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souls33k3r

Motorized focuser options for SCT & Refractor

33 posts in this topic

Posted (edited)

32 minutes ago, Davey-T said:

Aim where you're going to image and take a pic' find an average star mag 4 ish and draw a box round it using mouse / touchpad .

On the expose tab click on use subframe and set it looping on continuous, 

On the options tab select display large statistics, you will then get a continuous readout of the FWHM / 1/2 FD then you can adjust the focus to get the lowest reading which is basically what autofocus does for you.

If the Max reading goes over 64000 ie over exposed reduce your exposure time.

I can easily get down to under 2.0 FWHM on my Star71 using this method.

Don't forget to deselect subframe when you start imaging, a forgetful person could end up with a very small 30 minute sub :grin:

Dave

very nicely explained and than you so much for that.

Quick question, should i first use the bahtinov mask to get the proper bahtinov mask focused spikes and then take the bahtinov mask off and check the FWHM as well or will i just be doubling up the process? 

Edited by souls33k3r

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3 minutes ago, RayD said:

Nicely explained Sir.  

Just to clarify Dave, does the lowest FWHM level go up or down with better or worse seeing conditions, or would you expect to see this level whatever the conditions (within reason)?

I can generally get down to around 2 fwhm with the Star 71 and the WO110FLT, may be able to improve if the seeing was ever to get better.

Usually use autofocus  which achieves the same.

 I think this is better than using a BM as you don't need a bright star, you can find a star in your imaging FOV and it works in NB.

Dave

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2 minutes ago, souls33k3r said:

very nicely explained and than you so much for that.

Quick question, should i first use the bahtinov mask to get the proper bahtinov mask focused spikes and then take the bahtinov mask and check the FWHM as well or will i just be doubling up the process? 

Wouldn't bother with the BM if you're a long way out of focus to start with you can just loop and twiddle the focuser to get the stars smallish.

Dave

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1 minute ago, Davey-T said:

Wouldn't bother with the BM if you're a long way out of focus to start with you can just loop and twiddle the focuser to get the stars smallish.

Dave

thank you, i'll do just that then.

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1 minute ago, souls33k3r said:

thank you, i'll do just that then.

Good luck :)

Dave

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Just now, Davey-T said:

Good luck :)

Dave

Cheers Dave. Now i'm just waiting for my Star 71 to arrive and just in time for astro dark returning this week as well :)

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1 minute ago, souls33k3r said:

very nicely explained and than you so much for that.

Quick question, should i first use the bahtinov mask to get the proper bahtinov mask focused spikes and then take the bahtinov mask and check the FWHM as well or will i just be doubling up the process? 

I use my first alignmet star (indeed my only alignmet star) with the B mask at the very beginning of a session. That means that by the time all the other faffing has been done, and I'm ready to start the run, the scope will have cooled a little and may need a focus tweak, so I do an FWHM on it just before starting and then about every 90 minutes thereafter.

I find that, for a given setup, my FWHM varies considerably with the seeing. The Taks can be anywhere between O.85 to about 1.4. At 0.35"PP this doesn't matter all that much and I just get on with it. However the TEC/Atik 460 is working at just under 0.9"PP and varies far more than that, from say 1.1 to about 2.3. If the seeing is bad, as indicated by a poor FWHM value, I'll shoot colour but not luminance. Provided you get a sharp L layer at some point it doesn't matter that the colour is soft.

Olly

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I use manual focusing on my refractors but for the SCTs I really need a motorized focus or the star is wobbling all over the screen when I try to focus due to the long FL and my clumsy hands, so for my Edge HD scopes I use a JMI focuser. It does not have a stepper motor but it is probably about half prize compared to those with stepper motors. So, if you are not aiming at using it for auto focusing (need a stepper motor for that I think) a JMI focuser is a cheaper option:

http://www.jimsmobile.com/buy_motofocus.htm#chart1

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