36 replies to this topic

[KD5GIV]

My imaging train is Nexstar 6se, f6.3 focal reducer, T-adapter, T-ring, Canon T6. I don't think this is right although I don't know another way. If I unfocus a star with the focal reducer on the telescope isn't collimated, but without the focal reducer it seems fine. The whole image train is by Celestron products, except the camera. Doesn't it seem like something is fishy? I'm looking for advice on another way to connect the DSLR if this isn't right. My mount isn't suited for a focal length of 1500mm. Images on the edge this way are long but at the center they aren't elongated, as much.

[Iamhondo]

Why don't you think that's the correct arrangement? It seems perfect. There are other ways to connect the scope and camera. But they're less efficient and more complicated than the arrangement you list.

 

Why do you think the scope is suddenly not collimated with the FR installed? Optical modifiers (flatteners, reducers, barlows) don't change the collimation of a scope.

 

Seeing curved stars at the edge of your FOV is normal for long exposures with an alt-az mount. It's called "field rotation". It's due to the fact that the sky objects rotate according to Earth's axis while your telescope is rotating around a vertical access. Equatorial mounts are aligned to Earth's axis so they don't show field rotation. You can convert your mount to equatorial with a "wedge" and then used the equatorial alignment options on the hand controller to setup. Of course, you need to adjust the wedge so your azimuth axis is aligned with the north (or south) celestial pole.

 

A wedge is the technical solution for your curved stars. But it's a solution with mixed results. Some people swear by a wedge and many swear at it. The SE design and construction is primarily designed for visual use. It's still very good for solar system imaging. But for long exposure, deep-sky stuff its design is less than optimal. (Again, expect to hear a variety of opinions.)

 

One important thing: You need the correct T-adapter for your scope. Celestron makes two: one for the EdgeHD style scopes and another for all other SCTs. If you can post a photo of all your pieces connected, we can tell if it's right and if you have the right t-adapter. (The EdgeHD T-adapter is really just a standard SCT t-adapter with a removable extension tube.

 

HTH,

Joe

[bobzeq25]

I'm looking for advice on another way to connect the DSLR if this isn't right. My mount isn't suited for a focal length of 1500mm.

Bingo. 

 

Advice.  You need a much shorter scope, lighter is also good with the AVX.  Something like this would be a good choice.

 

https://www.astronom...fpl-53-f-6.html

 

There is no magic here.  Long focal lengths magnify very small errors.  Physics.  Shortening the focal length is the solution.

 

This was an 8, but the principle is the same.

 

"I regret spending the first 6 months trying to learn imaging with an 8" Edge on an AVX mount. I lucked out and got one of the good AVX's, but with that scope/mount combo it was a losing effort. Fortunately got a nice little refractor, and not only have the quality of my images improved but I'm actually enjoying the process of learning how to do it!"

[zxx]

My imaging train is Nexstar 6se, f6.3 focal reducer, T-adapter, T-ring, Canon T6. I don't think this is right although I don't know another way. If I unfocus a star with the focal reducer on the telescope isn't collimated, but without the focal reducer it seems fine. The whole image train is by Celestron products, except the camera. Doesn't it seem like something is fishy? I'm looking for advice on another way to connect the DSLR if this isn't right. My mount isn't suited for a focal length of 1500mm. Images on the edge this way are long but at the center they aren't elongated, as much.

Sure your distance between FR and camera sensor is correct ?

[KD5GIV]

I figured this was the right imaging train, but with stars in the edges being streaks I thought something might be wrong. I'll have to use this setup for a while because its all I will have money for for a long time. That astrotech scope looks nice but even after I could get it, I guess I would also need a flatter too. Of coarse it might be next year before that scope is in stock again. I have been looking at the 72mm Evostar refractor too at the same price. I'm watching here on cloudy nights for one on sale too.

 

I adjusted collimation and took some more test shots. I hope it wasn't a mistake because when the out of focus star donut was large I couldn't see it as out of collimation, but as I got close to focus it was obviously out of collimation. The stars looked better at the outer edges. But it could be better tracking, I'm using sharpcap for polar alignment now. I'll post some unedited images here after this reply.

[KD5GIV]

Here's an example of stars in the center being fine and on the edge streaks. I used this imagine train in the original post. It's a guided image of the dumbbell nebula. The focal length is 945mm with the focal reducer.

https://flic.kr/p/2gfzJ7n

[KD5GIV]

A test shot after collumation of the double cluster. The stars in the center aren't bad but still on the right side in the vignetting the stars are still streaks. It's like the vignetting is smashing the stars into streaks that follow the curvature of the scope. There's no editing on this shot, just a jpg version out of the camera. I was guiding with phd2 at the time and the total error on phd2 to the left of the graph was 1.3 arcsec.

https://flic.kr/p/2gEuudH

[KD5GIV]

Sure your distance between FR and camera sensor is correct ?

 

 

I don't know about this. I just learned that all this stuff connected together was supposed to work, but I wouldn't know how to find out if it's the right distance and from what to what. The camera has a marking for where the sensor is but I don't know what to measure to from the marking."

