Fibre-optic star field for scope alignment

[symmetal]

My previous attempt at using ball bearings to simulate stars over the full frame was a bit of a failure as the 'stars' reflections all reflected off each other creating a mess.

I recently splashed out on an ASI6200 with 7 Astrodon 2" filters, 😲 along with the rather costly WO 68III flattener to use with my FLT98 but am having trouble setting up the spacing. The spacing distance for the FLT98 isn't specified on the WO documentation. Discontinued scopes seem to be ignored in the info available.  I spent two nights playing a week ago, and while CCD Inspector gave a 'good' report after fiddling with a M48 tilt adjuster I put in the imaging train, the stars still look horrible towards the edges. Maybe the scope isn't suitable. 😬. Having no weather to test for the last week I decided to retry doing an artificial starfield.

I thought fibre optics might be worth investigating. I bought a 25m patch cable off ebay, twin core, 50/125. That's a fibre core of 50 microns diameter in a 125 micron sleeve. These cores are then wrapped in a coloured sheath followed by many thin kevlar strands and then sheathed overall in plastic. I thought I could cut it up into around 50cm lengths with one end fitted through a black 5mm foam board to simulate the stars and the other ends bunched together pointing to a white light source. I found that bare cable transmits light along the sleeve as well as the core giving a defined yellow halo surronding the white core. Properly terminated cables seem to eliminate the sleeve transmission as long as one of the ends is terminated. Here's the cables and the foam board.

I used my 1600 and ZS61 scope to test as it can focus down to 3m. Below is a crop of the focused image of the 'stars' at full size. My test high brightness white led light souce was still too bright at 0.001s exposure so they are a bit bloated. For this first test the three bare ends were pointing at the small led and the termination and bare end poked through the board. As the light source doesn't need to be that bright, my next test tonight is having the termination and bare end pointing to a Huion A4 led panel and the other bare ends poked through the board as in the photo above.

The halo around the bare cable is very noticeable. I just cut the cable with wire cutters, (the kevlar is hard to cut), and the glass cable end most likely has jagged edges causing the small spikes I imagine. I've ordered a set of fibre optic polishing discs (very fine 'grit' paper) to see if flattening and polishing the ends gives a cleaner star.

Here's the cropped full size image using a bahtinov mask which gives nice clear spikes on the terminated cables. The bare cable spikes are not as sharp.

The depth of field is about 5mm at the 3m board distance. I had to move one cable back about 10mm before the bahtinov spike showed a noticeable error. At a greater board distance the distance from the board edge to the sensor compared to the board centre should not be an issue, I would think.

I'd already ordered a set of single core 'pigtails' terminated at one end from ebay. If the Huion light souce works, the terminations can be arranged in a simple frame to point at the huion panel with the bare ends poked through the foam board. It may be cheaper getting 1m patch cables (2 core duplex) and cutting them in two to give 4 'star' cores. Most patch cords are duplex as in my first photo.

I'll update after some more tests. 🙂

[symmetal]

The Huion led panel was not bright enough as a light source. Exposures had to be over second and the background level was so high that there was little contrast with the stars. Bahtinov spikes were very indistinct. I turned the high power led I used yesterday down very low and put one fibre cable over a bright spot and could get it to around 30000 ADU at 0.002s. At 0.005s it was just clipped and I took three images, moving the mount to put the fibre star over three areas of the sensor.

The result does show some aberration in the corners, more in the top right, and examining an actual previous recent star image from the camera the top right does show slight star elongation toward the centre and the bottom right doesn't. More testing needed to see if the fibre stars do match real ones.

I tried putting the ASI6200MM and FLT98 on the board as that was what I really wanted to test but it was too close to focus. If I mount it on the HEQ5 and point it down the hallway it should hopefully focus but would probably be easier putting it outside to try.

I was going to make a brighter led panel but it's easier to get a led video light for around £40 which is dimmable so I've ordered one of those. It has a 12V power input too.

