I can only applaud your dedication to this task, I also have a C6 and It really annoys me bad star shape. What I do is start at 105mm, focusing, then I capture an image and plate solve it (I also use APT). So far I have got the best (or the "least bad") results when focal length is around 1010(ish) mm. At 945mm focal length stars look terrible on my C6. I think I read somewhere on Cloudynights that this distance may vary from one scope to the other, so the only way is testing and adjusting. Or getting the Starizona SCT corrector, which is pricey but apparently improves star shape dramatically. All the best and I hope you can get the best out of your C6! Clear skies!
Thanks Sergio. I understand your pain! I think I finally found the best position and the correct collimation for mine. The missing piece for me was tilt I was not aware of due to my T-Adapter. That and the realization that auto-focus does not work nearly as well as manual focusing using a Bahtinov mask. Good luck to you, and Clear Skies!
Hello, I was happy to come across this video. I too have a 6SE (it's the AVX setup, so I think they give it a different name - and color scheme), an ASI294MC Pro, and the Celestron .63 reducer. Pretty much from day one, I have shot using this combination and the requisite 105mm, and things were ok. But a few weeks ago, I decided it would be fun to go after smaller targets. So I pulled the reducer off and went to Celestron's recommended 5" of backspacing. And things just haven't looked good. Really fat, bloated stars across the board, and those weird winged shapes on the outer edges if you zoom in enough. Details aren't crisp either, though that may be more of a guiding issue, since that's been cropping up on me now and then as well. I wanted to experiment with other backspace settings, but it's such a hassle that I just swapped the camera to my Orion 80ED and went small. Much easier to control those star shapes, but at the cost of almost 2/3 of my focal length. That's great for the big beautiful targets like Orion or the Rosette or the North America nebula (which alllllmost completely fits using a .8 reducer), but I do like those little guys like the Iris or Dumbell or Owl. So next time I'm feeling brave, I'm going to try nosepiece only on the 6SE and see what I get! Thanks again for putting in that legwork!
I was surprised by those results as well. I would have thought that there should have been at least room for a filter tray/wheel. Let me know how it works out for you. I was going to try shooting galaxies without the reducer, but I also want to be able to use the UV/IR Cut filter which makes a big difference in Bortle 8 light pollution, so there's a trade-off. Clear Skies!
Initially I'll admit I don't have direct experience with this as the only thing I shoot from the back of my C8 up to now is the planets. All my deep sky objects have been done with Hyperstar. However I'm planning on shooting from the back in the near future and I've talked with the staff at Starizona (I live in Tucson). Understand that normally if a shop recommends one of their products you'd be wary of the advice but Starizona is known for being straight shooters and giving good advice. I was discussing this with Dean (owner) and he told me the Celestron is a good reducer but not a good flattener, in fact he said that it actually can make the stars worse. They sell a reducer/flattener that has a very good reputation for flattening the field as well. It's more expensive than the Celestron but maybe worth a try.
Visually comparing the lensing distortion with and without the reducer, I can see that the reducer seems to flatten the field of view marginally, but because it also increases the FOV, the distortions are almost amplified because you see them where before you were limited by the original focal length. Let me know how the Starizona reducer works out for you!
Thanks @yvangarcia3535, I appreciate the comment! Just so you know, I'm going to be making a follow-up Back Focus + Collimation '301' video as the two topics are very much related. It is possible to get even better back focus if you understand how the star shape morphs from 'too close' to 'too far'. Clear Skies!
Thanks Zax, if you check back later this week, I'll finally be posting a review of the 6SE with wedge for imaging, summarizing what I've found over the last few years. Clear Skies!
Hi Michael, since making this video I also learned that tightening the collimation screws pushes out the secondary mirror, which changes the focal distance and results in significant lensing. You need to use a feather touch on the collimation screws. Clear Skies!
Very interesting, I’m surprised that using results from focusing only 25 feet away would be relevant for imaging at infinity for real stars. The optical light paths are quite different when you’re so close. Did you compare actual star photos for different back spacings? In any case my Meade 10” SCT has a minimum focusing distance of about 150-200 ft, which I tested once across a parking lot.
