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C8 Collimation issues arising from diagonal, and fixing them

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#1 InductorMan

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Posted 07 March 2021 - 10:08 PM

I thought this post might be helpful to anyone experiencing the same issue as me.

 

I recently got frustrated with the fact that my C8 produces nowhere near collimated images when the original cheap "Celestron model #94115-A" diagonal in it (ca 1999 vintage), unless it's been collimated with the diagonal mounted at exactly that same angle.

 

Collimating with the eyepiece in the visual back means that the image in the diagonal is always out of collimation. Collimating with the diagonal in means the image is fine if you don't touch anything, but then it's even more out of collimation if the diagonal is rotated.

 

I always figured this was due to some eccentricity of the clamp screws, of the visual back, or an angular miscollimation of the diagonal. But today I read on CN that angular miscollimation of the diagonal causes mostly focal plane tilt, not collimation issues (since it's so close to the focal plane it more tilts the wavefront than moves the coma-free center of the image). Someone else (https://www.cloudyni...l/#entry5465272) also explained it in a way that clicked: the reflecting surface can be displaced, not just tilted. That's what really moves the center of the image away from the center of the eyepiece. And I knew there was eccentricity between the eyepiece and the reflected image of the OTA's axis because when I rotate the diagonal in the visual back, the image rotates around a point not even in the FOV with a 12mm/60deg EP.

 

Ok, so what's up with this diagonal then? Well, I actually finally looked at the thing critically! 

 

IMG 3485

 

Oh. My. God. What is going on here?!? No wonder the darn thing is rotation sensitive! The reflected image must be translated by at least 5-6mm!

 

Ok, can this be fixed? People talked about padding/shimming prisms and mirrors to fix these sorts of things.Carefully opening the thing up (and making sure to only touch the edges of the prism, and that with gloved hands) I found that the prism is spring-loaded inwards.

 

IMG 3488 2

 

Well, ok: which direction to push it? Working it out, the prism needs to move inwards. Dang how's that supposed to work? This is the direction the spring's already pushing it! Well turns out the thing was mis-assembled or assembled with faulty parts. Both of the threaded ends of the nosepiece and the (other thing?) protrude well into the plastic body, as seen below. That's apparently not where the prism is supposed to go: after screwing around with things, I determined it's supposed to rest on the plastic housing.

 

IMG 3486

 

With the two threaded interfaces unscrewed far enough that they weren't pushing on the prism anymore, the image looks like this:

 

IMG 3489 2

 

Better! But still not perfect, and I saw that there was a burr raised in the plastic from the threading: 

 

 

 
IMG 3495

 

Cut that out with a utility knife blade (in progress above). Now the prism can seat flat. I also noticed that the set screws which hold the prism from rattling around from the sides were super tight. In fact you can see that the pressure actually spalled the glass near one of them:

 

IMG 3496

 

Having allowed the prism to move inwards and getting it as good as I could, I now couldn't leave well enough alone! While I was definitely planning on tightening those set screws, I wanted the spring pressure higher. Now that the diagonal had moved the spring was more relaxed and applying less pressure. I tried to bend it to increase the pressure...

 

IMG 3497
 
Doh! Don't do that! Well, luckily I had some blued spring steel strip. But yeah, don't do that. 
 
Now that the nose piece/(other thing) screw threads were backed out, they were prone to rotation. One was way loose, the other pretty tight. I actually accidentally swapped them and this resulted in both threaded interfaces being pretty decently snug but still able to rotate. To address this I employed a very high quality, durable, precise method of construction:
 
IMG 3498

 

Hot melt glue! My favorite low quality adhesive! Appropriate here, I think, because I think I may need to get back in there at some point. I may choose to add set screws so that the threaded interfaces can be adjusted but then locked in place, to push the prism around in there in an adjustable way. 

 
Heck I think the moral of this story is that anyone who has the same problem doesn't need to do any of the mucking around that I did! If you have the same junky diagonal and the image is displaced outwards, it's possibly the same issue of improper parts/improper assembly. All one would have to do is loosen both of the small side set screws that keep the prism from rattling around, unscrew the threads until the prism is in about the right place, and then tighten the set screws back up again (to a sane tension). In retrospect I don't think the diagonal even really needed to be opened.
 
