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Putting A Modern Focuser On A Cave Astrola 8" f/4.5 RFT

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#1 JOEinCO

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Posted 13 October 2019 - 02:28 PM

I picked up a Cave Astrola 8" f/4.5 R.F.T. a couple of months back, and I have truly enjoyed using it. I was fortunate enough to find a scope that had the mirror refigured by Steve Swayze and recoated by Spectrum Coatings in late 2018. By all casual "testing" and perhaps more importantly to me, eyepiece results, this is a very good mirror now. 

 

But there's one problem. The focuser has been replaced with some mediocre old single-speed focuser. I love the mirror. I love the scope on my CGEM. I LOVE the rotating tube rings. But I hate the focuser. 

 

Besides it's awful "feel", I fear a far more significant problem: I think the drawtube is too long. Even without a collimating eyepiece in the 2" focuser drawtube - with nothing in the drawtube - I cannot see the edges or clips of the primary. Near as I can tell, I have an effective-6" mirror. shocked.gif 

 

Then again, I know just enough about this to hurt myself.... 

 

I know it's probably heresy, but the mirror is nice enough for me to warrant spending up for a Moonlite focuser or similar, and it was already a functional restoration, not an authentic restoration. With the rotating rings and a focuser with precision feel, everything I physically touch while observing will be "very nice" grin.gif . But does anyone have experience or suggestions on what works as far as drawtube length and such? 

 

Some technical information that I know: The mirror's been refigured at 36-1/8" (f/4.52), from the original 36-1/16" (f/4.51). It has an Astro-Systems 1.83" secondary. Am I foolish to think the starting point is as simple as removing the focuser and mocking up a cardboard drawtube to hold in the OTA hole? To see what I can get away with? Also, part of my fear is that the secondary is the wrong size, but figuring that is above my pay grade. tongue2.gif  

 

What say ye, collective brain? I'm all ears.


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#2 Thaeland

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Posted 13 October 2019 - 03:57 PM

I believe your secondary may be a bit too small. Look at the link below.

Jon

http://www.loptics.c.../diagonals.html


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#3 JOEinCO

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Posted 13 October 2019 - 04:10 PM

I believe your secondary may be a bit too small. Look at the link below.
Jon

http://www.loptics.c.../diagonals.html

Using the "easy formula" from that page, I get 1.78". So rounding up goes to the 1.83" secondary I have. 

 

Looking at stuff like this online makes me think it's the long drawtube. And I've felt all along that I can't imagine Randy at Astro-Systems selling him the wrong size secondary,



#4 Thaeland

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Posted 13 October 2019 - 04:30 PM

The problem is 1.83 is not the recommended 1/4" to 1/2" larger than your 1.77" minimum. It's recommended to go to the next size up.

Jon

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Edited by Thaeland, 13 October 2019 - 04:32 PM.

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#5 JOEinCO

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Posted 13 October 2019 - 04:48 PM

Ahhhh!  Dang devilish details.....



#6 Thaeland

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Posted 13 October 2019 - 05:02 PM

The secondary should also be slightly offset. How does it look through the focuser?

Jon

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#7 Thaeland

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Posted 13 October 2019 - 05:10 PM

Read this....
https://www.lcas-ast...gory=telescopes


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#8 rigelsys

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Posted 13 October 2019 - 06:06 PM

there is another alternative ....

 

https://www.cloudyni...+cave +rigelsys


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#9 apfever

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Posted 13 October 2019 - 07:02 PM

Another alternative is to mount the base inside the tube instead of outside.  This requires a larger hole that can clear the focuser body. Focus knobs may be close to the tube. Vignetting of off axis rays from inside the tube should be trivial for  a 10" mirror, you'd want to make sure the focuser body does not extend directly over the mirror. The larger the mirror, the more practical this becomes.



#10 JOEinCO

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Posted 13 October 2019 - 07:16 PM

Thanks guys, but as the title says: "Modern Focuser". So Rigel's old school focuser or Ap's inside-the-tube install aren't really in the running since I don't believe the base of a Moonlite or similar could be installed inside the tube.

 

And I'm fairly sure offset isn't the problem (yet). There is a gross error happening. Offset won't keep me from seeing the whole primary. The primary looks good in the secondary; I just can't see the outer inch all the way around.

 

Does anyone have thoughts on the current focuser's 5.3"-long drawtube and it's effect on my "clipped" view of the primary??



#11 Garyth64

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Posted 13 October 2019 - 08:31 PM

What is the diameter of your tube?