I had also ordered a 3m length of 64 core fibre optic light cable to try but it hasn't arrived yet. It's a lot cheaper than the glass fibre optic cable having solid plastic cores but at 0.25mm core diameter it will probably be too wide unless it's mounted a long way from the scope. Worth a try though. 😀

Alan

Edited July 19, 2020 by symmetal

[discardedastro]

13 hours ago, symmetal said:

I thought fibre optics might be worth investigating. I bought a 25m patch cable off ebay, twin core, 50/125. That's a fibre core of 50 microns diameter in a 125 micron sleeve.

What you want is single mode OS2 fibre - this will be 9/125, i.e. ~9um core in 125um cladding. The cladding is bonded to the fibre core - the protective sleeve (furcation tubing, for its proper name) over the top is optically separate. Singlemode is cheap as chips and easily gotten (e.g. https://www.fs.com/uk/products/96103.html ). You might also want to try an APC finish - this is angled 8 degrees at the connector to reduce reflections into the fibre from unterminated connections but might also help cut down on any light other than the primary mode.

The mode field diameter of most OS2 cables is ~8-9um at 1310nm (infrared) and will be in the same ballpark for visible spectra. You won't get points any smaller than this without external optics.

[symmetal]

3 hours ago, discardedastro said:

What you want is single mode OS2 fibre - this will be 9/125, i.e. ~9um core in 125um cladding. The cladding is bonded to the fibre core - the protective sleeve (furcation tubing, for its proper name) over the top is optically separate. Singlemode is cheap as chips and easily gotten (e.g. https://www.fs.com/uk/products/96103.html ). You might also want to try an APC finish - this is angled 8 degrees at the connector to reduce reflections into the fibre from unterminated connections but might also help cut down on any light other than the primary mode.

The mode field diameter of most OS2 cables is ~8-9um at 1310nm (infrared) and will be in the same ballpark for visible spectra. You won't get points any smaller than this without external optics.

Thanks for the information discardedastro. I saw that 9/125 and 50/125 were the common sizes and thought 50/125 may be a bit more robust when cutting it up so bought that. Single cables seem more common in 9/125 and the terminated end can be poked through the foam board to give a 'cleaner' star compared to the bare ends which can then be bunched together to point at the light source. I'll order some 9/125 single patch cables to try. 😀

Alan

[The Admiral]

I've thought about using artificial stars myself for adjusting the backfocus distance, but it's concerned me that under astro conditions the scope would be focused on infinity, whereas with an artificial star that's not so. With a shorter focal distance, the light cone entering the reducer or flattener will be more acute, and I've wondered if this might alter the optimum backfocus.

Any thoughts?

Ian

[symmetal]

54 minutes ago, The Admiral said:

I've thought about using artificial stars myself for adjusting the backfocus distance, but it's concerned me that under astro conditions the scope would be focused on infinity, whereas with an artificial star that's not so. With a shorter focal distance, the light cone entering the reducer or flattener will be more acute, and I've wondered if this might alter the optimum backfocus.

Any thoughts?

Ian

I raised the same question Ian, when I was trying my failed ball bearing design for an artificial star field. Flatteners sold to work with different scopes have different optimal spacing distances depending on the focal length. I tried graphing the specified spacing with focal length and spacing was less for longer focal lengths but it didn't seem to be a simple relationship. Some members with more optical knowhow than me 😀 said it probably wouldn't matter. Trying it for real is the only way to find out. 😉 At least you can use the artificial field to correct tilt errors which are normally more time consuming than straight distance.

Alan

[The Admiral]

58 minutes ago, symmetal said:

I raised the same question Ian

I wonder if we are talking about the same thing?

This is what I mean

If you focus on a real star the rays will converge at the focal length of the objective. On the other hand, if you are focusing on something not at infinity, the focus point is further away. For example, for a scope with a focal length of 700mm, if you focus on an artificial star 5 metres away, the image will now be at about 815mm from the objective, if my calculations are correct. So my concern is that if you adjust the backfocus to give round stars across the field, when using an artificial star only 5m away, is this also going to be correct for a real star? I would have thought that to the flattener it appears as though the scope has a focal length of 815mm, not 700mm, and therefore wouldn't be optimum. But may be I'm just talking out of my derrière! Certainly to avoid any potential issues it seems that the artificial star should be as far away as possible. That's one advantage of ball bearings in that because they are highly convex, the 'star' appears a lot further away than it actually is.

If anyone could confirm or otherwise my theory, please chip in!