Edited by KD5GIV, 22 July 2019 - 01:58 PM.

[zxx]

Not sure with your camera my guess is 50 to 55mm. Your image looks great in the center , how long of exposure?

Was your PA good? Looks like field rotation but not sure a 60s exposure would show that.

Edited by zxx, 22 July 2019 - 02:38 PM.

[KD5GIV]

Thanks it was 45 lights at 30s each. The polar alignment was ok I think. Probably not as good as now with SharpCap. I used all star polar alignment. This was for the M27 image.

 

The double cluster was 60sec I think.

Edited by KD5GIV, 22 July 2019 - 02:48 PM.

[zxx]

Thanks it was 45 lights at 30s each. The polar alignment was ok I think. Probably not as good as now with SharpCap. I used all star polar alignment.

I was looking at the single 60s exposure. I'm sure someone will have a fix for the problem.

[einarin]

If it's Celestron reduced the distance (from reducer shoulder to sensor) should be about 105mm.

Meade and others (Antares) may have other.

From the images I'd say your sensor is too far away from the reducer.

[zxx]

 You have a VX mount ? not the AVX, It is most likely field rotation. Or is it the AVX ?

Edited by zxx, 22 July 2019 - 04:30 PM.

[Alen K]

Unless your guidestar was right in the middle of the frames in those exposures above, it's not field rotation. In both images the arcs are symmetrical around the frame center. Furthermore, at the focal length you used your PA would have to be quite bad -worse than you could get from using a polar scope - to see field rotation of that magnitude in only 60 seconds, much less 30. Personally, I would bet on the spacing being incorrect. 

[KD5GIV]

Thanks. I have the Celestron advanced VX. I'll measure the distance but if it's wrong I have no idea how to fix that.

[zxx]

Thanks. I have the Celestron advanced VX. I'll measure the distance but if it's wrong I have no idea how to fix that.

Can you post a picture of your setup, mount,camera,scope.

[KD5GIV]

Hi,

 

I took measured 2 configurations I have and took them apart to show their configurations.

 

https://flic.kr/p/2gF7FvE

This one is 12.5cm from end to sensor. 10.2cm from back of focal reducer to sensor.

 

And

https://flic.kr/p/2gF7KjB

This one is 13.5cm from front of one end to sensor. 11.2 from back of focal reducer to sensor. This is the one I've been using. I may try the first setup above, the next time out. The first link setup might have more vignetting, but its closer to the focal reducer.

 

From these pictures I just attach the focal reducer to the back of the OTA.

Edited by KD5GIV, 23 July 2019 - 10:00 AM.

[zxx]

Hi,

 

I took measured 2 configurations I have and took them apart to show their configurations.

 

https://flic.kr/p/2gF7FvE

This one is 12.5cm from end to sensor. 10.2cm from back of focal reducer to sensor.

 

And

https://flic.kr/p/2gF7KjB

This one is 13.5cm from front of one end to sensor. 11.2 from back of focal reducer to sensor. This is the one I've been using. I may try the first setup above, the next time out. The first link setup might have more vignetting, but its closer to the focal reducer.

 

From these pictures I just attach the focal reducer to the back of the OTA.

 einarin commented 105mm , the 10.2cm configuration may be it.

Edited by zxx, 23 July 2019 - 10:30 AM.

[KD5GIV]

Thanks, I'll definitely try it next time out.

[TrustyChords]

I've been following this thread passively (since I don't have the exact answer), but wanted to chime in that I'm using the same image train on a 5SE (I use the threaded T-adapter with the celestron reducer as well), and see similar radially elongated stars around the edges as your M27 picture. Mine may not be quite as bad, but it's very close.

 

I've been suspecting that it isn't quite the proper distance (maybe off a mm or 3?), although everywhere I read (here and elsewhere) says 105 mm, and that the Celestron products you listed (reducer+T-Adapter+Ring+canon) are indeed the proper distance. Didn't verify myself, which I probably should have. Again though, be aware that I'm just guessing, and haven't tested this scientifically myself.

 

Also should note that the distance to sensor on these Canons does appear to be 44mm (I believe the T6, as well as my 1100D). So to get to 105, we'd need 61 mm for T-Adapter+ring.

Canon flange focal distance info:

https://en.wikipedia..._of_lens_mounts

 

Also from what I read is that you may get these type of elongated stars when the distance is too long for the reducer. So, I may also try the shorter setup with the non-threaded adapter that goes into the visual back itself, although I absolutely hate the sketchy loose connection it creates. I believe you're right and this will be a bit shorter. Even if that one is the proper distance, I may stick with the threaded T-Adapter still, and settle with some bad stars and just crop them. I'd rather not have the Camera jiggling around.

 

Let us know how it goes!

Edited by TrustyChords, 23 July 2019 - 03:59 PM.