 

How far outside the tube is the focal point?

 

Can you see the edge of your secondary?

 

I have a homemade 8" f/4.5.  The secondary is 8.5" inside the focus, my tube is 10" in diameter.  I am using a 2.14" secondary, and I can see the entire 8" primary.  The fif of .32", which is ½ degree, or the size of the Moon.


Edited by Garyth64, 14 October 2019 - 07:56 AM.


#12 JOEinCO

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Posted 14 October 2019 - 09:50 AM

What is the diameter of your tube?

 

How far outside the tube is the focal point?

 

Can you see the edge of your secondary?

 

I have a homemade 8" f/4.5.  The secondary is 8.5" inside the focus, my tube is 10" in diameter.  I am using a 2.14" secondary, and I can see the entire 8" primary.  The fif of .32", which is ½ degree, or the size of the Moon.

Tube inner diameter is 9-3/8", and the outer diameter is 9-3/4". 

 

The top of the drawtube (top of eyepiece's chrome barrel/the step on eyepiece) is 5" to 5-1/4" from the outside of the tube depending on eyepiece. 

 

I can see the edge of the secondary, and then some. Close to 1/2" on all sides.



#13 Garyth64

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Posted 14 October 2019 - 12:12 PM

"The top of the drawtube (top of eyepiece's chrome barrel/the step on eyepiece) is 5" to 5-1/4" from the outside of the tube depending on eyepiece."

 

but where exactly is the focus on the outside of the tube?  This is important.  Is it located inside the drawtube, or outside the drawtube?

 

If, the focus is at eg 4.5" outside the tube, then 4.5" + 4.875" = 9.375".

 

So, if the secondary is 9.375" inside the focus, and you have a fif of .5", then you need a 2.4" secondary. 

Using a fif of .25", then a 2.26" secondary would be needed. 

 

This is why to determine the correct diagonal, that distance of the secondary inside the focus is critical.  And that is why we need to know where the focus is outside the tube.  smile.gif

 

If, the focus is at 3.5" outside the tube, then the secondary is 8.375" inside the focus, and using a fif of .25", then you need a secondary of 2.05".

 

In a formula that I have used for long time, it calls for using the diameter of the Moon as the diameter of the image plane.  That can be determined by multiplying .009 times the focal length.  So for a f.l. of 36" the fif becomes .32".  And that I have used for all my homemade scopes.


Edited by Garyth64, 14 October 2019 - 12:28 PM.

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#14 JOEinCO

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Posted 14 October 2019 - 03:10 PM

but where exactly is the focus on the outside of the tube?.....

This is where I see how much I do not know. bow.gif 

 

Is there an easy way to do this? Will holding a small piece of translucent paper in the draw tube show me an image of the Moon at the point of focus?



#15 Garyth64

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Posted 14 October 2019 - 03:46 PM

With no eyepiece in the focuser, you can use that, or even an index card, just about anything as long it's not black.

 

If the focus is inside the draw tube, yes, then a small translucent piece of paper will have to be inserted.


Edited by Garyth64, 14 October 2019 - 03:48 PM.


#16 Garyth64

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Posted 14 October 2019 - 06:35 PM

The formula to figure out the size of the secondary you will need is this:

 

Secondary minor axis =  (D - a)  x  d   +   a

                                          f.l.

 

D = diameter of the primary

d = distance of the secondary inside the focus

f.l. = focal length of the primary

a = fully illuminated field  (fif in the program)

 

In the formula, I have always used the value of "a" as the diameter of the focal plane.  This diameter changes according to the focal length of the primary.  The original formula that I found, uses the angular diameter of the Moon, which is .31'.  I believe that it is the tangent of .31' that comes out to be .009. 

Multiplying .009 times the focal length will give you the diameter of the Moon at your focal plane.

In this case, it is .32".

 

For my 6" f/10, the diameter of the focal plane, using the Moon, is .54".  On my 8" f/18 Gregorian, the diameter of the image plane is 1.3".

 

So we just plug in all out numbers into the formula to find the size secondary that is needed.  It's almost easier to do this by hand that to go to that website and look up your scope.  After you do it a couple of times, you will see how easy it really is.

 

For example, let's say I have a 28" f/5 mirror.  It's f.l. is 140".  The diameter of the Moon will be 1.26" at the focal plane.  If the secondary is to be place 20" inside the focal plane, then a secondary of 5.08" should be used.  But the website says only a 4.43" secondary, with a .5" fif, could be used.  So I would go with my formula, and use a  5" secondary.

