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FFFR or FF + FR?

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#1 aroughroad

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Posted 03 March 2021 - 08:35 PM

Newbie here looking for some guidance on my optical train for best imaging results...

 

I am about to purchase an ES 127mm F/7.5 scope.  My instinct is to purchase a field flattener and focal reducer as separate items.  The logic is that different targets will be better framed with a 0.7x reducer, a 0.8x reducer, no reducer, or an extender!  But you want a field flattener in every case.  So, bundling the FF and the FR together into one item limits you to only one reducer (to say nothing of focal extenders/barlows!)

 

And yet, almost everyone seems to sell them / use them as one bundled item.  What gives?  Could someone shed some light on the best way to maximize flexibility on one's FOV so as to maximize the number of potential targets?  Do people just swap out different reducers/barlows to optimize the FOV based on the target?  And doesn't doing that necessitate having the FF and FR as separate items?

 

Next, is there a brand of FF/FRs considered to be the best?  I have read it's generally best to stick with the brand of your scope (Explore Scientific, in my case), but I'm not sure about that, and I don't think ES even offers anything except an 0.7x reducer (no 0.8x).

 

Finally, FFFRs connect to the focuser, correct?  I was planning on not using the focuser that comes with my scope and purchasing a better one separately (such as moonlite?), so any guidance on making sure the FFFR and focuser are compatible is appreciated!



#2 Astrola72

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Posted 03 March 2021 - 09:25 PM

If you buy a Moonlite focuser, Moonlite sells adapters for just about any thread out there, so you'll be ok no matter what FF, FR or FF/FR you choose.



#3 aroughroad

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Posted 03 March 2021 - 09:33 PM

If you buy a Moonlite focuser, Moonlite sells adapters for just about any thread out there, so you'll be ok no matter what FF, FR or FF/FR you choose.

Thanks, good to know.  But do you bless my logic on getting the FF and FR as separate components?  Why doesn't everyone do it this way since it offers more flexibility?  Is there no downside to having them as separate components in your optical train?



#4 ryanha

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Posted 03 March 2021 - 10:38 PM

In theory, having two pieces of glass or two lenses or two items will have worse performance than one so FFFR should be better that FF + FR.

 

I have a FF (925mm FL @F7) and a FFFR (675FL @F5).

 

In my test images with two different configurations (FF and FFFR) I was not able to see any difference in actual resolution which led me to believe that I am sky limited at 132mm at 600 or so mm FL.  Here is the thread with details: link.

 

So in my situation I use the FFFR configuration even when doing small targets since I found no benefit to switching lenses.  

 

That said, your mileage may vary.

 

And yes, these go between the focuser and the camera.

 

Good luck!

 

--Ryan



#5 Stelios

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Posted 04 March 2021 - 04:01 AM

Thanks, good to know.  But do you bless my logic on getting the FF and FR as separate components?  Why doesn't everyone do it this way since it offers more flexibility?  Is there no downside to having them as separate components in your optical train?

It offers zero additional flexibility. Nobody wants *just* a reducer (that's not a flattener), so all reducers must be flatteners as well. 

 

What most people do is get both a flattener and a reducer/flattener to give flexibility of two different F/ratios. Typically you want ONLY two values--the largest (just a flattener) and the smallest (maximum reduction that allows covering the full sensor of your camera with a flat field). 

 

Your idea appears to be based on the false premises that

1) reducers that do not flatten are useful (they are not)

2) that reducers that do not flatten would represent a significant cost savings (*very* doubtful)

3) that it would be trivial to combine/daisy chain non-flattening reducer and a 1x flattener (doubtful). 


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#6 aroughroad

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Posted 05 March 2021 - 12:08 PM

It offers zero additional flexibility. Nobody wants *just* a reducer (that's not a flattener), so all reducers must be flatteners as well. 

 

What most people do is get both a flattener and a reducer/flattener to give flexibility of two different F/ratios. Typically you want ONLY two values--the largest (just a flattener) and the smallest (maximum reduction that allows covering the full sensor of your camera with a flat field). 

 

Your idea appears to be based on the false premises that

1) reducers that do not flatten are useful (they are not)

2) that reducers that do not flatten would represent a significant cost savings (*very* doubtful)

3) that it would be trivial to combine/daisy chain non-flattening reducer and a 1x flattener (doubtful). 

Ok very clear, thank you! 

 

One clarification, please...if given the choice between a FFFR that reduces the f/ratio by a factor of 0.8x / 0.7x / 0.6x, etc...., how can I tell whether I'm picking the reducer that maximizes reduction but still allows covering the full sensor with a flat field?  In other words, is it possible that a 0.7x reducer would achieve this but a 0.6x reducer might be reducing the ratio "too much" given my specific camera?

 

Many thanks!




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