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Hello, I have an F/4 Newtonian with a ZWO ASI 1600 + Filter wheel + coma corrector. Before the ZWO ASI camera I had a Canon 600D that always gave me nice round stars up to the corners. Since I started using the ZWO Camera I started getting these sort of "hollow" stars - most evident on the bright ones - and also I have strong residual coma with the stars at the corners (see image; Full size version here) .  I'm attaching also an image of the collimation I did last weekend, which looks decent to me, but I might be wrong:  I believe that there are two effects acting simultaneously: the hollow stars are likely caused by flexure of the optical train due to the heavy weight and larger torque moment of the imaging system (camera + filter wheel + coma corrector) compared to the Canon 600D (lighter and also more compact, with a smaller moment); the coma in the corners I suspect is caused instead by the camera back focus being not precisely 55mm as in the Canon 600D, hence causing the residual coma. Any thoughts and suggestion on how to tackle these issues are very much welcome! Clear skyes, Michele |
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Whoa... From what I can see, you have a lot of field curvature plus a huge amount of astigmatism with maybe some coma that's being overwhelmed by everything else. Assuming that you are using the correct components, you've almost certainly got a spacing problem. That may be combined with an alignment problem but the spacing problem looks like it's overwhelming everything else. If you had this set up working well with a larger sensor, what changed when you swapped cameras? Have you double checked the mechanical spacings and orientation of the components? John |
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I suspect the problem is reflections from the filter. I don't think is has anything to to with collimation, spacing or flexure. It's a really bad case. Edit: Sorry. I was looking at the wrong thing. I guess it is spacing plus some collimation error. |
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| It looks more astigmatic field curvature than coma to me (hard to judge without looking at the original raw) but still related to spacing. I doubt that the weight of the imaging train has anything to do with the "hollowing" of the stars. If I recall right there is an issue with the ASI1600 Panasonic sensor, often discussed in Astrobin, which may cause this effect. If you want to check the collimation you should take a shot (without correctors) at an out-of-focus bright star right on the optical axis and that would tell all there is to know. |
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Thanks everybody for the kind answers...I cannot believe I could be so carelss...the good thing about discussions is that they trigger ideas and thoughts perhaps buried in one's mind...In wanted to double check that I have the spacers assembled in the right way and I just realised that I have the filter wheel assembled the wrong way round...the filters don't sit precisely in the middle of the EFW so I suppose that this has an effect on possible reflections; moreover I remember spending some time trying to find the "right" side of each of the filters, so on top of everything I've used the filters in wrong orientations. I'm not sure this error explains all the problems I have... perhaps I still need working on the correct spacing  |
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Changing the spacing of the FW will affect the stray reflections around the bright stars but with the same spacers that won't change the field aberrations. I think that you might still have a problem with the spacing between the camera and the coma corrector. I suggest checking: 1) That the coma corrector is properly oriented (i.e. not flipped front to back). 2) That the spacing between the coma corrector and the camera is correct to within a millimeter. 3) That the telescope is properly aligned. When everything is correct, you should see round, pinpoint stars into the corners of the sensor. John |
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Hi Michele, From your description, you're explicitly stating that you are using a coma corrector with the ZWO. For the Canon you don't explicitly say it but I assume you're also using it with the Canon, right? The back focus of the ZWO you have is 17.5mm measured from the beginning of the T2-adapter (I assume you use the T2). So to get the 55 of the Canon, you need some more distance but your filter wheel might take quite some space already, so you shouldn't need a lot. The distortion of the stars is radial. The stars in the center look good. It's not coma from a bad collimation. If it were, you'd also see it in the center of the image. CS! Björn |
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I think the collimation is quite good. The aberrations are quite symmetric around the center. No wonder you get large reflections if you mountet the filters the wrong way. Narrowband filters are often mirror-like on one side. I agree that the abberrations look like off axis astigmatism and not coma, but in either case, the spacing must be way off. |
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Erlend Langsrud: The optimum orientation of thin film filters seems to be a point of widespread confusion so allow me to explain this in some detail. The key issue centers on how the filter might generate stray light, which normally comes from light reflected from the sensor itself. Strays can also come from the other components between the filter and the sensor such as a cover glass, but here I'm going to focus just on the stray light coming from a bounce off of the sensor surface. Since the sensor surface sits precisely at the image plane, the size of any stray will be determined by the separation between the filter and the sensor along with the focal ratio of the telescope. The larger the separation, the larger the stray. As the stray gets bigger, the irradiance of the stray decreases making it less of a problem. We can look at how much optical power gets dumped into this stray reflection pretty easily. Each surface has three things that it can do to an incident beam. It can reflect it, transmit it, or absorb it. No surface is perfect so absorption is never identically zero but for most common optical coatings the absorption values are very small, so for this calculation we are going to ignore absorption and consider only reflection and transmission. Under these circumstances, the conservation of energy requires that T=1-R. This reduces the task of computing the transmitted power to a matter of simple book keeping. The figure below shows two configurations--one for the thin film (shiny side) up and the other for the thin film down (closest to the sensor). Here, we simply consider the amount of optical power that makes it into the stray from a bounce from the thin film layer. I've picked fairly typical values for the performance of the thin film coating and the AR coating on the other side. I'm also assuming a reflectivity of the sensor to be roughly 40% for a typical sensor but that will vary with the type of sensor, fill factor, and coatings on a micro lens array. Either way, that value won't change the relative results for our comparison. As you can see, in this case, the orientation of the filter does indeed make a small difference in the total power going into the stray. The ratio of the differences is driven by a factor of T2^2 (98%). So, putting the shiny side down puts a little more power into the stray. Furthermore, the size of the stray will be a bit smaller in size, which will increase the irradiance. How much so depends on the focal ratio of the telescope. In this case, placing the shiny side up is preferable.  But wait a minute, we aren't done yet! This first calculation only shows the effect of the single stray from the thin film coating. What happens if we add the stray light reflected from the AR coated side of the filter? This second diagram (below) shows that situation. In this case, we can see that the power going into the two (mostly) overlapping strays is nearly identical at 2.2% of the total incident power. So which orientation is better? The answer is that they are both very close; however, there is still a very small benefit to putting the shiny side up simply because it will make the stray reflection slightly bigger. Since the optical power is nearly the same, spreading that power over a slightly larger area will reduce the irradiance by a small amount and that's what counts. However most filters are pretty thin (commonly 3 mm) so in practical terms, you'll be very hard pressed to see much of a difference between the two orientations. Conclusion: Filter orientation won't make much difference.  |
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Hi all, thanks again everyone for the suggestions, the technical analysis (special mention for @John Hayes , fantastic post about reflections!) and for helping me identifying the issues with the image. So, as Shakespeare would say, "Much ado for nothing": it turns out I was missing a 16.5mm M48-T2 spacer which is supplied by ZWO with the camera (it was literally sitting in front of my eyes!) and it is also suggested by ZWO in their guide about reaching 55mm backfocus! |
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Well, there you go! As we all learn, this is a pastime that requires a lot of attention to detail. I'm glad to hear that you found the problem. John |