Definitely don't try to bend that spring! lol.gif

Edited by InductorMan, 07 March 2021 - 10:19 PM.

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#2 Oort Cloud

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Posted 07 March 2021 - 11:47 PM

Great write up!  When I started imaging a few months ago, I was trying to use the diagonal to allow for proper back-spacing on my 6se.  Couldn't figure out for the life of me why I was getting such uneven illumination once I tried taking flats for the first time.  Turns out, my diagonal (same model as yours, but circa 2012) was off by at least several degrees.  I contemplated taking it apart, but at this point, I don't see a point in ever going back to visual.  Seeing the live feed on my phone screen is much more enjoyable for me than trying to keep my eyeball steady in front of an eyepiece.  Still, it's nice to see how it's constructed inside so I know what to expect if I ever decide to try and fix it, so thanks for that!  I also think the hot glue was a good choice, for the same reason you listed - it's really only semi-permanent.

 

CS!



#3 InductorMan

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Posted 08 March 2021 - 05:33 PM

Cool! So wait, are you doing afocal imaging (with an eyepiece still installed and the iphone 6se stuck to it)? What's back spacing mean in that context?

 

Honestly... I know where you're coming from. I live in absurd light pollution. Sometimes the EP view is pretty garbage.

 

I was looking for NGC2903 the other day, and I sorta kinda maybe saw it but then I accidentally flipped the star map backwards in my head, and went off to photograph it with a single 30s acquisition using NightCap. But on the wrong side of the small star formation I had identified. Looking back on the images later, I saw that it actually was there in the original place I was looking! So I had indeed been able to see it through the EP, but barely perceptible with averted vision and just wasn't sure I actually saw it.

 

Are you using some sort of live stacking app? What're you using?

 

I sorta still have a different feeling personally when I can find something by eye. But... there's also yet another feeling to capture something photographically! Even if not very well. 

 

NGC2903 histo
 
That's about as good as I can get out of 10 minute's worth of exposure in NightCap on an iPhone X. Probably about the crappiest picture of that galaxy on the internet, but satisfying to take!

 

I would be very interested to experiment with live stacking.



#4 Andrew Brown

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Posted 08 March 2021 - 05:52 PM

Or you could do what everyone else does...

 

DSCN0485.JPG.71260b60eb4f4a96afad0ffd3f0



#5 Oort Cloud

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Posted 08 March 2021 - 09:10 PM

Cool! So wait, are you doing afocal imaging (with an eyepiece still installed and the iphone 6se stuck to it)? What's back spacing mean in that context?

 

Honestly... I know where you're coming from. I live in absurd light pollution. Sometimes the EP view is pretty garbage.

 

I was looking for NGC2903 the other day, and I sorta kinda maybe saw it but then I accidentally flipped the star map backwards in my head, and went off to photograph it with a single 30s acquisition using NightCap. But on the wrong side of the small star formation I had identified. Looking back on the images later, I saw that it actually was there in the original place I was looking! So I had indeed been able to see it through the EP, but barely perceptible with averted vision and just wasn't sure I actually saw it.

 

Are you using some sort of live stacking app? What're you using?

 

I sorta still have a different feeling personally when I can find something by eye. But... there's also yet another feeling to capture something photographically! Even if not very well. 

 

 
 
That's about as good as I can get out of 10 minute's worth of exposure in NightCap on an iPhone X. Probably about the crappiest picture of that galaxy on the internet, but satisfying to take!

 

I would be very interested to experiment with live stacking.

I was referring to the 6se in my signature.  It's a 6" SCT, from Celestron, just like yours but smaller.  The Nexstar series, I think, is just newer and may have more advanced coatings, or something of that nature.  But it came with a 1.25" diagonal with the same part number as yours, just about a decade newer.  Still the same junk construction though.  The reason I was talking about backspacing is because I'm using a focal reducer (2, actually), which calls for the sensor to be at a specific distance behind it.  In order to still clear the mount, I originally included the diagonal, but was having all sorts of problems, collimation included.  Once I discovered that I couldn't/shouldn't include the diagonal, I bought a 2nd focal reducer, and came up with a way to stack them that provides a much wider FOV (still quite narrow because it's an SCT, and the camera sensor is very small), which allowed me to finally do plate solving.  There is heavy vignetting, but only at the corners, and even though I've essentially thrown the entire concept of backspacing out the window, I still get a pretty decent image, and it exposes FAST.  The camera has a pretty high QE, and the F ratio is now 3.6.  By setting the gain to 200, I can get usable data on most targets in 5 seconds.  Stacking the 2 focal reducers close together, and giving about 40-50mm from the 2nd reducer to the sensor, and moving the scope all the way forward on the mount allows me to still shoot at the zenith.  It took me a little less than 6 months to get this all sorted (and learn DSS & GIMP) to the point where I can now effectively work around my mount's alt-azimuth shortcomings.  It did help that I did not have to learn the scope, as I've been using it for about 10 years now.

