Christmas Tree Star Shapes!

[WolfieGlos]

Well it is the festive time of year, but nobody likes odd star shapes!

Last night I had a brief window in the clouds for a first trial run pairing my 585MC with a Canon 200mm f2.8 prime lens, primarily since it will gather data much faster than my main scope so I plan on using this for widefield imaging of nebulae.

I've had the lens for a few years, used it for astro when I first started out and it was fairly easily to achieve focus with a DSLR, although the focus adjuster is VERY sensitive to move. I've had the 585mc for a few months, but this was first light with the lens. It worked fine with both my 72ED and 102ED. So, separately, both the lens and camera have worked - but this is the first time they have been used together.

So after getting it all manually in focus last night, I was surprised to see my images producing star shapes like christmas trees (and no, I didn't leave the bahtinov mask on!)

This was not in a corner, this was across the whole field. It did seem to improve slightly if I binned 2x2 and more with 3x3 but it was still there. Example fits files attached.

I've been trying to understand the reasoning for this, it cannot be bad tracking or wind since these were 5s subs and it was riding on top of my SF102 whilst that was taking 120s subs....and the stars in those subs are round.

The pixel size of the 585mc is 2.9µm , so with the lens I would definitely be undersampling - but is this what stars look like when undersampled?

My only other thought is that I had a 2" Astronomik L2 UV/IR cut filter in the imaging train.....and after the session I found I still had a Hoya UV filter mistakenly screwed onto the front of the lens - could this actually be the culprit?

My current forecast has no clear skies for a while, so any input appreciated so I can try next time 🙂

2023-12-15_01-52-56__8.30_5.00s_0000 1x1.fits 2023-12-15_01-49-44__8.70_5.00s_0000 3x3.fits 2023-12-15_01-50-17__8.60_5.00s_0000 2x2.fits

[Elp]

Have you ever imaged stars as points with this lens prior?

I'd suggest try stopping the aperture down but this won't help if it's across the whole fov. Some lenses can have internal lens elements decontacted which can cause something like this (also due to poor quality control), it's very common with camera lenses and people who do astro regular with them check them straight away for the star shapes they produce. It could also be tilt but again, across the whole fov is unlikely.

[WolfieGlos]

14 hours ago, Elp said:

Have you ever imaged stars as points with this lens prior?

Thanks Elp.

Yes, and with the hoya filter on ironically. It worked with a DSLR.

14 hours ago, Elp said:

I'd suggest try stopping the aperture down but this won't help if it's across the whole fov. Some lenses can have internal lens elements decontacted which can cause something like this (also due to poor quality control), it's very common with camera lenses and people who do astro regular with them check them straight away for the star shapes they produce. It could also be tilt but again, across the whole fov is unlikely.

You might be on to something here, I didn't consider stopping the lens down (can you do this in NINA?)

Last time I used the lens I was learning AP some 2 years ago and produced several images with it - all learner images, and I deleted all the subs so I just have the final images. I've been back and looked at them, and some at f/2.8 do show similar christmas tree type shapes - although not quite as bad as this.

One of the last images I used it for was shot at f/3.5, and to my eye the star shapes look OK, but they have diffraction spikes to it's hard to be 100% sure.

However, one of the images shot at f/2.8 doesn't show the same issue. Back then I didn't know what a bahtinov mask was, so focus was by eye on the LCD. I wonder if not quite achieving perfect focus could be causing this, or perhaps a combination of both.

[Elp]

I think the shape will still be around the stars whether in focus or not, they'll just be smaller or larger. Stopping the lens down manually if possible via the aperture ring if it has one is the way to do it, otherwise you can mount step down rings onto the front of the lens to reduce the aperture to achieve the same.

A the vast majority of lenses are prone to it from my experience at the edges of the frame anyway, the larger the imaging sensor the more you'll see it.

[Ratlet]

Not achieving focus might actually mask the issue rather than cause it as it will bloat the star which might hide the effect.

