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Please help. I am running out of Sonotube, money, patience, etc.

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#1 JimmyTrom

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Posted 09 October 2018 - 10:26 AM

History.  Bought a 8" "50" inch fl parabolic mirror for a new wide field project.  Tested the focus point, came out to 40" per John Dobson vid and other members.   Plugged my numbers into Newt for the web, cut tube, made frame, 2" focuser, 52mm diagonal cut holes finished tube sweet mounted and preliminary did the collimation.  Tested outside at power tower 3 miles away.  couldn't focus a thing.

Re tested  Image of flashlight in yard focused on sheet of paper at dead nuts 40"  O.K. f5, Plugged in again to Newt for web, re cut, no dice.

New tube remounted everything STILL nothing.

When I look at the secondary with the focuser removed, there is a perfectly clear image of stuff at least 1/2 mile away.

I am out of Aleve and need some expertise..

 

 



#2 J A VOLK

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Posted 09 October 2018 - 10:42 AM

maybe it actually is 50" - you can hold a low power eyepiece well outside the tube to see if you can focus. Just point where you think it should be and move around slowly - there will be an instant where you see the image. Easier is the moon in a few days. Point at the moon and hold a piece of paper beyond the focuser until you get an image and you will know the focal plane. In case the focus is a bit inside of where you calculated you can remove the focuser and similarly hold the eyepiece in the air pointed in the right direction to see if you can get an image.

Edited by J A VOLK, 09 October 2018 - 10:47 AM.


#3 perfessor

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Posted 09 October 2018 - 10:55 AM

Let me see if I understand your test:  You have a flashlight, some unspecified distance from the mirror, creating a focused image on a piece of paper 40" away - do I have that right?  With that setup, you won't get the info you need.  You need your light source to be at infinity (like the sun, but be careful); or at the focal point i.e. same distance as the paper you're focusing on.  In the first case, you are measuring the focal length directly.  In the second, you are at twice the focal length.  

 

There are other ways, but that should get you started.  Ideally, you should know your FL to within 1/8" before you start drilling holes in your tube.  If it's any consolation, many of us struggle with this problem - I know I did.

 

Let us know...


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#4 happylimpet

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Posted 09 October 2018 - 11:26 AM

Might be easiest, if clear skies are available, to just point up and see if any stars are visible holding the EP at a variety of distances.

 

This stuff is never easy. I calculated my **** off when I built a 51" FL 8" dob, and still got it wrong, and Ive got a PhD in this stuff (seriously). You'll get there.


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#5 JimmyTrom

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Posted 09 October 2018 - 11:52 AM

Is there any information I can derive out of the image from the exposed secondary being perfectly focused?

I don't want to screw this up again.



#6 gr5org

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Posted 09 October 2018 - 12:21 PM

Is there any information I can derive out of the image from the exposed secondary being perfectly focused?

I don't want to screw this up again.

You mean you took a photo without the eyepiece installed and saw stuff in the mirror that was in focus?  Is that what you mean?  I don't think that's at all useful, no.

 

Do what 2 people already said to do:  Point the telescope at something more than a mile away.  Like a bright star.  Then move the eyepiece to where you think the focal point should be - maybe be in the focuser - may be a foot away from the telescope.  Take your time until you see that star focused.

 

Or if you must do this during the day then point it at the farthest tree you can find and at least figure out where that focus point is (how many inches away from the telescope is the eyepiece when the tree is in focus).  Then you have a very rough idea.  The closer the object is you are trying to focus, the further the eyepiece needs to be from the mirror/telescope.



#7 Auburn80

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Posted 09 October 2018 - 12:43 PM

As others have stated, Focus on Infinity.
Easy way is to simply rest the tube on the ground and see if you can focus on a star, any star. There should be many available with an 8" looking straight up. If not, you should be able to tell which direction you're off.
I solved this problem with simple calculations and some shims that I used to adjust my tubes up & down over the main cell sitting on some stable blocks. Once all my eyepieces focused, I just used the height of the spacers, height of mirror above the ground (actually a 2x2 sheet of ply on my driveway) to determine where the holes should be drilled.
Good luck!

#8 roscoe

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Posted 09 October 2018 - 01:43 PM

Testing against the stars or the moon is best, but I concur that aiming it at the furthest fairly substantial thing you can see in the daytime will be easier and get you into the ballpark..... and stuff a mile away doesn't move while you're trying to figure it all out......



