10 replies to this topic

[VA3DSO]

I've got a little Sky-Watcher Mercury 705 and recently I took a critical eye to the star test and was pretty dismayed. The diffraction ring is more like a diffraction triangle (even with all the screws on the lens cell loosened to the point the objective could rattle around). Additionally there was kind of a smear in the diffraction ring on one of the triangle sides, and it followed the objective lens if I rotated it.

 

Upon further inspection, it looks like the little black separator pads between the lenses have migrated inwards and are now affecting the views. I tried to move them but they are stuck on there really well. Addtioinally the lens has a lot of little scratches on it - I suspect at one point I put it away and the lens cover fell off and some other gear was rubbing against it.

 

I decided to buy a relatively inexpensive 70mm doublet from China ($28 USD / $40 CAD) from a store on AliExpress called Sophisticated Optical Tools Store. They had a variety of achromatic doublets available and I bought the D72F500 lens set. They came well packaged and the lenses are indexed so you can line them up properly. The spacer is a thin clear plastic ring instead of the older style black pads. Actually it took two attempts for them to ship me the lens - the first time I got a bluetooth camera remote shutter release! But the store was very helpful and they couriered me the lens which I got today.

 

I popped it into my Sky-Watcher over lunch and had a quick look at a squirell nest across the street - I was actually pretty surprised with how sharp the view was. I compared back to back with my original lens, and I'd say the new lens is sharper and easier to find focus. The new lens appears to be 1 or 2 mm smaller in diameter than the original, and the height is the same - but the new lens set has a wider crown lens and thinner flint than the old set.

 

I'll see if I can find time to break out my artificial star tonight and do some A/B shots on what the Airy disc looks like in both. I'm hoping the new lens at least has round stars! As for coatings, I noticed the old lens has purple and red reflections of my window, and the new lens has the blue reflections, but instead of red, the reflection was white. That tells me some of the surfaces likely aren't coated - and it's just a single coating, where as the original appears to be fully coated on all surfaces (I saw no white reflections on the old lens). Please, someone correct me if that assumption is incorrect.

 

Here is the new lens in the Mercury - did this during my lunch break...

 

For the price, however, even without the same coatings, I will be very happy if I can have proper Airy discs in my little Mercury - that would be fab!

 

Clear skies!

 

Rick

[Polyphemos]

Nice report, Rick, and I’m looking forward to further comparisons and impressions. Thanks!

[John R.]

Yes. I want to follow this thread closely. Could produce some very interesting information. 

[Gonariu]

This is very interesting. After you have tried the telescope with the new doublet why don't you write a review? Personally I would like to buy a 120/1000 achromatic doublet because in my Konusuper 120 it is chipped laterally but on Aliexpress I have only seen 6" doublets.

[VA3DSO]

Okay, I had to last night and today to try out my new objective lens.

 

I was really hoping to get some pictures of the Airy disc and in + out of focus images, but none of my camera gear (or iPhone) wants to cooperate. 

 

With the original objective lens, the Airy disc and diffraction ring both had a very triangle nature to them - not round at all. it appeared the sides of the triangle coincided with the spacer pads between the crown and flint elements that look to have migrated inwards over time. Try as I might, I wasn't able to detach and relocate them without making things worse (although I may try again now that the replacement objective appears to be working well).

 

So, the new lens from China which cost me the princely sum of $28 looks to be way better than the original objective. The Airy disc and diffraction ring on it are ROUND!!! They look good - not as good as on my Celestron Omni AZ 102, but miles better than the original lens. When viewing terrestrial objects through my Plössl eyepieces, everything is sharp edge to edge. Nice crisp views. I would not hesitate to recommend this as a drop in replacement for a broken or damaged lens. Certainly worth the money (and hassle). 

 

Inside and outside of focus are not the same - but I'm not sure if that's just because I'm using an artificial star inside the house and I'm outside of the objective's optimal range. I had to use a draw tube extender to get it to focus with my eyepieces. I'm no expert, but when I compare it to Suiter's star test chart, it looks a lot like spherical aberration to me - but the images in my scope look much cleaner and better that Suiter's example:

 

 

What I mean by better is that both inside and outside have a bright outer ring, and the interior rings are brighter outside of focus than inside of focus, but the rings are all smooth and are the same vividness (I believe Suiter's example above is for an obstructed system, which explains the variance in brightness of the inner rings).

 

Chromatic aberration between the original and new lens seems pretty much the same (both are f/7.1). I won't know for sure that I have a winner until I get out under the stars, but my suspicion is that this new lens is going to perform fine - it's no APO, but it appears to be a decently functioning achromat for a very small price. I'd really like to back-to-back compare it with my Omni AZ 102 which I know has really good optics for an achromat (identical inside and outside of focus just like Suiter's last example but even better as everything is buttery smooth).