Hey Phil, good point, your scope has to be able to focus at the distance you put your laptop. I found I could focus at 20', but in the video the distance was closer to 35'. When you are too close, the pixels on the screen become too large to properly distinguish the distortion pattern. I would say that if you can use a laser collimator on your scope, this should work as well (I could be wrong). I did not record the back spacing results with actual stars, but I did try different focal lengths, that's what got me curious to see if I could run the tests indoors. I was getting distortions that no amount of collimation would fix, and swapping out seized rings at 2am and reslewing to frame a cluster to verify results was getting tedious! CS!
I've been looking for a way to make the star pattern you show on your screen. Did you do this yourself or was there some software? I'm talking about the one you used inside at a distance.
I used Paint in Windows. Black background, zoom in and colour individual pixels white. I started by making a grid in one corner, then copied and pasted across the rest of the image. It's important to make the image to be the same dimensions as your screen's resolution so that there is no stretching, and the "stars" will be a single pixel, ie the smallest size you can show.
Hi there, how do I find the star simulation you use in your laptop just in the minute 9:49 to try to find out my back focus? Thanks for the video, is just a master class on how to proceed.
Hi Reyak, I have added the files to an open Google Driver share: drive.google.com/file/d/1rC0tYoKo7RdWM2HA6O32OhZW6o3vkphP/view?usp=drive_link drive.google.com/file/d/1zb6UM152sIYTXctVIRvDFV2oATYfM7uc/view?usp=drive_link Let me know if you have trouble accessing. CS!
You are welcome! Quick tip: measure to make sure your screen center is at same height as the center of your telescope tube, and perpendicular (e.g. same distance from a wall to the side, or at right angle to a string)
Perhaps it’s my eyes and that I’m watching on any ipad but your images of fake and real starfields were difficult for me to see the stretching and differences between each. I am trying to understand all of this too, both with and without the 6.3 reducer AND when one uses a barlow in the train. After all, using a barlow must have an effect just as using a reducer but there doesn’t seem to be any knowledge online regarding this. I’ve been inserting my SVBony 305 into various trains both, fully and then pulling it out a little (not very scientific, I grant you) and while I believe it makes a difference, I cannot qualify or quantify it. So, yes, I crop. Good video and if you still work at it it would be good to see more on the topic. If you do, however, can you perhaps zoom into the fields to show the differences more clearly? Thanks for the effort taken to make this.
Hi Earthling, the differences are subtle until you really zoom in. I posted what the distortions look like when magnified at 106mm with a filter to my community tab, along with an image of NGC 40, the Bow Tie nebula, using a 2X barlow to give you an idea of how the stars get distorted. I'll post either a short or just the images of the other trials as well and let you know. Thanks for the feedback, and clear skies! www.youtube.com/@Felldisulfide/community
@@Felldisulfide Thank you. I just watched it and it was far clearer. It’s rather annoying to see that, where these manufacturers sell us field flatteners (even in our case with an SCT and the .63x reducer/flattener) they don’t do the job and these things are expensive. What’s your thoughts regarding the use of barlows which, like any other optical component (lens, filter) in the image train, must surely introduce a need for further adjusting back focus?
