11 replies to this topic

[Procyon]

I bought one of these from aliexpress for a buck 25 just now...(Daytsun model, Datsun?) to try on a binoviewer. But my brain can't seem to calculate this morning. Anyone know what focal length a 25mm eyepiece will turn into if I insert this 0.5x barlow/focal reducer into it? 

 

0.5X Focal Reducer Lens for M28x0.6 1.25" Telescope Eyepiece Ocular Barlow Lens

 

https://www.ebay.com...:Bk9SR_DB56WzZQ

Edited by Procyon, 15 March 2025 - 10:39 AM.

[fftulip]

It would be (approximately) 25mm/0.5 = 50mm.  You can also think of it as reducing the focal length (or f ratio) of your telescope by half.

Be aware that you might get vignetting with this.  It's intended for higher power eyepieces.

Weird that they call it a barlow lens when it reduces power.

[Procyon]

Thanks.

 

Hilarious.

[Olimad]

From my eyepiece collection, the only eyepiece that work with the 0,5x is the SVBony redline 6mm. I havn't tried It with my 82° eyepieces.

With my F4, It cleans the edge astigmatism.

[SeattleScott]

I think you went the wrong direction. Normally one uses an amplifier to reach focus with binoviewer, not a reducer. I would be fairly stunned if you are able to reach focus using this reducer in a binoviewer.

[SeattleScott]

From my eyepiece collection, the only eyepiece that work with the 0,5x is the SVBony redline 6mm. I havn't tried It with my 82° eyepieces.
With my F4, It cleans the edge astigmatism.

These reducers aren’t specific to an optical design, hence they are not optimized to provide good correction across the field. They can work well with small sensor cameras (their intended use) since those only use the center of the lens. I could see them also working with short focal length, narrow TFOV eyepieces, such as a 6 Redline. Of course, remember, the 6 Redline would then be operating as something like a 12mm. It might not be “cleaning up” the astigmatism as much as it is reducing the magnification, so the astigmatism isn’t as obvious because you aren’t zoomed in as much on it. That being said, it is certainly possible there is some kind of random symbiosis that causes the reducer to work well with that particular eyepiece in that particular scope.

[Olimad]

What I think It does, since the field stop of the smyth lens of the 6mm is small and gets close to the last lenses of the 0,5x reduce, It send the most central part toward the field stop of the smyth lens, expanding It through the eyepiece.

 

I have used It with barlow, and has given me one of my best views of planets/moon when I get what is till now my best seeing nights. I know you will tell me this is a lot of glasses, and not premium ones. But that was my experience so far.

 

And I have compared the reducer with the 6 mm to the UFF 10mm, and I got approx the same TFoV (compared seeing pleiades) but with no astigmatism in comparison to the UFF.

[SeattleScott]

What I think It does, since the field stop of the smyth lens of the 6mm is small and gets close to the last lenses of the 0,5x reduce, It send the most central part toward the field stop of the smyth lens, expanding It through the eyepiece.

I have used It with barlow, and has given me one of my best views of planets/moon when I get what is till now my best seeing nights. I know you will tell me this is a lot of glasses, and not premium ones. But that was my experience so far.

And I have compared the reducer with the 6 mm to the UFF 10mm, and I got approx the same TFoV (compared seeing pleiades) but with no astigmatism in comparison to the UFF.

Yes, but this just says the Redline has less astigmatism than the 10UFF. The 10UFF is only five elements and would be expected to struggle in a F4. The Redline has more elements and can better cope with the F4 light cone.

This doesn’t prove the reducer isn’t somehow cleaning up astigmatism in the Redline. It just doesn’t prove that it is. Certainly, one would not expect a reducer to clean up astigmatism in an eyepiece, so if that is occurring, it should be viewed as a happy coincidence rather than an expectation.

[triplemon]

Those reducers are pretty certain singlets, often even uncoated, just as most barlows under $5 on Ali are. At this price point, "optical glass" is the highest praise quality mark you can somewhat believe in any product description.

 

And yes, any reducer will certainly require significant infocus. The basic math is - if a reducer is located x mm from the focal plane reduces by a factor of y, it will require x * y infocus. Likely around half an inch in your case.

Edited by triplemon, 15 March 2025 - 02:28 PM.

[Olimad]

Yes, but this just says the Redline has less astigmatism than the 10UFF. The 10UFF is only five elements and would be expected to struggle in a F4. The Redline has more elements and can better cope with the F4 light cone.

This doesn’t prove the reducer isn’t somehow cleaning up astigmatism in the Redline. It just doesn’t prove that it is. Certainly, one would not expect a reducer to clean up astigmatism in an eyepiece, so if that is occurring, it should be viewed as a happy coincidence rather than an expectation.

The Redline 9mm has more astigmatism and edge aberrations than the UFF 10mm (after Ernest and seen by me at F4).

The 6mm also has some but you can not compared directly to the 10mm UFF because It offers less TFoV against the UFF.

Nevertheless I prefer to look at a stars field through the UFF10 against the Redline 6mm alone or 9mm.

With the 0,5x+6mm, then It is totally different.

 

It was not expected and only a trial and error with all my eyepieces. This positive behavior was only seen with the 6mm Redline. Not with the 4mm, 6mm, 8mm plössl, not with the 9mm Redline or 10mm UFF or BCO, and not with longer FL eyepieces.

Only with the 6mm. As you said an happy coincidence, astrophotographers would say an "lucky imaging" moment.

 

It is not exempt of any negative side, one of them is that the back of the sky is brighter for example.

But It was a cheap way to get close to pin point stars all over the field without corrector (in fact, the 0,5x was in a way a kind of lucky corrector). I have managed to get close to an equivalent of a 10mm eyepiece with the APM CC barlow, which gives better results. But this was one year after playing with the 0,5x FR + 6mm, and also way more expensive.

 

Again with other scopes, I do not know if this remain true.

Edited by Olimad, 15 March 2025 - 10:36 PM.

[SeattleScott]

Ah, the APM coma correcting barlow, a way to get coma correction in these fast little newts with only 1.25" focuser. I hadn't thought about that! Obviously doesn't work for low power, but it's something!

[Olimad]

Ah, the APM coma correcting barlow, a way to get coma correction in these fast little newts with only 1.25" focuser. I hadn't thought about that! Obviously doesn't work for low power, but it's something!

Indeed but you loose widefield....by playing with the barlow distance you can still get close to 2,3° of pretty well corrected field, but that is far from the 4° that this scope can offer. Always trade-off.