#9 JimmyTrom

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Posted 09 October 2018 - 02:07 PM

I did get  FLEETING image with a 32 Televue wide angle about 6" from the tube.

But, I set up a flashlight in the yard with the mirror out of the tube and got the image of the flashlight clear at ~54 inches

1/2 of that gives about 27 inch focal length.

Yowza.  That works out to about f3.9 !

So much for 50" focal length.

 

Rory Schider "Your gonna need a bigger diagonal"


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#10 hamishbarker

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Posted 09 October 2018 - 02:12 PM

what is your tube outside diameter? D

 

what is the distance from front surface of mirror to centre of focuser hole?  L

 

what is the height of top of the focuser above the outside surface of the tube when racked all the way in? H

 

measure the focal length by using flashlight and piece of paper side by side. Piece of paper needs to be at the same plane as the bulb. that will give you the radius of curvature which will be twice the focal length. You should be easily able to measure this to 5mm

 

focal length f must = L + (H + D/2) - difference between top of focusser and focal point of eyepiece in use (usually below top of focusser)

 

so L = f -H - D/2 + diff



#11 gr5org

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Posted 09 October 2018 - 02:35 PM

Jimmy you have been using the wrong formula.

 

You can find 2X the focal length which is what you must have been thinking about by putting a light source and a piece of paper at the same position and moving it towards and away from the mirror until you find that exact spot that is the "center of curvature" of the lens.  Focal length is half that distance.  If the flashlight is 100 feet away (1200 inches) then you need to use the lens equation:

1/LENS_FL = 1/OBJECT_DISTANCE + 1/FOCAL_PLANE_DISTANCE

 

In other words if the flashlight was EXACTLY 1200 inches (and you didn't tell us how far) and you saw it focus at 54 inches then the FL is not 27 - it's 1/(1/1200+1/54) or 51.6 inches.  But before you can trust that you need to measure the distance to the flashlight.

 

Or you can use one of the many other techniques.



#12 J A VOLK

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Posted 09 October 2018 - 03:51 PM

I did get  FLEETING image with a 32 Televue wide angle about 6" from the tube.

there is your answer!, retry with an object at infinity and you have your precise focal length.

Edited by J A VOLK, 09 October 2018 - 03:52 PM.


#13 JimmyTrom

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Posted 09 October 2018 - 05:07 PM

Merci Beau Coup to everybody.

 

1/LENS_FL = 1/OBJECT_DISTANCE + 1/FOCAL_PLANE_DISTANCE

 

In other words if the flashlight was EXACTLY 1200 inches (and you didn't tell us how far) and you saw it focus at 54 inches then the FL is not 27 - it's 1/(1/1200+1/54) or 51.6 inches.  But before you can trust that you need to measure the distance to the flashlight.

 

If I use a star or the moon to perform the test above, does the object distance become 0 or 1?



#14 JimmyTrom

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Posted 09 October 2018 - 06:21 PM

Went outside around sunset.

Got a piece of drywall to image against, tape measure on the ground, plumb bob in one hand, mirror cell in the other. Precarious I know.  Imaged the sun multiple times, rested bob string against primary face, dropped plumb bob slowly to ground.

I got 29" more than once.....So

 

1/29=0.0344827586206897



#15 jelloptic

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Posted 09 October 2018 - 08:06 PM

To answer your question in post#13, for an object at infinity, the  1/object-distance  term goes to zero, and the formula simplifies to :

 

lens-focal length  =  focal-plane distance.

 

From your recent post, it appears that the mirror's focal length is 29".   You can confirm that by setting a pen-light flashlight at the mirror's center-of curvature (equal to 2 x focal length = 58") and looking for an image on a screen.

Hint: poke a hole thru a white piece of cardstock, set 58" in front of mirror, shine the light thru to the mirror, and look for image. Piston the card/light until image is smallest, then re-measure distance to the mirror.  One-half of this is the focal length.

 

regards,  kbl



#16 gr5org

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Posted 09 October 2018 - 08:27 PM

Went outside around sunset.

Got a piece of drywall to image against, tape measure on the ground, plumb bob in one hand, mirror cell in the other. Precarious I know.  Imaged the sun multiple times, rested bob string against primary face, dropped plumb bob slowly to ground.