 

My desire to lose the eyepiece isn't really that the views weren't good.  They definitely could have been better, if my diagonal hadn't been hampering my ability to achieve good collimation.  Switching to imaging helped with that though too, because I can zoom in on the doughnut/diffraction rings during collimation, and because the app has a reticle, so I can get the star perfectly centered.  It really makes collimation way easier.  My views from the eyepiece were still enjoyable though, it was the being in uncomfortable positions, not being able to keep myself steady enough, and having to stay at the scope all night that drove me to imaging.  So really, I started out more interested in EAA so I could use my scope when it's cold, or when there's mosquitoes, etc. and just see a live feed on my phone over my home Wi-Fi.  Now, I spend a few minutes carrying the 1 piece outside, level it, point it at Polaris, focus, do a 1 star align on Polaris, and within 5 minutes I can be back inside if I want.  Then I pick a target, go-to, take a picture (usually takes a few before the backlash is taken up and the image stabilizes). Plate solve, sync, go-to.  Dead center every time.  I have used my scope more in the past few months than I did the entire 10 years prior that I've owned it, make it remote controlled really is a game changer.  If you want to see what an SCT set up this way can do photographically with small/mid-size DSOs, this is one of the best photos I've taken yet.  As I am still learning with every outing, it is also one of the most recent, about 2 days old.  It's the pic in my avatar, just higher quality (still from a 2.1MP camera though, so it's not gonna win any APOD awards or anything)

 

https://www.astrobin.com/d37qho/


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#6 Oort Cloud

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Posted 08 March 2021 - 09:31 PM

Or you could do what everyone else does...

 

DSCN0485.JPG.71260b60eb4f4a96afad0ffd3f0

Honestly, if I were to regain interest in visual, that is what I would want.  The diagonal is one of only 2 parts that I did not like with this scope when I got it.  The other is the RDS, also rubbish.  The rest of the scope, I guess I got lucky...I can cover the full range of focus without the FOV changing even slightly, it holds focus very well when slewing all over the sky, and it holds collimation even when I pack it in the car and drive it 30 miles round trip to a friend's house that has darker skies than I do (it's New Jersey, so every mile toward dark is worth it).  I can't see myself ever going back to visual though, not when I can view the camera feed on my TV While I slew the scope from the couch.



#7 InductorMan

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Posted 09 March 2021 - 02:20 AM

Quick report back: totally works. I'm so glad I did this. Managed to collimate through holes in the clouds, and do some random observing (honestly amazingly good seeing given the partial cloud cover). EP a 12mm/60deg X-Cel LX, with 2x Barlow from the same line. Collimated with EP/Barlow in the visual back, and then swapped them over to the diagonal. Still quite good! The star I'd been collimating on showed up maybe 3/4 of the way to the edge of the FOV. So assuming I'd not bumped the scope, the diagonal got the EP within say 2.5mm of the center of the optical system.

 

Definitely still not perfect, but much more workable. 

 

The only hiccough was that the hot melt glue blob now ends up hitting the visual back, rather than the plastic body. Well, not great for trying to maintain focus while rotating the diagonal. But easily solved by replacing the glue blob with either a flush blob, in the recessed area of the threads, or doing the added set screw thing.

 

I bought a 2nd focal reducer, and came up with a way to stack them that provides a much wider FOV

 

 

That's ridiculous! That is so cool that that works!

 

Is it not a field flattener too, though? I'd have thought it would end up "flattening" the field twice as much as it should, all the way to curved again, and you'd get soft focus off towards the edges. 

 

Beautiful picture!

 

Or you could do what everyone else does...

 

 

lol.gif  sure, sure: I will, eventually.

