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How do I simulate infinity focus by day?

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#1 Fivemileshigh

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Posted 11 August 2020 - 01:34 AM

I need to find out where the focal plane is on my dob but there will be a lot of messing with it in the garage before first light, so I need to find a way to focus the dob to infinity, artificially that is (without pointing at the sun or such thing)

 

I was thinking of pointing a finder at the mirror, should I be able to focus the dob on the finder eyepiece's reticule? The finder is already adjusted for infinity.

 

Thanks!

 

Dumitru

 



#2 sg6

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Posted 11 August 2020 - 02:10 AM

Have you sufficent distance/height to position a screen at what should be 2x focal length?

If so make a small hole, or big hole and reticule, put a light behind it then by whatever means get an image of the illuminated hole on the screen and that should be 2xfocal length.

 

Another needs a bit more optics. Make a collimated image and direct down onto the mirror and reflect back up where the image forms should be the focal plane.


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#3 davidc135

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Posted 11 August 2020 - 02:50 AM

I'd mask off all of the mirror leaving the .7r zone before measuring the RoC as sg6 suggests.

 

An illuminated target could be put at the estimated focus of the Dob and viewed through the mirror as you say with your finder. Focus is adjusted until target coincides with the reticle. I wonder that because the beam is so narrow and slow it might be imprecise but try both methods and see if they agree.

 

David


Edited by davidc135, 11 August 2020 - 02:59 AM.


#4 Fivemileshigh

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Posted 11 August 2020 - 04:18 AM

There is room for 2x focal, but I will try David’s idea first, that sounds workable, thanks guys!

#5 LU1AR

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Posted 11 August 2020 - 03:07 PM

After about 10,000 focal lengths; the difference in focus is negligible.
If you want to focus on something behind that distance, any adjustment will be minimal.
Regards.
Edgardo



#6 Star Shooter

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Posted 11 August 2020 - 05:39 PM

1. Tape a piece of paper to the end of the focuser.

2. Then point the telescope at the moon. 

3. Focus the moon on to the paper.

4. Lock the draw tube.

The focused image at the end of the focuser tube is your Focal Plane. Take what ever measurements you need.

You should be able to do this in the morning for the next few days. The moon is high in the sky as of noon EST. Approximately 40 degrees west of the meridian.

 

Glen


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#7 gregj888

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Posted 11 August 2020 - 07:46 PM

Put a mirror over the front of your scope and make up an eyepiece with with a ronchi or a fairly bright LED and screen at the same level.  You are basically doing a DPAC but with a marginal mirror.  Just inspect for focus not for mirror quality, but it will get you really close.

 

Front surface mirror is best but it should work with a plastic mirror as long as it's pretty flat.  You might get by with an old ferrotype photographic drying plate in a pinch. 

 

Thingiverse has a 3D printable Ronchi eyepiece.


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#8 Fivemileshigh

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Posted 14 August 2020 - 01:52 AM

1. Tape a piece of paper to the end of the focuser.

2. Then point the telescope at the moon. 

3. Focus the moon on to the paper.

 

The telescope hasn't been built yet grin.gif I have to cut the trusses to length but due to the construction method of the truss attachments, I can only shorten them in 13mm increments, and I'd like to get the focal plane in position more accurately than that.

 

Put a mirror over the front of your scope and make up an eyepiece with with a ronchi or a fairly bright LED and screen at the same level.  You are basically doing a DPAC but with a marginal mirror.  Just inspect for focus not for mirror quality, but it will get you really close.

 

Front surface mirror is best but it should work with a plastic mirror as long as it's pretty flat.  You might get by with an old ferrotype photographic drying plate in a pinch. 

 

Thingiverse has a 3D printable Ronchi eyepiece.

Very interesting option, I'll keep it in mind, thank you!



#9 Diego

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Posted 14 August 2020 - 07:09 AM

Hi Dimitru!

I went through the exact same process as you a few weeks ago.

 

At my location, I don't have any antennas or telephone poles far enough to focus at close to infinity.

 

What I did is mount 2 pairs of truss tubes to the exterior of the mirror box with U pipe clamps. I then propped the structure on crates and waited until moon rise.