[KD5GIV]

I'm going out Thursday or Friday this week to try a shorter image train to see how it goes. I'll post here the results. I know people might find this post through Google someday so I'll definitely make sure to post if it works or not.

[Iamhondo]

...

And

https://flic.kr/p/2gF7KjB

This one is 13.5cm from front of one end to sensor. 11.2 from back of focal reducer to sensor. This is the one I've been using. I may try the first setup above, the next time out. The first link setup might have more vignetting, but its closer to the focal reducer.

 

From these pictures I just attach the focal reducer to the back of the OTA.

This setup (reducer to adapter to t-ring to camera) is EXACTLY the prescribed setup for your scope. The spacing sets your sensor at the proper 55 mm distance from the reducer. You can confirm this with some daytime ground photos. The image should be very sharp to the corners.

 

If your spacing was wrong you would see distorted stars toward the corners -- seagulls, hour glasses, baby comets. Those are radial distortions aligned toward the center of the image. 

 

What your M27 shows is concentric distortions (not radial). You can confirm that your distortions are due to field rotation by multiplying the exposure length for a few subframes. If the arcs in the corner get longer with longer exposure, that validates the diagnosis of field rotation. Radial distortions (like bad spacing) wouldn't show longer arcing with longer exposures.

 

Also, the tell-tale signs of imprecise collimation is a more blah-all-over sense to the photo. Even the center of an image with poorly collimated scope will show crummy stars.

 

Finally, do your best to avoid push-fit connections for cameras. For long exposures, the tiniest of wiggles will mess your images. Try for a camera connection with 100% threaded connections -- just like your #2 setup. It's not always possible, but it's a worthy goal.

 

HTH, Joe

[KD5GIV]

Hi,

 

From what I can tell the focal distance from the camera to the focal reducer doesn't seem to make a difference. You might as well use the more stable setup with the T adapter from celestron, so the setup from the original post seems to work the best. People here already told me that. There are still star streaks though, which won't be going away with the 945mm focal length on this mount (Advanced VX). The weird thing is, using PHD2 for guiding even at 240sec of exposure the streaks in stars are the same as 30sec or 120sec. I'm using SharpCap for polar alignment and it gave an excellent alignment score when I finished it. PHD also showed polar alignment error as 0.1" because I was running guiding assistant. During the exposures in this post (below) total guiding error was around 1.40 arcsecs. I'm about to give it all up anyway, because I have no way for now to get away from light pollution that makes a bortle sky of 9 look like being on the dark side of the moon. None of these images below were processed in any way. They are all the JPG version of the shots last night downloaded from the camera.

 

This image of the Dumbbell Nebula is at ISO 800 @30s. With shorter focal distance with the camera at 12.5cm from end to sensor. 10.2cm from back of focal reducer to sensor.

https://flic.kr/p/2gHtDHh

 

This image of the Ring Nebula is at ISO 400 @120s (2 minute) with the Celestron T adapter with the longer focal distance of 13.5cm from front of one end to sensor. 11.2 from back of focal reducer to sensor.

https://flic.kr/p/2gHuvnD

 

This an image of the Ring Nebula is at ISO 200 @240s (4 minute) with the Celestron T adapter with the longer focal distance of 13.5cm from front of one end to sensor. 11.2 from back of focal reducer to sensor.

https://flic.kr/p/2gHtF5v

[zxx]

Hi,

 

From what I can tell the focal distance from the camera to the focal reducer doesn't seem to make a difference. You might as well use the more stable setup with the T adapter from celestron, so the setup from the original post seems to work the best. People here already told me that. There are still star streaks though, which won't be going away with the 945mm focal length on this mount (Advanced VX). The weird thing is, using PHD2 for guiding even at 240sec of exposure the streaks in stars are the same as 30sec or 120sec. I'm using SharpCap for polar alignment and it gave an excellent alignment score when I finished it. PHD also showed polar alignment error as 0.1" because I was running guiding assistant. During the exposures in this post (below) total guiding error was around 1.40 arcsecs. I'm about to give it all up anyway, because I have no way for now to get away from light pollution that makes a bortle sky of 9 look like being on the dark side of the moon. None of these images below were processed in any way. They are all the JPG version of the shots last night downloaded from the camera.

 

This image of the Dumbbell Nebula is at ISO 800 @30s. With shorter focal distance with the camera at 12.5cm from end to sensor. 10.2cm from back of focal reducer to sensor.

https://flic.kr/p/2gHtDHh

 

This image of the Ring Nebula is at ISO 400 @120s (2 minute) with the Celestron T adapter with the longer focal distance of 13.5cm from front of one end to sensor. 11.2 from back of focal reducer to sensor.

https://flic.kr/p/2gHuvnD

 

This an image of the Ring Nebula is at ISO 200 @240s (4 minute) with the Celestron T adapter with the longer focal distance of 13.5cm from front of one end to sensor. 11.2 from back of focal reducer to sensor.

https://flic.kr/p/2gHtF5v

That is strange, maybe a bad FR. I would start a new thread. Don't give up.....

[Alen K]

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.