So far I've only broken out the barlow to view or image planets and the moon. It's hard to tell if surface features on the moon are stretched when you are looking visually, and planets are still very small, even with the barlow, so there's not much stretching happening there either as I typically keep them centered while imaging. I was going to try the barlow on a galaxy, maybe M51 in the spring, but might also use it on a globular cluster where lensing would really be an issue. I like the write-up on ASG's website (not related to barlow): www.asgastronomy.com/collimation-tilt-and-backfocus/
When you are not using a field flattener you can just sliding the camera at that point back focus distance doesn’t exist because you don’t have a field flatener lens and focusing the image is based only on the focusing mechanism of the telescope and that is variable distance depending on object your focusing on. Adding spacer at that time can only act as reference point because your inter focusing mechanism does not have unlimited travel. It is a totally different story when you are using a field flattener. With field flattener the image is focused in 2 steps the back focus is fixed focal distance to the sensor and the telescope focusing mechanism is actually focusing the image in to the field flattener. Consider it as 2 optical groups independent from each other where the flattener has a fixed focal disntance to the sensor and the telescope focussing mechanism is floating. If your back focus is out everything will go wrong. In theory you could simply test your telescope during the day by focusing on an object during day lite. If your back focus distance is wrong you will see it immediately because your image will be soft and feels like is never on focus. So in order to adjust the distance correctly you need to take defence image without a flattener . Than compare the images to the refrence image . The reference image will be sharp in the centre your images with the flattener should look equally sharp as the centre of the reference image but across the hole image not only in the centre of the back focus is correct because this is what the flatter is doing correcting for chromatic and spherical abbreviation. If the image with flattener is not as sharp you need to increase or reduce the back focus distance until you achieve sharp image. The change needs to be small maybe around 1 mm from the recommended value by the manufacturer unless you have assembled something wrong. If you are 1 mm off this will make a big difference in your image with flattener. Try doing it during the day at night your chance for success is much smaller. There are adjustable backfocuser by Askar they offer a scale and adjustment of +_ 2 mm with a micro scale this should help you solved the problem good luck. I hope this helps you.
@MickyMouseLimited that's a really great way of putting it, and conforms to what I was seeing. A challenge with the Celestron flattener, is that it never fully flattens the image, and you always get some lensing distortion and CA. Between this video, and the analysis video which follows, it looks like there's an 'as good as it gets' distance. I have another follow up video on collimation which uses ASTAP. One nice thing about ASTAP is that the inspection gives you the deflection distance across your field of view, so you can see the drop-off in focus as you get further from the center. The better the back-focus, the less deflection there would be. Unfortunately, it just goes to support what I was experiencing, which is that you never get a fully flat field of view. Thank you for the write-up, it is very informative. CS!
I can only applaud your dedication to this task, I also have a C6 and It really annoys me bad star shape. What I do is start at 105mm, focusing, then I capture an image and plate solve it (I also use APT). So far I have got the best (or the "least bad") results when focal length is around 1010(ish) mm. At 945mm focal length stars look terrible on my C6. I think I read somewhere on Cloudynights that this distance may vary from one scope to the other, so the only way is testing and adjusting. Or getting the Starizona SCT corrector, which is pricey but apparently improves star shape dramatically. All the best and I hope you can get the best out of your C6! Clear skies!
Thanks Sergio. I understand your pain! I think I finally found the best position and the correct collimation for mine. The missing piece for me was tilt I was not aware of due to my T-Adapter. That and the realization that auto-focus does not work nearly as well as manual focusing using a Bahtinov mask. Good luck to you, and Clear Skies!
Hello, I was happy to come across this video. I too have a 6SE (it's the AVX setup, so I think they give it a different name - and color scheme), an ASI294MC Pro, and the Celestron .63 reducer. Pretty much from day one, I have shot using this combination and the requisite 105mm, and things were ok. But a few weeks ago, I decided it would be fun to go after smaller targets. So I pulled the reducer off and went to Celestron's recommended 5" of backspacing. And things just haven't looked good. Really fat, bloated stars across the board, and those weird winged shapes on the outer edges if you zoom in enough. Details aren't crisp either, though that may be more of a guiding issue, since that's been cropping up on me now and then as well. I wanted to experiment with other backspace settings, but it's such a hassle that I just swapped the camera to my Orion 80ED and went small. Much easier to control those star shapes, but at the cost of almost 2/3 of my focal length. That's great for the big beautiful targets like Orion or the Rosette or the North America nebula (which alllllmost completely fits using a .8 reducer), but I do like those little guys like the Iris or Dumbell or Owl. So next time I'm feeling brave, I'm going to try nosepiece only on the 6SE and see what I get! Thanks again for putting in that legwork!
I was surprised by those results as well. I would have thought that there should have been at least room for a filter tray/wheel. Let me know how it works out for you. I was going to try shooting galaxies without the reducer, but I also want to be able to use the UV/IR Cut filter which makes a big difference in Bortle 8 light pollution, so there's a trade-off. Clear Skies!