 

You can calculate a rough estimation of the focal plane with the mirrors focal length and the telescopes dimensions. I found my calculations to be fairly accurate to my goal of 2" - 3" tall focuser. In came in at 2.5"

 

See below my picture. Note this is nowhere near a finished telescope. Just a temporary and very crude set up with the sole purpose of finding the focal plane and determining the final length of the truss tubes. I still need to make upper and lower truss clamps, spider, sand, paint, etc

 

Hope this helps!! Good luck!!

 

 

 

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#10 Fivemileshigh

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Posted 15 August 2020 - 02:18 PM

I'm overthinking this. If I leave the struts 2in oversize, that still leaves me at least 3 cuts available to get it right. Besides, the relationship between the focal plane location and the truss length is 1/cosine of half the truss included angle. I'm just going to build it, try it on the moon like Diego says, measure and cut a couple of times.  Thanks guys!


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#11 MKV

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Posted 16 August 2020 - 09:18 AM

I was thinking of pointing a finder at the mirror, should I be able to focus the dob on the finder eyepiece's reticule? The finder is already adjusted for infinity.

Yes. Shine a parallel beam of light into the finder's eyepiece and point it at your mirror.  You can generate a parallel beam of light using three lenses as shown here, or an adjustable flashlight; the green dot is the light source.

 

collimated beam.png

 

cross hairs projection.jpg

 

Note: I didn't have a very good parallel beam, so the beam expanded. If you have a good parallel beam the projected image should be the same diameter as the finder's lens.


Edited by MKV, 16 August 2020 - 09:19 AM.

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#12 Fivemileshigh

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Posted 18 August 2020 - 04:38 AM

I tried the reverse finder idea on a different telescope. It doesn't work, the indicated focus position is way further outside the tube than where it is under the stars, about 2 if not 3 inches' worth. I think it's because I'm slightly near sighted and I adjusted the finder's eyepiece focus so I can sight without glasses. I'll have to think of something else, perhaps Greg's DPAC idea above. It may also turn out to be easiest to jury rig something on the moon.

 

Cheers guys, thanks,

 

Dumitru



#13 MKV

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Posted 18 August 2020 - 07:40 AM

Dumitru,

 

You must have done something wrong, but I am partly to blame.

 

First, the finder itself has no focus at infinity. The focus is provided by your eye.

 

My drawing wasn't very accurate. I said three lenses, basically ()  () () but I drew only one (), because the principle is the same no matter how many lenses you use. Hopefully thisexample of a finder will make it clearer.

 

finder.png

 

The parallel (collimated) light (1) comes from infinity, and is focused by the objective glass (OG) at fOG (2), and the eyepiece (EP) is placed so that its focal length (EP, fEP) is at the same focus as OG's.

 

From there, the EP relays the light by converting it again to a parallel (collimated) beam (3). The collimated beam then enters the observer's eye (OE), where it forms the image of the object at infinity on the retina.(4)

 

In optics if it works in one direction, it must work in the opposite direction, i.e. from 4 --->3--->2--->1. If you replace the eye with a light source (a pinhole) at the focus of a lens the light will be leave the lens as collimated (parallel) beam and enters the finder's EP, which is then focused  at the common focus for OG and EP at (3), and proceeds to enter the OG from where it leaves as a collimated beam again. 

 

If you have a flashlight with a fairly collimated beam then you don't an extra lens. You can just shine directly into the finder's eyepiece and that beam should exit at the objective end as a parallel beam. If there's a crosshair reticle at the OG/EP focus, the image of that reticle will be projected out and will be visible in another telescope if its eyepiece is at its infinity focus position. This is how you simulate an infinity source for collimaiton and focusing purposes.

 

You cna get small flahslights such as this one https://www.google.c...Qr4kDegUIARD3Aw  with adjustable beam that's sufficiently parallel. Some are quite cheap.

 

Good luck!


