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A focusing question

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#1 Sidney

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Posted 18 December 2004 - 01:34 PM

In another thread it is mentioned that focusing a 35 mm camera is perhaps best achieved using a knife edge or Rhonci focuser. Yet I see folks using high mignification or right-angle finders on their 35mm camera bodies when attached to the scope.

Is there a significant difference between the two methods?

#2 raydar

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Posted 19 December 2004 - 01:23 PM

I have only learnt about all this myself. There is a huge difference between the two methods.

A right angle viewer relies on your own eyes to focus an object. Your eye has a lens in it, so it can make an "out of focus" object look "in focus". So this method can be unreliable.

Whereas a knife edge focuser or Ronchi screen, relies on a clever process of optical physics, which i'm still trying to come to terms with, lol.

I think right angle viewers may be simply used to frame the image properly.

At higher magnifications, the image becomes fainter, so focusing a faint object with a right angle viewer, may prove even harder.

I'm still relatively new to astrophotography, but I know all the pro's use knife edge focusers or ronchi screens.

Because of the way ronchi screens and knife edge focusers work, it is almost impossible to be out of focus.

I'm sure the pro's in here will shed more light on this than I can.

And whilst we're on the subject, has anyone used both (knife edge and ronchi), whats better and why?

Ta

#3 Suk Lee

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Posted 19 December 2004 - 07:59 PM

A right angle viewer relies on your own eyes to focus an object. Your eye has a lens in it, so it can make an "out of focus" object look "in focus". So this method can be unreliable.

Whereas a knife edge focuser or Ronchi screen, relies on a clever process of optical physics, which i'm still trying to come to terms with, lol.


Take a look at the photo, it illustrates the light cone coming from a star and what's going on with a knife-edge and Ronchi focuser.

With the knife-edge, you place the edge outside the cone and move it slowly across the star by moving the telescope. Imagine you're inside focus. You're obstructing the light cone with an edge - for a grossly out-of-focus star you'll see a circle of light (or a donut for an obstructed telescope) with a dark edge moving across it. For a very close to in-focus star, you'll see a small dot, but as the edge moves across the dot you'll see it slowly fade out as you cut across the tiny disk. When the knife edge is RIGHT at the focal plane, you'll see the dot just wink out all of a sudden because you're cutting a tiny point of light instead of a small disk. So the method of using the knife edge is: move the edge across the light beam, observe behavior, adjust focus, and repeat until you've found the point where the star just winks out.

The Ronchi screen applies some of the same principles but is very different visually and in use. The Ronchi screen is a series of parallel lines etched into a piece of glass and then blackened. The picture illustrates the "bars" of the screen seen edge on. Imagine you're gross out-of-focus, inside of focus again. You'll see a disk (or donut...) with a bunch of black lines superimposed across. Now as you move closer to focus, the number of "lines" cutting the cone gets smaller, so you see fewer, bigger lines, spaced farther apart (a Ronchi screen focuser uses a magnifying lens system to magnify the cone so that even in focus you see a disk rather than a dot...). As you get closer to focus, the lines will spread apart, until either you've got the focus point completely obscured by one line, or the focus point will land between two lines. So either you'll see a completely obscured disk or a completely clear disk. In practice you rack the focuser in and out and see the lines expand/contract as you move the focuser.

If you think about the light cone falling "between" the bars of the screen, you'll realize that the fundamental accuracy of a Ronchi screen focuser is determined by the spacing of the bars - the tighter the spacing, the more accurate the focuser will be. In the case of the knife-edge there's no such limitation, an edge is an edge.

I personally find the knife edge less ambiguous, you can really tell when you've got focus. It's a little harder to do because you have to move the telescope - with the Ronchi all you have to do is rack the focuser in and out.

Either way is the best way to get precise focus, the downside being that your knife edge/Ronchi focuser has to be calibrated to the specific lens flange/focal plane distance of your camera.

Commercial knife edge focusers: http://www.sciencece...sub/focuser.htm

Commercial Ronchi focusers: http://www.stellar-international.com

Cheers,
Suk

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#4 raydar

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Posted 20 December 2004 - 04:05 AM

Cheers again Suk

#5 Bill G.

