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#1 Cazka

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Posted 04 May 2013 - 08:12 PM

I'm a beginner to the whole astrophotography scene, and am just picking out my telescope and camera, and have become confused about back focus, namely, what it exactly is.

The telescope I'm looking at is the Explore Scientific ED80 triplet refractor, and the camera I'm looking at is the Atik 320E one-shot-colour ccd camera.

Now, according to the specs, the ED80 has a back focus of 139mm from end of eyepiece holder. Does that mean that the chip of the 320E has to be 139mm away to bring the image into proper focus? If so, considering the 320E has a backfocus distance of 12mm (I'm assuming that's distance between chip and the front of the camera), that leaves 127mm left. That seems like a lot to make up. Do I have to use a whole heap of connectors that add up to 127mm? To me that seems a very inconvenient setup.

Sorry, if these questions sound stupid, but I haven't been able to find a clear definition of what the back focus number is, whether it's acceptable to have the camera chip within that distance, or whether it has to be exactly that far away to achieve focus.

#2 JoseBorrero

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Posted 04 May 2013 - 08:15 PM

you will need an 2" extension adapter,therwise you consider a focal reducer/filed flattener or only a field flatter that can also substitute an extension adapter, not both.

#3 Madratter

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Posted 04 May 2013 - 08:31 PM

You probably will need a 2" extension tube to reach focus. I say probably because we don't know how much travel the focus unit has. I'm dubious it has the 127mm of travel (5") needed, and I'm dubious that even if it does, things would be rigid enough with it extended that far.

#4 Cazka

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Posted 04 May 2013 - 08:55 PM

By focus unit, I presume you mean the focuser? The one included with the telescopes (Crayford 2" 10:1 dual-speed focuser) has 2-3/4" (70mm) travel according to the specs.

So the focuser is part of the back focus distance too? That was one other thing I wasn't sure about, where the back focus distance is measured from. Judging from your comment, it's measured from the focuser

#5 astrodog73

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Posted 05 May 2013 - 02:18 AM

Refractors are generally designed with the assumption you will view through a diagnonal, which adds a fair but of distance, which you replace with a straight extension...

Don't complain too much, having a lot of back focus is a good thing, wait until you start trying AP with a newt.... :)

#6 Madratter

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Posted 05 May 2013 - 07:25 AM

By focus unit, I presume you mean the focuser? The one included with the telescopes (Crayford 2" 10:1 dual-speed focuser) has 2-3/4" (70mm) travel according to the specs.

So the focuser is part of the back focus distance too? That was one other thing I wasn't sure about, where the back focus distance is measured from. Judging from your comment, it's measured from the focuser


If the focuser has 70mm of travel, that means you are 127mm - 70mm (at its furthest point) = 57mm from reaching focus. You will need the extender as anticipated.

#7 CounterWeight

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Posted 05 May 2013 - 10:25 AM

I've owned a few 80mm refractors and needed a 2" or ~50mm extension tube on each. I was always able to reach focus somewhere without having the focuser racked too far out.

One thing it's important to know is if you will need a field flattener for the size of imaging chip. I'd expect at f/6 it might be necessary but the 320 chip is not that large so you might try it without and look in the corners and decide from there.

#8 Cazka

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Posted 05 May 2013 - 08:37 PM

Thanks for the replies everyone, I think I understand this a lot better now. It should become even clearer once I get the scope, which I've now ordered, so hopefully it won't be too long away.

I'll definitely look into investing into one of those extension tubes. I was going to test out the camera with a few images before deciding on whether a field flattener is needed, but from what I've heard, it's probably going to be necessary with the ED80. That would even complicate the back focus matter more. Just part of the fun of getting into astrophotography I suppose

#9 CounterWeight

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Posted 06 May 2013 - 12:11 AM

It seems more complex that it really is. Just make sure to keep a small amount of $ set aside for spacer ring sets and and you are all set, just be ready to experiment a little.






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