If you want round stars whilst stepping down then you'll need to use Stepdown rings, otherwise you'll get some number of spikes.  

If you want to do some testing, if suggest getting a torch, poke a tiny hole in some tinfoil with a sewing needle and then cover the torch so you can only see light through the tiny hole.  If you set this as far away as possible from the lens you can use it as an artificial star.  You can use this to help diagnose the issue.

I'd start by removing everything from the imaging train and then checking how the star image in and out of focus (both sides of focus).  Check with it wide open and stepped down (either with rings or via the lens).

I used this method to diagnose astigmatism in a tair 3 lens.

[WolfieGlos]

6 hours ago, Elp said:

I think the shape will still be around the stars whether in focus or not, they'll just be smaller or larger. Stopping the lens down manually if possible via the aperture ring if it has one is the way to do it, otherwise you can mount step down rings onto the front of the lens to reduce the aperture to achieve the same.

A the vast majority of lenses are prone to it from my experience at the edges of the frame anyway, the larger the imaging sensor the more you'll see it.

I might have to invest in some step down rings then, there's no way to manually change the aperture on the lens itself.

Hook up a DSLR to it and of course you can in the camera settings, but that's it.

Must say I'm surprised by your experience there, given this is an L series lens and I thought the 200mm f/2.8 was supposed to be a good one.

4 hours ago, Ratlet said:

Not achieving focus might actually mask the issue rather than cause it as it will bloat the star which might hide the effect.

If you want round stars whilst stepping down then you'll need to use Stepdown rings, otherwise you'll get some number of spikes.  

If you want to do some testing, if suggest getting a torch, poke a tiny hole in some tinfoil with a sewing needle and then cover the torch so you can only see light through the tiny hole.  If you set this as far away as possible from the lens you can use it as an artificial star.  You can use this to help diagnose the issue.

I'd start by removing everything from the imaging train and then checking how the star image in and out of focus (both sides of focus).  Check with it wide open and stepped down (either with rings or via the lens).

I used this method to diagnose astigmatism in a tair 3 lens.

Thanks Ratlet. Yes, I discovered stepping down the aperture on the camera on one of my first ever AP sessions....for some reason I tried f/8 with this very lens....it made Webb's diffraction spikes look like nothing haha.

That's a good idea regarding the tin foil, that sounds like a great way of testing this out actually. How far away are we talking; length of a living room or bottom of the garden?

[bosun21]

6 hours ago, WolfieGlos said:

How far away are we talking; length of a living room or bottom of the garden?

Bottom of the garden will suffice assuming it's not a small garden.

Edited December 17, 2023 by bosun21
Typo

[Ratlet]

7 hours ago, WolfieGlos said:

I might have to invest in some step down rings then, there's no way to manually change the aperture on the lens itself.

Hook up a DSLR to it and of course you can in the camera settings, but that's it.

Must say I'm surprised by your experience there, given this is an L series lens and I thought the 200mm f/2.8 was supposed to be a good one.

Thanks Ratlet. Yes, I discovered stepping down the aperture on the camera on one of my first ever AP sessions....for some reason I tried f/8 with this very lens....it made Webb's diffraction spikes look like nothing haha.

That's a good idea regarding the tin foil, that sounds like a great way of testing this out actually. How far away are we talking; length of a living room or bottom of the garden?

The further away the easier it is, to a point.  You are trying to get the light to look stellar (ie like a point).  If it is too close or the pin hole too large then they will look like disks.

I did mine in the house, got maybe 5m distance and managed to diagnose my particular issue.  I think this sort of distance would work for you given the short focal length of the lens.  

[Elp]

The Canon 200mm L USM is supposed to be a good lens, especially the 2 series. With camera controlled apertures you can sometimes stop down whilst on the camera body then remove the lens and the aperture is still at the same position.

I haven't used this lens myself as the Samyang 135 more than justifies not needing any other lens even though I have a few others.

[happy-kat]

I wouldn't change anything in your next test other than ensure you have removed both the filters discovered to have been fitted at the same time when you got the Christmas tree shaped stars. I'd get a clear control test in before evaluating any other change.

