Collimating your camera

[gregbradley]

I have been doing a lot of squaring up of my cameras with my RHA which really shows up poor squareness on the stars especially in the corners.

The difference between unsquared and square is very pronounced. Stars are way smaller, look more firey and the overall look to the image is much like the difference between poor seeing and great seeing.

I am now wondering how much of the sharpness are we giving away with slower scopes where this nonsquareness is less visible but still there.

So if you are spending a lot of time collimating your compound scope to get it really spot on spend some time also inspecting tilt in your camera. Not all sensors are laid dead level either.

It may sharpen up your images and get round stars to the corners. Some of that star elongation may not be tracking error at all but lack of squareness of your camera. Its caught me out a few times thinking I was getting tracking errors when it was errors in orthogonality.

Greg.

[peter_4059]

Greg. How are you determining whether the camera is square or not and how are you adjusting the tilt?

[Somnium]

it sounds like you have had a fun time of late, camera alignment issues, computer crashing, skyx pointing your scope to the core, i hope you get it all sorted out quickly :prey:

[LewisM]

Squaring my CCD is being a pain getting it JUST right. Real pain. It does make an ENORMOUS difference.

And now I have to try it all over again seeing I have a new scope... back to square (hopefully pun intended!) one.

[gregbradley]

I look at the 4 corners of an image and pack the corner that is off. A bit of trial and error. You can focus the out of focus corner and see if you have to move the focus in or out. That tells you which way to pack.

The image on the screen is the same orientation as looking at the camera from behind it. So top right of screen is top right of camera looking at it from behind.

I also use CCDinspector although that is more of a guide and it can vary but its still useful.

I made a little tilt adapter today. I already have an adapter plate that is wider than the camera so it sits flat on the filter wheel face. I drilled and tapped 4 holes and used a fine threaded smallish screw in each. Now I can simply loosen the camera in the filter wheel and quickly adjust the corner that needs to be adjusted. Then take another image and then measure it in CCDInspector.

Then a 5 or 10 min exposure and measure again. Longer exposures seem to show different results to a short focus type exposure.

Its a bit of a tedious process.

Greg.

[gregbradley]

Yes it has been a bit of a chore but I am getting on top of it. Tracking is good and I am imaging as well.

Greg.

[SpaceNoob]

I spent many hours trying to sort this out with the FSQ and 16803. The tilt tolerance is pretty tight on this damn thing. End result was 0.2mm in one corner and 0.1mm in the adjacent corner. The best way I found to dial in the error was to use a bahtinov mask as it clearly shows the inside focus and outside focus error and the degree to which it is out. I then shimmed it out with square bits of coke can (0.1mm thick I think). Very fast way to shim out the error with very little guesswork involved.

To determine axis of error, rotate the mask 90degrees ;-)
Shim it out and confirm each axis again by a final 90degree rotation.

[RobF]

Its a shame there aren't better adapter solutions available to help with shimming/adjusting squareness.

Do the more expensive cameras come with this sort of capability?
Spacenoob, where were you putting the aluminium slivers exactly?

I can get by at F/5 on the FSQ, but suspect the QHY9 isn't square enough at F/3.7, even with all screw-in attachments to the scope.

Has anyone had good experiences with any of these?
http://www.gerdneumann.net/english/astrofotografie-parts-astrophotography/ctu-camera-tilting-unit.html
https://www.myastroshop.com.au/products/details.asp?id=MAS-005K4
http://www.teleskop-express.de/shop/product_info.php/language/en/info/p4745_TS-T2-collimator---compensation-for-field-tipping-in-astrophoto.html

[SpaceNoob]

I'm not using a stock focuser on the FSQ, I just added a 3.5" feathertouch this week to solve sag issues. The shims were placed between the precise parts adapter I had made up and the MOAG. It worked well under first light. Round stars across the field.

[gregbradley]

That's clever Chris. So what was you procedure when using the Bahtinov Mask?

When you say it shows the axis off error and also in or out how do you determine those?

I haven't used a Bahtinov mask before.

I've been using CCDinspector and the pinky coloured areas and lighter blues show the area that is out of focus also the tilt direction. I found that the image on the screen translates to the same corner when looking at the camera from the back of it. So top right of screen is the top right of the back of the camera when standing behind it and looking at it.

Yes the amount needing to be packed is often very little. Something around .3mm would be a large adjustment. I also notice it seems to be one corner that is out on my Trius 694 so I suspect the sensor is not properly levelled when the camera was made but I could be wrong.

Greg.

[gregbradley]

[QUOTE=RobF;1202108]Its a shame there aren't better adapter solutions available to help with shimming/adjusting squareness.

Do the more expensive cameras come with this sort of capability?
Spacenoob, where were you putting the aluminium slivers exactly?

I can get by at F/5 on the FSQ, but suspect the QHY9 isn't square enough at F/3.7, even with all screw-in attachments to the scope.


I can almost guarantee you your camera will show tilt at F3.7. It can be a very small tilt and it will cause elongated stars in the corners or odd shaped stars and just a general look of lack of sharpness in one corner. I can find bright stars in the corners will look fine but the dim ones show up the error quite clearly.

My only comment about tilt/tip adapters is that they would need to be easy to adjust rather than remove them, adjust a couple of screws retighten and then retest. That would be tedious.

So something that can adjusted without removing things would be ideal.

Also I am not sure how those 3 point adjusters work. It seems to me that they need to be 4 points not 3 as how do you adjust for the top right corner of the side with only one adjuster? Perhaps you can but it seems like it lacks fine control.