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#14 davidc135

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Posted 18 August 2020 - 11:34 AM

Would it be a bad idea to mask off all of the aperture save say three well spaced 1 inch holes and use the sun? Again, the image would be formed on a supported piece of paper (I used a microscope slide). and it only takes a few seconds.  David


Edited by davidc135, 18 August 2020 - 11:35 AM.

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#15 Fivemileshigh

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Posted 18 August 2020 - 02:14 PM

If you replace the eye with a light source (a pinhole) at the focus of a lens


I think that's the crux of the problem. I can focus with the main eyepiece on the finder's crosshair, greatly magnified. However, because that crosshairs is NOT at the focus of the finder's objective, it throws off the focus position of the main EP. I have no (easy) means of adjusting the finder so that the crosshair is at the correct point. Well I could devise something, but David above has the easiest idea yet. I don't observe the sun so this did not occur to me. Maybe I should start smile.gif .

 

Cheers,

 

Dumitru



#16 petertinkerer

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Posted 18 August 2020 - 08:23 PM

I need to find out where the focal plane is on my dob but there will be a lot of messing with it in the garage before first light, so I need to find a way to focus the dob to infinity, artificially that is (without pointing at the sun or such thing)

 

I was thinking of pointing a finder at the mirror, should I be able to focus the dob on the finder eyepiece's reticule? The finder is already adjusted for infinity.

 

Thanks!

 

Dumitru

Your need to find the infinity focus on your Dob inspired me to build the following device this afternoon since I have often thought about how convenient it would be to have the ability to adjust the mechanics of a prototype telescope build while having it focused at an effective infinity.  It would be particularly convenient if you are building binoscopes of different designs where the optical paths need to be matched very precisely to have the eyepieces at exactly the same height.

 

The basic principle is  one close to suggestions already made, i.e., to have a very small, bright object placed at the focal point of a small refractor telescope and point this telescope at the telescope under test.   As you will see from the following description there are quite a few steps to realize this accurately enough to achieve the desired purpose.

 

Let's start with the very small bright object.  You start by buying a very cheap, red, laser module, shown in photo #1.  These cost about 50 cents each and they are often sold by packets of 10 for about $6 or 20 for  $12 (Amazon). Next you carefully remove the actual laser board, see photo#2.   The "head" on spot  (photo#3) is very small ~10 microns and is great for checking airy discs of the completed telescope but we are going to use the "side on" light pattern (photo#4).  Photo#5 shows this side on light under a microscope.  The two dots are 200 microns apart and the two lines are about 150 microns apart.  This side pattern is great for testing the resolving power of your completed telescope in the basement, free of all atmospheric disturbance.  The black square between the two dots and two lines subtends just 2 arc seconds at 20 meters and if your telescope can "see" this pattern at 20 meters away, really clearly, at 200X then it is going to work very nicely under the real sky.

 

To be continued.....(need to up load more photos)

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#17 petertinkerer

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Posted 18 August 2020 - 09:08 PM

continued from above....

 

You need to mount the red laser module so that it will be easy to handle.  I took a very cheap (unused) eyepiece, it was a 6mm from an inexpensive refractor kit and removed the  glass lenses and glued the laser module to the center of it, see photo#1 so that the side light showed through the empty eyepiece, see photo#2.  In a moment we will install this in the refractor #1 that will become the test vehicle for the prototype telescope.  However we need to adjust the focus of refractor#1 to have the laser side light exactly at the focal point.  To achieve this you need another small inexpensive refractor telescope, see photo#3.  Take this refractor#2 outside the garage and focus on some clouds if possible or some distant object.  Now bring back refractor #2 into the garage and point it at refractor#1, see photo#4.  

 

To be continued.....

 

 

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#18 petertinkerer

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Posted 18 August 2020 - 09:24 PM

Continued from above....

 

 

Now looking into refractor#2 (which is focused at infinity) you are going to adjust the focus of refractor#1 until you see a focussed image in the eyepiece, see photo#1.  Refractor#1 is now projecting a very precise parallel beam which can be used to find the infinity focus of your Dob under construction.  I pointed refractor#1 at a newly completed binoscope to show this step, see photo#2.   You now adjust the focus of the telescope/binoscope under construction until you see a sharp image of the light pattern, see photo#3.....you have now achieved infinity focus for your Dob.

