96 replies to this topic

[eblanken]

Hi Chris (aka "C") & All,

 

I get that your name is Chris, but I didn't know if you wanted that spread in public forum, until now . . .

 

Whoa that's quite a trick - no clear nights here for a while, but I'll do the first chance I get.

 

I don't quite understand "Racked all the way in is 9.375" ". I would have said, "The AP focuser gives 4.875" of travel", because that's the maximum distance the focuser can move. You might be adding the diagonal to that, I guess, but could you clarify? Thanks!

 

(Chris, my name is Chris :-) )

 

Yes, TOMDEY's "tricks" are often like Ocham's Razors, they cut to the heart of the matter. It this case, to make obvious to tell "where is" the Focal Plane (really a curved surface) can be accessed or not !!! I suspect that this little "tape trick" would reveal to you where "exactly" is the focal plane of your RC scope is "exactly" and clear up any confusion. Please know that the eyepiece is just a fancy "magnifying glass or loupe" that makes the real image "bigger" in AFoV and has a "field stop" that sets how much of the image plane (TFoV) is passed downstream.

 

Now back to the 9.375 inches: In post #56, I read (and summed up) the components of this from the photo you supplied in post #36 in response to Jon's prompting. This is an accounting of the "fully-racked-in" case when using the AP spacer and the AP focuser and the 2.5 to 2 inch adapter and the Meade 2 inch Diagonal. This is referenced from the RC's back surface "reference plane" you supplied in your photo OUT TO THE EXIT FLANGE OF THE Meade Diagonal. If you also added the 4.875 inches of AP focus travel from "racked-in" to "racked-out", you should get 9.375 inches + 4.875 inches = 14.25 inches, right ??? Simple accounting of all the pieces . . . of the RC and ALL THAT IS NOT EYEPIECE . . . Ranges from 9.375 inches to 14.25 inches . . . SO what is the value at "infinity" focus ???

 

I hope this helps,

 

Ed

 

P.S. I will continue to be a resource to you here and "off-line" in PM and email, etc. and we can talk about "how you got to here" offline if you want . . . 

Edited by eblanken, 14 November 2024 - 12:43 AM.

[triplemon]

 I just had a close look at that image - I would say any home machinist worth his salt will see almost an full inch of reduced backspacing right there:

The (not anodized) aluminum adapter between the focuser and what looks like the 4.8" hole in the scopes back, apparently held with 3 screws, looks like its a circular dovetail connection to the scope. It would not have to be anywhere near that long. Have some machinist turn it down or make a new, shorter one. IMO it could be so short that nothing of the un-anodized aluminum is visible. If a new ring is made - the whole change is entirely reversible, i.e. zero risk. If the drawtube hits something inside the scope after that when fully racked in - cut it down, too.

The drawtube is 2.7". You could fit the entire 2" compression adapter inside, rather than screw it on its end. Just need a small hole drilled into the drawtube to fit the setscrews through, so you can still operate it. Plus optionally mill a small recess into the focuser body to clear that setscrew when fully racked in. Basically how JMI low profile newtonian focusers are built:
http://www.jimsmobil...mages/ev_1c.jpg

Edited by triplemon, 14 November 2024 - 02:02 AM.

[chemisted]

One more thought - often the eyepiece holder on the diagonal is longer than all of the eyepiece barrels in actual use. Machining off a bit on that one would give some small reduction in optical path length.

I did this by unscrewing that sleeve, machining off its threaded and (so the eyepiece end with the compression rings etc is left untouched) and then epoxyied it back on. Was too lazy to cut a thread into it and also allowed me to utilize the threaded hole in the diagonal housing to let the EP barrel get _really_ close to the mirror. When reworking a diagonal that way - make sure there is some form of stop left to prevent any longer EP barrel to hit the mirror.

This is an excellent suggestion.  The 1.75" length of the current diagonal is unusually long.  I have multiple 2" diagonals and they all measure 1.5" or less in that dimension.  The added path length, in addition to the 11/32" gained from the new 2" adapter, could very well allow all of Chris' eyepieces to come to focus.

 

Alternatively, a completely new diagonal, such as a 2" Baader Prism with Click-lock, would be a temptation for me personally , ,  ,

Edited by chemisted, 14 November 2024 - 05:25 AM.

[luxo II]

Clicklock is 45-48mm tall, this will eat into the light path, not reduce it.

