2" 0.5x focal reducer - suitability?

[furrysocks2]

There is a 0.5x 2" focal reducer in the for sale section at the moment and I'm trying to work out whether or not it would suit my set up.

 

I bought my first 2" EP earlier this month, a 35mm Skywatcher Aero. I bought this primarily for a Skywatcher Startravel 102 f/5 which I also acquired recently. First impressions were great, at something like 4.8 degrees TFOV. I also have an 8.5" f/7.6 DIY newt on a dobsonian base. I have just installed a 2" focuser on the dob and have had first light through the 35mm Aero though have not had time to test it thoroughly but I think it gives me ~1.46 TFOV (4.6mm exit pupil).

I imagine that a 0.5x reducer would be too much for the f/5 w/ 35mm EP? I'm already at 7mm exit pupil with that scope and from what I've read, EPs get pushed hard at such fast focal ratios and cheap EPs don't do so well. I have a cheap 0.5x 1.25" focal reducer that I got for a webcam and tried it in my 30mm Vixen NPL on the dob, but while it does reduce the magnification it reduces the AFOV considerably such that I essentially have the same TFOV in a tighter space. I don't know the focal length of the reducer or where it sits wrt the focal point of the EP, but I'm not intending on using it again for visual - I don't know whether that's vignetting due to the aperture of the FR or the limit of illumination.

But I wonder whether I'd be able to get a good result from the Aero 35mm + 0.5x reducer in the dob. How do I determine this without buying and trying?

 

I imagine that the illuminated field of my scope is critical here... I've no idea what my original calcs were but putting numbers into newt-web, I get 75% at 1 degree... suggesting that the un-reduced 35mm is already beyond the scope's ability to fully illuminate the EP. As I said, I have not had time to test this thoroughly.

I have often read that a barlow can double your effective EP collection and that with a barlow in the case, new EPs should be chosen accordingly to cover off various magnifications. Is it the same for focal reducers? And does the usefulness of focal reducers drop off as you get up towards the wider/lowmag EPs?

I'm not intending to increase my 2" collection much - for higher mags in the dob 1.25" I think is more common anyway.

 

Can anyone offer me some insight here? I'm not expecting wide-field from the dob, but as wide as possible would be a nice to have I guess and I'm not sure yet how far it can/should be pushed.

[YKSE]

Since you've not got any answer, I'll throw out some maybe €0.02 worth widl guesses and ideas (I have no experience with any reducer)

1. Comparing to a barlow.

A barlow diverges the light cone from the scope, makes it easier for eyepiece to get sharper stars in the edge.

Conversely, a reducer converge the light cone faster the scope's f-ratio, therefore (purely my guess) make the stars in the outer edge more aberrated than without focuser, some obviously vignetting are very possible.

2.There're some dedicated reducers for astrograph, or for SCT (f/6.3, e.g), majority of them have smaller fully illuminated FOV than without the reducer, despite most of these dedicated astrograph reducers are much more expensive, conclusion? the cheap 0.5x reducers will most likely vignette.

3. You test with 30mm NPL is kind of expected, the limiting factor there is the 1.25" barrel, it's very close to the field stop of 30mm NPL, therefore no wider FOV.

4. Since you have the 1.25" 0.5x reducer, you can easily test it with you other shorter 1.25" EPs, comparing it to what you see without the reducer.

I'm looking forward to reading your test results.

[Louis D]

I bought a used 2" 0.5x focal reducer and tried it in multiple configurations.  Either I couldn't get it to come to focus or it just aberrated the field of view horribly.  I really should sell it since I couldn't find a good use for it.

[furrysocks2]

Just now, Louis D said:

I bought a used 2" 0.5x focal reducer and tried it in multiple configurations.  Either I couldn't get it to come to focus or it just aberrated the field of view horribly.  I really should sell it since I couldn't find a good use for it.

Doesn't sound good. :/

What scope and what sort of focal lengths were you trying with? I should have plenty spare in-focus with my 2".

[Louis D]

12 minutes ago, furrysocks2 said:

Doesn't sound good. :/

What scope and what sort of focal lengths were you trying with? I should have plenty spare in-focus with my 2".

