Help my flats on my newtonian are uneven...

[Andyy]

Help my flats are uneven with newtonian. The flats are darker on right side than left side (horizontally) but flips when I turn my camera 180 degrees. What does that say me?

I need Help thinking I can't get me head around

 

 

Edited January 21, 2022 by Andyy

[Craig a]

Could be a a couple of things, can you post an image of your flats that may help people diagnose what the problem may be if there is a problem, the idea of flats is to correct any uneven feild illumination so they may be correct, does the problem still show on a calibrated light frame? 

[Andyy]

As seen on laptop screen:

Camera and coma corrector is rotated together.

I can also see the same vignetting pattern on a singel light frame. I use filters with filterwheel. Pattern is same for all filters.

Edited January 21, 2022 by Andyy

[andrew s]

This first thing I would do is check the secondary is correctly centered under the focuser. Regards Andrew 

[Andyy]

28 minutes ago, andrew s said:

This first thing I would do is check the secondary is correctly centered under the focuser. Regards Andrew 

If the secondary mirror is the problem, wouldn’t the vignetting pattern stay the same even when camera is rotated?

Edited January 21, 2022 by Andyy

[alacant]

26 minutes ago, Andyy said:

As seen on laptop screen:

Hi

360º, so you're back to where you started. The vignetting should be the same. Same for all filters, so not the filters. Not tested without coma corrector, so we don't know if that's the issue...

Our money is on the camera not being replaced at the same angle after rotation; we've never come across a Newtonian focuser out of the box which didn't tilt, threaded connections or not.

But you wouldn't rotate the camera after an imaging session before taking flat frames so if the flats are working...

Cheers

[Andyy]

14 minutes ago, alacant said:

Hi

360º, so you're back to where you started. The vignetting should be the same. Same for all filters, so not the filters. Not tested without coma corrector, so we don't know if that's the issue...

Our money is on the camera not being replaced at the same angle after rotation; we've never come across a Newtonian focuser out of the box which didn't tilt, threaded connections or not.

But you wouldn't rotate the camera after an imaging session before taking flat frames so if the flats are working...

Cheers

Sorry, correction camera is rotated 180 degrees of course. My bad, it was late…

I think the problem is camera-side (most likely coma corrector) not collimation.

Edited January 21, 2022 by Andyy

[alacant]

3 minutes ago, Andyy said:

rotated 180 degrees

All OK then:)

[Andyy]

4 minutes ago, alacant said:

All OK then:)

The vignetting pattern is still stronger (darker) on one side than the other. If you see my drawing there is to different shades of gray (top and bottom).

 

Pattern is following the top part of the sensor as it turn around 180 degrees.

Edited January 21, 2022 by Andyy

[alacant]

5 minutes ago, Andyy said:

still stronger (darker) on one side than the other.

The vignetting should be central so something is not square. Collimate, tighten, dismantle, secure main mirror...

Unless you have patience, the flats are working and you have decent stars, probably best to just leave it.

Cheers

[Andyy]

7 minutes ago, alacant said:

The vignetting should be central so something is not square. Collimate, tighten, dismantle, secure main mirror...

Unless you have patience, the flats are working and you have decent stars, probably best to just leave it.

Cheers

Lets say the the secondary is tilted causing vignetting on one side of sensor then wouldn’t the vignetting be stationary on that same side/place independent of sensor orientation?

Edited January 21, 2022 by Andyy

[ONIKKINEN]

Can you post a raw flat frame, rotated and not?

Not sure i understand the issue but would like to have a look if possible.

[alacant]

33 minutes ago, Andyy said:

stationary on that same side/place independent of sensor orientatio

You need to adjust tilt and collimation until the vignetting is more evenly distributed; central with dark corners is good.

Does your software rotate the images?

 

 

Edited January 21, 2022 by alacant

[Andyy]

4 minutes ago, alacant said:

You need to adjust tilt and collimation until the vignetting is more evenly distributed; central with dark corners is good.

Does your software rotate the images?

 

 

Collimation looks perfect but the vignetting is still uneven. I’ll give collimation another try!

