Detecting Tilt in Flats

[Atmos (Colin)]

I know I have some tilt in my image train at the moment, runs on a diagonal from top right to bottom left.
Someone with a bit more knowledge may be able to tell me whether tilt can be diagnosed from flat frames?

Ideally, I'd likely to be able to reduce it at home without having to waste dark clear skies Pretty much assume I'll need star tests but hoping flats and CCDInspector may fix the situation.

Has it been done before? Couldn't find anything in a Google search.

[tim.stephens (Tim)]

This might sound a little confusing but I have found on my RC8 that an off-centre flat, appropriately stretched and acquired can imply that you should look for tilt in your images. However, I find it generally misleading to use to collimate a telescope correctly - this should be done with a star field, collimation tool etc.... as you say. Following from that, the opposite can also be true - a perfectly centred flat can be obtained from a poorly collimated scope in the case of the RC8.

So, if the flat is not symmetrical, probably a good idea to look for tilt in that particular direction but if your star field is perfect, then don't bother and keep shooting. CCDinspector can be useful in this situation.

This can also be caused by imaging train/focuser tilt. This is much easier to see and fix first I would think...

Hope that helps!

Tim

Edit: I used to think the same about wasting good nights but I found it was worse to try to collimate without a star field (daytime) and then discover multiple nights were wasted by poor collimation. Better to dedicate a few nights upfront as 'collimation/bug fixing sessions' and then be happy for months after and produce high quality images consistently and easily.

[Atmos (Colin)]

Thanks for that info Tim, I've got a refractor so luckily I don't have to worry about collimation (unless I do something nasty!). The stars in the top left and bottom right are good, the ones in the top right and bottom left need some attending.

[peter_4059 (Peter)]

Might be worth checking if you get the same tilt when the camera is rotated 90 deg in the focuser. If so it would point to the tilt being in the CCD rather than the OTA/focuser. Does the QHY22 come with the tilt adjusting ring?

[Atmos (Colin)]

Originally Posted by peter_4059

Might be worth checking if you get the same tilt when the camera is rotated 90 deg in the focuser. If so it would point to the tilt being in the CCD rather than the OTA/focuser. Does the QHY22 come with the tilt adjusting ring?

That's something I'll check tonight, give it a rotate and see what happens. The QHY22 doesn't have any tilt adjustment on it but the focuser does so I can compensate that way. I place the camera on the same way every time so it would still end up being a one time adjustment.

[peter_4059 (Peter)]

You should check with Theo to see if the adjustment rings he has fit your camera. That would save messing with the focuser if the tilt is in the camera.

[gregbradley]

I doubt it. Its too vague. CCDInspector measures the stars. Handling tilt is a precise action and flats are too vague to tell you anything.

Here's my procedure for handling tilt as I have had to do it a lot over the last year and gotten some good results.

0. Make sure all your adapters are snug, that they are all the way in and not stuck with a gap on one side or hitting something that is preventing one side going all the way in (early on with my CDK the MMOAG guide camera section was blocking an adapter from engaging fully by perhaps only .5mm and hard to notice. It wrecked images badly). Replace any obvious weak points with solid screw in adapters. I use Precise Parts as they are well made, easy to order and they work. Expensive but worth it. This a physical examination step. Grab the camera and try to rock it, the filter wheel, any focuser movement etc. See how much force (without damaging anything of course) it takes to make something flex. Make sure the guide camera is well seated and can't rock. A full physical check of connections and strength is what this step is about. Does you filter wheel rock or are the filter carousels or filters have slop?

1. You need to work out how the image relates to the physical camera. I orient from looking at the physical camera from the behind it.

Now take a focus image say 5 seconds and put in a delay of say 7 seconds so you have time to get in front of the scope. With a not too bright torch dimmed down with some fabric put the torch on the upper part of the OTA objective.

Now go look at the resulting image. Is the bright patch on the top of the image or the bottom? In my case with a Honders Riccardi and CDK scopes they both have 2 mirrors and a corrector. The top and bottom are reversed.
So top of the scope for me is the bottom of the image on the screen.