 

 

It all sounds a bit complicated and you do need two inexpensive refractors and some effort to build the laser light module but it's well worth it.  You need to mark on the focuser of refractor#1 the setting which achieves this parallel beam so it can be used again for this calibration purpose without having to go through the intermediate steps.

 

Thanks for reading....

 

Peter

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#19 Bob4BVM

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Posted 19 August 2020 - 03:40 PM

Very nice Peter, thx for sharing that so well !

Definitely looks worth the trouble to build as a great tool for ATM'g.

I am at the same point as the OP, trying to determine a close structure length, except on a binoscope.

Your tool looks to be especially useful for b-scopes.

Cheers

Bob



#20 Fivemileshigh

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Posted 20 August 2020 - 01:22 AM

Take this refractor#2 outside the garage and focus on some clouds if possible or some distant object.  N
 
To be continued.....



Wow Peter, thank you very much for the effort in explaining all this, and all that work. I was always curious about those laser diodes, your post explains a few things. I would also dearly like to know more about your binoscope too. I dream of building a 2x22" myself lol.gif
 
But I digress....
 
Your idea, while miles more refined than mine, appears to suffer from the same flaw as mine: It requires me to focus one refractor at infinity using my eye. I have already done that with the finder, and because I'm nearsighted, the adjustment is off, as evidenced by the main telescope focal plane being nowhere near where it's supposed to be. By that I mean this is an already completed, well used telescope, and I know where its focal plane is. It's 2-3 inches further inward than the finder setup shows it to be. I need a way to generate an infinity target without relying on my own eye. I think the ideas presented in posts #14 and #7, in order of time effectiveness to make stuff, are likely the best way to go.

 

But, thanks again a million for the effort and pictures especially!



#21 MKV

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Posted 20 August 2020 - 08:02 AM

There's no need to go throgh all this. It's possible to make a simple device with a small laser module (without any modification) that fits over your eyepiece. As I have illustrated in #13 ( https://www.cloudyni...day/?p=10430641 ) when observing at infinity the light that comes into the telescope is collimated (parallel), and when it comes out of the eyepiece it's also collimated. The eye or camera then bring it to a focus.

 

A laser diode emits a collimated beam of light. When that beam enters the eyepiece, it's focused on the EP's focal plane. When a telescope is pointed at a very distant object (functional infinity), and is brought to a sharp focus, the focal plane of the eyepiece will coincide exactly with the focal plane of the scope, as illustrated earlier (post #13). The eyepiece position then needs to be locked. 

 

A small cap that fits over the eye lens end of the EP used for the infinity focus needs to be made or 3D printed in such a way that it can accept a laser diode or a small flashlight (as illustrated below). It's a good idea to also place a small pinhole centered over the EP's iris diaphragm (which is at its focal plane) to keep stray light out. Then Insert a small laser diode or a small flashlight  into the cap as shown below, and then insert the eyepiece and the laser cap with the laser diode/flashlight into the focuser locked at the infinity position. Done!

 

I made my lens cap out of two glued blocks of wood (yellow in the drawing below) and used a Forstner bit for an accurate 1.25" flat bottom hole for a tight fit over my Orthoscopic eyepiece. The entire process took about 15 minutes, and most of it was glue setting time.

 

artstar.JPG IMAG0732_LR.jpg IMAG0734_LR.jpg IMAG0742_LR.jpg

 

The infinity light source can be used in any telescope. I use only the weakest laser diodes for this (1 mW or less), and I cannot emphasize enough not to star at the laser signal. I also use a potentiometer to vary the brightness as close as possible to a non-lasing mode.

 

You can replace a laser module with a small Maglite flashlight of this type https://cdn.shopify....pg?v=1464432869. It's safer.

 

The device produces an Airy-disc size point source when monochrome light source (such as lasers or narrow bandpass filter) is used. 

 

Here I'm using a C8 as an inifinity collimator for a 6-inch f/8 Newtonian.

 

art star collimator_a.jpg

 

 

 



#22 MKV

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Posted 20 August 2020 - 08:07 AM

...It requires me to focus one refractor at infinity using my eye. I have already done that with the finder, and because I'm nearsighted, the adjustment is off...

