21 replies to this topic

[sunrag]

Tonight is clear and not breezy, so I decided to try out my Surplus Shed Wollensak 127mm based refractor for the first time. I have been working on this for past 3 weeks and finally got it completed last week.

 

I aimed for the moon. Right away I noticed the first problem - I could not achieve focus until I had the focuser racked all the way in (I designed it to be at focus at the mid-range of the draw-tube). Even then the focus was not good. The moon was mushy and showed a distinct second outline. I removed the diagonal to give me more room to focus. When I finally attained focus, the image was quite poor. Although I could see small craters, it was not a sharp image and was very sensitive. If I defocussed by a just a 1/4 turn it was enough to go out of focus. I measured the focal length, and it came out to be 1140 mm, not the specified 1200 mm.

 

I know that my collimation is not perfect but this is my 5th telescope and I have gotten better at it. With all my previous telescopes of similar focal lengths I found the images of the moon very sharp and clearly defined outer edge.

 

Next, I looked at Arcturus. At my best focus, it looked like a bright small disc in the middle surrounded by a perfectly symmetrical and big halo. The halo covered 1/2 the FOV of my 26 mm ep. I have never seen a star image like this. I tried a few fainter stars and they also showed a halo but the halo was not very bright.

 

This was quite unexpected because I was expecting nice sharp stars as in my previous builds. So I am confused. Only explanation I could think of was that the Objective could have been incorrectly assembled like crown and flint elements flipped or even backwards. Another clue is that the measured focal length is 1140 mm instead of 1200 mm.

 

 

Thanks for reading and for any suggestions!

 

[Jim Davis]

I would contact Fred at Surplus Shed. He really knows his stuff. You may just have gotten a bad one. Or he may have an idea of what is wrong.

[UT_JimW]

Please post an update when you figure things out. I also have a Surplus Shed Wollensak 127mm 1200mmfl objective I was planning on getting around to building this summer [I keep resetting to 0 as I debate the tube material/cost/construction.] I know there was talk in the past about whether the elements were reversed or seemed reversed to what people were expecting. The elements are reversible in my cell. I have been meaning to look up how to tell the correct orientation of a achro doublet.

Good luck!

[wrnchhead]

I like SS stuff. I jave read that sometimes the elements come flipped. I bet a search would turn something up.

[TOMDEY]

It sounds like something is flipped. With just two elements, could just try various combos of front-back and flipping elements over... One of those other seven should deliver obviously great imagery. All others should stink... like the one you are experiencing now!    Tom

[MKV]

This was quite unexpected because I was expecting nice sharp stars as in my previous builds. So I am confused. Only explanation I could think of was that the Objective could have been incorrectly assembled like crown and flint elements flipped or even backwards. Another clue is that the measured focal length is 1140 mm instead of 1200 mm.

 

I was asked once to test a lens for someone and the reverse order of elements was one of the problems. That was obvious as soon as the lens was put in front to vertical patio blinds. The blinds were obviously bowed.

 

 

The second one was that the elements themselves were also reversed (!), and the third problem was a broken nylon spacing spacer ring with thick jagged edges. 

 

 

Reversing the lenses was easy, but the broken spacer was irreparable. To add insult to injury, the spacer ring itself was not of uniform thickness. This, plus the ragged edges of the ring cause the element to be wedged, resulting in on-axis coma.

 

 

I measured the lens radii of curvature, and reassembled the lenses in the correct order and orientation and replaced the ring with proper shims of equal thickness, and the lens turned out to be quite good in an autocollimatrion test using a 133 lpi Ronchi screen.

 

 

All things considered, not bad, especially considering the special-order price or less than $200!

 

Surplus Shed offered to "fix" the lens, so the lens was returned with a request to fix it (with warranty). Unfortunately, Surplus Shed dropped and shattered the lens during the disassembly. Of course, SS has excellent customer service, and offered to issue a full refund or, as a bonus, replace the lens at no extra cost with a 153 mm (6-inch) objective when the next shipment arrived.

 

The party whose lens was broken agreed to the second offer and obtained the 6-inch lens six months later, with a promise that the lens will be "checked" before shipping, but this lens was not as good as the 127 mm objective by any stretch

 

...continued below

[Benach]

I agree that something is probably flipped. Okay, try the following as a first attempt, checking the objective is arranged as this. If not, contact Surplusshed: the biconvex lens should be mounted the furthest away from the focal plane and the inner two radii are nearly confocal i.e.: similar radii but opposite sign.

