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

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#26 JOEinCO

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Posted 15 October 2019 - 02:47 PM

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?

I'm curious about the diameter of your secondary; is it 1.55" like the Cave catalog indicated? And perhaps you can post a picture of the stock focuser? 

 

Thanks!



#27 Garyth64

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Posted 15 October 2019 - 02:49 PM

Just as a reminder, the size of the secondary depends on it's placement inside the focal point of the primary.

 

I would be curious to know how far outside the tube the focuses (foci) are on the last two scopes mentioned.


Edited by Garyth64, 15 October 2019 - 02:51 PM.


#28 JOEinCO

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Posted 15 October 2019 - 02:52 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.

Can you see the edges of the primary mirror in the secondary when you look into the focuser drawtube? 

 

Also, can you confirm it is an 8" f/5? I don't see an f/5 in Cave's catalog.



#29 Vic Menard

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Posted 15 October 2019 - 03:53 PM

Try plugging your numbers in this handy calculator:  https://www.bbastrod...om/diagonal.htm

 

If you can pull the primary mirror back an inch (Diagonal to focal plane distance = 8.2-inches), you can (just barely) use a 1.83 minor axis secondary mirror. Otherwise, you can evaluate various sizes (for RFT, larger fully illuminated field is better, but watch the central illumination too...)


Edited by Vic Menard, 15 October 2019 - 05:10 PM.

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#30 JOEinCO

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Posted 15 October 2019 - 05:54 PM

The Man himself. smile.gif 

 

I'll look into that, Vic. Thanks. Though off the top of my head, there isn't an inch available. But I'll look.....



#31 JOEinCO

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Posted 15 October 2019 - 06:09 PM

OK. Happy Time in Colorado. 

 

I can move it an inch, maybe 1.25", but that's getting awfully close to the end of the fiberglass tube. After my quick look, I'd say 1" makes me feel better. 

 

I may also be able to gain about 1/8" if I can source stiffer, shorter springs for the 3 "pull" screws on the cell, allowing me to decrease the distance between the two pieces of the cell. 


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#32 JOEinCO

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Posted 15 October 2019 - 06:37 PM

I see, perhaps, a better solution....

 

I think I can recreate the half of the cell that attaches to the tube. I have plenty of 3/4" and 1/2" Baltic Birch scraps in the shop. I saw a cell for sale online where the tube portion of the cell has adjustments via sliding metal end plates. Basically, the piece I make can hang as low as possible in the tube and still use the Cave tube holes.

 

This will also give me a chance to make a better primary collimating mechanism. Knobs and springs so it's tool-less. And low-profile knobs to get the mirror back as far as the tube allows.

 

But first, some math.....


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#33 wfj

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Posted 15 October 2019 - 07:50 PM

Can you see the edges of the primary mirror in the secondary when you look into the focuser drawtube? 

 

Also, can you confirm it is an 8" f/5? I don't see an f/5 in Cave's catalog.

Confirmed, can see the edges.

 

It is a 8" f/5. They did custom f/'s. It's in the Cave registry here.



#34 Garyth64

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Posted 15 October 2019 - 08:12 PM

Yes, to get your 1.83" secondary to work, we can do more math, (simple algebra), but this number, 1.83"  is the exact diameter of the cone of light.  at that separation, it should be just a bit larger.

 

8/36.125 = 1.83/x ,   x will be 8.26"

 

So, you have to get inside that number, 8.26".  At that separation, 1.83" is the diameter of the cone of light. 

 

If you can squeeze out another inch, and get that separation to 7.26", you would be all set, and be able to use your existing secondary.

 

Remaking the mirror cell is a great idea.


Edited by Garyth64, 15 October 2019 - 08:13 PM.


#35 wfj

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Posted 16 October 2019 - 04:25 PM

It occurs to me that perhaps few have ever seen a low profile focuser for a Cave (an option from the past, I've seen it last in Kenneth Novak's catalog).

 

Mine's less than 2" racked in.

 

IMG_6574.jpg


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#36 JOEinCO

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Posted 16 October 2019 - 09:57 PM

Nice! 

Where does it come to focus with most eyepieces? Does the drawtube stick into the OTA when racked in? If so, how far? 

 

I've kinda barely started my mirror cell "restructuring". I'll post some pictures and notes when I've actually got something to show. lol.gif 



#37 JOEinCO

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Posted 18 October 2019 - 03:50 AM

Had a little time to get started, and I've made my "drop brackets". Since the back of the drop brackets won't be accessible when I install my completed cell assembly, I welded blind nuts to the drop brackets. 

 

The first picture shows the Cave cell in the stock location.

 

The second picture shows the drop brackets installed at the stock mirror cell mounting holes. 

 

The third picture shows how the mirror cell portion will sit (roughly). The fixed half of the cell that I will make from Baltic Birch ply will attach on top of my drop brackets (further from secondary). Then the mirror half will be attached via it's collimation bolts with springs, and end up about 1/4" below (closer to secondary) than my drop brackets. 

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Attached Thumbnails

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#38 wfj

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Posted 18 October 2019 - 01:23 PM

Nice! 

