Help my flats on my newtonian are uneven...

[Vroobel]

Please let me join the thread.

I'm also a bit experienced in this hobby. Moreover, I thought I could properly collimate my budget Bresser Messier 10" F/5 Newtonian. I use a Farpoint triple collimation kit.

https://www.astroshop.eu/laser-pointers/farpoint-laser-pointers-650nm-cheshire-autocollimator-2-set/p,61481

A while ago I performed some modifications: a primary mirror mask and a focuser frame to make it stiffer on the thin tube:

 

 

Let's check if I do what I should do. Before I played with the collimation kit I used a string with a small load hanging in the middle of a central hole of a spider (with the secondary mirror unscrewed) to find whether the spider was placed perfectly over the ring mark. Unfortunately, it wasn't, the load hung rather over an edge of the ring than over its middle. It may happen that the mark wasn't placed perfectly in the middle of the primary mirror. I can use two pairs of four screws outside the Newtonian tube to pull the spider towards one of the four directions which causes the load hangs over the middle of the mark, but pairs of opposite arms of the spider are no longer in line then. It makes awful spikes which I cannot accept. I suspect that axes of pairs of holes used for screwing the spider aren't perpendicular. Taking the opportunity that the secondary mirror is unscrewed, I used the laser collimator to check whether the beam is perpendicular to the tube axis. Yes, it is. Moreover, the beam passes through the string which means it passes through the tube axis. I use a laser collimator to adjust the secondary mirror to aim the laser beam at a ring mark in the middle of the primary mirror. If it does, I adjust the primary mirror using a Cheshire collimator to see the mark inside a reflective ring placed at the end of the Cheshire collimator tube. Of course, I try to keep the offset, but it's not perfect. Then I use the laser collimator again to check if the beam comes back to its central point.

OK, it looks like the scope is collimated pretty well, let's take a flat frames then. This is how the flat frame looks like:

 

 

OK, so I used the secondary mirror collimation screw to find the best flat frame. I already know that the primary mirror is collimated pretty well. After plenty of attempts finally, I reached a perfect flat frame:

 

 

Then I used the laser collimator again to check what I made worse...

 

 

Well, the laser beam no longer aims at the ring mark and of course, it no longer comes back the the collimator central point.

 

Why does it happen? Is the primarily performed collimation proper? Maybe the even illumination is more important? I'm curious about your opinion.

I'm going to do a star test ASAP, this should give me the most trustful answer. 

 

[Astronomist]

That is puzzling @Vroobel, only thing I can think of is maybe the camera sensor tilt is not quite right?

 

[Vroobel]

I already checked the tilt. My ASI2600MC Pro has a tilt adjuster, whatever I do with it, the illumination stays the same. My conclusion is that this change doesn't affect the position of illuminated area. 

It changes when I loose two screws holding the focuser, then the heavy camera, MPCC and filter drawer pull the focuser down. The sensor moves significantly then. 

I hope it's clearer now. 

Edited May 6 by Vroobel

[bottletopburly]

3 hours ago, Vroobel said:

I used a string with a small load hanging in the middle of a central hole of a spider (with the secondary mirror unscrewed) to find whether the spider was placed perfectly over the ring mark

Assuming the ring is centred on the primary perfectly centred , my 200pds was 6mm out .

[Vroobel]

Did you check if your floor is leveled? I did, I had to use an old Dobsonian round base plate and level it before I started.