Focal extender and extension tubes

[zelonisv_bob]

I recently attempted imaging with my Explore Scientific (ES) ED127 refractor using a newly acquired ES 3X focal extender and my Canon T1i.  I was shocked by how many extension tubes I had to use to reach focus: a total length of 251.5 mm.!  Adding the 66.5 mm. length of the focal extender and 103 mm. of drawtube extension, put my camera a grand total of 421 mm. (16.6 inches) behind the rear end of the scope!  This arrangement seems very cumbersome and precarious(?) considering how many joints there are in the chain of pieces, each one relying on a tiny thumb screw or two.  There's flexure there, I'm assuming.  My questions: Does this seem like a needlessly excessive amount of extension to reach focus?  (by the way, I put the focal extender immediately behind the focuser-drawtube)  Do the 5X and 10X options in the Planetary Mode serve the same function as focal extenders and Barlow lenses?  Thanks.

 

Bob Z.     

[s3igell]

Yes, that is Hanging It Out There...  Especially with only Compression Ring Thumbscrews to hold your DSLR to the end of it...

 

It's hard to say what the expected Backfocus Length of the 3x Focal Extender would be, as ES provides no such detail in their Spec Sheet.

 

BUT, you are using that Barlow (ok, Focal Extender by any other name) to achieve 2850mm of Effective Focal Length!!  Can you imagine a 3 Meter long Refractor??  These are the Focal Lengths that truly Justify the Compound Folded Optics of an SCT or MCT or RC.

 

Yes, the result will include an appreciable amount of Flexture (and probably some rather Severe Vignetting being that it is a 1.25in Optics).

 

What are you planning to Image at f/22.5 and 2850mm??  One would assume Planetary Video (especially as you asked about BYE Planetary Mode) - the one case where neither Flexture nor Vignetting nor SLOW Optics will be major concerns.

Does 5x LiveView Planetary Mode serve the same Function??  Yes and No...  5x LiveView will produce a similar "Visual Magnification" because of a limited FOV.  But that FOV is caused by reducing the Pixels Dimensions of the Image by 5x.  The 5x Mode does NOT increase the Resolution, as the individual Pixels are the same size in 5x Mode as when producing a Full-Sensor Still Image (just a whole lot less Pixels are participating) as is the Focal Length.  

The use of a Barlow will change the Focal Length, thereby changing the actual Resolution while producing a Narrower FOV.  And as the effects of 5x LiveView and 3x Barlow are Multiplicative, you can get some real tight FOVs in combination.

 

But, Aperture affects the Resolution Limiting effects of Diffraction, so there is truly "Only so much" that can be expected from 2850mm FL through only 127mm Aperture....  This is where 11-15in SCTs really strut their stuff - Focal Length AND Aperture...

 

The 10x LiveView is a product of Digital Magnification - making Fat Pixels - and doesn't hold the same Effects or Benefits.

[zelonisv_bob]

s3igell, thank you so much for all that info.  Yes, I purchased the 3X focal extender primarily for planetary imaging, but on the night that I first used it, my targets were M57 and M2.  First, I fine-focused on Vega with a Bahtinov mask.  The globular cluster came out better at 75 second subs @ ISO 1600; M57 was a fuzzy disappointment.  I understand about the 5X planetary mode in BYEOS and the best resolution obtained by that method.  I also have a 10" f/4.5 Newtonian reflector scope that is suitable for short exposure imaging (short only because my old Cave Astrola mount runs at only one speed: approx. sidereal).  I'll be trying that out on Jupiter and Mars on the next clear morning.

 

Bob Z.