Initially I'll admit I don't have direct experience with this as the only thing I shoot from the back of my C8 up to now is the planets. All my deep sky objects have been done with Hyperstar. However I'm planning on shooting from the back in the near future and I've talked with the staff at Starizona (I live in Tucson). Understand that normally if a shop recommends one of their products you'd be wary of the advice but Starizona is known for being straight shooters and giving good advice. I was discussing this with Dean (owner) and he told me the Celestron is a good reducer but not a good flattener, in fact he said that it actually can make the stars worse. They sell a reducer/flattener that has a very good reputation for flattening the field as well. It's more expensive than the Celestron but maybe worth a try.
Visually comparing the lensing distortion with and without the reducer, I can see that the reducer seems to flatten the field of view marginally, but because it also increases the FOV, the distortions are almost amplified because you see them where before you were limited by the original focal length. Let me know how the Starizona reducer works out for you!
Very interesting study which is going to help me to reach the correct backfocus. Fingers crossed. Great job. Thank you.
Thanks @yvangarcia3535, I appreciate the comment! Just so you know, I'm going to be making a follow-up Back Focus + Collimation '301' video as the two topics are very much related. It is possible to get even better back focus if you understand how the star shape morphs from 'too close' to 'too far'. Clear Skies!
@@Felldisulfide Great news. Clear skies
Great video. Am considering buying a 6se for astrophotography, and was wondering about back focus set up
Thanks Zax, if you check back later this week, I'll finally be posting a review of the 6SE with wedge for imaging, summarizing what I've found over the last few years. Clear Skies!
Thanks for this video. I'm going to try without the filter next clear night.
Hi Michael, since making this video I also learned that tightening the collimation screws pushes out the secondary mirror, which changes the focal distance and results in significant lensing. You need to use a feather touch on the collimation screws. Clear Skies!
Very interesting, I’m surprised that using results from focusing only 25 feet away would be relevant for imaging at infinity for real stars. The optical light paths are quite different when you’re so close. Did you compare actual star photos for different back spacings? In any case my Meade 10” SCT has a minimum focusing distance of about 150-200 ft, which I tested once across a parking lot.
Hey Phil, good point, your scope has to be able to focus at the distance you put your laptop. I found I could focus at 20', but in the video the distance was closer to 35'. When you are too close, the pixels on the screen become too large to properly distinguish the distortion pattern.
I would say that if you can use a laser collimator on your scope, this should work as well (I could be wrong).
I did not record the back spacing results with actual stars, but I did try different focal lengths, that's what got me curious to see if I could run the tests indoors. I was getting distortions that no amount of collimation would fix, and swapping out seized rings at 2am and reslewing to frame a cluster to verify results was getting tedious! CS!
I've been looking for a way to make the star pattern you show on your screen. Did you do this yourself or was there some software? I'm talking about the one you used inside at a distance.
I used Paint in Windows. Black background, zoom in and colour individual pixels white. I started by making a grid in one corner, then copied and pasted across the rest of the image. It's important to make the image to be the same dimensions as your screen's resolution so that there is no stretching, and the "stars" will be a single pixel, ie the smallest size you can show.
@@Felldisulfide Thanks. How many pixels separate each star?
You should pick spacing based on your field of view. Between 40 and 50 pixels work for me.
@@Felldisulfide Thanks!
Hi there, how do I find the star simulation you use in your laptop just in the minute 9:49 to try to find out my back focus?
Thanks for the video, is just a master class on how to proceed.
Hi Reyak, I have added the files to an open Google Driver share:
drive.google.com/file/d/1rC0tYoKo7RdWM2HA6O32OhZW6o3vkphP/view?usp=drive_link
drive.google.com/file/d/1zb6UM152sIYTXctVIRvDFV2oATYfM7uc/view?usp=drive_link
Let me know if you have trouble accessing. CS!