 

camera is rotated physically not with software.

[alacant]

2 hours ago, Andyy said:

rotated physically

Think of images either side of a meridian flip. Stars left before are stars right after. Or just take a photo of a tree frame right then rotate the camera about the same centre. Where's the tree now? Upside down and on the left? The problem is that many cameras' default is to rotate the image to what they think is the correct orientation. Best to disable this auto-rotate.

Edited January 21, 2022 by alacant

[ONIKKINEN]

I would recommend this for secondary collimation: https://www.firstlightoptics.com/other-collimation-tools/ts-concenter-2-adjustment-eyepiece-for-newtonian-telescopes.html

Very easy to use and you can immediately see where the problem is.

You dont mention what telescope it is, or what focuser you have (if you changed it) but if its a stock newtonian and not of a premium quality the focuser is probably not perfect. If you have any slop in your focuser it wont matter how much you collimate the scope because the camera position is not reproducible and so collimation changes depending on where you point in the sky or some other trivial reason.

You also dont mention what camera you have as there could be another reason for it, like shutter shadow if using a DSLR with a physical shutter. Unless you post a raw flat these are just guesses, but collimation is a pretty good guess whether related to actual collimation or focuser slop.

[Andyy]

Got all necessary collimation tools and collimation looks spot on. Vignetting  is remove with calibration. Star field look even and stars are round all the way to the corners. Probably just a hair off regarding tilt.

«Problem» is that I’m a perfectionist

[alacant]

1 hour ago, Andyy said:

I’m a perfectionist

Astrophotography and perfection? 

I'll have to check my English, but I think it's called 'mutually exclusive'!

[andrew s]

4 hours ago, Andyy said:

If the secondary mirror is the problem, wouldn’t the vignetting pattern stay the same even when camera is rotated?

No it would rotate relative to the camera. A 90 rotation if the camera would produce a 90 rotation of the pattern.

Regards Andrew 

Edited January 21, 2022 by andrew s

[Andyy]

Used a mirror and inspected the secondary stalk and found the secondary mirror was slightly tilted upwards towards the focuser. This might explain the uneven field.

I’ve spent a lot of time trying to center the secondary under the focuser with an offset. I’m not sure if I’m quite there, and may have tilted the secondary mirror because of this (to compensate).

Feel free to chime in if you have a good technique for centering the secondary mirror with an offset without causing tilt!

 

[alacant]

Remember that other than reflection, the secondary has no optical properties. It's just a flat mirror.

Correct offset is inherent when the secondary is correctly centered on modern Newtonians. You don't need to set it.

Be sure to read the collimation myths. Both telia and seronik tell it exactly as it is.

Cheers

[Andyy]

27 minutes ago, alacant said:

Remember that other than reflection, the secondary has no optical properties. It's just a flat mirror.

Correct offset is inherent when the secondary is correctly centered on modern Newtonians. You don't need to set it.

Be sure to read the collimation myths. Both telia and seronik tell it exactly as it is.

Cheers

@alacant The Telia-linkis broken…

I tilted the mirror upwards to center the reflection in the mirror under the focuser. Isn’t it best to keep the secondary mirror stalk straight and «paralell»to the primary mirror?

…or it will result in this, either left or right. In my case; up or down.

Edited January 21, 2022 by Andyy

[alacant]

Telia. Try:

https://docs.google.com/document/d/189O1EzG4VDspu-okPl5rIYlmp1JSgepeKN58NmrF-GI/edit?usp=drivesdk

 

[alacant]

18 minutes ago, Andyy said:

tilted the mirror upwards

Small amounts are fine.

Post a shot

- looking centrally down the focuser

- through your Cheshire crosshairs

 

[Andyy]

15 hours ago, alacant said:

Small amounts are fine.

Post a shot

- looking centrally down the focuser

- through your Cheshire crosshairs

 

Thanks! I’ll start all over with the secondary tonight. It’s almost there now! I’m not a beginner in this hobby, just beginner with newtonians

Thanks again @alacant you are a great resource here!

Edited January 22, 2022 by Andyy