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Are GSO 0.5x focal reducers any good?

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#1 johngwheeler

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Posted 05 April 2016 - 11:13 PM

I have an f/7.4 refractor that I'm using for EAA, but have read that this is a bit slow for optimum results.

The "recommended" FR for my telescope costs $995 (Takahashi) - which seems too much to spend for my level of experience. I'm looking for a cheaper option that is at least "OK" for EAA (if not for visual).

Are the GSO FRs any good at all, or just cheap junk? If not, then is there any mid-tier FR for 600-800mm refractors that could work in my scope?

Thanks!


John.

#2 Dwight J

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Posted 06 April 2016 - 12:18 AM

Hi John:  I have one of those.  They are acceptable if you do not use the extension tube with them but thread them directly onto the camera nosepiece.  Using the extension gives excessive coma, at least in SCT's that I have.  Antares makes a 0.5X one that others on this forum describe as a better performer for about the same price as the GSO.  If you want a better quality one that gives you a variable ratio, look at the Mallincam MFR-5 which uses two elements and a variety of spacers to get optimum reduction with minimal aberrations.  If your camera has a small chip 1/3 to 1/2 inch type, vignetting and coma should largely be outside the field.



#3 GlennLeDrew

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Posted 06 April 2016 - 03:09 AM

This reducer and others like it are really the same as a 25mm f/4 objective as used in a binocular. These cemented achromats are a standard 'off the shelf' item churned out by the container ship load, and are of very low cost to make. There will be some variation in quality, depending on the vendor's requirements. But no matter how well made, such a lens designed for one of its conjugates at infinity is not optically corrected for the significantly non-parallel light entering it when installed as a reducer behind an objective. Under-corrected spherical aberration is therefore present--for all telescopes types.

 

And so one can push the reduction factor only so far before aberrations become detracting, that limit being about 0.5X. The principal aberrations we note--at non-aggressive reduction factors--are astigmatism and field curvature. What coma is imparted by the reducer is dominated by astigmatism. With rather aggressive reduction spherical aberration becomes the worst offender, especially as it marrs the central image too.

 

If such a reducer is found to perform worse than expected, it can be replaced by the objective taken from a 10X25 bino that's otherwise broken. Such an objective must be well enough figured to work at all well in that role, and so can be counted upon to be as good as can be for use as a reducer.

 

Actually, if one were lucky enough to find such an objective to be 'over-corrected' compared to the nominal spherical form, this would reduce the spherical aberration . But this would probably be an exceedingly rare beast, for 'under-correction', in the form of a turned down edge, would be the more likely state, and which would exacerbate spherical aberration. This could be the main cause of poorer reducer quality encountered...



#4 GlennLeDrew

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Posted 06 April 2016 - 03:10 AM

The other two identical threads on this topic should be vaporized. ;)



#5 johngwheeler

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Posted 06 April 2016 - 03:20 AM

This reducer and others like it are really the same as a 25mm f/4 objective as used in a binocular. These cemented achromats are a standard 'off the shelf' item churned out by the container ship load, and are of very low cost to make. There will be some variation in quality, depending on the vendor's requirements. But no matter how well made, such a lens designed for one of its conjugates at infinity is not optically corrected for the significantly non-parallel light entering it when installed as a reducer behind an objective. Under-corrected spherical aberration is therefore present--for all telescopes types.
 
And so one can push the reduction factor only so far before aberrations become detracting, that limit being about 0.5X. The principal aberrations we note--at non-aggressive reduction factors--are astigmatism and field curvature. What coma is imparted by the reducer is dominated by astigmatism. With rather aggressive reduction spherical aberration becomes the worst offender, especially as it marrs the central image too.
 
If such a reducer is found to perform worse than expected, it can be replaced by the objective taken from a 10X25 bino that's otherwise broken. Such an objective must be well enough figured to work at all well in that role, and so can be counted upon to be as good as can be for use as a reducer.
 
