Uncentered flats - what does it mean

[MarkInSpace]

I have a new used 10” Newtonian telescope.
What does it mean if the flat is extremely uncentered?

[MarkInSpace]

Btw, I have collimated the scope.

[Startrek]

Hmm……

I solely image with Newts 6” , 8” and 10”

Maybe camera not sitting square in focuser or maybe some tilt ?

Is focuser travelling true both infocus and outfocus ?

Is your collimator and camera adapter the same diameter, say 2”

Some collimators are 1.25 and if your camera is 2” then some offset might creep in

Are your subs centred ok and were your flats taken straight after your subs or next day ??

Can’t think of anything else

[Rerouter]

Did you collimate by eye / cap / cheshire or a laser? (I have some beef with laser collimators)

Its clearly off or you have a lot of tilt on your camera,

[Joshua Bunn]

Is your flat source coming in on axis, I mean is it evenly lighting the aperture?
I can not imagine camera tilt would cause this.

[multiweb]

It's very likely your secondary is not perfectly centered looking down your focuser barrel. Get a draw tube and start with that then re collimate. I reckon it's either too close or too far from your primary.

[MarkInSpace]

Marc, what you say makes sense, but I don’t understand how I check the secondary mirror’s position…

[Startrek]

Mark,
This Astro baby site for collimation is pretty good
I’m sure you can set your secondary from these instructions
The main thing is to ensure you have a concentric donut ring and can see the 3 clips and that the distance is even all the way around when you view down the OTA to the primary mirror
The orientation of your secondary mirror must positioned correctly with respect to the end of the focuser tube and OTA , the Astro baby site shows you this

https://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/

Hope this helps

Martin

[multiweb]

Looking down the barrel of your focuser does the secondary look like a circle evenly spaced from the edges of your focuser?

[mura_gadi]

I cover the EP holder all together with a pin hole in the centre to check alignment. The old plastic film canister are custom made for the job, right down to the centre hole being marked.

Look through the pin hole at the secondary, if the pin hole is centred you can check alignment very well by checking for an even gap as per multiweb's comment.

*The pinhole lets you know your looking in along the centre line and gives you a tight frame. The centre line can be a little harder to find if you have the full open focuser in front of you and the edge gap will larger and change easily if you move head position.

PS. You can also just insert a carboard tube down the focuser till its just about resting on the secondary and align from that.

[MarkInSpace]

Here’s the latest flat - using a homemade centering cap, then a barlowed laser, I’ve reset the middle screw of the mirror and re collimated the mirrors, not much better, but a bit better.
Any idea what can still be wrong?

[multiweb]

Whatever you did, keep doing it until the mirror is centered. Just need to go a little further in the direction you moved the secondary :thumbsup:

[mura_gadi]

I took your latest image and drew a circle over the light section in paint and the lighted area appears to be slightly oval.

So camera tilt might be an issue as suggested previously.

or you may need to tilt the main mirror if your running out of room to adjust the secondary.

PS. If your mucking around a lot with the collimation, drop the main mirror out and check the mirror cell clamps. I recently got an old silver tube Bintel 8" and the mirror clamps were so tightly screwed down the foam had squeezed out from under the clamps and out over the mirror. Your oval might be from pinching on the main mirror.

[Dave882]

I’ve seen this come up a few times (but I’m a sct guy so may not be so relevant). I can say absolutely it can be affected by collimation and tilt in the train, but one time recently I had a similar offset and when I adjusted the flat panel on top of the scope it completely resolved. Worth checking its sitting square and level. Just my 2c.

[pt1280]

It can be due to a number of reasons, but most likely as prior threads discussed, the height of the secondary mirror. Try pushing or pulling the secondary, collimate, flat test iteratively and you'll be able to find the right secondary height. You could also try using OCAL ELECTRONIC COLLIMATOR to determine the proper height too. I have found it incredibly useful to collimate my Sharpstar 150 f2.8 newt :).