36 replies to this topic

[KD5GIV]

I _did_ say it wasn't field rotation, for a couple of reasons. (Some people were fixated on that.) Since the spacing was apparently correct (for a Celestron FR - that _is_ what you have, right?) and changing it didn't change the artifact, there isn't much left other than a poor FR. 

Yes you did. A Facebook group I'm in also said it was field rotation, but I was thinking "then why have a GEM mount". As for a bad focal reducer I can't do anything about it since I bought it on cloudy nights and I have no way to prove it to be bad. I can't see anything wrong with it by just looking at it either. From what I can see there's no missing glass or scratches. It's a celestron focal reducer too, just to reiterate what it is. Maybe I can just crop out the center and call it even. But it doesn't change my bortle 25 skies.

[Alen K]

Then again maybe the image quality is variable from unit to unit and this one is on the edge of the "acceptable" tolerance (acceptable to the undoubtedly-Chinese manufacturer). I have seen some good images from SCTs with Celestron reducers but I have also seen some bad ones. You might be interested in this test of various FRs and flatteners on an SCT. Note in particular the second image, using the Celestron FR. Look familiar? 

[Iamhondo]

...streaks in stars are the same as 30sec or 120sec...

 

If that's the case, then it's not field rotation. It's kind of hard to buy the "bad FR" theory. But unfortunately, it's currently the most likely option.

 

Joe

[TrustyChords]

First of all, thanks KD5GIV and everyone else contributing here. I want to chime in, again, I’m using the identical imaging train as OP and see this nearly identical symptom with all of my images. I’m pretty well collimated, aligned, etc. Note I’m using a NexStar 5SE on an EQ6-R Pro, and this too occurs with even very short length exposures, with my tracking nails on center for hours— it’s not field rotation.

Anyways after reading this all I suspect this is just more or less just the effect of the FR on these SCT’s, particularly since the vignetting is also quite extreme (at least in my setup), and the stretched stars coincide with the darkened vignetted areas almost exactly. I’m sure someone has some more scientific explanation, but it’s my understanding that this vignetting is caused by the field being expanded a bit past where most of the light can freely pass through the whole system (so we’re getting way less light and probably light bouncing around the edges of baffle tubes, etc?).

Also I suspect that a lot of the images you’ll see with folks using this setup are just cropped out (which is exactly what I do). The drop off is pretty clear cut so you can easily crop to get crisp stars more toward the center of view. Else; perhaps we both have poor FR hardware instances.

Edited by TrustyChords, 27 July 2019 - 01:46 PM.

[KD5GIV]

Then again maybe the image quality is variable from unit to unit and this one is on the edge of the "acceptable" tolerance (acceptable to the undoubtedly-Chinese manufacturer). I have seen some good images from SCTs with Celestron reducers but I have also seen some bad ones. You might be interested in this test of various FRs and flatteners on an SCT. Note in particular the second image, using the Celestron FR. Look familiar? 

Yep that looks very familiar. It's nice it's not just me. From what I heard the Advanced VX isn't much on the high dollar mounts but it seems to be ok. But with what people were saying I thought this was proof that the mount isn't very good. You just have to know it's limits. I'll just have to crop the corners, and maybe in a few years I can get a refractor. My focal reducer was from China.

 

First of all, thanks KD5GIV and everyone else contributing here. I want to chime in, again, I’m using the identical imaging train as OP and see this nearly identical symptom with all of my images. I’m pretty well collimated, aligned, etc. Note I’m using a NexStar 5SE on an EQ6-R Pro, and this too occurs with even very short length exposures, with my tracking nails on center for hours— it’s not field rotation.

Anyways after reading this all I suspect this is just more or less just the effect of the FR on these SCT’s, particularly since the vignetting is also quite extreme (at least in my setup), and the stretched stars coincide with the darkened vignetted areas almost exactly. I’m sure someone has some more scientific explanation, but it’s my understanding that this vignetting is caused by the field being expanded a bit past where most of the light can freely pass through the whole system (so we’re getting way less light and probably light bouncing around the edges of baffle tubes, etc?).

Also I suspect that a lot of the images you’ll see with folks using this setup are just cropped out (which is exactly what I do). The drop off is pretty clear cut so you can easily crop to get crisp stars more toward the center of view. Else; perhaps we both have poor FR hardware instances.

You're welcome. Thanks for your contribution since the 5se and 6se are probably very close. The vignetting seems to be mostly on the sides of the image and maybe that has something to do with a square sensor in a round tube. I don't know about that.

