Low Cost x0.5 Focal Reducer With SCT: Findings

[cloudsweeper]

My 8" SCT gives at best a TFOV of about 1.34deg, and this cannot I believe be improved on.

There is lot reported about Focal Reducers, and the consensus is that for visual, they are not effective with long focal lengths (of EPs), and produce vignetting etc., so I would not expect any benefit from one.

Then I spotted a x0.5 SVBONY 1.25" Focal Reducer at £4.99 (post free), from China, and thought it was worth experimenting with at that price.

It arrived today, and being keen to do a test, I set up the 8SE, and aimed it at a rooftop, figuring that getting ridge tiles across the centre of the field of view would give a reasonable indication of changing FOV.  The Reducer fits straight on the EP, and my only 1.25" EPs are 24, 14, 12, 11mm (and lower).

Findings from the "Ridge Tile Test":

The 10, 11, and 12mm EPs easily give at least 50% more FOV, but the 14mm gives about 33%, and the 24mm actually gives a drop (of about 17%) in FOV.

Conclusions:

In this simple daylight test, I did not notice any vignetting, and the reported increase in FOV appeared not to incur any other unwanted effects.  The increased FOV seemed to work well enough with up to 12mm focal length, but simply using the 24mm EP on its own gives more FOV anyway, thus rendering the Focal Reducer redundant!

Reported issues of lack of effectiveness with longer focal lengths appear vindicated.

Granted, I have only used a very low cost Reducer, and a better one - perhaps placed in a different position - might perform differently.  Would a 2" Reducer do any better, especially at even longer FLs??  I would tend to doubt it, but if I ever see one for £4.99, I'd be prepared to try it out!

Doug.

[John]

Interesting report

The only focal reducer that I've used with an SCT was the Celestron F/6.3 one which I used with a C5 and later a C8. It seemed to do a decent job with 1.25" plossl eyepieces up to around 25 mm in focal length. In longer foacl length eyepieces I can recall seeing a blurred and narrower field edge. I didn't try it with 2" eyepieces because I'd read that the vignetting would be worse with the larger format eyepieces.

Looking at the one that you have tried out, it looks from the photo that the clear aperture of the reducer optics might be a fair bit smaller than the aperture of the field stop of a 24mm 68 degree AFoV eyepiece ?

 

 

[cloudsweeper]

3 minutes ago, John said:

Looking at the one that you have tried out, it looks from the photo that the clear aperture of the reducer optics might be a fair bit smaller than the aperture of the field stop of a 24mm 68 degree AFoV eyepiece ?

 

 

Yes, John - that would explain the drop in FOV!

Doug.

[domstar]

Very interesting. Alas I have to wait til Christmas to play with mine.

[Louis D]

I guess I'll have to dig out my 2" 0.5x reducer and try it in my 125 Mak with 2" eyepieces someday.  I have yet to find a good use for it.  Perhaps if I was an astrophotographer it might have use for me.

[cloudsweeper]

11 minutes ago, Louis D said:

I guess I'll have to dig out my 2" 0.5x reducer and try it in my 125 Mak with 2" eyepieces someday.  I have yet to find a good use for it.  Perhaps if I was an astrophotographer it might have use for me.

Do it, Louis - I'd like to know how that one fares with long FL 2" EPs!  (But I don't expect any startling findings!!) 

Doug.

[cloudsweeper]

1 hour ago, domstar said:

Very interesting. Alas I have to wait til Christmas to play with mine.

Eagerly awaiting your report for comparison! 

Doug.

[SpaceWatcher]

Thanks, I’m also 8” SCT user, this has saved me ordering one to try as I was only looking at one for bigger focal lengths

Looks like the Antares 6.3 will be the best option

👍