[WolfieGlos]

Thanks all, I’ll have a test session and see if I can work it out.

I forgot to say that the camera works fine on stars with my 72ed and 102ed with a 1.25” ZWO UV/IR cut filter, so thinking about it now, I suspect it lies in either the 2” filter or, more likely, the lens as described. I’ll get a set of tests together and try stopping down the lens too. I was hoping to use it at f2.8 for the speed of gathering data, oh well.

[Adam J]

On 15/12/2023 at 23:31, WolfieGlos said:

Well it is the festive time of year, but nobody likes odd star shapes!

Last night I had a brief window in the clouds for a first trial run pairing my 585MC with a Canon 200mm f2.8 prime lens, primarily since it will gather data much faster than my main scope so I plan on using this for widefield imaging of nebulae.

I've had the lens for a few years, used it for astro when I first started out and it was fairly easily to achieve focus with a DSLR, although the focus adjuster is VERY sensitive to move. I've had the 585mc for a few months, but this was first light with the lens. It worked fine with both my 72ED and 102ED. So, separately, both the lens and camera have worked - but this is the first time they have been used together.

So after getting it all manually in focus last night, I was surprised to see my images producing star shapes like christmas trees (and no, I didn't leave the bahtinov mask on!)

This was not in a corner, this was across the whole field. It did seem to improve slightly if I binned 2x2 and more with 3x3 but it was still there. Example fits files attached.

I've been trying to understand the reasoning for this, it cannot be bad tracking or wind since these were 5s subs and it was riding on top of my SF102 whilst that was taking 120s subs....and the stars in those subs are round.

The pixel size of the 585mc is 2.9µm , so with the lens I would definitely be undersampling - but is this what stars look like when undersampled?

My only other thought is that I had a 2" Astronomik L2 UV/IR cut filter in the imaging train.....and after the session I found I still had a Hoya UV filter mistakenly screwed onto the front of the lens - could this actually be the culprit?

My current forecast has no clear skies for a while, so any input appreciated so I can try next time 🙂

2023-12-15_01-52-56__8.30_5.00s_0000 1x1.fits 15.82 MB · 4 downloads 2023-12-15_01-49-44__8.70_5.00s_0000 3x3.fits 1.76 MB · 6 downloads 2023-12-15_01-50-17__8.60_5.00s_0000 2x2.fits 3.96 MB · 4 downloads

Do a Star test, post a image of a bright out of focus star in the centre of the FOV, show between three and five rings. Will let me diagnose the issue. 

[WolfieGlos]

Well I finally had a test session under some clear skies two nights ago. What with Christmas I haven’t had any opportunities to try during the day.

After setting up my main scope, I then set up my Star Adventurer along with the ASIAIR (which I bought in November…first time to test it as well!) and hooked it up to the 585mc and 200mm f2.8 lens. Sadly by the time I got it all sorted I could see the clouds on the horizon and it didn’t leave me long to try a thorough test before the clouds rolled in.

Knowing that f2.8 produced the poor star shapes previously, I decided to go for broke and stopped down the lens with my DSLR to f4, removed the DSLR, attached the lens to the 585mc (with a 2” Astronomik L2 filter - no UV protection on the front this time), and grabbed some test shots.

On 28/12/2023 at 03:41, Adam J said:

Do a Star test, post an image of a bright out of focus star in the centre of the FOV, show between three and five rings. Will let me diagnose the issue. 

Thanks - here are some shots at f4, one out of focus (both sides of being infocus), one with a bahtinov mask infocus…and again without the mask. All 5s subs on gain 252:

What I do notice from the rings when out of focus is that they all look concentric.

With the mask on, the lines all go through the middle but I do notice there’s another 4th line near the top? Local to the star only. I don’t know what this is.

