs of images are perfectly stacked with no error.
I'm gratified to see the central pupil of the autocollimator also shows a more perfect alignment, with a jet black background.

[Jason D]

It is interesting how the autocollimator reflections interact with the vision of different individuals. Now I need to figure out how an optometrist can make use of the autocollimator for examination!!!