I got 29" more than once.....So

 

1/29=0.0344827586206897

So the focal length would be 29 inches but that disagrees with your other measurements.  I don't understand how you did this.  If you imaged the sun then you burned something, right?  I'm imagining the mirror in one hand and the image of the sun is... where?  This sounds very dangerous by the way.

 

I think you should stick to something safer like the suggestion in the post above with the light and the card with the hole.  This is a pretty easy test and finds the center of curvature which is double the focal length.  Even better that other test you described where the eyepiece was several inches away from the telescope to get a good focus - repeat that except on the moon or starts or planets.



#17 JimmyTrom

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Posted 09 October 2018 - 09:03 PM

No, Didn't burn anything.  Just imaged the sun just before it set.  Image of the sun was clear as were trees a mile or so away.



#18 gr5org

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Posted 09 October 2018 - 09:39 PM

Okay you are definitely doing it wrong if nothing caught fire.  You need to avoid the sun until you know what you are doing.  You can really hurt yourself and in particular go blind.  The verb "image" has special meaning in optics.  Let's move on.

 

Do you have a flashlight and some cardstock or cardboard?  I think you should stick to that method.

 

Get a flashlight and some cardboard (preferably white but brown is fine) put a small hole in it - maybe 2mm across - maybe 1/8 inch.  That should be accurate enough (you can get greater accuracy with a smaller hole but that's more difficult - start with 1/8 inch or so).

 

Tape the flashlight to the cardstock/cardboard.  Place it roughly double the focal length (around 100 inches) from the mirror.  move it around until you see the return light on the paper.  Then move it towards and away from the mirror until the return dot is as small as possible and also right next to the hole.  Now the paper is exactly twice as far from the mirror as the focal length.  Probably around 100 inches since you have a 50 inch mirror it sounds like.  Now repeat but with a smaller hole (it will be much more difficult but give you a more accurate position).

 

coc2.jpg

 

In case you can't read my writing from left to right it says:

FLASHLIGHT TAPE CARDBOARD EYE MIRROR


Edited by gr5org, 09 October 2018 - 09:41 PM.

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#19 Bob4BVM

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Posted 09 October 2018 - 10:45 PM

Surprised no one mentioned it yet, but there is a simple way to nail down FL without any math of guesswork.

Build a simple "board scope"

Arrange the top end so the diagonal & focuser can move up & down and clamp into place at any height

If you make the sec support rod the same distance as your tube scope radius, then all your dimensions will transfer directly to your tube build:

 

CS

Bob

 

Board Scope.png


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#20 JimmyTrom

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Posted 10 October 2018 - 07:13 AM

You guys are the best
I will try both methods and report
Thanks for the help

#21 JimmyTrom

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Posted 10 October 2018 - 05:18 PM

SUCCESS!!
I USED a cutoff piece if sonotube and mounted my diagonal and focuser in it.
Used my Ryobi miter saw stand as a test bench. Made a fixture to hold the primary in its cell at the end of the bench.
Aimed at a power tower about a mile or so away, put the focuser about midway in travel and then raised the end if the stand with some boards to approx direction.
Slid the cutoff back and forth and got focus with my TV wide angle 32 at 20" from mirror face. 20 + 5 +3.5 + 1/2= 29" fl!!!!
Finally!!!
Gratsi everybody
p.s....my Edmund 28 RKE is practical with the TV as a bonus!

#22 GShaffer

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Posted 10 October 2018 - 06:04 PM

So how did the "50" figure into this that you mentioned in the 1st post? Was it something written on the mirror or told to you and by whom?



#23 JimmyTrom

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Posted 10 October 2018 - 06:33 PM

The guy I bought the mirror from said he thought it was 50"
Piginnapoke
100 bucks
Diagonal,spider, focuser, 12' of sonotube, paint,lots of time, dovetail,etc.etc.etc.
Should have jumped on the 8" Breeder for 300 on CN a while back!!!

#24 JimmyTrom

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Posted 10 October 2018 - 07:55 PM

Bresser Not Breeder.

I live in Louisville and keep thinking about the nags running in this years Breeders Cup!


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#25 hamishbarker

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Posted 11 October 2018 - 01:52 AM

At that focal length, I wouldn't be surprised if it's spherical not parabolic.

 

Have you done a ronchi test?




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