Ian

[symmetal]

Yes, that's what I meant to agree with Ian, but didn't explain myself too well. 🙂 The back spacing depends on focal length but does focusing closer 'appear' as a change in focal length as far as the flattener is concerned.

I bought some A2 size foam boards and I worked out it should fill the FLT98 frame at around 17m which I hope is enough to make any 'apparant' focal length change not enough to matter. Testing is the only way. I've started so I may as well finish. 

Alan

Edited July 19, 2020 by symmetal

[Davey-T]

Be looking forward to your results Alan, I made the ball bearing version but didn't have any success with it indoors.

Dave

[discardedastro]

20 hours ago, symmetal said:

Thanks for the information discardedastro. I saw that 9/125 and 50/125 were the common sizes and thought 50/125 may be a bit more robust when cutting it up so bought that. Single cables seem more common in 9/125 and the terminated end can be poked through the foam board to give a 'cleaner' star compared to the bare ends which can then be bunched together to point at the light source. I'll order some 9/125 single patch cables to try. 

Since they both have 125um fibre diameters, they'll both be pretty robust - look for G.657.A1/A2/B3 fibre if you want robustness to bending, this is the stuff commonly used in FTTH networks and the like and will tolate bends as low as 25mm without any loss.

Pretty much all of these will have at least a 250um cladding around the outside. Removing this will help - you can do this easily with some 250/125um strippers like https://www.millsltd.com/tri-hole-miller-stripper-ripley-80677.html which are pretty inexpensive.

For shining light into the far end, I'd keep the far end terminated in a connector and get something like a "visual fault locator" - these will give you a red light, but will typically have a non-contact mounting flange for a 2.5mm ferrule as you'd find on an SC connector, e.g. https://www.ebay.co.uk/itm/10mW-10KM-Visual-Fault-Locator-Fiber-Optic-Laser-Cable-Tester-Test-Equipment/261476281179?epid=1374228066&hash=item3ce1336b5b:g:pjMAAOxyMjJTlov6 - alternatively, you can just dangle your SC/LC ends into a box with a good bright light source. You ideally want the fibres to be pointing at the bulb, so getting some SC or LC couplers might be helpful mechanically.

If you want to further constrict the exit aperture of the glass, I'm not sure how to go about that. Glass processors for fibre exist but are super expensive (e.g. https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_ID=11031 - and when Thor tells you to call for price, given they frequently list stuff in the £50-100k bracket, you know it's going to be cheap). We normally use fusion splicers (which are still expensive - halfway decent ones are 3-4k with a cleaver/stripper etc) to make joints in fibre. Down at 9um you're into proper microscopes to do anything much with manual manipulation.

Edit: Almost forgot, you might do well with a fibre cleaver. You can get cheap ones off eBay/Aliexpress. These will make, using a tension-and-score method, a very clean cut of the fibre (within 0.5 degrees of flat, typically). That'll do you a much cleaner edge than snapping or cutting with scissors (we typically use scissors/diamond snips if we want a bad endface - this is often beneficial to provide a low-reflectance termination (since there won't be a flat end to bounce the light down the core).

Edited July 20, 2020 by discardedastro

[symmetal]

Thanks again for the further detailed information discardedastro. 😀

5 hours ago, discardedastro said:

Pretty much all of these will have at least a 250um cladding around the outside. Removing this will help - you can do this easily with some 250/125um strippers like https://www.millsltd.com/tri-hole-miller-stripper-ripley-80677.html which are pretty inexpensive.

I presume this cladding is responsible for the halo around the bare end to bare end fibre image in the second picture of this thread as it is separated from the 50um light core by the 125um coating. Perhaps using the strippers you mentioned on the last few cm of the fible and bending the fibre end slightly so the cladding is behind the foam board and isn't in line of sight with the core will stop it showing.

If I can get away with bare ends to bare ends it'll be the cheapest option. 😁

Actual star images aren't sharp points of light due to the atmosphere, so a clean cut end to the fibre may not be such an issue in this application. The fibres may have brightness variations but that shouldn't matter. Whether it causes excessive scatter may be an issue. When the 9/125 patch cords arrive I'll experiment with different combinations of terminations and bare ends and see what works. 🙂

I'll buy a stripper and use (and ruin) a cheap pair of scissors to cut the cable at first. If necessary I can get a cleaver off ebay. There are some second hand cleavers available quite cheaply which should be fine for this use.

