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Basic Focus Question

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9 replies to this topic

#1 Rac19

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Posted 14 June 2018 - 08:01 AM

I have seen mention that it is good practice to initially focus on a bright star (preferably with a Bahtinov mask) as a basis for the rest of the session.

 

How much (if at all) is it likely that the focus wouid need to be adjusted after that, especially for planets and the moon? Is everything out there at “infinity” focus? It may be a silly question, but I would be interested to hear any opinions, or definitive advice.

 

Having achieved focus on one object, should the urge to make further adjustments be resisted?



#2 bobzeq25

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Posted 14 June 2018 - 08:16 AM

Infinity is infinity, but...

 

The main problem you run into is temperature almost inevitably changes focus.   Most everyone focuses periodically during a session because of that.  Changing filters can also change focus, depends on the filters.

 

This is a major reason why a lot of imagers, especially those doing DSOs,  set up autofocus systems.


Edited by bobzeq25, 14 June 2018 - 08:17 AM.


#3 baron555

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Posted 14 June 2018 - 08:34 AM

I set up my imaging sequences such that I refocus about every 30-40 minutes.  I have filter presets already figured and built in so no need to redo those as I change filters.



#4 Jeff Struve

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Posted 14 June 2018 - 09:01 AM

It can also depend on the type of scope... for instance, an SCT has a primary mirror that may shift for various reasons...


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#5 drmikevt

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Posted 14 June 2018 - 09:16 AM

 

Having achieved focus on one object, should the urge to make further adjustments be resisted?

Focus is focus and once you have focus on one object, it will be the same for all with the exception of the moon and planets. As people were saying, focus can and does change over time.  But, initially, if you focus on a bright star with a mask then you should be good for at least an hour, or until the temp changes, or until you change a filter, or if the seeing changes...


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#6 Stelios

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Posted 14 June 2018 - 10:36 AM

Focus is focus and once you have focus on one object, it will be the same for all with the exception of the moon and planets. 

True, but misleading. Although focus for the moon is different than focus for infinity, the difference is of the order of angstroms. It would be impossible even for the finest amateur focuser to be appropriately adjusted, and certainly the difference would be millions of times smaller than the likely deviation from true perfect focus even with the highest power eye-balling of the Bahtinov mask. And should such a fantastically accurate focusing system be invented, we would be totally unable to tell the difference.

 

However *radical* focus changes even in a few minutes are very possible, so do refocus, especially if temperature is dropping. 


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#7 APshooter

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Posted 14 June 2018 - 12:23 PM

The metal OTA in your telescope will contact during the night as the temperature drops. How much depends on the rate of cooling. This means you'll have to refocus several times during the night.

Planets are best focused by eye, as the seeing changes moment by moment... and you have to wait for the seeing to become very sharp, allowing you to focus for that critical seeing.

Edited by APshooter, 14 June 2018 - 12:24 PM.


#8 Rac19

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Posted 14 June 2018 - 03:32 PM

Thanks for all the replies, very helpful. I forgot to mention that I have an Evolution 8 so mirror movement is definitely a factor. The point is taken on temperature changes also.

 

For solar system targets, it sounds as though I have been doing the right thing. I tend to pulse back and forth through focus, watching for “lucky” in focus live images.



#9 Alex McConahay

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Posted 14 June 2018 - 08:31 PM

>>>>> For solar system targets, it sounds as though I have been doing the right thing. I tend to pulse back and forth through focus, watching for “lucky” in focus live images.

 

The lack of focus due to seeing is so quick that it really is not an issue. Now, it may be true that when a front passes through, seeing changes, and then you should refocus because you have better seeing. But seeing moves things on the scale of several times a second. No time to change focus for seeing itself. 

Alex



#10 Rac19

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Posted 14 June 2018 - 09:27 PM

>>>>> For solar system targets, it sounds as though I have been doing the right thing. I tend to pulse back and forth through focus, watching for “lucky” in focus live images.

 

The lack of focus due to seeing is so quick that it really is not an issue. Now, it may be true that when a front passes through, seeing changes, and then you should refocus because you have better seeing. But seeing moves things on the scale of several times a second. No time to change focus for seeing itself. 

Alex

I probably didn't explain myself clearly. What I do is step through very slightly different focus positions, pause and watch to see how good the focus is at its best. There are usually fames (in live view) with good focus between other frames that are blurred.




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