 

But right now, honestly, tinkering with the scope is as much a hobby as astronomy! I love tinkering with stuff. And while I'm not exactly trying to save money (although that pleases me too), it gives me great satisfaction to fix things rather than buying new stuff. Although I also don't like to work with bad tools either, so if I ever do anything where the diagonal quality is a serious impediment I'd definitely just buy a better one. 


Edited by InductorMan, 09 March 2021 - 02:23 AM.


#8 Redbetter

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Posted 09 March 2021 - 04:16 AM

Or you could do what everyone else does...

Just remember to buy or have some extra caps for the diagonal.  For some inexplicable reason Takahashi sends them out without those (unless they changed that in the last two years or so.)  I bought two at different times in recent years and both lacked caps/end plugs.  I was so surprised that I checked with the U.S. Tak rep. and they said that was the way they came from Japan.  

 

I don't use them with my C8, though.  That has a 2" TeleVue on it.


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#9 InductorMan

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Posted 13 March 2021 - 03:20 PM

 Quick update on this fix. It was working OK. Not great. But I had to do more tinkering!

 

The blob of glue in the way of the mate between the diagonal body and visual back really was quite annoying. Obviously I could have fixed that with more carefully applied glue. But the alignment/collimation was still sucky. When rotating the diagonal around, I think I figured that I was still 2.5mm off axis. Not very good.

 

I realized two things: 

 

First, the crappy threading in the plastic body wasn't quite straight. Even though there's not much of a flange around the nosepiece, when threaded in all the way so that the flange seats on the plastics instead of just the threads trying to hold things in place, it seated flatter. The eyepiece holder end has a much nicer flange and definitely seats flatter when screwed in all the way than when it's just hanging out part-way engaged.

 

Second, I could tell that the prism was still not sitting square. I couldn't get it to sit so that the view of the nosepiece through the eyepiece holder looked coaxial. Obviously they may not be at true right angles to each other to begin with, but the angle of the reflection looked way more severe than any deviation from right angle I could detect by just looking at the nosepiece and eyepiece holder.

 

What I ended up doing instead of gluing them in place part-way engaged was sticking both the nosepiece and the eyepiece holder in my lathe and facing off enough thread length that the threaded ends didn't protrude into the plastic body's cavity even when fully threaded in. Obviously the right move, if the tools are at hand. A file would probably work too, a lathe was just faster and meant that I didn't have to do any work to keep the face straight.

 

I was left with barely more than a full thread of engagement though. The gutter at the base of these threads (the unthreaded part between the start of the threads and the flange) is way too wide compared to the plastic wall thickness, and by the time they're threaded in, barely any thread is still in the plastic, most is sticking out the other side! And that's what I have to cut off. Oh well, it's a garbage diagonal. One thread of engagement of aluminum/brass into plastic is... not great. I think it will hold up... it had better not shear apart and drop any of my eyepieces, is all I gotta say!

 

Then to address the prism not seating properly, I took another critical look in the body. What I realized was that the right angle edge of the prism joining the entrance and exit faces is barely beveled at all. Looking in the body, the plastic cavity has a reasonably broad radius in the interior corner where this edge needs to rest. Too much plastic in there, not enough room for the almost sharp edge. And there was even a bump on the plastic in there, taller even that the radius, from a dent in the injection mold! Pretty sure the prism ended up sitting on its edge rather than on the faces. 

 

So I took my pocket knife and dug out that radius until the plastic was recessed at that corner, to be sure that the two right angle optical faces of the prism would be what rests on the plastic, not their common edge.

 

Is it better? Yeah, I think. Some quick experimentation shows that the image is maybe 1.7mm off axis now. That's an improvement for sure. But I don't think this qualifies as "great" by any stretch!

 

However it at least means that the field center to which I collimated, with the EP in the visual back, is only shifted 1/2 of the way out to the edge of the FOV by the prism diagonal worst case. That's when I've got 12mm EP+2x Barlow in there, which is about as much magnification as this scope can stand from what I've seen.

 

my C8 ... has a 2" TeleVue on it

 

 

I have yet to bite the bullet and start buying 2" stuff... ultimately, as my dad recently gave me this scope after years of sad closet occupation, I do sorta I have this "sunk cost" fallacy where I'm trying to get my good out of it, on his behalf. Haven't run outa good yet! But I can imagine that I may feel limited at some point soon.

