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Old 04-08-2016, 04:09 PM
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codemonkey (Lee)
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Quote:
Originally Posted by gregbradley View Post
I am assuming you are using your ASI1600 camera and a Skywatcher 120mm refractor.

How solid is the focuser? Does it move when you grab it and shove it a bit one way and then the other?

How is your camera connected to the filter wheel? The filter wheel to the focuser or any other item in between?
Thanks so much for your comprehensive post, Greg. Very much appreciate it. I'll do some further analysis to try and work out why I get different results over the course of the night... everything appears tight so I don't know why this is, yet. Great idea re the spark plug feeler gauges and thanks for the tips in diagnosing the tilt, they'll definitely come in handy.

This is indeed the Esprit and the ASI1600 that I'm having issues with.

I've got a Moonlite on the Esprit now. It does have some wiggle, but not a lot, and it requires a reasonable amount of force.

The camera is screwed directly into the filter wheel.

Quote:
Originally Posted by glend View Post
Lee, there are several type of T-thread tilt adjusters on the market, they work like the GSO focuser collination ring but are much smaller obviously. Teleskop-Express have a couple that would suit. With the 1600, the face plate is, well should be, flat and the sensor perpendicular. You might check to make sure the sensor is not raised on one corner, i have not heard of any mounting errors by ZWO but you never know. Good luck
Thanks Glen, fingers crossed it's not the sensor.
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  #22  
Old 26-11-2018, 05:34 PM
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SimmoW (SIMON)
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Epically good and useful reply Greg,
I wish this site stickied such useful posts!
Quote:
Originally Posted by gregbradley View Post
What you have here is a bad case of tilt. I have been doing a lot of work on tilt in my systems over the last year. That's exactly what it looks like.
In your case the tilt is severe.

Stars bottom right are perfect. Stars bottom left are quite good. Stars top right and top left are terrible. In particular the top right.

Tilt can also be flexure to some degree but in your case because it is so severe it must be the adapters, connections to the filter wheel perhaps the focuser itself tilts.

I am assuming you are using your ASI1600 camera and a Skywatcher 120mm refractor.

How solid is the focuser? Does it move when you grab it and shove it a bit one way and then the other?

How is your camera connected to the filter wheel? The filter wheel to the focuser or any other item in between?

With a small sensor you are getting a zoom effect so it multiplies your focal length. Like a crop digital camera. So the effect is magnified somewhat.

So you are chasing done the wrong thing. Its not spacing and its not guiding. Guiding errors gives elongation over the whole image not just parts of it. Spacing errors does the same. Tilt affects one corner or side compared to the other.

Here is my procedure for correcting tilt. Its similar really to a collimation error and you will see it in the image where it suddenly just looks sharper and snappier when you correct it.

1. You need to orient the image to the physical camera. So the top right corner of the computer image is which corner of the camera. I orient by referring to the camera from looking at it from behind it.

The way I ascertain this is to get a dim torch or put a tissue or rag over the end of the torch, set the camera to take a 15 second exposure which allows enough time to put the torch in front of the scope.

Now put the torch so its shining on the top half of the lens. Now look at what the image displays. Is it showing the top half of the image all white or the bottom half? Chances are it will be the bottom half. So now you know that the bottom of the computer image is actually the top half of the scope.

Do the same for left and right. Chances are they are also reversed.

So at the end of this step you now know for CERTAIN which corner of the image is which corner of the actual camera. In both of my mirrored scopes with correctors both top and bottom and left and right of the actual camera is the reverse of what is displayed on the monitor.

If you don't do this step well you will fail at correcting the tilt and it becomes hit and miss and its like doing a drift alignment when you forget which way you are supposed to adjust the mount for different drifts.

2. Now you know the orientation then get some brass shim material from a steel shop or a spark plug feeler gauge from an auto shop (that has little metal spacers in a set from very fine to a bit thicker.
Start with something like .10mm. Put it in the space between say your camera and the filter wheel and tighten the camera up so its locked in.

Now take another short exposure at 1x1 binning (2x2 tends to disguise the fine errors of tilt so always end with a 1x1 even if you start with 2x2).
Check the corner that you were attempting to correct. Is it better or worse or the same? It should be better (this assumes the worst corner needs to be packed out not pulled in).

You could add another step here which is to focus the camera so that worst corner improves and see if you had to rack focus in or out to improve it. Hopefully it was to rack it out. If you had to rack it in then you need to pack out the opposite corner right?

So simply repeat these steps until you have all 4 corners showing round stars with no elongation/distortions.

I could probably do it in 10 - 15 minutes as I have done it a lot lately.

You can also use CCDInspector to help but its not that intuitive as it doesn't really tell you which corner to pack out. But if you've done step 1 well and you know which corner of your camera is what corner of the monitor image then you can work it out from CCDinspectors graphic.

I bet you I could grab anyones setup here apart from a few and find it needs some minor tilt correction. I think its very common and easily mistaken for tracking errors. I made that error earlier this year and spent a lot of time chasing down "tracking errors" only to find that despite improving tracking I was not handling it.

Another clue is odd shaped stars like triangular but that probably only occurs on mirrored scopes.

What can make matters more complex though is if the problem is flexure and it only occurs or gets worse at certain angles then its going to be a difficult problem.

So really the first step is too make sure everything is strong and tight and nothing shifts easily when you try to physically rock or move your camera.
Watch the focuser - does it have some play.

The longer the focal length the more this comes into play. Also the faster the F ratio of the scope. Also the larger the sensor but it also affects small sensors as well due to the magnification factor.

Another possible source of tilt is the sensor is not set orthogonal in the camera. FLI features this as an item in their cameras that they guarantee orthogonality.

Greg.
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