Fibre-optic star field for scope alignment

[symmetal]

Clear tonight at the moment so here's an image which should hopefully agree with the test image above but on inspection it doesnt really. 😬 The actual image corner stars aren't as bad as the artificial star test showed and the smearing is generally in the opposite direction. Well that was a lot of money wasted. 😁

Alan

[The Admiral]

12 hours ago, symmetal said:

An easy way to get the panel orthogonal is needed for tilt adjustments. 

If you were to place a mirror in the centre of the board, then if things were orthogonal then what you see in the scope should just be the scope's objective in the centre, reflected from the mirror. I think that's how it would work. It's a technique used in photography to ensure that the camera sensor is parallel to a document you want to copy.

So your final image is an actual star field? Hmm, I find it difficult to compare but the false stars did exhibit smearing on the left and in the same direction, akin to the real star field. On that point, am I correct in that the smearing on the left isn't radial, but is in the same direction  to at least half way up the frame? Could there be something else going on, like tilt, giving those results, I'm not experienced enough to know these things?

It was an experiment worth embarking on. Where do you go from here?

Ian

Edited September 1, 2020 by The Admiral

[symmetal]

7 hours ago, The Admiral said:

If you were to place a mirror in the centre of the board, then if things were orthogonal then what you see in the scope should just be the scope's objective in the centre, reflected from the mirror. I think that's how it would work. It's a technique used in photography to ensure that the camera sensor is parallel to a document you want to copy.

That's a good idea Ian. I'd need two mirrors to go either side of the central star adapter or remove the centre star. As you'll only see the objective front and not the sides of the scope (unless it's a long way off) a ring of card or similar mounted further back along the dew shield should assist in getting it spot on.

I was semi-joking when I said it was money wasted as there is some correlation between the results. Real images through the atmosphere don't give sharp edges to stars while the board test images can do. On the board test a peak white pixel can sit next to a black pixel while there would be a transition over several pixels with real stars. Perhaps low pass filtering the board images (blurring) may give a better representation, though real star images are blurred before they hit the sensor. Maybe an anti-alias/low pass filter is needed in front of the scope for the board images to better imitate real stars. 🙂 Getting complicated. Or dim the board stars a lot more so that several second exposures are needed and have a heater in front the stars to atmospherically blur them. 😀

The indoor test with the wider FOV scope gave more promising (realistic) looking results so I'm not sure what's different here.

I did increase the FF spacing by a quarter turn on the adjustment (0.2mm) and it was better in the corners compared to the first real star image above but then the cloud came so I had to stop.

Alan

[Davey-T]

Still trying to get the Sharpstar HNT collimated, set it up indoors then tried outside last night but it was way out.

Thought it was a good idea to set it up with all ZWO stuff controlled by an ASIAIR Pro and iPad which proved a right PITA  as it insisted on latching onto the home network  rather then the ASIAIR hotspot, it's WiFi range is pants, supposed to be 20 meters bur seemed more like 5, was using ZWOASI2600mc but the ASIAIR image download is mind numbingly slow so going to use DSLR next time.

As far as I can see there is no way to get the mirrors aligned without rotating the secondary which means it's no longer aligned with the focuser, appears that either the spider or the focuser is drilled in the wrong place, wouldn't surprise me after the way the finder bracket was drilled way out of alignment.

Dave

Edited September 13, 2020 by Davey-T

[symmetal]

Having had a few clearish nights recently I had another go at sorting the spacing/tilt on the ASI6200 and think I've got it as good as it's going to be. Using auto focus it always gave best FWHM results top left and the centre was always softer and getting worse across the frame, due to tilt. I gave a big turn on each of the tilt adjuster screws individually just to see the result and was surprised that they had little to no effect.

I reset the tilt adjuster and took a series of images increasing the FF spacing adjustment by half a turn (about 0.3mm) until I got the minimum difference between min and max FWHM using CCD Inspector and then tried the Tilt adjuster again and this time it worked and I could get the best FWHM at the centre. The FF spacing needs to be reduced a little after this as the tilt adjuster pushes the sensor further away.

Here's the final result from CCDI

Here it is after a meridian flip showing a slight focuser flop but not enough to be too concerned. The FWHM difference between top and bottom is very small.

Here's a single 240s L image taken after the meridian flip. Corners not perfect but I'll learn to live with it. 😄 Image scale is 1.26"/pixel.

If I move the focuser out by 5 steps the centre does get sharper with little change at the edges so that would be optimum but the SGP autofocus doesn't agree. It must be averaging the whole image rather than centre weighting.

This means I'm not pressured into getting my fibre optic star field project done. I bought some adhesive plastic mirror tiles to stick around the centre of the board to ensure orthoganality (if that's a word 😄) An ND filter can easily be fitted in front of the leds to dim them which should give more realistic stars I think and not be clipped at 0.01s exposure.

On 13/09/2020 at 14:47, Davey-T said:

Still trying to get the Sharpstar HNT collimated, set it up indoors then tried outside last night but it was way out.

Thought it was a good idea to set it up with all ZWO stuff controlled by an ASIAIR Pro and iPad which proved a right PITA  as it insisted on latching onto the home network  rather then the ASIAIR hotspot, it's WiFi range is pants, supposed to be 20 meters bur seemed more like 5, was using ZWOASI2600mc but the ASIAIR image download is mind numbingly slow so going to use DSLR next time.

As far as I can see there is no way to get the mirrors aligned without rotating the secondary which means it's no longer aligned with the focuser, appears that either the spider or the focuser is drilled in the wrong place, wouldn't surprise me after the way the finder bracket was drilled way out of alignment.

Dave

Pity to hear you're still having problems Dave and were hoping the fibre star field might help you. I'd be happy to let you borrow it to see how it works for you, but it's a bit large and fragile to transport. 😟

The ASI6200 takes about 8s to download at 1x1 (116MB) to my Celeron based mini PC on the mount. I have to drop the driver USB speed down to 70% to guarantee getting it. At 80% it will very occasionally hang during downloading if you're also doing something else. All the other cameras work flawlessly at 100% download setting. I use the ethernet link to my main PC as the Wi-Fi is slower and transferring a block of files over Wi-Fi freezes the remote desktop. No effect if using gigabit ethernet.

Alan

[Davey-T]

Took the primary  mirror off again and set the adjusters to the same height using micrometer, centred the secondary under the focuser and centred the laser on my white card with exact centre hole on rear, replaced the mirror and the laser was some way off centre showing that when mounted square to the end of the tube the primary isn't central. its got six screws holding it on so may try rotating it sometime to see if there's a better position for it.

Had another go outside last night after my latest collimation attempt and seems to be a better at least I had  stars not seagulls 😂

Going out again shortly, replaced the DSLR with ASI2600MC.

Dave