Off-Center Flats a collimation issue? - Forums

Topic starter

Dec 27, 2020

Today I was looking a bit deeper into the flats I produced with my Takahasi Epsilon 130D and an astromodified Canon EOR R as full frame camera. What puzzeled me was that the peak/maximum brightness of the flat is not in the center of the image, but significantly above at row 2650. The image has a height of 4498 pixels, so I thought it should be at about row 2250. It means its ~400 pixels or ~2.1mm off. Can this be a collimation issue or maybe due to the eccentric secondary mirror of the Epsilon ? The Epsilon is collimated with a laser and the collimation procedure recommended by Tommy Nawratil from Lacerta Optics, and the stars are OK (not perfect).

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andrea tasselli

Frequent contributor

Dec 27, 2020

This might not have anything to do with collimation, just de-centering of the sensor with respect to the optical axis. The secondary itself needs to be off-centre with respect to the primary optical axis in order to produce even illumination across the field. Incidentally, you would need to use a barlowed collimated laser collimator to be dead sure your collimation is spot on. The only way to be sure is to use a high power EP on a bright star and look at the defocused diffraction pattern. If it OK for a centred star then you know that the issue is with the position of the sensor.

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Götz Golla

Topic starter

Dec 27, 2020

andrea tasselli:
This might not have anything to do with collimation, just de-centering of the sensor with respect to the optical axis. The secondary itself needs to be off-centre with respect to the primary optical axis in order to produce even illumination across the field. Incidentally, you would need to use a barlowed collimated laser collimator to be dead sure your collimation is spot on. The only way to be sure is to use a high power EP on a bright star and look at the defocused diffraction pattern. If it OK for a centred star then you know that the issue is with the position of the sensor.

Thanks Andrea,

I just found that playing with the collimation even by a lot did not change the "center" of the flat significantly. However rotating the camera alone, or rotating the camera plus focussing unit and corrector lens, did rotate the center of the flat around the center of the sensor. This is consistent with a de-centering of the sensor of the EOS R, but a 400 pixel de-centering is a lot. I read that people were already complaining about a 50 pixel de-centering of a Canon DSLR since it seems to affect the quality of daylight photos with a fisheye lens.

The de-centering certainly means that even for a small rotation of the camera new flats will have to be produced, and it also explains why the correction of some of my images with flats did not work well. Thats quite an important discovery

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Götz Golla

Topic starter

Dec 27, 2020

Here is the image with contour lines after rotation the camera by 90 degrees.

Götz Golla

Topic starter

Dec 28, 2020 · Edited: Dec 28, 2020

Actually the more I think about it right now the less I understand what I am seeing. Lets assume the optical axis of the camera and the optical axis of the telescope are aligned, but the sensor is de-centered. Then a rotation of the camera should in my opinion not change where the optical axis hits the sensor, and thus the "center of the flat", even though off-center on the sensor, should not change position on the sensor ?

andrea tasselli

Frequent contributor

Dec 28, 2020

I think the centre of the sensor is rotating about the optical axis as seen from the focuser connection point of view. De-centring in optical system like hyperbolic newtons with prime focus correctors like the Epsilon should lead to some noticeable on axis optical aberration, although to be sure I'd need to ray-trace it. If you don't see it than the issue is with the focuser or the attachment of the camera body. Personally, I wouldn't fuss much about if I can flat-field it. And I do take flats every time I move the camera in or out of the focuser so I am sure I get the flat nailed. And for DSLR I take flats before I turn them off and on again regardless if they have been mounted anew or never tuched in the first place.

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Götz Golla

Topic starter

Jan 10, 2021

andrea tasselli:
This might not have anything to do with collimation, just de-centering of the sensor with respect to the optical axis. The secondary itself needs to be off-centre with respect to the primary optical axis in order to produce even illumination across the field. Incidentally, you would need to use a barlowed collimated laser collimator to be dead sure your collimation is spot on. The only way to be sure is to use a high power EP on a bright star and look at the defocused diffraction pattern. If it OK for a centred star then you know that the issue is with the position of the sensor.

Meanwhile a received a 2x barlow lens and made a barlowed laser. Unfortunatelly the result has left me puzzled: I can adjust the primary mirror so that the central ring of the primary is projected exactly central to the exit point of the laser. As expected, this is also stable when I rotate the laser. However, when I rotate the focuser with the laser, or even the focuser only (i.e. while holding on the to laser), the ring becomes off-center. The center of the ring is rotating by about the radius of the ring !
Now if the focuser is not aligned with the optical axis I would expect that rotating the laser alone would already make the center of the ring rotate, which is doesnt. So I have no idea what is going here. It looks like the optical axis of the laser has an offset w/r to the optical axis of the focuser ?

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andrea tasselli

Frequent contributor

Jan 10, 2021

Not so strange if the focuser+ corrector is also tilted. Once collimated to the tilted plane the rotation of the laser won't change anything. But rotating the focuser will as it is rotating about the tilted plane normal, IMHO.

Götz Golla

Topic starter

Jan 10, 2021

so this probably means that the secondary needs adjustment ?

andrea tasselli

Frequent contributor

Jan 11, 2021

With a barlowed laser you should be able to.

Götz Golla

Topic starter

Jan 23, 2021

The mystery of the collimation has been solved. It turned out that the optical axis of the focuser is tilted w/r to its rotational axis. This amounts to 3mm at the backfocus distance, which is quite a bit. It probably is due to a tilted inner thread of the focuser where the camera or eyepiece adapter is connected.

Therefore, whenever the Epsilon is collimated, the collimation is gone when the focuser is rotated. That is including the adjustment of the secondary mirror. Thats why just by adjusting the primary I never managed to fix the collimation, even when the collimation was perfect the day before in another position of the focuser.

Currently I am arranging with my Tak dealer that the scope is sent back, possibly to Japan, for repair.

BTW: Apart from this problem I found that, after some practice, collimating an Epsilon with the Tak collimation eyepiece and tube is relatively easy, fast (20mins) and very precise.

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