Alan

[inFINNity Deck]

If you wish to use a fibre optic indoors as an artificial star then the best way to go is by creating yourself a collimator from any other OTA of approximately the same or slightly larger diameter. You can read about it in an article I wrote about that, including some maths about the wavefront. The article is about a 12" collimator (made of a Newton and a 9μm fibre-optic cable), used for 10" CSTs and 11" RCs. It is written in Dutch, but should fairly well translate in a browser like Chrome.

Nicolàs

 

[symmetal]

3 hours ago, inFINNity Deck said:

If you wish to use a fibre optic indoors as an artificial star then the best way to go is by creating yourself a collimator from any other OTA of approximately the same or slightly larger diameter. You can read about it in an article I wrote about that, including some maths about the wavefront. The article is about a 12" collimator (made of a Newton and a 9μm fibre-optic cable), used for 10" CSTs and 11" RCs. It is written in Dutch, but should fairly well translate in a browser like Chrome.

Nicolàs

 

Thanks Nicolàs. A very informative article, Chrome translated it no problem 🙂. Your system would need the star moving to each corner for analysis, which would be a bit time consuming. I'm hoping to simulate a star field in one go which just gives a useable indication of the edge performance rather than an exact measurement as your system would. How realistic the simulation actually is, will be discovered in due course. 😀

Alan

[symmetal]

Fibre strippers arrived today and stripping the 250um cladding around the 125um core eliminated the yellow halo so unterminated cables looks to be a possibility. Plain cut cable facing the light and about 2cm of sheath, kevlar and cladding removed from the other end and poked through a pinhole in the foam board. The yellow halo is still there 2 cm from the tip but should be hidden behind the foam board. Even if still slightly visible it will be out of focus so shouldn't present a problem. I'll do a test with a scope tomorrow. The 9um fibre patch cables are on their way so I'll try them then. 🙂

Alan

Edited July 22, 2020 by symmetal

[fwm891]

Why didn't you continue with the ball bearings and paint all but a small area facing the scope to avoid multiple reflections?

[discardedastro]

16 hours ago, symmetal said:

Fibre strippers arrived today and stripping the 250um cladding around the 125um core eliminated the yellow halo so unterminated cables looks to be a possibility. Plain cut cable facing the light and about 2cm of sheath, kevlar and cladding removed from the other end and poked through a pinhole in the foam board. The yellow halo is still there 2 cm from the tip but should be hidden behind the foam board. Even if still slightly visible it will be out of focus so shouldn't present a problem. I'll do a test with a scope tomorrow. The 9um fibre patch cables are on their way so I'll try them then. 🙂

Sounds good. I was thinking on this some more and going for a fibre specifically targeting a low mode field diameter at visual wavelengths may yield an even smaller point source, e.g. https://www.thorlabs.com/thorproduct.cfm?partnumber=SM450 is £5/m and has a MFD of about 3um from 488 - 633 nm. The 9um stuff (standard telecoms fare) is going to be around 8.6-9.2um at those wavelengths but I've no idea what the behaviour would be at visible wavelengths. Sadly I don't have MFD measurement kit (it's expensive stuff and we generally just push that onto the manufacturers to specify or pay NPL to check it).

[PeterW]

Nice SC patchcord, single mode would be better, but getting light in will be a royal pain due to the tiny area. The core and cladding are drawn out together in one go, single mode will be operating multi mode in the visible (unless you find visible single mode fibre which has an even smaller core!). There is a plastic jacket around the glass fibre for protection and then the Kevlar and outer plastic tube. I used to get through loads of them, used to lop then in half so I could add connectors to custom fibre devices. (Need a proper cutter and auto splicing machine).

I wouldn’t worry about APC, you are not working single mode, at high power or with a coherent light source. Neither 9 or 50micron fibres are any different mechanically, depends what you are trying to use them for.

The laser glass processor will cost you a big pile, but heating the glass fibre in a suitably hot flame and pulling to neck it down and then score and snap it could give you a very thin result, but you’d have to be very careful with little glass sharps! There are smaller core fibres available.

