Strange behavior of Sigma 40mm F/1.4 Art lens.

[Vroobel]

27 minutes ago, Elp said:

I would note the ZWO Ef adaptor likely isn't mounted square so will cause an element of tilt, I get it on all my SY135s, the weight of the lens doesn't help. A lot of people have to shim to get it right.

 

Oh, yes, the lens is 1.3kg heavy, so I bought a dedicated ring holding it while the Canon or the ASI camera (both around 700g) is just attached to the lens. I wouldn't be worried if I saw marks of the tilt: pinpoint stars along one edge and comas on the opposite edge, but that's not the case. 

Edited April 24 by Vroobel

[Vroobel]

7 minutes ago, Elp said:

Make sure you're measuring with a good quality Vernier caliper too either a manual one or digital, measure each component separately, then again when assembled.

 

There is not too much to measure, only the ZWO filter drawer. I base on the Sigma 44mm flange distance and the ASI 17.5mm back focus. The ZWO must be the remaining 26.5mm long and it is indeed.

I use a high-class vernier caliper which is older than me.  I love to use it, even if I have two digital ones.

[Elp]

Have you got any thin spacer rings to put onto the back of the lens adaptor prior to the camera, that's what I'd try next.

But before that, just because you don't need anything else to test it, try stopping the lens down at each f ratio and see if it improves.

Also I assume you've already checked your 2600 tilt adjustment plate is square to the body and not tilted in any way?

Edited April 24 by Elp

[Vroobel]

Yes, I have a lot of thin plastic spacers starting at 0.2mm and also three paper ones, around 0.1mm each, I described experiments with them earlier in this thread. I reached a compromise between the coma and radially elongated stars at 1.2mm spacer in total, but I used filter then, I think it was the L-eXtreme. I have to check it again without filters, but not tonight, the setup is at home now due to clouds. I'm happy to see that the Sigma lens is OK, I considered returning it under warranty... 

 

There is a very interesting method of setting the iris permanently:

Based on a following CN thread I attached the 6D body, set a BULB mode, pressed the shutter release button and... detached the body - the iris remains in the F/1.8 position. That's a great trick! 

https://www.cloudynights.com/topic/837147-sigma-art-40mm-14-canon-mount-no-aperture-ring/

Edited April 25 by Vroobel

[Vroobel]

34 minutes ago, Elp said:

Also I assume you've already checked your 2600 tilt adjustment plate is square to the body and not tilted in any way?

 

I've never had any bigger problems with tilt on this camera. The only tilt-like effects are related to a bent and it's always visible on the edge aimed down during the session. Here is a masterLight as example with an Askar FMA230: 

 

 

The stars aren't perfect, but take please into account that my Askar FMA230 has its back focus equal to 52.5mm instead of 55mm, so I stopped selecting spacers at an acceptable point which is fixable by a BlurXterminator.

 

Here is the same camera attached to an Altair 102 EDT Triplet APO with a dedicated x0.8 reducer, distortion is neglectable (at least for me). 

 

Edited April 24 by Vroobel

[Elp]

Have you got another astro camera you can try with the lens adaptor? I know the size won't necessarily match the 2600 but you'll likely start to see the distortion if it's a decent sized sensor, and if it reproduces the issue. Process of elimination.

Edited April 25 by Elp

[Vroobel]

Apart from ASI224MC and ASI678MC with their total sensors surface smaller than a half of the APS-C ( 😁 ) I have also a 2600MM Pro which is permanently attached to a filter wheel and used with bigger scopes. I think I have no choice and have to (reluctantly) disassemble it if I want to perform a proper test. 

[Elp]

It never occured for me to ask but does that AF lens have manual focus control when not connected to a camera body?

[Vroobel]

🙂 Yes, it has. It's always switched to the manual mode, even if used with the Canon body. I used a Bahtinov mask earlier, but now I rely on an EAF.

 

 

Edited April 26 by Vroobel

[Elp]

I'd do a test without filters, then try a few different ones. I'm thinking a bodied camera has a few pieces of glass mounted onto the sensor, with the 2600 you may have less though this doesn't explain if you have MF capability why its still causing coma at the edges. I find with my SY135s, if I keep the backspacing consistent, different filters will move the focal point away from the infinity point on the lens significantly.

[Vroobel]

40 minutes ago, Elp said:

I find with my SY135s, if I keep the backspacing consistent, different filters will move the focal point away from the infinity point on the lens significantly.

 

It happens to the Sigma as well, but I don't remember how much it moves. I'll try to record the values bot for the Canon and astro-cameras. 

Edited April 26 by Vroobel