[axlns]

I wanted to check with OP and the guys having same issue as OP.

Were you able to resolve this issue. I believe I have something very similar, I posted details in this thread - https://www.cloudyni...something-else/ - I really want to get rid of these radially elgongated stars 

[TrustyChords]

I wanted to check with OP and the guys having same issue as OP.

Were you able to resolve this issue. I believe I have something very similar, I posted details in this thread - https://www.cloudyni...something-else/ - I really want to get rid of these radially elgongated stars 

First of all I want to note the disclaimer that I'm using an SCT on an EQ mount, and have eliminated field rotation as the culprit.

 

I'm still under the assumption that in my case, the large sensor (Canon 1100D DSLR) coupled with the wide® field of view with the 0.63 focal reducer and extreme vignetting I see are primarily due to the narrow internals of the SCT and image train, the photons at the edges and especially diagonal corners have been interfered and/or deflected in a manner that physics just can't put back together with any sort of lens correction.

 

In other words, the corners of my images are sufficiently dark and photon-deprived that they wouldn't be of much use even if perfectly "sharp". So my workflow necessarily involves a heavy amount of cropping very early on in the processing (and often more later on).

 

Again I would hesitate to say whether your case is exactly the same as mine or not, so please just use this as anecdotal.

[KD5GIV]

I wanted to check with OP and the guys having same issue as OP.
Were you able to resolve this issue. I believe I have something very similar, I posted details in this thread - https://www.cloudyni...something-else/ - I really want to get rid of these radially elgongated stars 

Hi,
 
I'm the OP of this thread but I have pretty much given up on this telescope setup for now. I bought a Skywatcher Evostar 72mm and field flattener (which also has the elongated stars on the edges), but it's way better at imaging. I found a chart on a facebook page about astrophotography for beginners that seems to show the reason for this pattern on the Nexstar 6se is that the distance between the flattener and the sensor of the camera needs to be reduced. It actually says CCD to the flattener, but I assume a CMOS sensor is the same idea. I bought a ZWO OAG to try to use that scope more but I can't get focus on it or attach it to the field reducer. I'm not sure what to do with the SCT anymore as far as imaging. It's too bad because the SCT is better with planetary nebula than the refractor and if the stars would come out it might be really useful.

[TrustyChords]

Hi,
 
I'm the OP of this thread but I have pretty much given up on this telescope setup for now. I bought a Skywatcher Evostar 72mm and field flattener (which also has the elongated stars on the edges), but it's way better at imaging. I found a chart on a facebook page about astrophotography for beginners that seems to show the reason for this pattern on the Nexstar 6se is that the distance between the flattener and the sensor of the camera needs to be reduced. It actually says CCD to the flattener, but I assume a CMOS sensor is the same idea. I bought a ZWO OAG to try to use that scope more but I can't get focus on it or attach it to the field reducer. I'm not sure what to do with the SCT anymore as far as imaging. It's too bad because the SCT is better with planetary nebula than the refractor and if the stars would come out it might be really useful.

Any chance you have a link to this chart?

 

One thing I wanted to say about the SCT is don't forget that they can be effective for planetary nebulas, globular clusters, and double stars...without the focal reducer!  Granted you are adding to exposure time drastically-- but the effect may be worth it if you are trying to get in closer. YMMV.

 

Edit: additionally, the thing about planetary nebula is also that they are generally very small, so you can crop out all of the rough edges and still get the round stars in more than enough of the area you are interested in.

 

And then you have the 72mm for wide field and nebulas and all that good stuff..

Edited by TrustyChords, 17 September 2019 - 03:03 PM.

[KD5GIV]

I couldn't find it on the web and I also don't know the source of it but I uploaded it to my flicker page and here is the link. I'm not even sure it's right but I'm hoping it is.
https://flic.kr/p/2hgYZtk

[TrustyChords]

I couldn't find it on the web and I also don't know the source of it but I uploaded it to my flicker page and here is the link. I'm not even sure it's right but I'm hoping it is.
https://flic.kr/p/2hgYZtk

Awesome, thanks.

[axlns]

Thanks, guys, for your feedback.

I also have full-frame camera, so maybe its vingetting or something like this.

But I also see that pattern of concentric circles, whcih suggest the distance between sensor and coma corrector must be reduced - but I really cant reduce it further, and also it is per instructions  -55mm

I'll keep ivestigating/trying further.