I used to use these at work with a focused halogen bulb and a micro positioner   and amount of light launched was not a lot of nothing!

Having been stabbed by bits of fibre I would advise leaving the patch cords intact, no hazard and the ferrule end is easier to use than bare fibre, you could build a jig to hold one ferrule in contact with a high power LED.

If you are set on cutting then decent scissors will be needed for the Kevlar, a special coating stripper (bit like a wire stripper) to get the polymer off and then a specialist cleaver to cut the exposed fibre, as mentioned be very careful with any offcuts, you will need a sharps bin at a minimum. MFD measurement, one for Rob! 😉

(No one has mentioned numerical aperture yet, but I doubt Anyrhing we’ve mentioned will be affected)

You can buy precision apertures which have laser drilled holes in this size range, just pop them over a light source. The drawback is that the hole is in a thin (fragile) metal foil. There is always This https://www.firstlightoptics.com/other-collimation-tools/hubble-optics-5-star-artificial-star.html though larger than some things discussed here.

 

Peter

PS if you do buy a pile of stuff from Thorlabs they might throw in some “Lab snacks”!

[symmetal]

With the halo causing 250um cladding removed the fibre left is just too fragile to be left poked through the foam board on its own. It'll snap far too easily. If you lift the board with the fibres attached a lot of them will likely snap off. My next thought is to strip off just around 5mm of furcation tubing and 250um cladding from the end and push and glue the furcation tubing through the board until the tubing is flush with the front leaving the 5mm fibre core sticking out. A dab of black paint or similar around the base should then block the cladding halo. Just remember not to touch or place anything on the board or you'll break off your stars. If it does work a second sheet of 5mm foam board could be fixed to the front with wide holes where the fibres are to protect them but will need careful placing. 

Alternatively leave one end terminated on each fibre to avoid the halo. If you don't need too many stars this may be the easiest option and won't cost much more. Update to follow. 😀

49 minutes ago, discardedastro said:

Sounds good. I was thinking on this some more and going for a fibre specifically targeting a low mode field diameter at visual wavelengths may yield an even smaller point source, e.g. https://www.thorlabs.com/thorproduct.cfm?partnumber=SM450 is £5/m and has a MFD of about 3um from 488 - 633 nm. The 9um stuff (standard telecoms fare) is going to be around 8.6-9.2um at those wavelengths but I've no idea what the behaviour would be at visible wavelengths. Sadly I don't have MFD measurement kit (it's expensive stuff and we generally just push that onto the manufacturers to specify or pay NPL to check it).

Thanks for the extra thinking and info discardedastro. The 9um patch cables from FS are arriving tomorrow so we'll see how they fare.

1 hour ago, fwm891 said:

Why didn't you continue with the ball bearings and paint all but a small area facing the scope to avoid multiple reflections?

The reflected 'stars' were not all that small were not a uniform shape so didn't think they would be much use if the reflections were reduced.. I used about 200 5mm ball bearings stuck to an A2 board. Perhaps fewer larger ball bearings would have been better but the 5mm ones were very cheap. I did think of spraying the whole lot black and wiping a cloth dipped in white spirit over the top to reveal just the tip of the bearing but never tried it. 

I don't know what size bearings Davey-T used but his didn't work either.

Alan

[symmetal]

30 minutes ago, PeterW said:

Nice SC patchcord, single mode would be better, but getting light in will be a royal pain due to the tiny area. The core and cladding are drawn out together in one go, single mode will be operating multi mode in the visible (unless you find visible single mode fibre which has an even smaller core!). There is a plastic jacket around the glass fibre for protection and then the Kevlar and outer plastic tube. I used to get through loads of them, used to lop then in half so I could add connectors to custom fibre devices. (Need a proper cutter and auto splicing machine).

I wouldn’t worry about APC, you are not working single mode, at high power or with a coherent light source. Neither 9 or 50micron fibres are any different mechanically, depends what you are trying to use them for.

The laser glass processor will cost you a big pile, but heating the glass fibre in a suitably hot flame and pulling to neck it down and then score and snap it could give you a very thin result, but you’d have to be very careful with little glass sharps! There are smaller core fibres available.

