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Celestron C8 focusing

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#1 Michael Miles

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Posted 05 August 2014 - 03:31 PM

Hi folks:

 

My C8 is in pieces, and I'm trying to remember where the infinity focus is for the scope.  At infinity focus, is the main mirror near the back of the scope (away from the secondary), or further forward (toward the secondary)?  I tried to figure it out, but the brain keeps getting confused on the details...

 

Thanks,

 

Michael



#2 WarmWeatherGuy

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Posted 05 August 2014 - 09:03 PM

You can focus at infinity no matter where the mirror is. Turning the focus knob changes the point of focus (how far beyond the back of the scope the image comes to focus). Try focusing the Moon on a sheet of paper when you have no eyepiece. There is a point where the Moon will come to focus. Turning the focus knob will change where you have to hold the paper but you will always be able to find a place to focus the Moon on some paper.

 

Now let's assume you have an eyepiece or CCD at some fixed point. On the back of my focus knob is a marking that indicates that you get to infinity by turning the knob left. If you turn it too far "beyond infinity" then it won't focus on anything. If you turn it to the right of infinity then it will focus on objects closer in.

 

The infinity position of the mirror depends on how far back you have the eyepiece. If you have a diagonal then it is further back than if you don't.

 

I would just turn the knob clockwise until you can't go any further. This will move the mirror all the way to the back. Then turn the knob left while looking at something bright (a tree in the daytime, the Moon at night) until it comes into focus. You may have to turn the knob 20, 30, more? times to get focus.



#3 Michael Miles

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Posted 05 August 2014 - 11:52 PM

OK, Steve - I think that answers my question.  To rephrase:

 

1) If I have a particular eyepiece, move the mirror all the way forward (turn/rotate the standard focusing knob all the way to the left) and then move the eyepiece to focus at infinity (astronomical objects).  

 

2) Then that eyepiece in that location will have the broadest range of closer distances that it can focus on.

 

Did I interpret that correctly?

 

Thanks,  Michael



#4 WarmWeatherGuy

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Posted 06 August 2014 - 07:40 AM

OK, Steve - I think that answers my question.  To rephrase:

 

1) If I have a particular eyepiece, move the mirror all the way forward (turn/rotate the standard focusing knob all the way to the left) and then move the eyepiece to focus at infinity (astronomical objects).  

 

2) Then that eyepiece in that location will have the broadest range of closer distances that it can focus on.

 

Did I interpret that correctly?

 

Thanks,  Michael

I would not move the focus knob all the way to the left. That pushes the mirror all the way forward. The only thing stopping it is a rubber O-ring. If that fails then the mirror could become detached from the focus knob and, if you point the scope down the mirror would fall onto the corrector.

 

Typically you don't move the eyepiece (on an SCT) to focus unless you have an external focuser. If you have an external focuser and you want to have the ability to focus up close by using only the external focuser then you would want to have the eyepiece be close to the back end of the OTA (retracted, not extended) when you're focused at infinity. This is usually not a consideration. Telescopes are meant to focus on infinity.

 

I thought you originally just wanted to find focus. To do that I would turn the knob clockwise (to the right) as far as it will go. Then begin the long process of turning the knob left many turns while looking through the eyepiece until you have focus.



#5 cbwerner

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Posted 06 August 2014 - 10:11 AM

My C8 is in pieces

 

 

Steve,

 

I'm fuzzy on what Michael is trying to do but I think the issue is that he has disassembled the scope - is that right Michael?



#6 Michael Miles

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Posted 06 August 2014 - 03:18 PM

Hi Guys:

 

I actually gutted the scope to build a modified Schmidt-Newtonian-Hyperstar frankenscope:

 

http://www.cloudynig...-3#entry6149284

 

I needed to know whether to set the fixed focus for the Hyperstar when the mirror was closer to the back of the scope or the front.  Steve gave me enough info to answer my question as you can see here:

 

http://www.telescope...cs.net/SCT2.htm

 

If the eyepiece (Hyperstar) is at a fixed position, the primary mirror would have to be moved farther away from the secondary.  So the solution is to make the infinity focus of the scope when the main mirror is closer to the secondary.

 

Thanks for the help,

 

Michael



#7 Geo.

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Posted 06 August 2014 - 04:50 PM

Generally, near focus is with the mirror toward the rear of the baffle tube, far is closer to the secondary. Infinity is usually found somewhere between them :)








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