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How to focus ccd?

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#1 xbunnyraptorx


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Posted 13 September 2017 - 03:24 PM

So at my school's observatory my teacher and I went out to try to make a t-point model better using the automatic model maker in TheSkyX Professional with a paramount MX. Anyways, spent the whole time trying to get the ccd focused to do so, taking a 5 second exposure on a star, going in or out, and doing it again. It's an old an low res ccd too so if you're way off the star doesn't even show up. There is a motor driven focus, I tried to do the autofocus with that but it just ran the motor and slewed out all the way. So the situation I'm at right now is having iffy focus, or wasting another night trying to get it perfect. I don't have a Bahtinov Mask, but do you think that would be a better solution? 

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#2 Stelios


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Posted 13 September 2017 - 03:35 PM

A Bahtinov mask will get you great focus, and it's very inexpensive (you can even make your own from a cutout).


The motor-driven focuser--there are two kinds, you want the one that has a *stepper* motor as this can be positioned accurately. You then control this from software (such as SGP, FocusMax, etc.) 


Start with the Bahtinov mask, you'll have excellent focus in a couple of minutes. And you'll know that (even if there's issues with the camera) it's the best you can do.

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#3 KLWalsh


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Posted 13 September 2017 - 09:48 PM

If you're too far out of focus, the Bahtinov mask might not work. Start with a Hartmann Mask. It's much easier to make, gives a brighter useful image, and is easier to start with. Use 3 large holes at the corners of an equilateral triangle for the mask. You'll see 3 out-of-focus images when you start. As you approach focus the 3 images get smaller, brighter, and come together. When the 3 images converge you're at focus.
Here's a link (not my page):

The Bahtinov is great for super precise focus when you're already close.
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#4 ericsolo


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Posted 13 September 2017 - 10:42 PM

You could also try Nebulosity's "Frame and Focus" and "Fine Focus" routines, provided your camera is supported by ASCOM or one of the other drivers available within Nebulosity.  You can download a free version for focusing purposes.  Using Frame and Focus, I set the exposure time to 5 seconds, and Nebulosity downloads a new image after each exposure in a continuous loop.  This allows you to see the results of focusing.  Once I get near focus, I switch to Fine Focus to fine tune.  Initially, the stars may be very faint, or not visible at all, but with patience  as you progress through the focuser travel, they eventually show up.  Once focus is achieved, I mark focus with a piece of tape so I have a quick reference for the next time.



Nebulosity works well with Batinov masks as well.



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#5 rgsalinger



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Posted 14 September 2017 - 06:52 AM

1. You don't need terrific focus to make a terrific TPoint model using the Skyx. As long as the stars are reasonably focused you're going to be fine. I never bother to reset my focus to do a model, I just use whatever what the last focus point. (You do need reasonably accurate PA, though). So don't fret about that.


2. The @focus2 routine should work IF you have a focuser that has repeatable moves. There's no way that routine "runs all the way" in or out unless you actually cannot bring the scope to focus. If that's the case then a mask will not help you. Check with the maker of the focuser to see what the deal is.


3. You do need to calibrate the @focus2 routine with suitable stars. The idea is to find stars that will give around 20k adu when in focus. Then using the planetarium features work out what magnitude they are. You can tailor the star selection that way by modifying the advanced query that picks the stars.


4. Post the problem with details on the camera and the focuser over on the SB forum to get better answers or download a copy of the SkyX manual and give it a read. You do need to register over on the SB website but you don't need to be a registered customer to do so.



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#6 Phil Sherman

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Posted 14 September 2017 - 08:03 AM

One trick I use for quickly achieving focus, especially when starting at one end of the focuser travel is to use a bright object as the target. This allows using a short exposure time, decreasing the amount of time it takes to focus. The moon is the best target, followed by Venus, Jupiter, and Saturn. When none of these is visible, I use the brightest visible stars. If I set up before dark, I'll use a distant tree, hilltop, or other object.

Edited by Phil Sherman, 14 September 2017 - 08:13 AM.

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