Focus wearing your prescription eyeglasses.



#23 petertinkerer

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Posted 20 August 2020 - 09:43 AM

Wow Peter, thank you very much for the effort in explaining all this, and all that work. I was always curious about those laser diodes, your post explains a few things. I would also dearly like to know more about your binoscope too. I dream of building a 2x22" myself lol.gif
 
But I digress....
 
Your idea, while miles more refined than mine, appears to suffer from the same flaw as mine: It requires me to focus one refractor at infinity using my eye. I have already done that with the finder, and because I'm nearsighted, the adjustment is off, as evidenced by the main telescope focal plane being nowhere near where it's supposed to be. By that I mean this is an already completed, well used telescope, and I know where its focal plane is. It's 2-3 inches further inward than the finder setup shows it to be. I need a way to generate an infinity target without relying on my own eye. I think the ideas presented in posts #14 and #7, in order of time effectiveness to make stuff, are likely the best way to go.

 

But, thanks again a million for the effort and pictures especially!

Thanks for the comments,  I do find the disassembled laser module very useful for testing optics with its 2 arc second pattern available in the basement at 20 meters, without atmospheric aberrations.

 

I have tested the system I described above and by using refractor#2 as the intermediary for placing the light source exactly at the focal plane of refractor#1 the eye optics cancel out. Refractor #2 becomes a means (which includes the eye optics) fo bring to focus a parallel beam of light.  To prove this to myself I put on 3 pairs of reading glasses giving myself +6 diopter correction.  Using refractor#2 looking at a very distant object I needed to change its focus position because of the 3 pairs of glasses.  However when I looked into refractor#1 with refractor #2 while wearing the glasses I did not need to adjust the focus of refractor#1 showing that any  eye optics cancels out.

 

I then pointed the parallel beam coming from refractor#1 into my 6" F8 telescope, which I had just used looking at Saturn, so I knew it was focussed at infinity and when I looked into the eyepiece the small bright pattern was perfectly in focus.

 

I'm happy with my effective infinity object in the basement and I will continue to use it.  I'm also quite sure that the method described by MKV works fine as well as do the other methods described above.  Whichever one  you decide to go with I am pleased to have been motivated by this thread to add an "infinity" light source to my test equipment for my "basement astronomy" 

 

Peter


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#24 Fivemileshigh

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Posted 20 August 2020 - 02:13 PM

I'm happy with my effective infinity object in the basement and I will continue to use it.  I'm also quite sure that the method described by MKV works fine as well as do the other methods described above.  Whichever one  you decide to go with I am pleased to have been motivated by this thread to add an "infinity" light source to my test equipment for my "basement astronomy" 
 
Peter


I ended up using the "3 holes in a piece of cardboard and point it at the sun" idea. Luckily we could see the sun today. When I have a little time I'm going to revisit this page when I work on the 16", thanks again!

 

Dumitru


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#25 MKV

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Posted 20 August 2020 - 10:27 PM

I'm happy with my effective infinity object in the basement and I will continue to use it.  I'm also quite sure that the method described by MKV works fine as well as do the other methods described above.  Whichever one  you decide to go with I am pleased to have been motivated by this thread to add an "infinity" light source to my test equipment for my "basement astronomy" 

They both work -- guaranteed -- because they both create an Airy disc size point source. I used mine because it places the point course exactly at the infinity focus, and that it did 't  require sacrificing laser diodes. However, petertinerer's approach doesn't have to sacrifice a laser diode if one gets laser didoes with collimating lens caps, which can be simply  unscrewed. This is the kind of laser diode I use for interferometry. My method also allows using ordinary flashlight or even a collimated LED light, which is safer than laser light. Small wattage mini laser diodes can operate in non-lasing  mode, which is essentially no different than an LED.

 

Whichever approach works is irrelvant. The principle is the same in all of them: a point source in a focal plane of a collimating telescope-created wide collimated exist beam which will focus at the infinity focus in another telescope

 

Single-pass collimation test.png .




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