Repeat this process until you are certain of the relationship for top and bottom of the screen - is the top of the computer image the top or bottom of the camera is what you are trying to work out.

2. Repeat this process but now its orienting left and right. So a torch on the left of the scope and see where the bright patch is on the computer image - left or right?

Again in my case both my scopes are reversed left and right. So for me I have to reverse top and bottom and left and right on the computer displayed image to orient on the camera looking at the camera from behind.

So to adjust the bottom left corner on the computer image then I pack the top right corner of the camera.

This is similar to drift alignment but if you don't do this step then you are left with 4 possibilities when seeing a bad corner and you can get lost and spend hours (believe me, I've wasted hours and hours).

2. Now let's say the bottom left has eggy or radiating stars but the other 3 corners are nice and round. The next step really is to tell whether you need to pack it out and get it in compared to the other 3 corners (ie. pack out the other 3)..

Focus the image whilst looking only at the bad corner in the image. Do the stars get tighter when you move the focus in or out?

If the focus gets better moving the focus out then you simply need to pack out the correct corresponding corner on the camera. If the focus gets better moving the focuser in then the other 3 corners need to be equally packed out.

3. The amount of packing would vary with scopes and possibly focal length. Not sure about, perhaps not. But its usually a fairly small amount.

I got some .3mm brass shim from a steel supply shop. You can cut it with scissors into little rectangles that you can handle easily. I also got some thinner shim which is .08 for fine shimming.

As a guide at one point my Proline on the Honders required .32mm on the top left and .34mm on the top right. So its not huge amounts that cause tilt.

I hope this is of help to some as I had to work this procedure out for myself as not much is written on this topic yet its an image wrecker when not addressed.

When packing out I use a 5-10 second focus image with a luminance filter and check the 4 corners and the sides (sometimes a whole side is not in focus compared to the other side). Don't use 2x2 as it averages the tilt out and will hide a certain amount of tilt. 1x1 shows all the warts!

All this is done at night. Get adept at it and fully set up and you can do this procedure in perhaps 30 minutes and then its done.

I would also recommend taking notes about the position and the amount of packers once determined so if you take off the camera later then you know next time what the packers are and where without having to repeat this process.

CCDinspector can be useful as well but again you need to know which corner of the camera relates to the image and the direction of tilt it is referring to. I find that extra orientation step of working out the direction of tilt a bit unclear in CCDinspector so I prefer the above procedure which is very clear and certain.

Greg.

[Atmos (Colin)]

Originally Posted by peter_4059

You should check with Theo to see if the adjustment rings he has fit your camera. That would save messing with the focuser if the tilt is in the camera.

I'll send him off an email tomorrow and see what he thinks.

Originally Posted by gregbradley

Big wod of text

Greg.

Thanks for all that Greg, will have to read over that a couple of times to get it down pat in my noggin
I totally agree with you about Precise Parts, not cheap but definitely worth it. It seems like my first port of call is getting my imaging train as rigid as possible, something that I don't quite believe it is at the moment. Something else I'll have to contact Theo about.

I have some more calculations and testing to do before ordering another part from PP. It didn't take me long to figure out a couple of things with this scope. With it being a triplet front cell and a three element corrector rear cell, the distance from the corrector (in the focuser) to the CCD ultimately determines the amount of back focus (focuser draw tube) and therefore the focal length of the system. The first time I set it up I had to rack the focuser out about 50mm which lead to slop and a focal length of 695mm. Having now increased the distance from corrector to CCD, I am down to 15-16mm focuser travel and 669mm, much better. I have a feeling that the tilt is a small amount of flexure, work on this first before anything else.

[gregbradley]

The first time I set it up I had to rack the focuser out about 50mm which lead to slop and a focal length of 695mm. Having now increased the distance from corrector to CCD, I am down to 15-16mm focuser travel and 669mm, much better. I have a feeling that the tilt is a small amount of flexure, work on this first before anything else.[/QUOTE]

Yeah you have to eliminate variables. If you get your focuser in even more (not too much as you need some margin for focus travel) then that will either fix it or show its not it and keep looking elsewhere. Focuser flex is pretty common.

Greg.