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Celestron Evolution 8 EdgeHD + 0.7x Focal Reducer

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#1 Tume


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Posted 14 July 2020 - 05:17 PM

Not sure if I should post this on the beginners forum or here, but I think it might be more relevant here.


My equipment is very limited at the moment:

- Celestron Evolution 8 EdgeHD

- 2" ClickLock Visual Back

- 2" Diagonal with 1.25" adapter

- 20mm ES100 eyepiece

- The 40mm and 13mm Plossl eyepieces that came with the telescope

- 2.5x 2" Barlow with 1.25" adapter


I am thinking of getting one lower power 2" eyepiece, and maybe a higher power eyepiece.  I was wondering if it is better (and cheaper) to buy one eyepiece and a focal reducer, or multiple eyepieces.


My first question is, would the 0.7x reducer work with my setup for visual use?  From the white paper and the reducer manual, the 0.7x reducer would decrease the photographic back focus to 105mm for the 8".  But the light path of my 2" diagonal alone is already 112mm.  If the 0.7x reducer works for your 8" EdgeHD for visual use, what equipment (diagonal, eyepiece, visual back) are you using, and is there any effect to image quality?



Assume it works, then here are my pros and cons, but I am not sure if they are right or not.



- No need to get more eyepiece for different magnifications, as I can use a combination of barlow and reducer

- For a given wide AFOV, lower power eyepieces are generally more expensive

- I can try some astrophotography for now with the reducer (and get a wedge/EQ mount later)

- I am an OCD hoarder and would not sell anything unless I really have to.  So I try to be a minimalist.



- Image quality might be compromised when using the reducer

- Clumsy setup

- For beginner level astrophotography, it is better to start with an EQ mount + 4" refractor, than to deal with 8" SCT + reducer + wedge/EQ mount + frustration



Thanks and clear skies!


#2 Astrojedi



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Posted 14 July 2020 - 05:55 PM

It may not clear the base. The 0.7x reducer is quite long - once you put a 2" diagonal on it you may have issues. Also for visual you don't really need the reducer. With my EdgeHD 8 and a 30mm 82deg EP I can achieve the max fov possible with the EdgeHD 8. The reducer will not increase the fov beyond this - I know as I have tried.


For imaging, the Evolution mount is not the best but with a wedge it can be quite capable. I used to image with the wedge and a C8 on a concrete pier and it worked surprisingly well. So could be a starter setup to learn the ropes - but the weak link was the tripod. But eventually you will want to get an EQ mount for imaging.


Also for imaging I would recommend starting out with a small refractor. In fact even after 15 years of imaging I still find wide-field imaging with small refractors the most fun and least painless.

#3 gnowellsct



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Posted 14 July 2020 - 10:48 PM

The reducer flattens the field.  That is good for image quality (of star fields).

The reducer flattens the field.  That is bad for image quality (of planets).


For star fields you want the light to fall as evenly as possible on the imaging chip, creating minimal star distortion.


For imaging planets you want IMAGE SCALE which means a lot of magnification.  


With a reducer you will find that a 1.25 inch 32 mm eyepiece will give you the same field as a 1.25 inch 40 mm eyepiece.


Unless you REALLY want/need the flat field effect, you're likely to find you'll enjoy views better in two inch format.  That would require a visual back, a diagonal, and a Pan 41 or XW 40 (forexample).


Greg N

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