Actually, if one were lucky enough to find such an objective to be 'over-corrected' compared to the nominal spherical form, this would reduce the spherical aberration . But this would probably be an exceedingly rare beast, for 'under-correction', in the form of a turned down edge, would be the more likely state, and which would exacerbate spherical aberration. This could be the main cause of poorer reducer quality encountered...

 
Hmmm, so they don't sound like a very good piece of optical gear at all. Not the sort of thing I want to put on a premium scope :-( I also don't want to pay the Takahashi price for their reducer, so I'm looking for a middle ground.

The other two identical threads on this topic should be vaporized. ;)


Not sure how this happened - but I also can't see how I can delete a post myself....

#6 Dragon Man

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Posted 06 April 2016 - 04:44 AM

John, in a very basic way of saying what Glenn is sort of saying is that many many brands of Focal Reducer on the market all come out of the same factory from the same production line but with different brand names printed on them.

 

It's common practice with Astronomy products, and the majority of what we buy, regardless of the brand name is printed on it, comes mainly from 1 of 2 factories: GSO or Synta.

I have several different Focal reducers from different suppliers and regardless of the name on them they are all GSO's.

 

My favourite FR's are the ones I make myself from Binocular lenses.

Here's my video on how to do it if you are feeling adventurous: HERE



#7 mclewis1

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Posted 06 April 2016 - 08:05 AM

John, There's certainly a whole world in between the GSO and similar inexpensive .5x 1.25" reducers and the Tak unit.

 

Optical figure (design)

Optical quality (accuracy of the elements)

Surface quality (smoothness)

Coatings

Mechanical quality

Aperture

Reduction factor (.5 vs. ~.7x)

 

While I've never used the Tak reducers (visually or imaging) I'm sure they are wonderful. I have used a variety of lower priced reducers on a similar quality scope (my TMB) and if you are very critical in your viewing you will certainly be able to see a difference across a whole range of focal reducers (and then very likely also notice some differences with a camera).

 

As mentioned above, for EAA use with a small sensor camera the Tak reducers are overkill. You will be able to get more aggressive reduction from 3rd party reducers, but at the expense of some optical quality.

 

In general if you are using a camera with a type 1/3" sensor I think you'll be fine with the GSO (assuming a reasonable quality example). If you don't like the GSO reducer you might also consider what we've called "the Chinese reducer". This is an unbranded model at a similar low price point, and available from a variety of online sources (ebay, alibaba, etc.). The quick way to tell the difference (aside from no branding) is the body of the "Chinese" model is larger. Otherwise it's just another .5x 1.25" focal reducer that is used in exactly the same manner as the GSO model.

 

If you are using a camera with a type 1/2" or larger sensor (or soon plan to) you may be better off going for a different reducer. There are .5 and .7x basic 2" reducers (GSO, Antares), and a bunch of .7 and .8x (WO, TeleVue, etc.) models  that would be effective.

 

I was quite pleasantly surprised when using a .5x 2" Antares focal reducer with my Mallincam with it's type 1/2" sensor (as well as with similar cameras) and my TMB refractor. I've also used a William Optical .8x (type II) reducer with my TMB with very good results (but not as much reduction as with the .5x Antares).


Edited by mclewis1, 06 April 2016 - 08:06 AM.


#8 jimthompson

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Posted 06 April 2016 - 09:23 AM

Hi John,

 

By coincidence I have done a fair amount of testing recently using the 2" GSO focal reducer.  I have been able to demonstrate good performance with it on a 1/2" sensor down to around 0.5x.  I was also able to get quite good performance on a 28.4mm sensor (APS-C) with a reduced spacing, giving around 0.72x...the coma and vignetting free radius was approx. 26mm out of 28.4 (~83% of sensor area).  I did find during my testing that the FR was not assembled in the correct way to achieve optimum results.  As received the highly curved side of the lens was facing the camera (this particular lens has an almost flat side and a highly convex side).  I achieved much better results with the highly curved side facing the telescope.

 

In general my impression of GSO is that they manufacture good quality products.  I have a number of items made by them including my 10" RC carbon fiber telescope, which I am very happy with.