I used to use these at work with a focused halogen bulb and a micro positioner   and amount of light launched was not a lot of nothing!

Having been stabbed by bits of fibre I would advise leaving the patch cords intact, no hazard and the ferrule end is easier to use than bare fibre, you could build a jig to hold one ferrule in contact with a high power LED.

If you are set on cutting then decent scissors will be needed for the Kevlar, a special coating stripper (bit like a wire stripper) to get the polymer off and then a specialist cleaver to cut the exposed fibre, as mentioned be very careful with any offcuts, you will need a sharps bin at a minimum. MFD measurement, one for Rob! 😉

(No one has mentioned numerical aperture yet, but I doubt Anyrhing we’ve mentioned will be affected)

You can buy precision apertures which have laser drilled holes in this size range, just pop them over a light source. The drawback is that the hole is in a thin (fragile) metal foil. There is always This https://www.firstlightoptics.com/other-collimation-tools/hubble-optics-5-star-artificial-star.html though larger than some things discussed here.

 

Peter

PS if you do buy a pile of stuff from Thorlabs they might throw in some “Lab snacks”!

Thanks Peter for your extra insight. 🙂 I have the proper strippers, and have used cheap scissors to cut the cable. The kevlar is tough to cut though as you say. A cleaver may be worth getting if the ordinary cut ends cause a problem. The bare fibre cores which snapped off landed on the carpet somewhere so I'll have to be more careful in future, though I haven't stabbed myself yet.

I do have a hubble optics artificial star and the smallest hole is quite a bit larger than the fibre cable. Sticking 5 hubble optics to a board was one possibility. 😀

Alan

Edited July 22, 2020 by symmetal

[discardedastro]

51 minutes ago, symmetal said:

Thanks Peter for your extra insight. 🙂 I have the proper strippers, and have used cheap scissors to cut the cable. The kevlar is tough to cut though as you say. A cleaver may be worth getting if the ordinary cut ends cause a problem. The bare fibre cores which snapped off landed on the carpet somewhere so I'll have to be more careful in future, though I haven't stabbed myself yet.

I do have a hubble optics artificial star and the smallest hole is quite a bit larger than the fibre cable. Sticking 5 hubble optics to a board was one possibility. 😀

Alan

Getting a sharps bin is a fairly important thing if you're cutting fibre and leaving shards - not a good thing to have in your carpet, especially if you have pets/kids or might sell someday to someone who does...

Kevlar snips are a thing e.g. https://www.millsltd.com/mills-kevlar-scissors.html

My last Thor Labs order was five figures (optical breadboard, frame, etc, plus some piles of fibre and tools) and no lab snacks contained within. Gutted.

I'd definitely stick to ferrules/connectors for the launch end - if nothing else you're guaranteed a fairly flat end which will accept light from free space a bit more readily.

[PeterW]

You’ll know it when you find a sharp... hurt a lot more than you expect it to. you might want to protect the star end by adding one epoxy round the stripped bits (leaving the extreme ends sticking clear), extra strength and will tend to spread out any cladding light you may have.

My colleagues and I never worked out a guaranteed way to get Thorlabs Lab Snacks.. it’s only the box you want... some of the snacks aren’t much good 😉

Peter

[symmetal]

9um fibre patch leads arrived today.

The 50um fibres work easiest with the cable cut in half and the board end stripped down to the 1mm approx. furcation sleeve and poked through the holes in the board. The termination is left on the light box end to avoid the 250um cladding halo. Stripping off the 250um cladding makes them far to fragile. I broke off the two ends I had glued into the board with 5mm of 125um fibre exposed, so the test with them doesn't mean much.

9um fibres need more work done if you cut them. I cut 1 patch cord in two and neither half let light out the cut end. I had to strip it down to the 250um cladding and wipe the end a few times over the polishing sheet to get them to work. They also have to be fixed more accurately perpendicular to the board surface or the light output is reduced. The uncut patch cords are more tolerant of the angle they're mounted. For 9um cables I would just use the cables uncut with a termination at both ends. Fixing the termination to the board is a bit more tricky. I drilled a 9mm hole in the board, (real messy hole) and poked the termination through for the test. It may be preferable to use FC terminated patch cords rather than the SC terminated ones I bought as they have a longer exposed ferrule at the end so the whole termination doesn't have to pierce the board.