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Posted 12 January 2005 - 09:05 PM

Good stuff Suk. Thanks.
Bill

#6 ClownFish

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Posted 08 June 2005 - 11:35 AM

I just purchased a Hi-Res Stelletto focuser for my OM-1.
I was amazed at:
1. How easy it was to find the focus
2. How easy it was to be out-of-focus when I tried to focus without the Stilletto. I focussed the camera the way I did without the new focuser, and then went and checked it with the Stilletto. In all attempts, I was sure I was at focus, but the Stilletto showed I was off slightly in 9 out of 10 times (not much, but enough to realize how my eyes can be fooled)!
cf

#7 Guest_**DONOTDELETE**_*

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Posted 06 September 2005 - 12:19 PM

I sacrificed a 15mm eyepiece for focusing my Nikon F2, so far it has been great, just remember to refocus if you use any filters.

#8 lawrie

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Posted 23 May 2006 - 08:51 AM

Has anyone out there used the stiletto 1V to focus a CCD camera too, I know it works with a 35mm, but they say it will work with a CCD also.

Lawrie

#9 ClownFish

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Posted 23 May 2006 - 09:29 AM

As long as you can get the adapter to fit your camera.

The Stiletto helps you to focus your SCOPE, not the camera. You must be able to remove your camera lens, as if you are shooting at prime focus (no lens). In such a case, you do not focus the camera lens, but instead you are focusing with the scope, since that now acts as your camera lens.

The way the Stiletto works is that you attach the Stiletto INSTEAD of your camera, and focus on a bright star. Your camera is not used at all.
Then after you find the focus, you swap out the Stiletto with your camera, and now your camera is all focused. So the only thing is.. does the Stiletto manufacturer make an adapter that is the SAME style that you currently use between your scope and your camera.

If however, you have a CCD camera with a fixed lens, then you may have a problem.

As far as astro CCD cameras like S.B.I.G., Starlight XPress, Meade, SAC, and some others, then yes, it will work well.

Go HERE to view the company website.

CF

#10 lawrie

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Posted 23 May 2006 - 08:08 PM

Thanks CF, anybody out there tried using a stiletto with a CCD camera, it would be interesting to know how well it worked.

Lawrie :)

#11 Suk Lee

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Posted 24 May 2006 - 11:38 PM

Much as STI makes a great film focuser, I wouldn't use one for a CCD camera. Better to get critical focus directly using the CCD, it's only a couple of minutes. CCD is much less tolerant of focus errors than film because the focal depth of the sensor is measured in microns, not fractions of a millimeter like film.

In a sense, all knife-edges and ronchi focusers are indirect focusers - they can't tell you directly what's going on at the film sensor because film isn't a realtime medium. Since CCD can give you immediate results why take the risk - just check out exactly what's going on by taking some test shots and reading out focus quality.

Cheers,
Suk

#12 Nebhunter

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Posted 14 May 2007 - 09:26 PM

I have a chance to pick up a Stiletto IV deluxe with Ronchi screen 300. The Hartmann mask is just too difficult to use, and then I am not sure if it's on 100%. The Stilleto is going for $100 Canadian. I guess I should jump on this. It has never been used, as fits the OM1 camera. Speaking of OM1's, there are a couple on Ebay that are serviced and guaranteed in excellent condition. I guess I'm going to bid on them. My old Konica has will be used for wide field photo's. I have a Konica vari-focal lens (rare)that has increments from 35 - 50 - 70 - 85 - 100. Once you change power you must refocus. You can't tell the difference on photo's between a fixed lens and this lens. It is 82mm and F2.8 and cost $450 back in 1974.

I was out with my Equinox 80mm last night and was imaging M57 prime. I was glued to the illuminated eyepiece on the 150 F8 refractor when a bright light zoomed across it. I think I may have captured a meteor. The film is being processed and I will know tomorrow night. Thanks for listening.

#13 Bees

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Posted 03 June 2009 - 02:48 PM

So how fine of a focuser is recommended for each application? I have a Takahashi 10:1 microfocuser, will it be possible for me to achieve perfect focus if I'm just using my hands when adjusting (ie. no special computer motors to make fine adjustments).

#14 Nebhunter

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Posted 08 June 2009 - 01:58 PM

That focuser should work just fine. It's everything else that seems to be the problem. A heavy duty mount that allows fine focus without inducing the shakes. Achieving perfect sharp focus is very difficult without a aid like the Harmann mask or STI Stiletto device.

Let us know how you make out.

Igor

#15 jwk

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Posted 08 November 2015 - 03:10 PM

I am having a few difficultys getting very sharp focusing with my celestron nexstar slt 127 - does anyone have any solutions to this - take a look at my hobby site http://www.starbright.org.uk let me know if you see anything that I am doing wrong -




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