 

cheers,

 

Jim T.



#9 Alex Parker

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Posted 06 April 2016 - 09:26 AM

I have an f/7.4 refractor that I'm using for EAA, but have read that this is a bit slow for optimum results.

The "recommended" FR for my telescope costs $995 (Takahashi) - which seems too much to spend for my level of experience. I'm looking for a cheaper option that is at least "OK" for EAA (if not for visual).

Are the GSO FRs any good at all, or just cheap junk? If not, then is there any mid-tier FR for 600-800mm refractors that could work in my scope?

Thanks!


John.

For quite a while I used one of these GSO 0.5x reducers with my C8, but ultimately got rid of it and bought a better one because I got tired of the coma and vignetting it produced.  With my small refractor (72 mm f6) I use an 0.8x Orion FR that gives very good results and cost me $120.  This FR is described by Orion as being best for refractors in the f6-f8 range.  It might be a more affordable option for your Tak.

 

It is also worth noting that there is no "best" f-ratio for EAA, and faster does not necessarily mean better.  The goal in EAA is to have a system that makes decent quality images of the things you want to see very quickly.  The factors that matter here are quantum efficiency of your sensor, pixel scale (arcsec per pixel on the chip), and telescope aperture.  At long focal lengths (which you often need to magnify small objects) you will often get the best results by binning pixels to make the pixel scale larger (for example, having pixels that are 0.5 arcsec in scale makes no sense under most EAA conditions, especially with less than perfect seeing).  For a given pixel scale and QE, more aperture gets you more photons per unit time, gets you your image sooner.  Note that pixel binning only helps sensitivity ("speed") if it is true on-chip binning.  Merging pixels post-hoc in software does not help.



#10 kuba_mysluk

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Posted 06 April 2016 - 11:58 PM

Last night I tested GSO 1.25 0.5x on C 9.25, fitted on the nose of Infinity.
In this setup it was worth nothing. On quite good focus (FWHM about 2) I still see black dots inside faint stars. The bright ones was quite good.
I have no possibility for change distance from sensor, no data regarding correct one provided from the GSO.

Will stay with my Baader Alan Gee then :)

Kuba

#11 ChrisFC

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Posted 07 April 2016 - 12:36 AM

There must be a lot of variability. I have gso that's a few years old and it is terrible compared to the one that came with my revolution imager (which i presume is from the same source) ?

I find the Meade and optec (its a really old one) 3.3s really good. Use a shorter spacer so only doing about x0.5. They are reasonably priced second hand

Edited by ChrisFC, 07 April 2016 - 12:38 AM.


#12 GlennLeDrew

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Posted 07 April 2016 - 02:30 AM

Kuba,

There is no 'correct' working distance for such a simple reducer. Being not optimized for any telescope, they necessarily work best at minimal reduction/minimal separation. The aberrations pretty much always worsen as the separation between reducer and sensor, and hence aggressiveness of reduction, is increased.

 

The focal length for these ~25mm aperture reducers is about 95mm. For a 0.5X reduction factor the lens must be placed about 1/2 its focal length from the sensor, or some 47mm.



#13 kuba_mysluk

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Posted 07 April 2016 - 07:16 AM

Glenn,

Thanks for the info, will try set it about 47 mm, but course I dont expect any great results from single cheap glass. But Infinity sensor is small and it is worth of check.

My idea was stack it with Baader Alan Gee reducer (works excellent alone on distance 121 mm).

Regards

Kuba

#14 ccs_hello

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Posted 07 April 2016 - 07:56 AM

Use a large diameter F.R.

 

The inner part of teh optics is always the sweetspot of the optics.  You'll gain some by going 2".

 

P.S. my bino objective converted F.R. does have a large diameter.

 

http://www.cloudynig...dslrmirrorless/

http://www.cloudynig...-focal-reducer/

 

Clear Skies!

