'Artificial Star Field' to Help Adjust Sensor Tilt

[symmetal]

I've been having trouble with correcting sensor tilt on my ZS61 + FF + ASI1600MM. I thought it was due to the manual camera rotator which comes with the FF or the Baader variable T2 spacer. It's a lengthy job making adjustments putting it all back together slewing to a good star field, taking a few exposures, running them through CCD Inspector and repeating. The tilt seems to be random and doesn't follow any logic to the adjustments I thought I was making. The weather has been very poor for the past few weeks and I've managed just a few tests between the clouds in that time with no improvement.

I was wondering if it's feasible to make a back lit board with some foil sheets in front covered in tiny pin holes to simulate a star field. At the FOV of 2 degrees the board would have to be about 2m across at a distance of 30m. I don't know whether the field flattener would perform correctly with non-parallel rays entering so it might cause field curvature, but I though the tilt error would still show the same using this method. It would be much easier having the scope on a bench looking at the board at the end of the garden and could be done during the numerous dull cloudy days.

Does anyone have any views on whether this is a viable method.  CCD Inspector doesn't require pin point stars, just ones it can measure the 'roundness' on.

Alan

[AKB]

I’ve been wondering the same for adjusting my Hyperstar.  I think that for a wide FOV it might be better to use ball bearings stuck onto a black background and illuminated by a single offset light source. 

Haven't tried it yet!

[symmetal]

6 hours ago, AKB said:

I think that for a wide FOV it might be better to use ball bearings stuck onto a black background and illuminated by a single offset light source. 

Good idea, that seems a more practical solution.   Cover the board with black silicone sealant and pour a bucket of small ball bearings over it. Spread them out and before the sealant is fully dried gently go over the surface with a cloth dampened with white spirit to remove any sealant on the top surface of the balls. It would be nice and rigid with a thick backing board, and as a bonus, when you're not using it to test your setup you could hang it on the living room wall as an art piece. 

Also you'd only need to cover spot sections of the board and not the whole surface to get a usable result but it might not look so good on the living room wall then.  You could then use double sided black foam tape to stick the balls to the board.

Alan

Edited December 10, 2017 by symmetal

[newbie alert]

Think you can do it with flat files..i try to find the video I was watching..he was adjusting the focuser..

 

Edited December 10, 2017 by newbie alert
Added info

[symmetal]

Thanks newbie alert, but the video is using flat frames to centre the sensor whereas my sensor is tilted causing varying focus across the frame. Here's some examples using the 3D view in CCD Inspector as I was adjusting sensor distance to remove curvature by altering the Baader spacer and using the camera rotator to put the camera in the same orientation each time.

The curvature is not bad in the last one but the tilt is awful and unpredictable, with everything done up tight.

Here's a prototype diagram of the artificial star field at 1000 x 750mm size. This is OK at 20m for the ZS61 and ASI1600 (my initial calc was a bit off). I've ordered 500 5mm chrome ball bearings off ebay for £16.20 which seems a good deal and a large tube of black sealant to spot stick them down. The diagram needs 625 ball bearings but the 500 should tell me if it actually works and is worth pursuing.  My initial thought of flood filling it with ball bearings would require 30,000 bearings. 

Does anyone know if the field flattener will still produce a flat field if not focused at infinity so that I could use the star field to correct curvature as well as tilt?

Alan

 

Edited December 10, 2017 by symmetal

[Stub Mandrel]

I have a very old Universal Lens Testing Chart, now stuck to a sheet of ply. It has concentric circles, patterns of lines and radial patterns of different sizes all over the field and if photographed at the right distance you can find the resolution in lines per millimeter at several points across the frame. It would easily show up sensor tilt as reduced resolution.

I can't find my test photos... but google Universal Lens Testing Chart and you will find lots of images with similar charts and patterns you could print and paste onto board.

his is it;

But you could print some of these and stick them in the corners and centre of a board:

  

[Peter Drew]

The idea seems good in principle but at a distance of 30m how could you guarantee that the artificial array is square on to the camera? Stars at infinity presumably must be.  

[AKB]

3 hours ago, symmetal said:

Here's a prototype diagram of the artificial star field at 1000 x 750mm size

That seems like an awful lot of hard work?  I would have thought that a fairly random placement would work fine... it does for real stars.

 

[symmetal]

2 hours ago, Stub Mandrel said:

But you could print some of these and stick them in the corners and centre of a board:

  

Thanks 'Stub Mandrel' that would certainly work, but I was hoping to let CCD Inspector do the hard work by analysing the 'stars' reflected from the ball bearings located at the crosses in my diagram. 

46 minutes ago, Peter Drew said:

The idea seems good in principle but at a distance of 30m how could you guarantee that the artificial array is square on to the camera? Stars at infinity presumably must be.  

Thanks Peter, good point. My initial thoughts of a frame of perforated foil would certainly be tricky getting set up but the  thick board with the ball bearings stuck on suggested by AKB would hopefully be easier. A plumb line would do vertical and a non-stretch line for horizontal might work. I don't know what the distance accuracy is needed at that distance to still be within the depth of focus at the sensor.