 

The new lens is 72mm in diameter, whereas the original was closer to 74mm. The original fits very snugly in the plastic lens cell - the new one rattles a bit. I shimmed the new one with some black Bristol board and that centered things and firmed everything up.

 

ORIG LENS DIAMETER:

 

NEW LENS DIAMETER:

 

The new lens was indexed to ensure the crown and flint are properly aligned - the original was not. The thicknesses are slightly different as well...

 

ORIG LENS EDGE:

 

NEW LENS EDGE:

 

Finally, the coatings don't appear to be as robust on the new lens - but I might be mis-interpreting what I'm seeing. When I look at the almost white reflection, it does appear slightly yellow/green to me, so it may in fact have coatings on all the surfaces...

 

ORIG LENS REFLECTIONS:

 

NEW LENS REFLECTIONS:

 

In summary, for the price I feel I got more than I could have hoped for. The focal length on the new lens is very close to the original, it focuses with all my eyepieces, the terrestrial views are sharp edge to edge, and the in focus star shows a proper round Airy disc and a complete diffraction ring. I really can't ask for more than that for $28!

 

I hope this helps someone who is on the fence whether to buy an inexpensive replacement or not. Likely these are the same quality of lenses that are going in the super-cheap refractors you see on AliExpress, Amazon and eBay. Still... better than no lens! 

 

Clear skies!

 

Rick

Edited by Rick-T137, 17 April 2025 - 02:47 PM.

[RichA]

I've got a little Sky-Watcher Mercury 705 and recently I took a critical eye to the star test and was pretty dismayed. The diffraction ring is more like a diffraction triangle (even with all the screws on the lens cell loosened to the point the objective could rattle around). Additionally there was kind of a smear in the diffraction ring on one of the triangle sides, and it followed the objective lens if I rotated it.

 

Upon further inspection, it looks like the little black separator pads between the lenses have migrated inwards and are now affecting the views. I tried to move them but they are stuck on there really well. Addtioinally the lens has a lot of little scratches on it - I suspect at one point I put it away and the lens cover fell off and some other gear was rubbing against it.

 

I decided to buy a relatively inexpensive 70mm doublet from China ($28 USD / $40 CAD) from a store on AliExpress called Sophisticated Optical Tools Store. They had a variety of achromatic doublets available and I bought the D72F500 lens set. They came well packaged and the lenses are indexed so you can line them up properly. The spacer is a thin clear plastic ring instead of the older style black pads. Actually it took two attempts for them to ship me the lens - the first time I got a bluetooth camera remote shutter release! But the store was very helpful and they couriered me the lens which I got today.

 

I popped it into my Sky-Watcher over lunch and had a quick look at a squirell nest across the street - I was actually pretty surprised with how sharp the view was. I compared back to back with my original lens, and I'd say the new lens is sharper and easier to find focus. The new lens appears to be 1 or 2 mm smaller in diameter than the original, and the height is the same - but the new lens set has a wider crown lens and thinner flint than the old set.

 

I'll see if I can find time to break out my artificial star tonight and do some A/B shots on what the Airy disc looks like in both. I'm hoping the new lens at least has round stars! As for coatings, I noticed the old lens has purple and red reflections of my window, and the new lens has the blue reflections, but instead of red, the reflection was white. That tells me some of the surfaces likely aren't coated - and it's just a single coating, where as the original appears to be fully coated on all surfaces (I saw no white reflections on the old lens). Please, someone correct me if that assumption is incorrect.

 

Here is the new lens in the Mercury - did this during my lunch break...

 

 

For the price, however, even without the same coatings, I will be very happy if I can have proper Airy discs in my little Mercury - that would be fab!

 

Clear skies!

 

Rick

I just got a $25.00 pocket spotter and it is as good or better (at the edge) than a Zeiss Miniquick plus 2x the aperture.

 

[John R.]

Thanks for the report on the replacement lens. Looking forward to first light. 

[Polyphemos]

Great report, Rick. Thanks!

[VA3DSO]

After work today I went in the backyard and had a peek at the Sun - the views looked absolutely fine. Everything was sharp and the scope easily came to focus - actually noticeably easier than before. Likely due to this new lens not having as many issues as the original.

 

Tonight is supposed to be clear at first, so I'll try and get a look at some stars (and other things) before the clouds roll in. I'm looking forward to it, as it would appear I have brought my little 70mm back from the brink!

[Gonariu]

 

Okay, I had to last night and today to try out my new objective lens.

 

I was really hoping to get some pictures of the Airy disc and in + out of focus images, but none of my camera gear (or iPhone) wants to cooperate. 