If it is flipped, just replace the lenses like this and everything will come fine in a few hassles, if the lenses are already arranged like this, a lemon came through QC.

[MKV]

Continuation of the Post #6

 

The new 153 mm lens had unmistakable axial astigmatism, smack in the middle of the FOV. Unfortunately, I didn't have a chance to look for the cause. The owner decided to shelve and "think about it for a while." 

 

 

Interferometry shoed a highly astigmatic and oblate wavefront that looks almost like a Pringle chip!

 

 

The Ronchi test is so insensitive it almost doesn't seem that bad, but when used as a knife-edge (at focus), it's brutally honest that this lens was not good when it arrived as a replacement. 

 

 

Mladen

 

 

[MKV]

Anyone interested in reversing the lenses in their cells, this is how it should be done.

 

 

1. You'll need a stiff cardboard or plastic tube which woy nee to put upright on a flat surace (such as a kitchen table).
2. Cover the top with a trimmed napkin.
3. Remove the retaining ring form your lens cell and lift the lens cell verrtically over the cardboard/plastic tube. 
4. Lower the cell slowly on the tube, making sure the lens cell is centered on the tube.
5. Continue lowering the cell until it reaches the base, and leave it there. 
6. The lens will now rest on top of the tube.
7. Mark witness marks on the lens edges with a pencil.
8. Lift it vertically and flip it carefully, so the bottom lens is now on top.
9. Place it back on top of the tube, and make sure all witness marks coincide.
10. Now lift the cell from the bottom of the tube vertically and let it engage the lens. 
11. Continue lifting the cell with the lenses until they clear the tube.
12. Movie the lens cell to the side and put it down vertically on a flat surface.
13. Replace the retaining ring.

[Rutilus]

 

Next, I looked at Arcturus. At my best focus, it looked like a bright small disc in the middle surrounded by a perfectly symmetrical and big halo. The halo covered 1/2 the FOV of my 26 mm ep. I have never seen a star image like this. I tried a few fainter stars and they also showed a halo but the halo was not very bright.

 

This was quite unexpected because I was expecting nice sharp stars as in my previous builds. So I am confused. Only explanation I could think of was that the Objective could have been incorrectly assembled like crown and flint elements flipped or even backwards. Another clue is that the measured focal length is 1140 mm instead of 1200 mm.

 

 

Thanks for reading and for any suggestions!

IMG_8246.JPG

 

One of the lenses is flipped the wrong way around. A few years ago I did a test with several lenses to see how they looked with 

flipped lenses, this is just an extract from one of my test. I also had a very similar effect when the crown only was flipped as this too

produced a big halo around the airy disk.

 

"Focused  - Small white disc at centre, with several diffraction rings, the whole star is surrounded  by a circular Halo of diffuse light, the outer edge of the halo has a noticeable brighter ring.
Outward - The Halo does not disapear it simply gets larger, and there are no nocticeable
diffraction rings . The whole pattern is filled by a Lilac/ pale pink colouration.

Inward - The Halo disapears and there are strong diffraction rings from the centre ,
which get brighter as they move outwards. The outer diffraction ring has a noticeable
purple/red colour fringe to its outside edge".

Edited by Rutilus, 04 May 2020 - 08:32 AM.

[sunrag]

Thanks everyone for your very helpful suggestions!

 

One of the lenses is flipped the wrong way around. A few years ago I did a test with several lenses to see how they looked with 

flipped lenses, this is just an extract from one of my test. I also had a very similar effect when the crown only was flipped as this too

produced a big halo around the airy disk.

 

"Focused  - Small white disc at centre, with several diffraction rings, the whole star is surrounded  by a circular Halo of diffuse light, the outer edge of the halo has a noticeable brighter ring.
Outward - The Halo does not disapear it simply gets larger, and there are no nocticeable
diffraction rings . The whole pattern is filled by a Lilac/ pale pink colouration.

Inward - The Halo disapears and there are strong diffraction rings from the centre ,
which get brighter as they move outwards. The outer diffraction ring has a noticeable
purple/red colour fringe to its outside edge".

Rutilus, your description at focus accurately matches what I saw! When I moved the focuser inward or outward (I can't remember which), the halo disappeared and was replaced with very clear concentric rings. 

 

MKV, I now remember your post from before. Will definitely try the "vertical blind" test, but don't have any test equipment.