Where does it come to focus with most eyepieces? Does the drawtube stick into the OTA when racked in? If so, how far? 

 

I've kinda barely started my mirror cell "restructuring". I'll post some pictures and notes when I've actually got something to show. lol.gif

Volcano top ortho's (what it was originally used with) focus about 2" above the tube (more than 3/8" additional in focus). Modern short FL EP's are further out. Wide-field's of the old era require the drawtube all the way out.

 

The draw tube  doesn't occlude the mirror because the tube is much larger than the mirror - sticks in about an inch with a high magnification (4mm) ortho. With a low power EP, it doesn't enter the tube at all.

 

The benefit of this arrangement is to maximize contrast. This scope was carefully flocked, and various projections removed, and the tube extends well beyond the focuser. And, another effect of the small diagonal is to act as an "inverse aperture stop".

 

I have a 8" f/4 with a much larger diagonal (and a coma corrector) for a wide field.



#39 JOEinCO

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Posted 22 October 2019 - 09:24 PM

Got a chance to work on the mirror cell a little today. It's a work in progress, but here's the basic cell with the new Baltic Birch plate. 

 

I'm going to try to source some stiffer springs (Ace Hardware was no joy), but it looks like I'm going to end up with the mirror about 1-1/4" lower in the tube. And I haven't made a single non-reversible change to the tube or the cell. grin.gif 

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Attached Thumbnails

  • Cave_04.jpg
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#40 Garyth64

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Posted 23 October 2019 - 01:33 PM

How far from the end of the tube will the plywood be?



#41 wfj

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Posted 27 October 2019 - 03:58 PM

For stiffer springs, try internal combustion engine’s valve springs.


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#42 JOEinCO

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Posted 27 October 2019 - 06:33 PM

For stiffer springs, try internal combustion engine’s valve springs.

THAT small?? On the order of 1/4" inside- or 7/16" outside-diameter and 3/4" long?

 

How far from the end of the tube will the plywood be?

Outside surface about 1/2".



#43 Garyth64

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Posted 27 October 2019 - 06:42 PM

Moving the primary 1.25", as you said, will certainly help a lot.



#44 JOEinCO

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Posted 28 October 2019 - 08:22 AM

Snowy days in Colorado. Must mean "indoor astronomy"! 

 

I finished making the new mirror cell plate, and painted it yesterday....

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Attached Thumbnails

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#45 JOEinCO

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Posted 28 October 2019 - 08:25 AM

Then this morning, added the "components".... grin.gif

 

It's coming together now!

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Attached Thumbnails

  • Cave_11.jpg
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#46 Garyth64

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Posted 28 October 2019 - 08:47 AM

That looks great.

 

You also could have made that from 1/2" aluminum.  3 mounting screws drilled and tapped into the aluminum, and the plate flush to the end of the tube. 



#47 JOEinCO

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Posted 28 October 2019 - 08:51 AM

But I had the BB on hand. waytogo.gif 

 

Here's the two halves of the cell mated.... 

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Attached Thumbnails

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#48 JOEinCO

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Posted 28 October 2019 - 03:51 PM

The mirror cell is back in the optical tube. It looks good from the focuser tube, but all I've given it is a casual glance. The mirror is 1-3/8" lower in the tube, so the math is now saying 1.99" for the secondary. Not quite perfect, but closer to my 1.82" diagonal than the original 2.28".

 

My Baltic Birch plywood plate is smaller O.D. than the Cave tube's aluminum end ring's I.D., so as Gary said, I could drop the whole shebang quite easily by making three longer drop brackets. Another 3/4" wouldn't be terribly difficult, though I'd probably have to come up with a non-destructive optical tube extension of sorts since that will almost have the mirror outside the Cave optical tube. For now I'm going to collimate it and see how things end up.

 

The best part is that I haven't changed a single Cave part. The original cell could be put back together exactly as it was.

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  • Cave_19.jpg
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#49 Garyth64

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Posted 28 October 2019 - 04:03 PM

Hate to question the math, but was your original secondary placement, 8.26" inside the focus?  ( I may have to go back thru the thread and re-read some of them).

 

if so, then I come up with a secondary size of 1.78", for moving the mirror 1.375".

 

I'll go back and re-read.

 

Edit:

 

I did look, and you did say 8.26". So now being at 6.88", you may be looking pretty good.


Edited by Garyth64, 28 October 2019 - 04:07 PM.


#50 JOEinCO

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Posted 28 October 2019 - 05:34 PM

Hate to question the math, but was your original secondary placement, 8.26" inside the focus?  ( I may have to go back thru the thread and re-read some of them).

 

if so, then I come up with a secondary size of 1.78", for moving the mirror 1.375".

 

I'll go back and re-read.

 

Edit:

 

I did look, and you did say 8.26". So now being at 6.88", you may be looking pretty good.

No. The original distance, d, the distance of the secondary inside focus, was 9.219". Post #17. 

 

The new distance (unmeasured, but using the 1-3/8" the mirror moved), d, is 7.844" (9.219-1.375).


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