I have a couple of them.

Edited by luxo II, 14 November 2024 - 05:44 PM.

[eblanken]

Hi All,

 

I had a nice chat with Chris via phone today. He's exploring several options in parallel. There are others involved with a variety of opinions (just like here, LOL).

 

Ed

[cduston44]

cduston44 says of the factory alignment "the result is a dramatic improvement in optical quality"; they adjusted the PM-SM spacing to correct the spherical aberration. Fiddling with the alignment of the mirrors at this point would probably meaning sending this 20-inch RC right back to the factory!

 

I measured a bunch of commercial Star Diagonals. The 2-inch Lumicon LumiBrite Star diagonal is the shortest added path of all 2-inchers (95.5mm). I don't know if this will bring all of your eyepieces to focus, but it's certainly the best possible (both in theory and in practice) in the 2-inch format.     Tom

This thing appears to be out of stock in most places I can see it - do you know if it's being produced any longer?

 

The Baader seems to be 112 mm, or 71.5 mm without the click-lock (pg 21): https://www.baader-p...RGB-LR_0321.pdf

[Starman1]

This thing appears to be out of stock in most places I can see it - do you know if it's being produced any longer?

 

The Baader seems to be 112 mm, or 71.5 mm without the click-lock (pg 21): https://www.baader-p...RGB-LR_0321.pdf

Lumicon diagonals haven't been manufactured since 2012.

There were some in dealer stocks for a few years after that.

[eblanken]

Hi Chris & All,

 

Don, Jon, others, what is a current production, good quality 2 inch diagonal that has a shorter path length than the Meade which Chris is using ? Is there merit in going with a Prism or is a Mirror better for a 508mm (20 inch) x 4,064mm (f/8) RC with the AP Focuser and a shorter 2.5 inch to 2 inch adapter on the end ?

 

Best,

 

Ed

 

P.S. TOMDEY, others, do you have an opinion for a currently available diagonal (that is not a decade+ out-of-production) ?

Edited by eblanken, 14 November 2024 - 08:44 PM.

[chemisted]

The Baader 2" Prism Click-lock diagonal has a light path of 100 mm which is 12 mm shorter than the corresponding mirror diagonal.  Together with the thin 2" adapter this would give over 20 mm of in travel and would be a nice solution to the problem.

Edited by chemisted, 14 November 2024 - 09:35 PM.

[Starman1]

Hi Chris & All,

Don, Jon, others, what is a current production, good quality 2 inch diagonal that has a shorter path length than the Meade which Chris is using ? Is there merit in going with a Prism or is a Mirror better for a 508mm (20 inch) x 4,064mm (f/8) RC with the AP Focuser and a shorter 2.5 inch to 2 inch adapter on the end ?

Best,

Ed

P.S. TOMDEY, others, do you have an opinion for a currently available diagonal (that is not a decade+ out-of-production) ?

A prism would work fine at f/8 and shorten the lightpath.

Edited by Starman1, 14 November 2024 - 09:36 PM.

[TOMDEY]

I love that Lumicon. Glad I bought more than one. I even do that with shoes --- when I find something that is superior... I buy "extras" and just put them in storage.    Tom

[eblanken]

Hi TOMDEY,

 

I love that Lumicon. Glad I bought more than one. I even do that with shoes --- when I find something that is superior... I buy "extras" and just put them in storage.    Tom

 

"great minds think alike" is all I can say, LOL !!! I have two pairs of Nike Shoes brand new still in the boxes that are about 5 years old. They are (I am told) the same shoes that the founder wears (Cortez Classic) and are the only SKUs that have a 4-digit number (the rest have 6 digits).

 

Best,

 

Ed

[cduston44]

Hey All,

  I'll make individual replies at some point later, but I wanted to get this in here - I think I've taken all the various focal measurements suggested by folks here, and might have an answer for "how much distance do we need?" See the table:

 

 

Just some info - this is all of our eyepieces + saran wrap ("focal length 0") looking at the sky (focus at infinity) and a terrestrial object a few miles away.  I took the measurements by doing each one three times, so they are averages (and that's where the errors come from). I did not find the errors for the terrestrial case, because I think that does not address my problem directly enough.