Setup on the cooker. 😁 I used the video light panel though a brighter source would be better as while it was OK for the test, getting a good exposure for the bahtinov test was more tricky for the 9um.

Cropped image of the board in frame centre. The 9um terminated both ends gave a clear star, the cut ones a bit variable. The 2 50um ends that broke off gave unreliable results. The 50um plain cut end is a bigger star of course. Image stretched to reveal detail. None of the stars were clipped from the camera.

9um corner test. Stars show coma like distortion towards the centre. An actual recent star image shown after reveals no such features. This could be due to the incident light angle to the flattener being not parallel as 'The Admiral' was concerned about earlier leading to incorrect spacing for the 'apparant' focal length the flattener is seeing.

50um corner test. Very similar results to 9um. So a smaller star is not actually necessary to show aberrations. It could be that the 50um star being over a larger area than the 9um has a useful effect.

Actual recent star image. Slight elongation top right but no coma like problems. (ASI 1600 on ZS61 used for the tests.)

It appears that the board has to be quite a distance away to hopefully avoid the incident light angle problem. That's the next test then. I've got a number of 50um 'pigtails' arriving shortly so I could make two boards with different star sizes on each, or one board with different sizes like a 'real' star field. 🙂

Alan

Edited July 24, 2020 by symmetal

[symmetal]

Been giving more thought to how the cables can be fixed securely and accurately at each end and thought of using these LC - ST duplex 1m cables. The ST ends can be split apart enough to form two stars as the connectors are separate.

The light box can have one or more of these LC panels attached to attach the LC ends so that the other empty end points toward the light. They seem very low priced. It implies the panel does come with the adapters fitted and you don't have to buy them separately. Link to page

The star board can have these ST couplers fitted to form the stars. A circular hole makes it easy to attach and should ensure the cable is at 90 degrees to the board. The star should shine out the end nicely I assume and not be cut off from the scope as long as it's far enough away.

 

If discardedastro or PeterW are looking in can they say as to whether this looks a good solution before I order the bits. 😀 Might be able to order enough for free postage. 😁 It had better work after all this as long as the star board is far enough away. 😬

It also means the cables I've already bought, strippers and polishing kit aren't now needed. 😒

Alan

 

Edited July 24, 2020 by symmetal

[discardedastro]

11 hours ago, symmetal said:

 

If discardedastro or PeterW are looking in can they say as to whether this looks a good solution before I order the bits. 😀 Might be able to order enough for free postage. 😁 It had better work after all this as long as the star board is far enough away. 😬

It also means the cables I've already bought, strippers and polishing kit aren't now needed. 

That's an excellent set of result images you've got there!

The ST connectors are a bit oldschool but should work great for that application - as you say, the round hole is easy to make and you get a free clamp. The other option I'd consider is LC-LC alignment sleeves with flanges - these can be secured with wire/bolts but are a rectangular cut-out which is a bit trickier.

The polishing materials you've got there are meant to be used with a connector - the only way to make a "clean" end on a bare fibre is score-and-tension cleaving. You also need an alignment puck to hold the connector flat on the polishing sheet (which should be on something nice and flat - surface plate in a lab, piece of flat glass is the common field option). I would well imagine that on your 9um bare you'll not have a clean end. If you have a cheap USB microscope that'll actually do alright at showing the end.

This is a SC connector under a microscope (a Nikon SMZ18 at about 8x mag, if memory serves) with a single mode fibre. You can just about see the core in the middle of the ferrule (a tiny dark spot inside the dark grey spot in the middle).

This is what's in the back of a particular LC ferrule - this one's a bit odd (and failed) but from right to left is the outer jacket, an acetal binder, the 250um cladding, 125um coated fibre.

[The Admiral]

I've been trying to follow this but optical fibre technology is well outside my knowledge base. When it's all sorted and working I'd appreciate a summary of the setup, with a DIY parts list, please. 

I imagine that diffraction of the light coming out of the fibre end will enable the light to still be visible slightly off axis, so that the whole star field will be visible.

One other question, could you check and adjust the back-spacing with the artificial stars in daylight?

Ian

Edited July 25, 2020 by The Admiral