 

So, Joe, when you come up with how far your focal plane outside the tube, we can figure out the size secondary you should have.


Edited by Garyth64, 14 October 2019 - 06:39 PM.

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#17 JOEinCO

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Posted 14 October 2019 - 11:16 PM

Wow, Gary.....Thanks! 

 

Just came in from the moonlight. I took the eyepiece out and put a piece of Scotch Tape across the 2" drawtube.  The cloudy Scotch Tape worked great, and the Moon was easy to see and focus.

 

So here's the facts:

Tube outer diameter is 9-3/4".

Sharp image on tape is 4-11/32" from outside of tube.

So d = 9.219....

 

(8 - 0.32) x 9.219   +  0.32 

          36.125 

 

= 2.28" 

 

Quite a bit different from my 1.83" secondary. shocked.gif 


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#18 JOEinCO

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Posted 15 October 2019 - 04:06 AM

Looking at the 1972-1973 Cave catalog, it looks like they originally put a 1.55" secondary in the 8" f/4.5 RFT.

 

Holy macaroni.... Would Cave have really screwed up that badly??



#19 Garyth64

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Posted 15 October 2019 - 07:44 AM

That may be why you don't see your entire mirror.  smile.gif



#20 JOEinCO

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Posted 15 October 2019 - 08:01 AM

That may be why you don't see your entire mirror.  smile.gif

I don't have the original diagonal to look at, that's why I pulled up the old 1972-1973 catalog online. Mine has a 1.83" Astro-Systems secondary. Hence the Holy Macaroni regarding all those scopes Cave sold with a 1.55"er. That had to take the effective aperture of the stock 8" RFT down to almost 5 inches!! 

 

It sounds like, all-in-all, the 8" RFT should have had an optical tube about 2" longer.....

 

Any thoughts on secondary sources, Gary or others? Astro-Systems and several other sources I looked at jump from 2.14" to 2.60".


Edited by JOEinCO, 15 October 2019 - 08:02 AM.


#21 Garyth64

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Posted 15 October 2019 - 08:23 AM

Wow, a 8" rft with a 1.5" secondary?  What focuser was on those?

 

If it has a low profile focuser, and the image plane is 2" above the tube, and if the fif is zero, a 1.52" diagonal could be used.  But you'd get some fall off at the edge of the fov when using low power eyepieces.

 

I have no source for a secondary today.  I purchased mine a long time ago, and I don't remember from who.


Edited by Garyth64, 15 October 2019 - 04:45 PM.


#22 JOEinCO

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Posted 15 October 2019 - 08:31 AM

I bought this 8" RFT after a restoration. The guy I bought it from got it in derelict condition with no focuser and no secondary on the OTA. 

 

Anybody out there with a stock 8" RFT that can measure the secondary and the focuser Cave put in the scopes?


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#23 tim53

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Posted 15 October 2019 - 09:36 AM

I think you'd be fine with a 2.14", but it certainly wouldn't hurt to go with a 2.6".  Since it's an RFT, the small difference in contrast won't be noticeable visually.  And if you do any photography, especially with something like a full-frame digital camera, you won't have any vignetting issues with the larger secondary.  [edit:  I should say that I don't *think* you'll have vignetting issues with the larger secondary, as I'm still just an inch into my first cup of coffee and in no condition to calculate anything!  grin.gif ]

 

-Tim.


Edited by tim53, 15 October 2019 - 09:38 AM.

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#24 semiosteve

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Posted 15 October 2019 - 11:54 AM

I bought this 8" RFT after a restoration. The guy I bought it from got it in derelict condition with no focuser and no secondary on the OTA. 

 

Anybody out there with a stock 8" RFT that can measure the secondary and the focuser Cave put in the scopes?

Mine has the original secondary and original 1.25 helical focuser. Clouds coming in for next two days, but clear Friday (?).

 

What do you want measured for the  focuser?



#25 wfj

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Posted 15 October 2019 - 01:17 PM

I have a Cave 8" f/5 with an original low profile 1.25" R&P focuser with a 1.55" secondary. The original owner went to great lengths to minimize CO/diffraction, and sent the mirror back for refigure to better correction/fewer zones. Only used it for planetary, had no WF EP's, just an ancient 28mm war surplus EP in an aluminum sleeve. The ortho's he used were barely able to reach focus racked in.

 

Obviously not an RFT. I think he was short and had a compact scope/mount for purposes of low height and short moment arm on the GEM mount.

 

Not everyone wants to drink the large FOV.




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