@@Felldisulfide thanks a million 🙏🏼
You are welcome! Quick tip: measure to make sure your screen center is at same height as the center of your telescope tube, and perpendicular (e.g. same distance from a wall to the side, or at right angle to a string)
Perhaps it’s my eyes and that I’m watching on any ipad but your images of fake and real starfields were difficult for me to see the stretching and differences between each. I am trying to understand all of this too, both with and without the 6.3 reducer AND when one uses a barlow in the train. After all, using a barlow must have an effect just as using a reducer but there doesn’t seem to be any knowledge online regarding this. I’ve been inserting my SVBony 305 into various trains both, fully and then pulling it out a little (not very scientific, I grant you) and while I believe it makes a difference, I cannot qualify or quantify it. So, yes, I crop. Good video and if you still work at it it would be good to see more on the topic. If you do, however, can you perhaps zoom into the fields to show the differences more clearly? Thanks for the effort taken to make this.
Hi Earthling, the differences are subtle until you really zoom in. I posted what the distortions look like when magnified at 106mm with a filter to my community tab, along with an image of NGC 40, the Bow Tie nebula, using a 2X barlow to give you an idea of how the stars get distorted. I'll post either a short or just the images of the other trials as well and let you know. Thanks for the feedback, and clear skies!
www.youtube.com/@Felldisulfide/community
I put together a quick video describing the results in detail:
ua-cam.com/video/daCEZNRcgrU/v-deo.html
Clear Skies!
@@Felldisulfide Thank you. I just watched it and it was far clearer. It’s rather annoying to see that, where these manufacturers sell us field flatteners (even in our case with an SCT and the .63x reducer/flattener) they don’t do the job and these things are expensive. What’s your thoughts regarding the use of barlows which, like any other optical component (lens, filter) in the image train, must surely introduce a need for further adjusting back focus?
So far I've only broken out the barlow to view or image planets and the moon. It's hard to tell if surface features on the moon are stretched when you are looking visually, and planets are still very small, even with the barlow, so there's not much stretching happening there either as I typically keep them centered while imaging. I was going to try the barlow on a galaxy, maybe M51 in the spring, but might also use it on a globular cluster where lensing would really be an issue.
I like the write-up on ASG's website (not related to barlow):
www.asgastronomy.com/collimation-tilt-and-backfocus/
When you are not using a field flattener you can just sliding the camera at that point back focus distance doesn’t exist because you don’t have a field flatener lens and focusing the image is based only on the focusing mechanism of the telescope and that is variable distance depending on object your focusing on. Adding spacer at that time can only act as reference point because your inter focusing mechanism does not have unlimited travel. It is a totally different story when you are using a field flattener. With field flattener the image is focused in 2 steps the back focus is fixed focal distance to the sensor and the telescope focusing mechanism is actually focusing the image in to the field flattener. Consider it as 2 optical groups independent from each other where the flattener has a fixed focal disntance to the sensor and the telescope focussing mechanism is floating. If your back focus is out everything will go wrong. In theory you could simply test your telescope during the day by focusing on an object during day lite. If your back focus distance is wrong you will see it immediately because your image will be soft and feels like is never on focus. So in order to adjust the distance correctly you need to take defence image without a flattener . Than compare the images to the refrence image . The reference image will be sharp in the centre your images with the flattener should look equally sharp as the centre of the reference image but across the hole image not only in the centre of the back focus is correct because this is what the flatter is doing correcting for chromatic and spherical abbreviation. If the image with flattener is not as sharp you need to increase or reduce the back focus distance until you achieve sharp image. The change needs to be small maybe around 1 mm from the recommended value by the manufacturer unless you have assembled something wrong. If you are 1 mm off this will make a big difference in your image with flattener. Try doing it during the day at night your chance for success is much smaller. There are adjustable backfocuser by Askar they offer a scale and adjustment of +_ 2 mm with a micro scale this should help you solved the problem good luck. I hope this helps you.
@MickyMouseLimited that's a really great way of putting it, and conforms to what I was seeing. A challenge with the Celestron flattener, is that it never fully flattens the image, and you always get some lensing distortion and CA. Between this video, and the analysis video which follows, it looks like there's an 'as good as it gets' distance. I have another follow up video on collimation which uses ASTAP. One nice thing about ASTAP is that the inspection gives you the deflection distance across your field of view, so you can see the drop-off in focus as you get further from the center. The better the back-focus, the less deflection there would be. Unfortunately, it just goes to support what I was experiencing, which is that you never get a fully flat field of view. Thank you for the write-up, it is very informative. CS!