Is this flat file a sign my secondary mirror needs alignment?

[pipnina]

I've never paid too much attention to my secondary mirror as people said it wasn't too bad if it was out for visual, but taking a flat file with my D3200 it seems the light circle is offset somewhat. Should I try to collimate the secondary mirror?

I've avoided it until now also partly because the screws are really tight, and I'm afraid of damaging it.

[knobby]

How did you take the flat, it's probably the worst one I've ever seen 

Was the light source even ?

As for collimation you'd be better off looking at a slightly defocused star .

[knobby]

http://garyseronik.com/no-tools-telescope-collimation/

[pipnina]

6 minutes ago, knobby said:

How did you take the flat, it's probably the worst one I've ever seen 

Was the light source even ?

As for collimation you'd be better off looking at a slightly defocused star .

I have stretched the flat to make the imperfections more visible. The original would have looked like an almost solid white image. I probably should have clarified that!

I pointed it at my white ceiling, which is relatively even but also so close to the front of the scope I figured it wouldn't matter too much (not that i'd use it for real imaging). I don't have A4 or a screen big enough to do the method I use for my camera+lenses though so it's a bit tricky.

[vlaiv]

Might be focuser tilt / collimation issue. I guess it can be adjusted with secondary but try rotating camera first and doing another set of flats to see what happens.

[knobby]

13 minutes ago, pipnina said:

I have stretched the flat to make the imperfections more visible. The original would have looked like an almost solid white image. I probably should have clarified that!

I pointed it at my white ceiling, which is relatively even but also so close to the front of the scope I figured it wouldn't matter too much (not that i'd use it for real imaging). I don't have A4 or a screen big enough to do the method I use for my camera+lenses though so it's a bit tricky.

Stretch a white tee shirt over front of scope and aim at clear sky in the morning.

[pipnina]

2 minutes ago, knobby said:

Stretch a white tee shirt over front of scope and aim at clear sky in the morning.

Would something like a white bedsheet work as well?

[knobby]

12 minutes ago, pipnina said:

Would something like a white bedsheet work as well?

Should be ok if you can stretch it to get rid of rinkles or creases

[pipnina]

54 minutes ago, vlaiv said:

Might be focuser tilt / collimation issue. I guess it can be adjusted with secondary but try rotating camera first and doing another set of flats to see what happens.

I took them again (this time with bedroom lights on, hence the orange tint, but I used a white bedsheet like knobby suggested). First image is with same orientation second is rotated 90 degrees. Some of the dark spots are still there, perhaps dust in the camera somewhere?

[Tim]

Normally, with a nicely collimated scope, the illumination will be centred in your eyepiece or in this case, onto your camera chip.

As mentioned above, there can be many reasons that this is not the case.

At any rate, learning a bit about collimation will improve your relationship with your telescope, making it much easier to get it to perform at the peak of its individual ability.

This book by Vic Menard provides a comprehensive guide to the subject:

https://www.firstlightoptics.com/books/new-perspectives-on-newtonian-collimation-vic-menard.html

It is a little heavy in places  but provides the knowledge to enable anyone to achieve perfect optical alignment in a Newtonian, especially if assisted with the Cats Eye range of collimation tools.

Failing that, check out the videos on youtube

[vlaiv]

I've done some research and it turns out that flat field asymmetry can happen even if scope is collimated and focuser is squared and there is no tilt! (I know that this does not help much).

It turns out that those three while sometimes can have similar appearance of flats need to be addressed and diagnosed separately.

Here are some sources:

https://www.cloudynights.com/topic/549265-ccdinspector-flats-analysis-to-collimate/

http://web.telia.com/~u41105032/myths/myths.htm

So, to check collimation - best use star test.

To check tilt - examine FWHM of stars in focus - there should be "gradient" in one direction where FWHM changes (increases).

Even if two above are good you can still have uneven field illumination and this is handled by offsetting the secondary. Find collimation instructions for fast newtonians - it should include offset image / instructions.