 

It's interesting though; I feel like urban skies seem to favor narrow fields, so maybe it'll take a while before I need 2" stuff. I'm finding that I end up with quite short FL eyepieces much of the time even on DSOs, because the city glow is such garbage that wide fields kinda suck. Any DSO which would benefit from an even wider field just isn't there anymore: the only DSOs I can see are small/high surface brightness ones (excepting maybe andromeda and orion).

 

Not much loss of contrast/visibility with increasing magnification on these small/bright DSOs until you've magnified enough that the background glow approaches "dark" in the eyepiece. Just a subjective opinion, I'd love to hear about others' light polluted observing preferences. As an example I feel like Hubble's Variable Nebula looks better at 12mm than at 25mm (although this 12mm is also a higher quality EP). 

 

For context on the darksitefinder.com website I'm on the border of the highest light polution in the SF bay area. Solidly in the middle of the "bright red" category, if that gives any indication of what I'm usually looking through.


Edited by InductorMan, 13 March 2021 - 03:24 PM.


#10 charlesgeiger

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Posted 16 March 2021 - 12:31 AM

As a last resort, you might try putting some thick double sided tape inside the diagonal between the prism and the back plate; you don't need to take the paper off either side of the tape.  Just use tape that is thick enough to take up the space between the prism and the back plate plus a little more and then screw back on the back plate.  That should position the prism correctly.

At this point, you have put way more time into this thing then it could possibly be worth.  I would pick up a used 1 1/4" dielectric diagonal for $30 to $60 and call it good.  Keep this one if you feel it is part of the package that your Dad gave you.

The scope deserves better!

Charlie



#11 Redbetter

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Posted 16 March 2021 - 01:46 AM

I have seen this non-perpendicular/non-orthogonal threading problem of the nose piece and eyepiece holder of several mirror diagonals with plastic bodies, that is why I don't recommend the 1.25" diagonals with plastic bodies.  The exception is the Tak 1.25" prism which has a well made plastic body.  (Just be ready if it doesn't have caps for the diagonal.)  If you are going to get a dielectric mirror diagonal, get one with a good metal build quality like the Astro-Tech.  The AT is roughly half the price of the Tak, but still twice the price of the cheap plastic mirror diagonals that I have had ZERO luck with due to misalignment issues.  I won't get burned by the cheap ones again.

 

With the Celestar 8 on traditional mount, a 2" mirror diagonal works well.  I have the TeleVue (since the mid 90's), but it is not SCT threaded and uses a 2" TV visual back.  This, along with the "high hat" 1.25" adapter makes for a longer projection than one would have with an SCT threaded 2" mirror further increasing the focal length of the scope because of the longer optical path.  (Each extra mm requires moving the primary enough to result in about 3mm of additional focal length.)  The 2" mirror already moves it quite a bit relative to the 1.25" RA prism. 

 

A 2" diagonal is much more secure and my experience is that they are better aligned collimated compared to 1.25" (other than the Tak.)  However, on some of the newer SCT's length can be an issue passing between the base on fork.  The other issue with heavy eyepieces is balance.  That has not been a problem for me since I bought a dovetail weight system for my Celestar 8 shortly after getting the scope.  

 

There is a potential fork in the road if you want wider FOV.  You can either use a 2" diagonal, or the Celestron reducer/corrector that also flattens the field.  There are advantages and disadvantages to each. 

  • The 1.25" system is cheaper in terms of eyepieces and nebula filters.
  • The reducer/corrector flattens the field which should improve stars near the edge (I don't have the reducer corrector, so I can't attest to the impact...probably going to pick one up to experiment with.)
  • It probably isn't the most practical to be pulling the R/C off and on regularly for higher power viewing, so this means one will likely have to expand the short focal length eyepiece line.  
  • From what I have read, the threads can be tricky with the R/C's if you try to remove/reinstall in the field frequently.  They can bind at times.  My understanding is that this can usually be addressed, but sometimes not immediately (in the field.)
  • I prefer the wide field view offered by 2" eyepieces for low power, but like I said, they are heavy and spendy...and then one ends up with 2" filters...along with some 1.25"
  • The 2" diagonal doesn't rotate on me with heavy/long eyepiece and/or Barlow combinations as the 1.25" diagonal did.



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