TS 0.5x Focal Reducer

[Ags]

I bought one of these in 2011 as part of a master plan to turn an ST80 into 200mm F2.5 instrument for photography (of course with field curvature very little of the FOV would be usable). As it turned out it wouldn't reach focus. I also tried it as reducer for a 24mm Hyperion in an F13 Mak, but the view was very distorted so the reducer went into my bits box.

I tried it out this afternoon with various eyepieces and with up to three filters added as spacers. No comments about optical quality, as daytime observing is quite misleading, and for some combinations I was viewing through a polarizer, Wratten 8 and UHC! But I can at least say I didn't see anything that looked like an optical malfunction!

I measured true FOV by counting bricks so of course FOV is expressed in the ISO unit of qb (quarter bricks).

Scope: Skymax 102

Explore Scientific 24/68: 21qb

Speer WALER 10/82: 9qb

Speer WALER 10/82 + Reducer: 13qb

Speer WALER 10/82 + Reducer + 1 Filter: 15qb

Speer WALER 10/82 + Reducer + 2 Filter: 18qb

Speer WALER 10/82 + Reducer + 3 Filter: 20qb

TS  Nirvana16/82: 18qb

TS  Nirvana16/82 + Reducer: 21qb

TS  Nirvana16/82 + Reducer + 1Filter: 21qb + Vignetting

[vlaiv]

I'm surprised you could not reach focus with ST80 and this focal reducer, I had no trouble reaching focus with it and ST102. You might have in focus trouble (not enough in focus travel) if you mount reducer too far away from sensor.

Thing with this focal reducer is that it is very short focal length lens and you need to mount if fairly close to sensor to get proper x0.5 reduction.

It has estimated FL of about 102mm, so you need to mount it at about 50mm to sensor (this is measured somewhere from lens center and not from thread), or about 35-40mm from thread (If I remember correctly body of that reducer is about 12mm give or take).

We can see if this figure fits with your measurements - let's take SW EP as it has proper quarter brick counts - without reducer 9qb and with reducer and some spacers - 18qb. Don't know that particular EP, but I would say that 1.25" nose piece is about 25mm long. Filter cells are about 7mm of optical path (low profile ones?) that would mean that nosepiece + 2 x 7mm = 39mm or pretty much the same as my calc above.

One more note - I had to turn the lens in mine to get better results - might be that all are assembled reversed or my particular one was assembled reversed - I've read somewhere online that someone did that and it helped, so I tried and it indeed provided better image, so I left it like that - reversed.

[Ags]

1 hour ago, vlaiv said:

You might have in focus trouble (not enough in focus travel) if you mount reducer too far away from sensor.

I think that was the issue - the camera was a DSLR and the reducer was on the far end of a 1.25 nosepiece.

[vlaiv]

14 minutes ago, Ags said:

I think that was the issue - the camera was a DSLR and the reducer was on the far end of a 1.25 nosepiece.

This reducer is best used with small sensor cameras like guide cams, so if you have one, give it a go (but don't expect too much at F/2.5 )

[Ags]

I was expecting to throw away 90% of the image field 😀

Next clear night I will see how the eyepiece-reducer combinations work under the stars.

[Ags]

Reducer... Spacing... Must make zoom eyepiece!

https://www.teleskop-express.de/shop/product_info.php/info/p4339_TS-Optics-variable-T2-and-1-25--Extension---Helical-Focuser.html

Of course, it would NOT be parfocal.

[vlaiv]

Better way to do zoom EP is with barlow - view will be much sharper, but principle is the same, vary spacing and magnification will change.

It will not be parfocal unless you can find "double sided" helical focuser, one that moves both sides in/out (towards the center, away from center) - in exact ratio needed given EP optics and barlow optics . Hm, I think that is how they actually make zoom eyepieces

 

[Louis D]

14 hours ago, vlaiv said:

Better way to do zoom EP is with barlow - view will be much sharper, but principle is the same, vary spacing and magnification will change.

It will not be parfocal unless you can find "double sided" helical focuser, one that moves both sides in/out (towards the center, away from center) - in exact ratio needed given EP optics and barlow optics . Hm, I think that is how they actually make zoom eyepieces

 

The Speers Waler zooms (actually varifocal) eyepieces varied the distance between the negative and positive sections.  You can master turning the focuser in tandem with the zooming action to maintain focus.  Certainly having a double helical would be much better.

Most commercial zoom eyepieces move a middle group rather than the initial negative element.  I'm not really sure how that works, but first generation camera zoom lenses also generally moved a middle group to zoom.  They moved the front element to focus rather than moving the entire lens forward or back.  The back group never moved except in a specialized "macro" focus mode on some models.

[Ags]

Tried out the reducer with the Speer WALER 10mm on some stars. In the F13 Mak, it does a very good job of turning a 10mm EP into a 14mm EP, with only a small loss in sharpness. The view off axis was still pretty good.

I then transferred the combination to the ST80, and the result was as dreadful as you might expect. Not many eyepieces work well at effectively F2.5!

If I am ever travelling again with the Mak, I may well pop the reducer in the bag as a free extra eyepiece.