Greg.

[RobF]

Agree totally about your points of being able to adjust while in the light-train and 4points versus 3.

I guess its a niche market, and likely would require at least 2 aperture sizes to cover a multitude of rigs.


Chris, just to understand better - I would have thought the MOAG connections were threaded and wouldn't allow you to get the slivers in?

[Paul Haese]

I could always do with a bit sharper images Greg. :P

Seriously though the SBIG camera I have came with shims installed and I have replicated this when adding adapters. My QSI does not need shimming at all given the size of the sensor. I have not noticed a huge problem but I imagine that large format cameras and very fast systems will show issues more than slower systems with 35mm sensors or smaller as you point out.

[SpaceNoob]

The MOAG uses dovetails and pinch screws. The threaded parts are on the adapters facing telescope or camera side.

I'll post more on the procedures re the bahtinov mask tonight, assuming I'm still somewhat capable of operating a keyboard.... Currently at a food festival and there's lots of craft beer here lol

[SpaceNoob]

Ok so regarding the Bahtinov mask and fixing tilt issues, I simply placed the mask over the FSQ objective side.



Perform a quick test to determine the direction that the diffraction spikes move when at intra and extra focal positions.
After I had an idea of what I was expecting to see, I did a rough focus for center of field, then exposed a full field with plenty of bright stars spread across the whole field.
Check the corners to see if there are any observable errors, if none, I simply rotated the mask by 90 degrees and repeated the same exposure. The second attempt showed the error at each corner. One side was inside of focus, the other outside. (You may need to expose longer to show the spikes).
Shim the corner with the error to bring the spike as central as possible and the opposing corner should also align.
Once central I rotate the mask by 90 degrees again to see if I have induced a slight error in the adjacent corners, there was a slight error so I shimmed it out a little bit.
Another quick exposure to check that the spikes are central
Followed by a final 90degree rotation to check the previously altered corners and they were fine.

Job done, 10minutes or so in total compared to the hours of messing around on the previous night. There are more precise ways to determine inside and outside of focus but the mask is far easier and states the obvious.

Now I just need a fat mask for the CDK lol.

I've attached an image I found which shows what you'll see depending on the error, note that your central field will show focused, while the varied error across the field will alternate between intra and extra focal errors....

[RobF]

Thanks for sharing Chris. Clever approach.

[gregbradley]

Its not necessarily only the camera sensor being out of square although I think that is the bulk of it. It may also be the optical train typical sag even if slight.

The Trius is smaller than your QSI and it was way off. But then I took off the collimating ring a while ago trying to adapt the camera to a reducer and MMOAG. So the squareness it came with from the factory was lost. Although I thought by simply getting the ring flat to the camera would have stopped that error but apparently the sensor is not 100% level.

The Proline seems a bit easier as the sensor is very square which is what FLI promote that the sensor is accurately levelled.

Also Paul you mentioned the flattener you had was off but I have found that in my case the camera not being square was creating errors in the stars that looked a lot like scope out of collimation or even tracking errors. So I think it may be a bit hard to differentiate between what is a scope optical out of collimation error and what is the sensor not being square to the optics error. I suspect they look the same.

Greg.

[gregbradley]

Thanks Chris for your procedure. I may try doing that.

I did a test tonight as I adjusted it again the night before and got lost and it took several hours of wasted effort. I realised I had a wrong assumption and that's what was causing me trouble. So I devised a simple test to check my assumptions before attempting to collimate it again:

1. Use a torch with a cloth over it and expose an image with the torch lighting up the top part of your objective. Note where this is on the image on the screen. In my case the light was on the other side of the image compared to where I illuminated the scope. So the bottom of my image on the computer screen was the top of the camera looking at it from the back.

2. Do the same for left and right. Again my camera was reverse to the image displayed on the computer. So looking at the back of the camera right hand side of the camera = the left hand side of the image on the computer.

Steps 1 and 2 get you oriented and without that you won't know which way to adjust and it will take ages and you'll get lost.

3. Take an image and inspect all 4 corners for star distortions. In my case tonight the 4 corners weren't too bad but top and bottom of the image were not in focus at the same time.

4 Pick the side of the image that is out of focus compared to the other side of the image. Now focus it and note if you had to go in to focus or out.

5. Note how much the focus had to be moved to get that side into focus. Now adjust or pack out that side. (if it had to be moved out to go into focus pack out that side if it had to go in to be in focus then pack out the opposite side).

6. Do this back and forward until both sides are in focus at the same time and stars are round in each corner. The image seems to pop when its close.

7. Do the same for left and right sides if they are not both in focus at the same time.

This is very like drift alignment where if you don't know which way you adjust the mount for the drift you are seeing you can get lost and make it worse. But get oriented and do it right and its fairly fast. Not 10 minutes like Chris's but maybe 30 minutes to an hour.

I thought I would do a write up as I posted earlier in error that the computer image taken was the same orientation of the camera when looking at it from the rear. In my case (Riccardi Honders, a corrector plate, 2 mirrors and a triplet corrector) the image was reversed both top and bottom and left and right). I am not sure what other scopes do to the image so best to test so you know for certain.

You can buy brass shim in various thicknesses from a steel supplier or use spark plug feeler gauges from a car parts place like SuperCheap Auto.

I also use CCDInspector to show how its going but I also find CCDInspector can show varying results from the same setup. So treat it as a guide but you should see all 4 corners stars looking good, the overall image pops when its close and top and bottom left and right both are in the same focus point. I would treat that as senior to CCDInspector but its still a good tool.

Greg.