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Do you refocus with each RGB filter?

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#1 moxican

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Posted 12 August 2014 - 10:27 PM

The topic says it all. I have acquired an ASI120MM-S recently, and although I am happy with the camera monochrome imaging is a little new to me. I have googled the subject but all I could came up with is a formula of finding the dept of focus, not exactly what I was looking for. I read it somewhere that different colors of the spectrum has different focus point. I think it said that blue is the one to watch for in that matter.

I use a C9.25 with the camera and usually shoot aroung F20.

Do you guys refocus for each filter? If yes, what is your technique?

 



#2 Mike Phillips

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Posted 12 August 2014 - 11:29 PM

I do this, but only because of two reasons.

 

1) I have a motorized CFW AND focuser which enables me to be lazy, not bump or touch the ota/mount between filter change and focus attempts

2) The focuser (JMI) includes a digital readout and PC control which allows me to 'see' where the focus position I like is.  This means I fiddle with it to see where I think it's best in focus and then can compare it to the other filters.

 

Typically the R&G are nearly the same in avg and above conditions but the blue tends to be slightly out, typically no more than 2 micron/steps of the focuser.

 

So, without a motorized CFW OR focuser, I wouldn't bother, but it depends on your filters.  With a good CFW and focuser you can 'tell' how much difference there is which means a LOT just to know, much less control.

 

YMMV,

 

Mike 



#3 leviathan

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Posted 13 August 2014 - 01:29 AM

Yes, I do refocus from B->G->R->IR and the difference is significant on 5m focus. More than that, difference might be more or less depending on seeing. Least difference is indeed between G and R. Astronomik Typ 2c RGB and Baader 685nm that is.


Edited by leviathan, 13 August 2014 - 01:30 AM.


#4 yock1960

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Posted 13 August 2014 - 06:32 AM

As Mike said, red and green are close, blue is definitely a refocus. Motorized is probably the way to go, but I haven't added that complexity yet. I don't trust myself to focus on target (except the moon) and slew off to a star to use a focus mask. PIA? Yes, but you want the detail.

Steve

#5 John Boudreau

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Posted 13 August 2014 - 08:21 AM

I use a C9.25 with the camera and usually shoot aroung F20.

 

In case you are assuming your system's critical focus zone (CFZ) in that configuration to be that of a native f20 system, it's not. Your CFZ will still be that of the C9.25's native f10. Any time we use a scope with an auxiliary optic like a Barlow or eyepiece that will be the case. 



#6 Sunspot

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Posted 13 August 2014 - 09:30 AM

The topic says it all. I have acquired an ASI120MM-S recently, and although I am happy with the camera monochrome imaging is a little new to me. I have googled the subject but all I could came up with is a formula of finding the dept of focus, not exactly what I was looking for. I read it somewhere that different colors of the spectrum has different focus point. I think it said that blue is the one to watch for in that matter.

I use a C9.25 with the camera and usually shoot aroung F20.

Do you guys refocus for each filter? If yes, what is your technique?

 

I always plan on refocusing between filters, sometimes it just doesn't seem necessary. I'd definitely plan on refocusing the blue filter and when I use the IR filter, that is a must. Experimentation is a good plan but one of the very best amateur planetary imagers once told me that he always refocuses between filters.



#7 CharlesW

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Posted 13 August 2014 - 10:00 AM

I have a motorized focuser, an Optec, and I use a Bahtinov mask to get focused. Firecapture stores the offsets so when I change filters, it refocuses to the new position. 



#8 moxican

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Posted 13 August 2014 - 05:33 PM

Wow, lot of info here guys, I appreciate it. I use Baader LRGB CCD filter set. So, it is a good idea to refocus the blue filter, I am not imaging in IR yet. But when I refocus the blue I assume the focuser needs to travel inward, meaning that blue focuses at a shorter focal length a little bit then G and R. I do have a 10:1 dual speed focuser to adjust that small, althogh my filter wheel and focuser are not motorized.

The question is than, how do I know how much to adjust? Is there a way to calculate it and come up with a number say B needs to be readjusted X amount of microns?



#9 WarmWeatherGuy

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Posted 13 August 2014 - 06:37 PM

The question is than, how do I know how much to adjust? Is there a way to calculate it and come up with a number say B needs to be readjusted X amount of microns?

You could use a Bahtinov mask on a star with all the filters and write down the offsets for each color (from the previous color). Then use a star to focus the first color and then move by the offset amount after each color change. It is somewhat open loop but only the deltas, which should be small.



#10 John_K

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Posted 30 September 2015 - 11:10 PM

I have a motorized focuser, an Optec, and I use a Bahtinov mask to get focused. Firecapture stores the offsets so when I change filters, it refocuses to the new position. 

 

 

Just re-bumping this topic.

 

With a Shoestring adaptor and a JMI motorised focuser, does Firecapture know how to reset the focus point for a particular filter as previously set by the user? 


Edited by John_K, 01 October 2015 - 01:44 AM.


#11 JoseBorrero

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Posted 04 October 2015 - 08:27 AM

JMI motorised focuser can try, but since it doesn't have a stepper motor the position might not be accurate.




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