Author Topic: How to achieve the absolute best focus in a refractor?  (Read 21 times)

Rasheed Grayson

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How to achieve the absolute best focus in a refractor?
« on: December 24, 2017, 01:44:17 PM »
How can you attain the very best focus in a refractor for the visual use and AP?  Simplicity is a plus but best results are priority.



Richard Reed

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Re: How to achieve the absolute best focus in a refractor?
« Reply #1 on: December 24, 2017, 07:41:10 PM »
Bahtonov (sp?)  Hide I'd say for both programs....pretty simple, very exact.

Jerome Fountain

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Re: How to achieve the absolute best focus in a refractor?
« Reply #2 on: December 28, 2017, 05:02:45 AM »
There's a number of ways. If you have a crayford style, or a very precise R&P focuser you can do this with a DSLR:Create an index scale for your focuser - the scale should be calibrated to 10 or 20 microns. A circular paper disc will work.

Find the best visual focus. record the temperature.

move the focuser very slightly OOF

take a short (~2sec) test exposure

repeat the process 10 or 20 times, changing focus at regular intervals (10 or 20 microns per exposure) until slightly OOF on the other side of focus.

examine the images and pick the one with best focus and record the time and temperature. Reset focus to best focuser position.

If the star size changes noticeably between focus increments, the repeat the process with finer increments on either side of best focus.

repeat the process at a different temp. Work out the focus shift per temp change and you can then dial in focus based upon temp as long as the image train remains the same length (note some scopes will not have a constant focus change per unit of temperature, but most do).

blacosticna

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Re: How to achieve the absolute best focus in a refractor?
« Reply #3 on: December 28, 2017, 08:26:30 PM »

tidutamar

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Re: How to achieve the absolute best focus in a refractor?
« Reply #4 on: December 30, 2017, 11:39:16 AM »
For visual, if you had doubt then you did not hit the best focus (or the seeing was very bad)...move a bit here, a tad there, and then, wow! (it is called also "snap test").

When imaging, learned ones adjust the focus through the FWHM parameter of stars, but use also robotic focusers capable to cope with temperature shifts; however I found a Bahtinov mask a quick and accurate way to focus through the live-view of the DSLR.

EDIT the mask works also for visual, but IME can be useful only with rather "soft" telescopes

propdiagairil

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Re: How to achieve the absolute best focus in a refractor?
« Reply #5 on: December 31, 2017, 07:17:52 AM »
When you get a Bahtinov mask do you buy one based on aperture or diameter of the OTA and how is it attached to the telescope?

kerolero

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Re: How to achieve the absolute best focus in a refractor?
« Reply #6 on: December 31, 2017, 05:26:49 PM »
A Bahtinov mask for imaging, can be used for visual but I suspect it will be little use for visual. Reason for that is simple - I don't know of anyone that does not try to tweek the focus just a little when the put their eye to an eyepiece. It seems to be second nature or automatic that people do it. In which case the focus you achieved with the mask is then lost.

Also you have to set focus with it on the remove it. Somewhere will be the thought of "Did it move????" So you adjust the focus just in case it did and thats it gone.

You get a mask based on the focal ratio and the diameter. You basically place it at the front of the scope and it produces a sort of "Y" shape when at ideal focus. You do need a nice bright star to do it all with and it has to be a sort of isolated bright star - I doubt that using the Pleiades would be mush use. Rigel should, maybe Betelgeuese but not overly sure of something like Aldebaren as the Hyades are surrounding it.

xenjavabve

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Re: How to achieve the absolute best focus in a refractor?
« Reply #7 on: January 01, 2018, 08:02:22 AM »
If you do not want to make your won mask (need only a printer, cardboard and a cutter; or a 3d printer; or a CNC cutter; or...or...) can purchase an "industrial" sample like those by FarPoint Astro.

The "industrial" mask basically come in 2 ways: those for SCTs&co, donut-shaped, and those for all the other scopes. These are a sort of plastic round grid with 3 pins, to hold the mask on the dewshield (in the case of refractors).
There are broad "sizes" (mine works for both the 66/400 and the 100/740), if the mask is larger than the dewshield the pins will grip on the outer face of it; if the mask is smaller, on the inner face.