Finally, the last image without the mask, to my eyes, looks to be in focus - but each star appears to have a bit of a red ring around it? Could this be related to the extra line from the mask? I do seem to recall reading somewhere (I can’t find it now) that the lens focuses the red channel differently to blue/green and it might be best to have it slightly out of focus.

The focuser on this lens is extremely sensitive; a movement of a millimetre makes a huge difference so if I can get this working I think I’d be investing in an EAF for this.

Edited December 31, 2023 by WolfieGlos

[Adam J]

1 hour ago, WolfieGlos said:

Well I finally had a test session under some clear skies two nights ago. What with Christmas I haven’t had any opportunities to try during the day.

After setting up my main scope, I then set up my Star Adventurer along with the ASIAIR (which I bought in November…first time to test it as well!) and hooked it up to the 585mc and 200mm f2.8 lens. Sadly by the time I got it all sorted I could see the clouds on the horizon and it didn’t leave me long to try a thorough test before the clouds rolled in.

Knowing that f2.8 produced the poor star shapes previously, I decided to go for broke and stopped down the lens with my DSLR to f4, removed the DSLR, attached the lens to the 585mc (with a 2” Astronomik L2 filter - no UV protection on the front this time), and grabbed some test shots.

Thanks - here are some shots at f4, one out of focus (both sides of being infocus), one with a bahtinov mask infocus…and again without the mask. All 5s subs on gain 252:

What I do notice from the rings when out of focus is that they all look concentric.

With the mask on, the lines all go through the middle but I do notice there’s another 4th line near the top? Local to the star only. I don’t know what this is.

Finally, the last image without the mask, to my eyes, looks to be in focus - but each star appears to have a bit of a red ring around it? Could this be related to the extra line from the mask? I do seem to recall reading somewhere (I can’t find it now) that the lens focuses the red channel differently to blue/green and it might be best to have it slightly out of focus.

The focuser on this lens is extremely sensitive; a movement of a millimetre makes a huge difference so if I can get this working I think I’d be investing in an EAF for this.

So for me the first shot is not too bad but the second is showing a collimation issue as the rings infact don't appear concentric to me. The center of the pattern appears offset to the 10oclock position. It would only be worse at 2.8. That will be your odd star shapes.  So essentially the center of the optical axis is not passing through the centre of the sensor although that in itself can be an oversimplification in these lenses with multiple elements / groupings. You might improve it a little by trying to adjust sensor tilt something that the small sensor is going to allow you to get away with a little more than a larger one but in the end you will be fighting one problem by introducing another. 

The lens is not perfectly corrected either and the red is clearly focusing to a different point than both green and blue. 

But to be honest the cannon 200mm prime was never the best astro lens, more an ok for the money solution that you should not expect perfection from. 

Adam

 

Edited December 31, 2023 by Adam J

[WolfieGlos]

1 hour ago, Adam J said:

So for me the first shot is not too bad but the second is showing a collimation issue as the rings infact don't appear concentric to me. The center of the pattern appears offset to the 10oclock position. It would only be worse at 2.8. That will be your odd star shapes.  So essentially the center of the optical axis is not passing through the centre of the sensor although that in itself can be an oversimplification in these lenses with multiple elements / groupings. You might improve it a little by trying to adjust sensor tilt something that the small sensor is going to allow you to get away with a little more than a larger one but in the end you will be fighting one problem by introducing another. 

The lens is not perfectly corrected either and the red is clearly focusing to a different point than both green and blue. 

But to be honest the cannon 200mm prime was never the best astro lens, more an ok for the money solution that you should not expect perfection from. 

Adam

 

Thanks for the quick reply Adam.

I must admit I don’t see the same collimation issue in the second image that you’re seeing (even zooming right in on the bright star), but I’ll take your word on it.

I’ve had the lens for a few years, not long before I started Astro actually, and initially I had read it was a pretty good lens for it, maybe that was just due to low cost and the fast aperture.

I might try stopping it down again and seeing if its any better at f4.5 or f5, but at that point it’s speed is basically lost.