In my Newt, my low profile focuser only has 1/2" of in-focus left with most eyepieces.  I couldn't get it to come to focus with the FR screwed into the bottom of any of my 2" eyepieces.  That includes the Meade 40mm 5000 SWA and ES 30mm 82 series.

In my 72mm ED refractor, I could get focus in the above configuration, but it looked horrible (blurry starting from the center).  Even on a 17mm Nagler T4, it looked bad.  I couldn't find anything it paired well with. If I screwed it onto the front of my diagonal, I couldn't reach focus.  I even tried pairing it with a 2" 2x barlow for kicks, but again, it looked bad.  I haven't tried it for photography, so I can't comment on that usage.

Since I only paid $20 used for it, I wrote it off as a learning experience.

[furrysocks2]

Refractor is f/6? I wonder if 0.5x is too much for that... making me wonder if 0.5x is too much for f/7.6, also, even if I do have enough in-focus.

[furrysocks2]

Clear skies during twilight this morning, so I got a test on the Moon and Jupiter which were only a degree and a half apart so I could flick between them.

The 1.25" 18mm with 0.5x reducer gave an image which appeared clear enough but I think I've some reflections appearing in the FOV. Interesting to try the 18mm w/reducer on M42 next time I get a chance. I didn't have time to try the 30mm NPL w/reducer again, because I was supposed to be watching the kids.

My 2" EP focuses at +50mm so presumably plenty in-focus if I were to get a 2" reducer, but still performance of that combination unknown.

 

[Dave In Vermont]

I'd ask a few questions here, if I may?

1. What did you plan/think to use a .50X focal-reducer for? Visual? Photo?

2. What make (brand) is this FR?

3. How would you place it in your optical-pathway?

YKSE is correct about 'vignetting.' Using these FR's in already 'fast' telescopes - such as an F/5 - for visual applications can result in one looking into a 'bowl-of-comets' where once resided nice & sharp pinpoint stars.

The Pdf. I'm attaching below was written around their use in video-photography with a MallinCam Video-Cam. But the principles are the same across-the-board:

Focal Reduction For Dummies.pdf

A Barlow would be a good thing though.

Enjoy,

Dave

 

[furrysocks2]

1 hour ago, Dave In Vermont said:

I'd ask a few questions here, if I may?

1. What did you plan/think to use a .50X focal-reducer for? Visual? Photo?

2. What make (brand) is this FR?

3. How would you place it in your optical-pathway?

YKSE is correct about 'vignetting.' Using these FR's in already 'fast' telescopes - such as an F/5 - for visual applications can result in one looking into a 'bowl-of-comets' where once resided nice & sharp pinpoint stars.

The Pdf. I'm attaching below was written around their use in video-photography with a MallinCam Video-Cam. But the principles are the same across-the-board:

Focal Reduction For Dummies.pdf

A Barlow would be a good thing though.

Enjoy,

Dave

 

Cheers, Dave.

A bit of a mixed bag of intent, to be honest.

1. 1.25" FR photo - 2" FR visual/photo

2. 1.25" ebay cheapies - 2" Antares

3. 1.25" on webcam directly - 2" either on EP for visual, or on the front of a T-2" or 1.25"-2" adaptor with webcam behind.

 

I've got a 2x 1.25" barlow coming. The 1.25" reducers are intended pretty much only for webcam/lunar on my dob - I get <1/8th of a degree TFOV without one. I've got a couple, whether 1.25" or C-mount. Trying it in my EPs was just for fun.

The 2" FR for sale is an Antares. Originally thought it would be for visual only on my 35mm EP to try to get more FOV out of the dob, but I'm not convinced I've got enough illumination to light it up, and unsure how the EP would perform at f/3.8. I'm not intending it to go near the f/5. I've since thought it might be possible to install the 2" FR at the front of a T to 2" of 1.25" to 2" nosepiece, and have filter/cam on the back of that.

 

So it's really just trying to understand if a 0.5x 2" reducer is going to perform with my 2" 35mm EP for as-wide-a-field-as-possible in my f/7.6 dob, or if that's a non-starter due to illumination or EP performance at f/3.8-ish.

Then to work out whether a 2" FR in front or 1.25" cam/filters would work too - that might just be a convenience thing, if I'm swapping out cameras or whatnot.

[furrysocks2]

It's sold anyway.