 

37 minutes ago, AKB said:

That seems like an awful lot of hard work?  I would have thought that a fairly random placement would work fine... it does for real stars.

Random would work AKB, but I believe CCD Inspector needs a good sample of stars in an area to get a good value on the actual focus. The 25 in the 100x100mm squares may still be too far apart for this. Randomly sticking them on would require lots more ball bearings to get a good density and the cost would shoot up. Spending £20 or so to make this prototype will I hope show if the idea does at least work. It shouldn't take too long to make and I certainly have plenty of time at the moment.

main()

{ do

       { cloud(); rain(); wind();

       }

  while (moon != blue);

}  

Alan

Edited December 10, 2017 by symmetal

[symmetal]

Giving further thought, the centre of the board is closer to the scope than the corners so it will cause 'field curvature' of sorts. A curved board is too difficult to make. Tilt should still be visible though with the flat board.

Squareness of the board to the scope could be checked by mounting the scope on a tripod and panning to the corners and checking they are the same focus.

Alan

 

[Shibby]

On 10/12/2017 at 23:17, symmetal said:

I don't know what the distance accuracy is needed at that distance to still be within the depth of focus at the sensor.

Well over a metre, the DOF with your 'scope at 30m; you should be OK!

It's odd what you say about the tilt seeming random. Do you use the same star field between adjustments? If not, then there could still be something a bit loose affecting your results. Perhaps try moving around the sky and see if the tilt changes? (If you can experience enough clear sky!)

[symmetal]

Hi Shibby, yes I checked on a photography site and the DOF is stated as about 1m as you say. They use 'circles of confusion' as a limit for being in focus. I would have thought that a star would be seen as out of focus before the circle of confusion limit but the actual limit should hopefully be manageable.

I did use the same star field for each test centred on Shedar in Cassiopeia as there are a mass of Milky Way stars to use as a test, 3000 stars on a 10 second exposure. I have tried it before and after a meridian flip and the tilt was the same in both tests implying everything was tight and not flopping under gravity. I've measured with digital calipers around the rotator and the Baader spacer and there was a slight error in spacing around their circumference of about 0.2mm in some tests. I think all the errors in all the spacers are on the same side at the moment so they're all adding up.

The ball bearings arrived today so I can do a test indoors at about 10m to see if it works. The draw tube is only half way out at 10m so that shouldn't be too floppy when testing. Can't do it outside as pouring rain and 50mph winds for the next 24 hours.

Alan

[Shibby]

Sounds like you need to just keep on tweaking until you get the right result then. Let us know how it goes! If well, I'll be ordering some ball bearings

 

[nkondrashov]

Any update? Did the board work?

[symmetal]

Can't really say. I tried it indoors but there were too many reflections off the walls etc. to get a meaningful result. I tried it at the end of the garden but the scopes on the fixed pier couldn't get a view due to obstructions. I took the scopes off and had them on the ground looking at the board but it was such a pain trying to get it all lined up running back and forth many times, and the board blowing over in the wind, that I gave up. Getting a tilt adjuster for the scope let me adjust it reasonably quickly on real stars anyway so I never tried again with the board.

Here's the board I made. I used 5mm ball bearings as they were cheap for a few hundred but I think larger balls might give a more pronounced bright spot. The matt black paint I used was more satin black (should have used blackboard paint) so created 'hot spots' of its own with a torch shining on it and the overall brightness of the background was still fairly high, and I think the many reflections tended to 'muddy' the overall results. I believe using fewer larger ball bearings and real matt black paint would have given a better result. 

Alan

[david_taurus83]

I setup my laptop about 6m from my scope in the hallway one night and loaded a blank black screen on Autocad. I generated a hatched area with small dots to replicate stars. I twiddled and tweaked all night trying to get a perfect flat field. Looking at the actual field test  results I got on the second page below, I wish I'd left the bloody thing alone after that compared to what I'm currently getting in the corners!!! Its a road to madness! I'd rather image the Squid Nebula!

 

https://stargazerslounge.com/topic/325405-field-flatness-of-refractor/

[symmetal]

3 hours ago, david_taurus83 said:

I setup my laptop about 6m from my scope in the hallway one night and loaded a blank black screen on Autocad. I generated a hatched area with small dots to replicate stars. I twiddled and tweaked all night trying to get a perfect flat field. Looking at the actual field test  results I got on the second page below, I wish I'd left the bloody thing alone after that compared to what I'm currently getting in the corners!!! Its a road to madness! I'd rather image the Squid Nebula!

So your method with the dot pattern on an LCD screen seemed to give good results which is certainly easier than sticking ball bearings to a board. Less reflections to contend with too.

I found after adjusting the tilt adjuster with my Flt98 with flattener that I'd get good round stars all over on one night, then the next night or even the next target they would be slightly elongated in some corners. I found it was just due to slight focus differences. If SGP autofocus gave me funny corner stars I just shift the focus position out by 10 steps. Not enough to change the HFR of the stars by any significant amount (sometimes it would report as slightly better) but the stars would then be be round all over. I'm not sure what weighting (if any) SGP autofocus gives to best overall average HFR values compared to just best anywhere, but it often ends up with it being focused in one corner rather than nearer the centre.

Alan