Would you say it’s worth pursuing using the lens at all? Initially I wanted to use it for the fast aperture but if it’s only usable at slower f stops then I’d probably look to sell it and put the funds towards a redcat 51 or SY135. 

[Adam J]

5 hours ago, WolfieGlos said:

Thanks for the quick reply Adam.

I must admit I don’t see the same collimation issue in the second image that you’re seeing (even zooming right in on the bright star), but I’ll take your word on it.

I’ve had the lens for a few years, not long before I started Astro actually, and initially I had read it was a pretty good lens for it, maybe that was just due to low cost and the fast aperture.

I might try stopping it down again and seeing if its any better at f4.5 or f5, but at that point it’s speed is basically lost.

Would you say it’s worth pursuing using the lens at all? Initially I wanted to use it for the fast aperture but if it’s only usable at slower f stops then I’d probably look to sell it and put the funds towards a redcat 51 or SY135. 

There is nothing wrong with using this lens for astro many do, just don't expect perfection from it...or to different degrees anything else. It just depends on how fussy you are really and what software tools you have to mitigate where it falls short. Some people rave about blur X terminator and how it helps with stars, I tend to think it's cheating a bit and can look artificial to be honest, but many don't see it that way. But you can't get something from nothing. 

Its not a bad lens it's just some are better like the Samyang you mentioned. But personally I would move away from lenses due to the connection type and not being optimized for shooting at infinity.

For wide shots I use the FMA180 as I value image quality over speed. I find F4.5 sufficiently fast for my own use but I am not taking an image in a single night but over multiple nights so I don't need it to be. The new pro seems better than the original. The FMA 230 also seems to do well. 

Collimation of the optics really doesn't have to out by much at all at the speeds you are shooting at and it's difficult to see when using a OSC camera. I tend to star test with a mono camera and a green filter when looking for collimation. 

If you are removing stars.in processing then why not shoot with a fast f-ratio for the target data and then add stars in shot at a slower f-ratio in processing? Best of both worlds. 

Adam 

Edited January 1 by Adam J

[WolfieGlos]

Cheers Adam.

I did think of shooting at f2.8 for the background, and say f4 for stars to combine them but it would require a lot of faff having to connect a DSLR to stop down the lens - which obviously uses a different connection compared to the 585mc I intend to pair it with. Refocusing would also be a pain - so I’ve ruled this out. Another option would be to pair it with a DSLR in the first place I guess.

I like the idea of the FMA 180, I’ve just been looking at it and the FMA135 and they are well priced - much more affordable than a Redcat which I’ve always struggled to justify for the price for a similar FOV and aperture. I’ve just had a quote from Wex for the 200/2.8 and I can get £275 for it in a trade in (or cash) - pretty good considering I paid around about £350 for it. I’ll check a few other places but as a ballpark figure that will basically pay for either of those in the Askar range. Although I’m saving for an astrocam later this year (533mm or 2600mc), I can probably justify this and it seems to be the best option to get a working wide field view.

[Adam J]

1 hour ago, WolfieGlos said:

Cheers Adam.

I did think of shooting at f2.8 for the background, and say f4 for stars to combine them but it would require a lot of faff having to connect a DSLR to stop down the lens - which obviously uses a different connection compared to the 585mc I intend to pair it with. Refocusing would also be a pain - so I’ve ruled this out. Another option would be to pair it with a DSLR in the first place I guess.

I like the idea of the FMA 180, I’ve just been looking at it and the FMA135 and they are well priced - much more affordable than a Redcat which I’ve always struggled to justify for the price for a similar FOV and aperture. I’ve just had a quote from Wex for the 200/2.8 and I can get £275 for it in a trade in (or cash) - pretty good considering I paid around about £350 for it. I’ll check a few other places but as a ballpark figure that will basically pay for either of those in the Askar range. Although I’m saving for an astrocam later this year (533mm or 2600mc), I can probably justify this and it seems to be the best option to get a working wide field view.

I would go with the 180 once you have a larger sensor you would regret not having a little more focal length.