 

With the original objective lens, the Airy disc and diffraction ring both had a very triangle nature to them - not round at all. it appeared the sides of the triangle coincided with the spacer pads between the crown and flint elements that look to have migrated inwards over time. Try as I might, I wasn't able to detach and relocate them without making things worse (although I may try again now that the replacement objective appears to be working well).

 

So, the new lens from China which cost me the princely sum of $28 looks to be way better than the original objective. The Airy disc and diffraction ring on it are ROUND!!! They look good - not as good as on my Celestron Omni AZ 102, but miles better than the original lens. When viewing terrestrial objects through my Plössl eyepieces, everything is sharp edge to edge. Nice crisp views. I would not hesitate to recommend this as a drop in replacement for a broken or damaged lens. Certainly worth the money (and hassle). 

 

Inside and outside of focus are not the same - but I'm not sure if that's just because I'm using an artificial star inside the house and I'm outside of the objective's optimal range. I had to use a draw tube extender to get it to focus with my eyepieces. I'm no expert, but when I compare it to Suiter's star test chart, it looks a lot like spherical aberration to me - but the images in my scope look much cleaner and better that Suiter's example:

 

 

What I mean by better is that both inside and outside have a bright outer ring, and the interior rings are brighter outside of focus than inside of focus, but the rings are all smooth and are the same vividness (I believe Suiter's example above is for an obstructed system, which explains the variance in brightness of the inner rings).

 

Chromatic aberration between the original and new lens seems pretty much the same (both are f/7.1). I won't know for sure that I have a winner until I get out under the stars, but my suspicion is that this new lens is going to perform fine - it's no APO, but it appears to be a decently functioning achromat for a very small price. I'd really like to back-to-back compare it with my Omni AZ 102 which I know has really good optics for an achromat (identical inside and outside of focus just like Suiter's last example but even better as everything is buttery smooth).

 

The new lens is 72mm in diameter, whereas the original was closer to 74mm. The original fits very snugly in the plastic lens cell - the new one rattles a bit. I shimmed the new one with some black Bristol board and that centered things and firmed everything up.

 

ORIG LENS DIAMETER:

 

 

NEW LENS DIAMETER:

 

 

The new lens was indexed to ensure the crown and flint are properly aligned - the original was not. The thicknesses are slightly different as well...

 

ORIG LENS EDGE:

 

 

NEW LENS EDGE:

 

 

Finally, the coatings don't appear to be as robust on the new lens - but I might be mis-interpreting what I'm seeing. When I look at the almost white reflection, it does appear slightly yellow/green to me, so it may in fact have coatings on all the surfaces...

 

ORIG LENS REFLECTIONS:

 

 

NEW LENS REFLECTIONS:

 

 

In summary, for the price I feel I got more than I could have hoped for. The focal length on the new lens is very close to the original, it focuses with all my eyepieces, the terrestrial views are sharp edge to edge, and the in focus star shows a proper round Airy disc and a complete diffraction ring. I really can't ask for more than that for $28!

 

I hope this helps someone who is on the fence whether to buy an inexpensive replacement or not. Likely these are the same quality of lenses that are going in the super-cheap refractors you see on AliExpress, Amazon and eBay. Still... better than no lens! 

 

Clear skies!

 

Rick

 

Being a 70/500 achromatic I think the result is more than positive. I have the Celestron Travelscope 70, bought about ten years ago, one day it fell to the ground and the achromatic doublet chipped on the edge. I bought it again from someone in Italy who is quite good at fixing optics with problems and I must say that it is better, before the replacement, seeing Saturn too low you couldn't distinguish the rings, with the new doublet it is no longer like that and at 80X it gives an image of the ringed planet that is not bad for a 70/400 achromatic. As I said before, I would like to change the laterally chipped doublet of the Konusuper 120 with another new achromatic doublet, in this Easter holiday period (I am a teacher) I will look carefully on the internet to see if there is a 120 mm achromatic doublet. I will say that I have half an idea to transform it into an apochromatic, a 120/1000 ED doublet that I think has good correction of chromatic and spherical aberration.

[VA3DSO]

Before the clouds rolled in last night I did get to peek at Arcturus and I verified my previous star test (with the artificial star) that this new lens does have a bit of spherical aberration (but not as much as what I saw on the artificial star). In focus, I had a nice Airy disc enclosed in a diffraction ring (which tells me my collimation is okay).

 

I'm gonna call this a success. The optics in my Omni AZ 102 are better, so that scope will be staying long term, but now I have a working little grab 'n' go / travel scope for when I need it. I'm really happy it all worked out.

 

Clear skies!

 

Rick