 

Terrestrial: Although I was able to get each eyepiece in focus with the diagonal, you can see e.g. that the difference in the 35 mm to 55 mm (~1") was smaller than the apparent difference between when looking at the sky (over 1.73", since the 35 mm could not come to focus). I think that the closer object distance would modify this spacing (thin lens formula etc), so I think that just tells me "the 35 mm is almost in focus" - but I kinda knew that. Exploring this more would require equation hoping - which is fine with me, if the sky one didn't seem more positive.

 

Sky: Using the saran wrap I think we've found that the difference between the focal plane in the diagonal vs direct (just a note - to get the diagonal in focus I had to remove the nosepiece, which was a length of 1.75". That's not included in the chart, that's just a measurement of the draw tube length). So, it looks like the difference is

 

(4.852 +/- 0.037) - (1.860 +/- 0.177) = 2.992 +/- 0.181 inches,

 

but the diagonal is actually 1.75" longer than that, or

 

4.742" +/- 0.181".

 

That's actually kinda interesting, because it suggests that the 35 mm will come into focus, using the diagonal, at a draw tube length of

 

(4.881 +/- 0.010) - (4.742 +/- 0.181) = 0.139" +/- 0.181"

 

So this feels like quite a bit of OVER-analysis, but it's actually consistent with my experience - that the 35 mm is *just* not able to come into focus. To be absolutely sure (assuming my errors are good estimates...), I should check out the 1sigma, 2sigma, 3 sigma values....

 

0.139 - 0.181 = -0.042"

0.139 - 2*0.181 = -0.223"

0.139 - 3*0.181 = -0.404"

 

So this adapter: https://www.astro-physics.com/ada20132 will get us 0.344", which covers the first two sigma, and I actually found a diagonal in our dome which has a 1.659" nosepiece, gets us another 0.091" actually covers the 3 sigma.

 

So, maybe that's enough! Thoughts? I'll reply to everyone else shortly....

 

EDIT: I guess I'll estimate what we would need to get the other eyepieces also in focus:

 

31mm : 0.457" more than the 35 mm, so that's -0.318 +/- .181, with sigma levels (-0.499, -0.680, -0.861)

21mm: 0.396" more than the 35 mm, so that's -0.257" +/- .181, with sigma levels (-0.438, -0.619, -0.800)

20mm: No estimate there, since it did not find focus with either diagonal or direct.

 

Again, I'm sure this is over-analysis and there are other things going on, but if I can get a prediction to check that helps guide the scale of the problem.

Anyway, adapters on the way, I'll keep everyone updated!

Edited by cduston44, 20 November 2024 - 12:05 PM.

[chemisted]

I assume you have the Astro-Physics adapter on order. When do you expect it to arrive to try with the additional diagonal that you found?

[cduston44]

Hi Ed et al.

 

Yes! I typically use Scotch "Magic Mending Tape", which has a nice matte finish for use as a high-gain rear projection screen and leaves no residue when you peel it off. Any other equivalent tape should work as well. This RP Screen takes 1.00s to build, 1.00s to install, and costs 2.6ȼ (each, inflation adjusted Y2024, USA currency). Comes on a nice spool of 100 disposable Rear Projection Screens >>>    Tom

I wish I had read this before working with Saran Wrap - it was a nightmare!

[cduston44]

One more thought - often the eyepiece holder on the diagonal is longer than all of the eyepiece barrels in actual use. Machining off a bit on that one would give some small reduction in optical path length.

I did this by unscrewing that sleeve, machining off its threaded and (so the eyepiece end with the compression rings etc is left untouched) and then epoxyied it back on. Was too lazy to cut a thread into it and also allowed me to utilize the threaded hole in the diagonal housing to let the EP barrel get _really_ close to the mirror. When reworking a diagonal that way - make sure there is some form of stop left to prevent any longer EP barrel to hit the mirror.

Similar could possibly be done to the compression ring adapter on the focuser or any adapter connecting the focuser to the OTA.

Yeah I like this idea too - although I might try to get something from precise parts rather then machining it. (no doubt it's easy, and I guess I think our guys could handle it.....meh I dunno.)

[cduston44]

Hi Chris (aka "C") & All,

 

I get that your name is Chris, but I didn't know if you wanted that spread in public forum, until now . . .

 

 

Yes, TOMDEY's "tricks" are often like Ocham's Razors, they cut to the heart of the matter. It this case, to make obvious to tell "where is" the Focal Plane (really a curved surface) can be accessed or not !!! I suspect that this little "tape trick" would reveal to you where "exactly" is the focal plane of your RC scope is "exactly" and clear up any confusion. Please know that the eyepiece is just a fancy "magnifying glass or loupe" that makes the real image "bigger" in AFoV and has a "field stop" that sets how much of the image plane (TFoV) is passed downstream.

