Strange bahtinov mask pattern

[Mushroomrice]

I am new to dso imaging. Last couple of imaging sessions I have been seeing a strange curved bahtinov mask pattern. I am using an evostar 72ed with an asi 183 camera. I tried swapping the camera for an asi120mcs which I also have and this seems to show a normal mask pattern. I managed to focus the 183 by eye and took some images which seem to have reasonable focus. I wondered if anyone had any ideas what could be causing the curvature seen in the mask pattern?

[Tomatobro]

There is a post on SGL

Search on and take a look at           Bahtinov mask "Banana" spikes

[Tomatobro]

There is also a lot of discussion on the Cloudy Nights Forum and reading through it no firm conclusion was reached

Cloudy nights search..........Strange Bahtinov Pattern

[Mushroomrice]

Thanks. I have read both of these discussions and they seem rather inconclusive. Suggesting varying possibilities such as collimation, spacing and atmospheric conditions. Neither post seem to have been resolved. Was hoping somebody had experienced this issue and manged to resolve it.

[Tomatobro]

Out of interest do you have a reducer/corrector in the ED72?

[Mushroomrice]

3 hours ago, Tomatobro said:

Out of interest do you have a reducer/corrector in the ED72?

No that is my next item to buy. I did wonder if it was being caused by field curvature, if that is the cause not sure why I don't see it when using the asi120mc-s, I know the chip size is a lot smaller but you're only looking at one star. 

[fozzybear]

Could be back focus issues as the 2 camera's have different settings the 

asi183 is 6.5mm

asi120mcs is12.5mm 

how are you attaching the camera's to your scope?

[Mushroomrice]

18 minutes ago, fozzybear said:

Could be back focus issues as the 2 camera's have different settings the 

asi183 is 6.5mm

asi120mcs is12.5mm 

how are you attaching the camera's to your scope?

I did adjust the focus between the 2 cameras. I have a flip mirror connected to the back of the scope as I still like to do a bit of visual, the camera is then fixed to the flip mirror. The flip mirror also seems to give sufficient back focus for the camera to come to focus. 

[fozzybear]

6 minutes ago, Mushroomrice said:

I did adjust the focus between the 2 cameras. I have a flip mirror connected to the back of the scope as I still like to do a bit of visual, the camera is then fixed to the flip mirror. The flip mirror also seems to give sufficient back focus for the camera to come to focus. 

 Could be the flip mirror causing the problem can you try without it and using extensions?

[Mushroomrice]

39 minutes ago, fozzybear said:

 Could be the flip mirror causing the problem can you try without it and using extensions?

Thanks for the advice. Removing the flip mirror and replacing with extensions was going to be my next try . I am a little concerned I may be getting some flexure. Seems strange it only shows on the asi183, suppose it is heavier though. Will give it a try once I finally get a cloud free night. 

[fozzybear]

3 minutes ago, Mushroomrice said:

Thanks for the advice. Removing the flip mirror and replacing with extensions was going to be my next try . I am a little concerned I may be getting some flexure. Seems strange it only shows on the asi183, suppose it is heavier though. Will give it a try once I finally get a cloud free night. 

just a thought hope you get sorted

[Mushroomrice]

Still getting my strange banana shaped bahtinov mask pattern. Thought I would try checking the collimation. When slightly off focus I am getting the pattern shown in the image. I am new to using a refractor but this appears strange to me. Can anyone confirm that it looks like a collimation issue?

[michael8554]

On 30/08/2021 at 10:28, Mushroomrice said:

I am using an evostar 72ed with an asi 183 camera. I tried swapping the camera for an asi120mcs which I also have and this seems to show a normal mask pattern.

Then it's a camera problem, not scope collimation ?

Michael

[Mushroomrice]

I agree that the fact the issue disappeared when using another camera would suggest it's an issue with the 183. However I am confused to what sort of camera issue could cause this effect. 

[michael8554]

Maybe tilted sensor ?

Is the camera sealed by a clear glass slip that has become dislodged ?

No expert on collimation, but shouldn't all the Airy Rings be offset, not just the inner rings ?

Michael

[dodgerroger]

I had the exact same issue with my 183mm although it is a Altair camera. I never got to the bottom of it and now use an electronic focuser. 

[Mushroomrice]

6 minutes ago, michael8554 said:

Maybe tilted sensor ?

Is the camera sealed by a clear glass slip that has become dislodged ?

No expert on collimation, but shouldn't all the Airy Rings be offset, not just the inner rings ?

Michael

I will take a closer look at the camera to see if I can identify any issue. I am new to collimation and thought the centre ring being so offset was strange 

[Mushroomrice]

1 minute ago, dodgerroger said:

I had the exact same issue with my 183mm although it is a Altair camera. I never got to the bottom of it and now use an electronic focuser. 

Thats interesting. I do seem to be achieving a reasonable focus and have taken some okay images. Would really like to know the cause as it's bugging me. 

[bottletopburly]

Have you tried a different bahtinov preferably not a clear one which I’m assuming the Williams optics one is could it be if it is the clear type light refracting off the plastic edge ? Maybe 🤔 

[dodgerroger]

If I recall, the spikes looked straight when checked with my nb filters but with rgb filters the spikes would look wonky. 

[Mushroomrice]

8 minutes ago, bottletopburly said:

Have you tried a different bahtinov preferably not a clear one which I’m assuming the Williams optics one is could it be if it is the clear type light refracting off the plastic edge ? Maybe 🤔 

Unfortunately I only have the 1 mask. It isn't a clear one though. It's made from black card from FLO. 

[Mushroomrice]

9 minutes ago, dodgerroger said:

If I recall, the spikes looked straight when checked with my nb filters but with rgb filters the spikes would look wonky. 

I am using a colour camera so sounds like the same issue. I wondered if it could be due to the fact the scope is a doublet.

[dodgerroger]

I don’t think so, I started with a 72ED with the same camera and have progressed to a GT71. It was still the same. I thought it might be over saturation and or the high pixel count that is causing it. I found it annoying as it is very difficult to determine correct focus