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The Unicorn Project

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#1 Mike Spooner

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Posted 15 January 2020 - 01:07 AM

I have an 8" telescope mirror that has the optical and mechanical centers offset by about 3/32". The blank isn't wedged and I'm usually careful about attaching the drive plate well centered on the mirror for grinding and polishing MOT so I was surprised to find this when I was testing; hence my Unicorn project.grin.gif  When offset to the optical center for testing the correction looks pretty good so I thought "What the heck!" I'll get it coated and make myself a compact 8" OTA and see how it performs. I decided to do photos of the various steps in the build so hopefully they'll have a bit of interest here and there. I have quite a few photos starting with some uncoated tests using the mechanical center and after coating tests where I was pretty well centered on the optical center. I've had the raw materials for a while so the past week I decided to put my ATM hat on and go for it. It'll be nice to have a solid tube scope that fits nicely in the back seat if need be. First photos are the test shots in autocollimation. Compare the first two with the next two where I was closer to the optical center (still may need a bit of alignment but much better.)

This is 100LPI inside focus mechanical center:

8inch883mm100lpiBeforeInside.jpg

This is 100LPI outside focus mechanical center

8inch883mm100lpiBefore.jpg

This is 100LPI inside focus optical center

8inch883mm100lpiAfterInside.jpg

This is 100LPI outside focus optical center

8inch883mm100lpiAfter.jpg

 

Here is the null:

8inch883mm100lpiNull.jpg

 


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#2 Mike Spooner

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Posted 15 January 2020 - 01:20 AM

So the next step was to get the cell and mirror mount ready. My cell for small mirrors is an aluminum Y with collimation bolts and the assembly siliconed to the mirror. I'm still using rubber fuel line instead of springs and once set I rarely need to collimate. Hopefully the photos explain the major details.

Cell.jpg

CellA.jpg

Cellsupport.jpg

CellsupportD.jpg

 



#3 Mike Spooner

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Posted 15 January 2020 - 01:29 AM

Preparing the aluminum tube is the dirtiest part due to metal shavings and the amount of cleanup needed. Cutting the tube square and to length and then cutting the focuser hole is all my least favorite part opf the build. On the other hand it's one of the more satisfying when it's finished - go figure! 

 

Using a paper wrap to mark for a square cut:

cutOTA.jpg

cutOTAa.jpg

 

Here I wash the tube with dish soap and a brush:

TubeClean.jpg

 

Then to embellish the outside with conditioning discs on an angle grinder.

TubeCleanA.jpg

 



#4 Mike Spooner

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Posted 15 January 2020 - 01:40 AM

Okay I skipped a couple of steps so I'll post them here.

 

Cutting the slots for the mirror cell mount that slides into the tube.

Note that the vent holes are in the shadow of the primary so light doesn't shine up from the tube bottom - this is a big deal for solar viewing with a white light filter.

The slots allow the mirror to be removed for washing and replaced without losing collimation (or only needing a tiny tweak). Note the x mark - I always line this with the focuser though the spacing of the slots is usually off a bit so helps with indexing if I happen to paint over the x. The plywood helps keep the tube round and dampens vibrations, too. 

CellMountA.jpg

CellMountC.jpg

CellMountE.jpg

FocuserB.jpg

 

 



#5 Mike Spooner

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Posted 15 January 2020 - 01:47 AM

After drilling the hole for the focuser I decided to machine a ring to accept a Feathertouch focuser and then just TIG weld it to the tube. I don't recommend this unless you've made a few scopes and are confident you know how to collimate without the ability to square the focuser. 

FocuserC.jpg

FocuserF.jpg

FocuserE.jpg

FocuserWeld.jpg


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#6 Mike Spooner

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Posted 15 January 2020 - 01:55 AM

Continuing with the focuser and then to the circular spider and the end is in sight! The secondary mirror is attached with silicone and uses vacuum hose "springs" like the fuel hose used on the primary. Doesn't shift like real springs and is nice to transport and setup without the need to collimate. The circular spider takes time and care to get positioned and attached correctly... a lot of time - and care.tongue2.gif

FocuserWeldA.jpg

Spider.jpg

SpiderA.jpg

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  • SecondaryMount.jpg

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#7 Mike Spooner

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Posted 15 January 2020 - 02:04 AM

Once the spider and secondary are ready, the focuser is attached and once the spider is confirmed to be in the correct spot it can be drilled and bolted in.

FocuserWeldB.jpg

SpiderB.jpg

 

I started making adapters with an angle plastic screw and it works extremely well on both stepped and tapered barrels - much less frustrating than clamp rings or straight in screws.

Adapter.jpg

 

A couple of photos from the collimation point of view.

Collimation.jpg

CollimationA.jpg

 

 

 


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#8 Mike Spooner

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Posted 15 January 2020 - 02:10 AM

Last post with photos of the finished OTA.

If the clouds don't preclude it, I might get to star test the rig this week! I'm excited to have a fairly compact 8" again. It has f.l. of 883mm and has a 1.83" secondary from the late Bob Goff. And Jon Isaacs will be happy to know I'll be trying a Paracorr (type 1) with it!!lol.gif

 

UnicornA.jpg

Unicorn.jpg


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#9 greenstars3

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Posted 15 January 2020 - 02:23 AM

Nice job Mike, looks good,

 

Robert


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#10 jtsenghas

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Posted 15 January 2020 - 06:37 AM

Nice.  I like your adapter with the angled plastic screw.  I'll file that one away for future use. 