Once the mask is set in place aim the telescope at some stars, zoom the liveview (5x or 10x) and adjust the focus until you see a symmetrical, asterisk-shaped pattern (brighter the star/s, more evident the spikes). If you do not have a field flattener, be sure to focus right in the middle of the field.
Since the focus will change with the telescope cooling (the FC can change its focus by few mm), make sure to check the focus, especially if feel the air temperature drop or if are not sure about thermal stabilization of the telescope

John Fimbres

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Re: How to achieve the absolute best focus in a refractor?
« Reply #8 on: January 03, 2018, 01:45:31 PM »
Whenever I want to be sure that I have set the best possible focus I always use a Bahtinov mask. I find it to be a big help to stop the maddening practice of endlessly tweaking the focus. These masks are fairly easy to make and I have one for each of my telescopes.

zbermecasa

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Re: How to achieve the absolute best focus in a refractor?
« Reply #9 on: January 09, 2018, 08:50:38 PM »
Quote
A Bahtinov mask for imaging, can be used for visual but I suspect it will be little use for visual. Reason for that is simple - I don't know of anyone that does not try to tweek the focus just a little when the put their eye to an eyepiece. It seems to be second nature or automatic that people do it. In which case the focus you achieved with the mask is then lost.

Also you have to set focus with it on the remove it. Somewhere will be the thought of "Did it move????" So you adjust the focus just in case it did and thats it gone.

You get a mask based on the focal ratio and the diameter. You basically place it at the front of the scope and it produces a sort of "Y" shape when at ideal focus. You do need a nice bright star to do it all with and it has to be a sort of isolated bright star - I doubt that using the Pleiades would be mush use. Rigel should, maybe Betelgeuese but not overly sure of something like Aldebaren as the Hyades are surrounding it.

I cannot imagine trying to use a mask visually, that would require refocusing with the mask each time I swapped eyepieces. Visually, the eye is the final authority. Sharp focus takes a solid mount, a decent focuser and some practice.

Jon

rackramasca

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Re: How to achieve the absolute best focus in a refractor?
« Reply #10 on: January 10, 2018, 12:33:53 AM »
I turn the twirly knobs  .

I've found with the seeing conditions, many times on planets, I refocused a lot...now I'm learning to just wait for those seconds of clarity.

John Newsom

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Re: How to achieve the absolute best focus in a refractor?
« Reply #11 on: January 10, 2018, 03:23:08 AM »
Ive found that for AP, a stepper motor driven focuser and autofocus is by far the quickest and most accurate method. There is an upfront investment in time and money to get it set up but once it is done, focus is a one click and 1.5min proposition regardless of seeing. That is much faster than using a mask or liveview etc. You can also add temp compensation (with a temp probe) and reduce the number of focus checks per night.
For visual, I just loosen the clutch on the motor and focus by eye.
Not the simplest method, but for me at least, autofocus equates to an extra hour of imaging time per night and sharper images than I can get manually. Normally, I'm not in favor of the "buy this widget and your troubles are over" type of advice but for focusing, the results are well worth the hassle of initial setup.
Check out rigelsys for focuser retrofit kits. Leon is on CN frequently and is a great guy to work with.
Jeremy

awesrasa

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Re: How to achieve the absolute best focus in a refractor?
« Reply #12 on: January 11, 2018, 02:32:12 AM »
If you can reach prime-focus at your telescope's tail-piece, usually by racking the focusser in as far as necessary, it's fun to put a knife-edge in, using the end of the focusser as a steady-rest, and cut it into the beam from a bright star (not a planet) while tracking the star. The knide-edge need not be a razor-blade (caution!). A thin metal straightedge (say... ) will do.

Just perform the Foucault test this way, as you change the longitudinal distance ("focus") of the knife-edge from the primary (its position along the optical axis, as you cut the knife in and out). There's a big literature on the Foucault test in ATM sources as a guide to what to look for.

Of course, you'll lose the focus point as soon as you do anything else  , e.g., use an eyepiece, or a camera. So, this is just for "fun".

And, it may tell you something about your primary (objective). Sometimes, more than you want to know.

best!,

--Joe

haigeoredis

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Re: How to achieve the absolute best focus in a refractor?
« Reply #13 on: January 11, 2018, 01:34:35 PM »
I actually discovered that my window screen acts pretty good as a substitute for a Bahtinov mask. I cut a small piece and put it on my refractor lens cover. That small opening in the middle, I put the mesh screen on there and now when I want to focus, I just put my lens cover on my scope, open the middle cover of the lens cover itself which has the mesh on it already, and I can focus. I then remove the cover and I am good to go. It's cheap and readily available and seems to work just fine. You just need to find a star that shine nicely and you will see the difference in focus.

behelphyri

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Re: How to achieve the absolute best focus in a refractor?
« Reply #14 on: January 12, 2018, 01:05:11 AM »
It is cumbersome to use a focusing mask for each and every eyepiece and ir is not necessary to do so. However, on those occassions wher I want to be absolutely sure that I have the best possible focus, I use a mask. This has become particularly helpful as my eyes have aged and they just ain't what they used to be. Another situation where a mask is very helpful is setting the independent focus of binoviewers. Once set, these should not change.