Strange Bahtinov pattern

[Spleenus]

It appears I have come across an issue which I cannot seem to find on the internet. I was wondering why all of my recent planetary videos were just not playing ball and never seemed focused despite my best efforts. So tonight I defocused on a star and noticed it appeared a little unsymmetrical.

Then I put on my trusty bahtinov mask and I found the following issue. This mask I have used before and it has not given me this issue before. These are shown in the attached pictures. Even more frustrating was it appeared to be one of those fantastic nights too!

Two of the pictures are when I got as close to focus as I could (i.e the lines intersected as best they could). But it appears that one of the lines is curved in the wrong direction. This also leans me to think the collimation has gone.

The telescope is a 150mm Saxon Maksutov - so the information out there is limited :( Maybe it's because I ordered a nice 9.25 EdgeHD which is on it's way and it just isn't happy! :)

Does anyone have any different ideas on what may be wrong or has come across this before?

[pmrid]

It would probably be more informative to see some inside-outside focus star images. That’s where collimation issues would be clearer.

[JA]

Hi Darren,

You mentioned that your recent planetary videos have had focus issues: Suggesting that perhaps previous videos were OK? If so, what has changed? A new Bahtinov mask? Another remote possibility - is the Bahtinov mask OK, any damage, scrapes etc? Were you previously focusing without a mask and now with? etc....



It may be some coma &/or astigmatism seen through the Bahtinov mask &/or some collimation issues. Any bumps/jolts or falls for the scope lately? What does the star look like without the mask? Is it perfectly symmetrical? Is the star you used in the centre of the frame / on the optical axis? It might be telling to compare the same test you have done, with a star at the centre (on axis) and one towards the edge of the frame. That would help to rule in or out coma/astigmatism issues and point you towards collimation issues.


Best
JA

[Spleenus]

Hi JA, Some good questions for me there to look at. In all honesty, the planetary ones have never been that good in focus but are good enough for the eyepiece. I thought it was just me not getting things right.

The bahtinov mask is in good condition, and I don't recall any jolts or anything to the scope. I've previously used the bahtinov mask when I trialed some DSO's (at f12 I was asking for trouble :rofl:) and it worked fine then. That was probably a few months ago now.

I'll see if there is a particular axis which is off and see over the field of view. But if I don't have the bahtinov mask on the stars don't appear to be out of shape (at least visually) and I can split doubles no problem as well as not noticing any colour fringing visually.

It's all a little bit of a conundrum. Maybe it is slightly out of collimation - do the effects of this get worse at longer focal lengths? I will definitely defocus a star and see what happens - unless the edgeHD just gets me too excited and I forget :)

But thanks both for your suggestions and thoughts.

[multiweb]

This can happen when the mask is tilted and/or not concentric with the scope aperture and offset.