 

ccs_hello



#15 jimthompson

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Posted 07 April 2016 - 09:25 AM

Last night I tested GSO 1.25 0.5x on C 9.25, fitted on the nose of Infinity.
In this setup it was worth nothing. On quite good focus (FWHM about 2) I still see black dots inside faint stars. The bright ones was quite good.
I have no possibility for change distance from sensor, no data regarding correct one provided from the GSO.

Will stay with my Baader Alan Gee then :)

Kuba

Hi Kuba,

 

If you have a lens spanner, I recommend testing the FR out with the glass element as received and then flipped around.  The difference in image quality that I found between as-received and with glass flipped was dramatic.  I am not sure why the FR would not have been assembled in the best orientation...perhaps the FR is configured from the manufacturer for eyepiece use? (ie. curved focus plane instead of flat)

 

I have attached some screen captures to illustrate the difference.  I discovered the sensitivity to lens orientation when playing with some generic achromats of varying focal length.  The results from the 105mm f.l. achromat show very dramatically the difference lens orientation has.  The images were captured with a Mallincam Universe, sensor dimensions 28.4mm diagonal), staring at an image of a star field on a laptop across the room through a William Optics FLT98.

 

cheers,

 

Jim T.

Attached Thumbnails

  • generic105fl-curvetowards.jpg
  • generic105fl-curveaway.jpg


#16 jimthompson

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Posted 07 April 2016 - 09:27 AM

My focus was off a little with the 2" GSO 0.5x FR, but you can still see that there is an improvement in the image with the highly curved side of the lens towards the telescope (curveaway).

 

cheers,

 

Jim T.

 

Attached Thumbnails

  • GSO 0.50x-curvetowards.jpg
  • GSO 0.50x-curveaway.jpg


#17 kuba_mysluk

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Posted 07 April 2016 - 11:06 AM


Thanks for great suggestions, will return this small 1,25' and try 2', I have Badeer T2 to 2" nose, can use 2' easilly.

To be honest, Im very happy with quality of Alan G image but fitting it on the field is kind a nightmare.. :)

Additionally: just want try more aggressive reduction by stacking reducers, because the bigger distance on AG made "comets" from the stars.

It need more trial&error tests..

Kuba

#18 GlennLeDrew

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Posted 07 April 2016 - 04:04 PM

ccs_hello,

Aperture alone is not the fix. If, say, the central 80% of a 25mm f/4 reducer is used when the reduction factor is 0.5X, just about the same central 80% is used for a 50mm f/4 reducer at 0.5X. And so the way to avoid potential edge issues (such as turned edge due to poor and aggressive lens polishing) is to use a reducer of faster f/ratio. Then at given reduction factor a smaller portion of the reducer aperture is used.

 

Jim,

Any lens is designed to work properly for a defined pair of conjugates. For example, imaging objectives (including these generic 0.5X reducers) are designed with one conjugate at infinity and the other necessarily at 1 focal length. Transfer lenses, as used in copying systems, might have both conjugates at 2 focal lengths (as for a 1:1 system). Between these two extremes are the range of design possibilities for the imaging of real world objects.

 

For best performance, the conjugates should match the object and focus distances. If the object and focus do not match the conjugates, the lens should at least be oriented such that its conjugates are in the correct sense, and not 'crossed.' Clearly, these 2" reducers were mounted so that the conjugates were crossed.

 

As can be appreciated, a reducer works in a somewhat 'unique' environment in one respect. Its conjugates have one located 'beyond infinity', due to the convergent light entering it, and the other at less than 1 focal length. This is why a 'standard' lens having infinite/1 f.l. conjugates introduces spherical aberration. The reducer ideally should be designed for the 'beyond infinity'/<1 f.l. conjugates, which naturally is not for the imaging of real objects *by itself* (but which the common reducer lens is designed to do.)

 

Reversing a lens having infinite/1 f.l. conjugates, when used as a reducer, is even worse than when reversing it when used to image a scene at optical infinity. The crossed conjugates are even more divergent.


Edited by GlennLeDrew, 07 April 2016 - 04:15 PM.



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