 

I will contact SS, but it appears that the 127mm/1200FL objectives are out-of-stock. I don't want a refund unless I find that the lenses themselves are of bad quality. So will try checking the assembly first.

[TOMDEY]

Continuation of the Post #6

 

The new 153 mm lens had unmistakable axial astigmatism, smack in the middle of the FOV. Unfortunately, I didn't have a chance to look for the cause. The owner decided to shelve and "think about it for a while." 

 

SS 153 F 7.8 Ach.jpg

 

Interferometry shoed a highly astigmatic and oblate wavefront that looks almost like a Pringle chip!

 

SAM_4640 (2)_LR.JPGastigmatism2.jpg

 

The Ronchi test is so insensitive it almost doesn't seem that bad, but when used as a knife-edge (at focus), it's brutally honest that this lens was not good when it arrived as a replacement. 

 

D 153 f1200 SS Ronchi_ke.jpg

 

Mladen

Can one element be clocked to (try to) minimize that? Well, can keep it as a textbook perfect example of axial astigmatism! And that output graphic sure shows why we call it "potato chip"!    Tom

[MKV]

Can one element be clocked to (try to) minimize that? Well, can keep it as a textbook perfect example of axial astigmatism! And that output graphic sure shows why we call it "potato chip"!  

Certainly it would have been on my to-do list, but the owner decided to "shelve" the project "for a while.". 

[MKV]

One of the lenses is flipped the wrong way around. A few years ago I did a test with several lenses to see how they looked with 

flipped lenses, this is just an extract from one of my test. I also had a very similar effect when the crown only was flipped as this too

produced a big halo around the airy disk.

The 127 mm I tested  with both lenses installed in reverse order and flipped produced these deformation in the Ronchi test. 

 

 

Normally, turning the cell aorund produces correct results, but in this case this is what came up on the other side

 

 

Makes you wonder who's doing quality control. How can they sell a lens like this?!?

Edited by MKV, 04 May 2020 - 12:29 PM.

[sunrag]

I measured the FL again carefully. It came out to 1160 mm. While doing the FL measurement, I noticed that the image was sharp but had a halo as well.

 

Then I took apart the cell/lens assembly. Sure enough, the crown was reversed. The stronger curvature (R2) was facing outside. I also noticed that the two alignment marks were not aligned at all. They were apart by almost 120 degrees. The retainer ring was loose, so it is possible that the two lenses rotated out of alignment.

 

Another issue I noticed was that the thin plastic separator was creased and bent in two places (probably got pinched sometime?).

 

I straightened the plastic separator as best as I could and reassembled the lenses this time making sure that R1 side was facing outside.

 

Then did a FL measurement again, and this time I got 1200mm +/- 5 mm. Also the halo disappeared! So maybe it is fixed now.

 

Will post another update after I use it on a clear night.

 

Thanks everyone !

[Benach]

Sunrag: can you please upload a photo of the objective in the cell? Makes all guessing so much easier.

[MKV]

These lenses are reversed for shipping because of the way the cell is designed. Otherwise the front lens protrudes beyond the cell and could get damaged in shipping. 

[sunrag]

Sunrag: can you please upload a photo of the objective in the cell? Makes all guessing so much easier.

Sure. Here are some pics.

This is after removing lenses from cell. The thicker lens on the top is the flint which faces the focuser. The thinner lens on the bottom is the crown and faces the sky. The thinner lens is backwards. Its sky facing surface (R1) is actually in the middle, facing the flint's R3 surface.

 

This is after I reversed the crown and put the lenses back in the cell. Front of the cell (sky-facing side) is shown.

 

This pic is of the cell looking from the back. Note the threads on the ID which are used to attach to the telescope tube. 

 

I don't have an aluminium tube with OD of 145mm, so I installed it inside a 152 mm ID Sonotube. Works better I think because the larger ID of the Sonotube lets you baffle near the objective.

[sunrag]

One more pic. This is the cell mounted inside a 152mm Sonotube.

 

 

[dave brock]

No, the engraved part is supposed to be facing the sky.

Which is what he said.

[MKV]

Which is what he said.

Duly noted.

[sunrag]

Just a quick note to update you all on the corrected lens. 

 

After I reversed the crown, I tried it out last night. The views were superb !! All stars looked like nice sharp points. I could see many binary stars.

 

Is Polaris's binary star easily visible at 40X? I thought I saw it last night.

 

Thanks again everyone for your help!