 

Out of curiosity,  did you figure out how you ended up a couple of millimeters off center with the primary? Are you assuming that you hadn't centered well for polishing and figuring, or can you think of another possible cause? 



#11 Jon Isaacs

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Posted 15 January 2020 - 06:39 AM

Mike:

 

Interesting story.  I always wonder just how close to the mechanical center of the mirror the optical center is.  Most of us are relying on the optical center coinciding with the mechanical center of the mirror for collimation but I wonder how valid that assumption is.

 

And I particularly like the last two photos.. Nothing quite like an automatic transmission to sharper up the view and make swapping eyepieces easier.  :)

 

And I think you will like the Paracorr.. 

 

Jon


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#12 Mike Spooner

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Posted 15 January 2020 - 10:06 AM

Nice.  I like your adapter with the angled plastic screw.  I'll file that one away for future use. 

 

Out of curiosity,  did you figure out how you ended up a couple of millimeters off center with the primary? Are you assuming that you hadn't centered well for polishing and figuring, or can you think of another possible cause? 

The adapter is a pain to make (manual lathe) but they work so well I'll be making them for some of my other scopes when I don't have ambition for bigger projects. smile.gif By pushing down at an angle (and drilled to the depth that it intercepts the inset so as not to lift the eyepiece) it traps the eyepiece against the adapter wall. I suppose it decenters the eyepiece a tiny bit but the brass clamp rings of commercial designs often barely grip the top of some eyepieces and I've had them pop up occasionally. 

 

I may have mismeasured the center of the back of the blank or perhaps the result was from hand figuring being off center but I don't recall if I did this on this mirror as it was part of a run of 5 or 6 I made at that time. The others turned out with centers aligned. I don't mind having a "Unicorn" if it performs well. cool.gif

 

Mike



#13 Mike Spooner

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Posted 15 January 2020 - 10:22 AM

Mike:

 

Interesting story.  I always wonder just how close to the mechanical center of the mirror the optical center is.  Most of us are relying on the optical center coinciding with the mechanical center of the mirror for collimation but I wonder how valid that assumption is.

 

And I particularly like the last two photos.. Nothing quite like an automatic transmission to sharper up the view and make swapping eyepieces easier.  smile.gif

 

And I think you will like the Paracorr.. 

 

Jon

I try to center mirrors as close as possible - especially after testing when figuring tool on top and putting the mirror back on the machine. I think most mirrors have centers aligned pretty close but I'm thinking faster optics would cause more collimation problems since they're a bit less forgiving. 

 

Yeah, I rebuilt the 700r4 a few years ago to go in the 67 Camaro but it's waiting for a window of opportunity - along with other projects. 

 

There is an 11" F/3.4 Dob in the background that has convinced me the Paracorr is useful but it's needing a flatter secondary to get it tuned up. I tried the Paracorr on an 8" Dob but really didn't see any improvement for my eyes. Of course I rarely use double digit eyepiece focal lengths so maybe I don't have enough experience.lol.gif With the 11" I can see where the type 2 would be good but even the type 1 is pleasant compared to no Paracorr. 

 

Mike



#14 wells_c

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Posted 15 January 2020 - 10:07 PM

Question: collimating with a star test would align the scope to the optical center of the primary (and result in optimal collimation whether or not the 2 centers were coincident), correct?
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#15 Mike Spooner

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Posted 15 January 2020 - 11:17 PM

Question: collimating with a star test would align the scope to the optical center of the primary (and result in optimal collimation whether or not the 2 centers were coincident), correct?

Great question! Since it's only offset about 3/32" I'm not sure how noticeable it will be. After all the secondary offset on fast mirrors doesn't seem to cause a problem. Is collimating with the star test geometric or diffractive in nature? The in focus star image should show a brighter star as an Airy disk and 1st ring by diffraction within a certain FOV. Off axis paraboloids (which I guess technically this is slightly) show good images when collimated but the FOV is affected as part of a larger mirror. It's likely the greater problem would be using collimation tools on the mechanical center as Jon was hinting at earlier as these are geometrical. I've marked the optical center as closely as I could using DPAC so I'm starting there until I check it on a star. 


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#16 BGRE

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Posted 15 January 2020 - 11:27 PM

The field coma exhibited by off axis stars can be used to identify the axis of a paraboloid and if its out of the FOV the direction in which the axis lies.


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#17 jtsenghas

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Posted 16 January 2020 - 07:00 AM

Here's an example where a specific method of star collimation that I like to use may be particularly handy. 

 

After getting collimation as close as possible using tools,  set up to look at an appropriately bright slow moving star (If tracking isn't used), ideally Polaris for those of us north enough. 

 

Find the position in field of view in which out of focused images give symmetrical circular diffraction patterns. 

 

Without moving scope axes, tweak collimation to center the star. If the first adjustment is large this may be a bit iterative

 

This method,  I find,  is less fiddly than centering the star,  adjusting collimation,  recentering the star, adjusting collimation....

 

Repeat the process with ever increasing power as needed,  finishing with a short focal length eyepiece. If initial results are way off, drop down to a longer focal length eyepiece for the first iteration. If the image is nearly round towards one edge of the field of view, start there rather then go to a longer eyepiece to maximize the size of the diffraction images. 


Edited by jtsenghas, 16 January 2020 - 08:06 AM.


#18 calypsob

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Posted 17 January 2020 - 02:57 PM

What do the special springs look like?


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