Spacing issues?

[Phillyo]

Good afternoon all,

So I managed to get out very quickly last night under not ideal circumstances but I wanted to try out my new set up and get the teething issues out of the way. I'm wondering if anyone could tell me if this is sensor tilt or spacing issues maybe? All the stars point towards the centre of the frame, but CCD Inspector doesn't show what I expected to see? There was some very thin high cloud so it wasn't perfect but wanted to give it a shot.

ASI533 with Samyang 135/2 @f2.8 connected with the ZWO Filter Drawer for Canon EOS lenses.

Thank you,

Phil.

[symmetal]

The image does indicate that the spacing is slightly incorrect. Stars pointing towards the centre generally suggests the spacing needs to be increased, but this isn't always the case.

CCD Inspector may be getting confused by the bayer matrix where the star brightness is varying between one pixel and the next due to the filters and so there isn't a steady fall off in brightness away from the star centre.

Alan

Edited October 23, 2020 by symmetal

[Adreneline]

My understanding is that connecting a Canon fit lens to this adapter with the 11mm spacer between the adapter and the camera will give an end of lens to sensor distance of 44mm - theoretically the correct spacing.

My experience with Canon/Samyang lenses combined with a filter is that 44mm is a good starting point. With my Canon 200mm combined with Astronomik filters my spacing is 44.5mm which allows me to focus my Ha, OIII and SII filters within the 'L' marker on the lens. With my Samyang 135mm the spacing needs to be 44.75mm to get the focus within the 'L' mark. If I then switch to my Baader LRGB-C filters the focus point is outside the 'L' mark because ideal spacing needs to be increased by a fraction of a millimetre because the filters are thicker - and we are talking changes of .2 mm making all the difference. One day I will change my LRGB-C filters to Astronomik so they are all the same thickness.

I have spent hours and hours trying to find the right spacing to get the best shaped stars I can whilst maintaining the focus position within the 'L' mark.

Good luck!

[old_eyes]

I have often wondered whether stars should be exactly in focus when lens is at infinity, rather than somewhere near infinity. Does it matter if the field is not showing curvature? Is the field only flat when the lens focuses exactly at infinity?

[Adreneline]

1 hour ago, old_eyes said:

I have often wondered whether stars should be exactly in focus when lens is at infinity, rather than somewhere near infinity.

I have read somewhere - can't exactly remember where now - that modern lenses focus "beyond infinity" although I'm really not sure where that is!

Whatever, the same article stated that when focussing at infinity the focus mark should be within the confines of the 'L' mark on the lens. My personal view is unless you are very, very lucky you will not get perfect shaped stars all the way to the extremities of the image because camera lenses are not specifically designed for astrophotography whereas astrograph telescopes are.

The stars in the middle region of my images are acceptably round with both the Samyang and the Canon to the extent that I can tell recognising the significant undersampling that is going on with the ASI1600 sensor. There is some distortion toward the edges and in the extreme corners which I cannot get rid of although I believe I have reduced as much as I can by getting the spacing as optimal as I can. In many cases I end up cropping the image to some extent which gets rid of the worst of the elongated/oval stars.

1 hour ago, old_eyes said:

Does it matter if the field is not showing curvature? Is the field only flat when the lens focuses exactly at infinity?

Not sure I am qualified to answer this - I can't help but feel this is more a function of the quality of the optics in the lens. We need to keep in mind that the amount of curvature we find unacceptable in astrophotography is essentially imperceptible in terrestial photography. You get what you pay for and I can't afford or justify a Takahashi!

Adrian

 

[Phillyo]

I would be more inclined to accept the eggy stars at the edge of the field if I was using a full frame sensor, but a 1" sensor that 'crops' in so much, I would've expected a flatter field. I will mess with the spacings as I think I have some delrin spacers somewhere and see if it improves things next time I have a clear sky. Maybe sometime in 2021.

Thanks all for the input.

Edited October 23, 2020 by Phillyo
typo