 

Now back to the 9.375 inches: In post #56, I read (and summed up) the components of this from the photo you supplied in post #36 in response to Jon's prompting. This is an accounting of the "fully-racked-in" case when using the AP spacer and the AP focuser and the 2.5 to 2 inch adapter and the Meade 2 inch Diagonal. This is referenced from the RC's back surface "reference plane" you supplied in your photo OUT TO THE EXIT FLANGE OF THE Meade Diagonal. If you also added the 4.875 inches of AP focus travel from "racked-in" to "racked-out", you should get 9.375 inches + 4.875 inches = 14.25 inches, right ??? Simple accounting of all the pieces . . . of the RC and ALL THAT IS NOT EYEPIECE . . . Ranges from 9.375 inches to 14.25 inches . . . SO what is the value at "infinity" focus ???

 

I hope this helps,

 

Ed

 

P.S. I will continue to be a resource to you here and "off-line" in PM and email, etc. and we can talk about "how you got to here" offline if you want . . . 

Got it, thanks Ed. I had been thinking about just measuring the racking distance, but yes of course, if you want to bring in the idea of changing focusers, the distance you're quoting is the relevant one.

[cduston44]

 I just had a close look at that image - I would say any home machinist worth his salt will see almost an full inch of reduced backspacing right there:

The (not anodized) aluminum adapter between the focuser and what looks like the 4.8" hole in the scopes back, apparently held with 3 screws, looks like its a circular dovetail connection to the scope. It would not have to be anywhere near that long. Have some machinist turn it down or make a new, shorter one. IMO it could be so short that nothing of the un-anodized aluminum is visible. If a new ring is made - the whole change is entirely reversible, i.e. zero risk. If the drawtube hits something inside the scope after that when fully racked in - cut it down, too.

The drawtube is 2.7". You could fit the entire 2" compression adapter inside, rather than screw it on its end. Just need a small hole drilled into the drawtube to fit the setscrews through, so you can still operate it. Plus optionally mill a small recess into the focuser body to clear that setscrew when fully racked in. Basically how JMI low profile newtonian focusers are built:
http://www.jimsmobil...mages/ev_1c.jpg

Yes to all this. I do think it's a "circular dovetail connection to the scope", although I might have said "it's got a ridge instead of a thread"....I think we are saying the same thing.

 

I don't understand how you can do any of this "reversable" - I think you're talking about cutting into the silver bit ("un-anodized"?). But maybe you think the silver bit can be removed, and a new low-profile silver bit made. Sure, that'd be great.

 

A 2" compression adapter would be perfect - best I can find is this: https://www.astro-physics.com/ada20132. That's what you mean, correct?

[triplemon]

I'm pretty sure the silver ring is removable. It clearly is a tad wider than the adjacent anodized focuser body, so it was manufactured as a separate piece and therefore should be separatable again. Could be just screwed onto the focuser.

 

And wrt the compression adapter - I do not mean any commercially available adapter. Its a custom machined part that fits entirely inside the drawtube, not protruding at all. You could use a generic compression adapter to make one as a starting point, if you can find one that has a large enough outer diameter to start with. But for any machinist it would likely be easier to turn one from scratch from a chunk of round aluminum stock.

Edited by triplemon, 19 November 2024 - 04:44 PM.

[cduston44]

I've come to report success!

 

The shorter adapter + shorter diagonal got both our 35 mm and 21 mm into focus! So, what seemed like a small disaster turned out to require a fair amount of time, learning, measurement, and planning....but an $80 fix (since we randomly had a slightly shorter diagonal in a bin already).

 

I would not have had the confidence to explore and diagnose these issues without the help of folks here - Ed, TOMDEY, chemisted, Starman, everyone else who contributed, I very much appreciate it! (And, dozens of members of the public who will use the instrument over the next few months appreciate it too, but they won't know you did anything...)

[Rick-T137]

That is fantastic! Congrats!

[eblanken]

Hi Chris & All,

 

I am glad that this issue got resolved and well resolved: The Factory Mirror spacing is now correct AND the "workhorse" eyepieces come to focus as well.

 

All-the-Best,

 

Ed