Sensor centrality problem bugging me (is it even a problem?)

[BrendanC]

TL:DR - my sensor isn't central, is it the secondary screw, if so how does that impact collimation and do I even need to fix it?

Right, this has been bugging me for a while and I've decided to at least try and understand it, if not fix it.

My 130PDS is now perfectly collimated (ideal pattern from tribahtinov, Cheshire, and own method using mobile phone and Al's Collimation Aid), and ASTAP is showing that I have virtually no tilt. However, my flats look like this, from a Lacerta panel. I think that, when I look carefully at my subs, they do too.

Clearly, the sensor isn't central. It's probably always been like this but it's now bothering me.

I wondered whether the fix could be to use the central screw to move the secondary up/down the tube, and I've had responses elsewhere from fairly knowledgeable people who think so too. However, when I think about this some more, I get confused. 

My understanding is that, according to pretty much every collimation guide I've read, the secondary mirror should be centrally aligned relative to the focuser tube, and that for a fast (above F5) scope, this produces the offset collimation pattern when viewed through a collimation cap/Cheshire. So, central mirror produces offset pattern.

However, if I'm going to be moving the secondary up/down the tube, isn't that going to affect this? In other words, it implies that the secondary should be physically offset, which implies I would then see a central collimation pattern. So, offset mirror produces central pattern?

If neither of these, then what should I be expecting to see through the Cheshire if I physically offset the mirror? Or, is this such a small adjustment, given the size of the camera's sensor, that the secondary will still appear central?

I've created a quick diagram illustrating what I see as the options. Which one applies, do you think?

Thing is, flats seem to calibrate it out, so is it even worth looking into? I'd like to fix it, but not at the expense of a ton of bother. 

Any takers? Can anyone offer a fairly confident diagnosis of the problem, and a fix, and explain how it impacts collimation? Or should I leave well alone if it calibrates out?

Thanks
Brendan

Edited April 27, 2023 by BrendanC

[ONIKKINEN]

7 hours ago, BrendanC said:

Or should I leave well alone if it calibrates out?

Thanks
Brendan

This is what i would do, as a newtonian imager a "dont try to fix what is not broken" mentality is good. You mention that everything is perfect in terms of collimation and tilt so is this really important? You have a real chance here to make things worse with fiddling - just leave it be and enjoy a well collimated newtonian.

But anyway, looks like you just need to center the secondary under the focuser like you already concluded at first. The only thing this achieves is that you have less vignetting. Simple to do but again i would not bother.

A centered secondary is no requirement for collimation by the way. Its just good to have since vignetting is lost light.

[BrendanC]

Thanks. I think I will leave it for all the reasons you give. But I just know it'll continue to bug me!

Out of interest, which of the two options in the graphic do you think are applicable? I always thought the mirror should be central, which yields the offset pattern. But, if the mirror needs to be offset, what happens them? Or, if this is really a very small adjustment, then perhaps the mirror will still appear central?

[The Lazy Astronomer]

Is the vignetting even really that severe? Assume the above is stretched so it makes it look bad, but what are the actual differences in pixel values between the bright and dark regions? 

[BrendanC]

OK so this has uncovered yet another issue. I spent quite some time using the APT flats aid to get my flats to 30K ADU. Yet when I measure this in APT, I get from top left to bottom left, going clockwise: 40015, 36117, 37160, 41142, and in the centre, I get 43347.

So I'm probably going to have to revisit this too, although (again) calibration seems to work OK. 

But, ignoring that for now, I wouldn't even really know whether these are acceptable figures. All I can say is that the difference between the highest and lowest values is about 17%.

Edited April 27, 2023 by BrendanC

[ONIKKINEN]

53 minutes ago, BrendanC said:

Thanks. I think I will leave it for all the reasons you give. But I just know it'll continue to bug me!

Out of interest, which of the two options in the graphic do you think are applicable? I always thought the mirror should be central, which yields the offset pattern. But, if the mirror needs to be offset, what happens them? Or, if this is really a very small adjustment, then perhaps the mirror will still appear central?

I will admit i have not paid any attention to trying to get a specific offset pattern with mine since i collimate via laser. I only check the secondary is roughly central when i have removed and reinstalled the secondary for some reason, but the top image looks more familiar to me.

[BrendanC]

I just discovered this cool feature in ASTAP which shows the proportional differences. But again, I don't know how to interpret them. I guess it sort of means I'm losing a small percent of light that I could, in theory, actually calculate if somehow I could export these figures.

Edited April 27, 2023 by BrendanC

[BrendanC]

OK, a bit of hackneyed data processing and maths...

Process the above image so it has a dark background, upload it to Google Photos, grab the text from Google Lens, copy it to my PC (without the dark background Google Lens doesn't detect some of the text).

Paste the values into Word, then convert to table with 19 columns.

Paste that table into Excel and sum all the values. Comes to 345.384. Interestingly, this implies that if I had no vignetting, and all the values were 1, that would come to 361 (ie 19 columns x 19 rows), so my current setup is 345.384/361=95.7% efficient given the current vignetting.

Paste them again but rejig the table so that the values 'reflect' either side and top to bottom, as if the sensor were dead centre, and sum that. Slightly subjective but roughly right. Comes to 348.218.

So, it seems to me that my off-centre sensor is collecting 345.384/348.218=99.2% of the light that it would were the sensor to be absolutely centered. (This could probably also all have been done using averages).

I think I'll leave it.

Edited April 27, 2023 by BrendanC