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Imaging with Nexstar 8se question

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#1 gatehealing

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Posted 18 April 2021 - 07:43 PM

While I have another rig for imaging DSO, my 8se reflector seems like an easier option to start with bc the mount is smaller and the scope is lighter.

 

My question pertains to the order of stuff on the tube. Does this look correct?

Scope > F6.3 reducer > Celestron T adapter/ring > Extender(s)** > asi224mc cool camera (NOT 294mc cool)

 

**I have 2 extenders: one is 16.5mm, the other is 21mm. I can use one or both, but I have to use at least one at this point so the camera can be attached. So my 2nd question is under what circumstances would I use the 16.5mm, the 21mm, and both?

 

Thanks y'all!

Jon



#2 Mr. Pepap

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Posted 18 April 2021 - 08:08 PM

First of all, the 8SE is not a reflector, it is a Schmidt-Cassegrain Telescope, or SCT for short. It has a really long focal length, making it suitable only for smaller DSOs.

 

Contrary to popular beliefs held on this site, SCTs are NOT bad scopes for imaging, they are bad to LEARN with. You will find that refractors are MUCH easier to learn with, and you will get better quality images as a result.

 

Now, let's just say you decide to continue with the C8. If you are serious about doing any sort of DSO AP, you'll need a lot of equipment, and the cost can get really high really fast. Be sure you are making informed purchases! I'm not the kind of guy to go around recommending a bunch of stuff because it costs money, so take what I'm about to say with a grain of salt. 

 

If you are using the mount that came out of the box, you need to change that ASAP. With imaging, the single most important piece of equipment in your toolkit is the mount. If you plan on doing any serious deep sky imaging, you need an equatorial mount. A regular Alt-Az mount will limit you to extremely short exposures (usually 30 seconds or less), which is no good for some of the fainter DSOs. By the looks of things, you already have an EQ6-R Pro. This is the exact mount that I have, and it is great!

 

Now that's settled, next up is your imaging train. So far it looks OK, F/6.3 reducer is definitely a priority, obviously something to fit your camera, etc. You'll need an autoguiding solution to go with all this as well. I like OAGs, they fit nicely onto the C8 and pair well with the ASI174MM-Mini. Highly recommend. You'll need to bring your PC along with you and hook it up to PHD2 or some other guiding software. 

 

--------

 

OK, now after that rant, I'll get to your main question: the extenders. Let's assume for a moment that you're going with the f/6.3 reducer, OAG, and ASI224MC. The thing you need to look out for is backfocus, which is the distance between the camera sensor and the reducer. There is no formula for this, It's a process that takes time to learn. However, you can get a good estimate. Generally speaking, the backfocus distance for the f/6.3 reducer is 105mm. The OAG's body length is 29mm, and the distance between the sensor and the lens of the ASI224MC is 12.5mm, meaning that somehow you're going to have to come up with 63.5mm of spacing. Celestron's OAG comes with some spacers so you can play around with those and the extension tubes to get yourself to 105mm. 

 

Clear skies and good luck!


Edited by Mr. Pepap, 19 April 2021 - 06:29 AM.

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#3 barbarosa

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Posted 18 April 2021 - 08:17 PM

You are looking for about 105mm from reducer to the sensor. You won't notice it if the actual spacing is a bit less or more. You should measure the T adapter but I think it is about 45mm. So you want 105 - 45- 17.5 (sensor recess) =42.5. So you need from 40 to 45mm in additional T rings. You have 37mm (16.5 + 21 = 37). That will is close enough for use on a C-8 SCT. All that it means is that  your won't have exactly .63 reduction.

 

If you plan to use a filter holder subtract that thickness.


Edited by barbarosa, 18 April 2021 - 08:19 PM.


#4 jupiter122

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Posted 18 April 2021 - 08:17 PM

Sorry Mr. Pepap, but a “reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image.” The SCT is a type of reflector.

#5 Mr. Pepap

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Posted 18 April 2021 - 08:23 PM

Sorry Mr. Pepap, but a “reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image.” The SCT is a type of reflector.

Thank you for the correction, jupiter122! Of course, it is a type of reflector, but that's kind of the point. Since the SCT is a type of reflector that uses a combination of curved mirrors that you mentioned, it has a really large focal length, making it ideal for smaller DSOs, but would not be suitable for imaging larger objects such as M31 or the Veil Nebula. 



#6 EdDixon

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Posted 18 April 2021 - 08:32 PM

I too started with an 8SE.  It’s a good scope for moon and planetary images.  For DSO things, not so much, especially with the Alt/Az mount.  Combine that with the ASI224MC camera, which is higher mag, makes it even harder.

 

Doing DSO requires longer exposures.  You will need a GEM mount of some sort to get that, as well as a guide camera.  It would also be better to have a wider field camera, like the ASI294MC, for DSO.

 

What I have today, for the 8SE, is a Celestron AVX mount, ASI120mm-S guide camera, and the ASI295MC Pro camera..  I also have an iPolar scope from iOptron which makes getting a good PA much faster and easier.  SharpCap can also do this, which I also have.  Using the reducer also helps when starting out as the wider FOV is more forgiving.



#7 Justin Fuller

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Posted 18 April 2021 - 08:46 PM

While I have another rig for imaging DSO, my 8se reflector seems like an easier option to start with bc the mount is smaller and the scope is lighter.

My question pertains to the order of stuff on the tube. Does this look correct?
Scope > F6.3 reducer > Celestron T adapter/ring > Extender(s)** > asi224mc cool camera (NOT 294mc cool)

**I have 2 extenders: one is 16.5mm, the other is 21mm. I can use one or both, but I have to use at least one at this point so the camera can be attached. So my 2nd question is under what circumstances would I use the 16.5mm, the 21mm, and both?

Thanks y'all!
Jon

What brand is the f/6.3 reducer? These can have different working distances for optimal correction.

As mentioned by Mr. Pepap, you'll want to use your EQ-6 for imaging. Imaging with the smaller Nexstar SE mount will in fact making things harder for you. I tried my hand at astrophotography with the Nexstar SE mount on a wedge, and it's definitely just luck of the draw if you get good results, no matter how well you polar align, the mount is just not a precision piece of gear and autoguiding (despite having an autoguide port) will not help you much, as there seems to be a lot of hysteresis in both dec. and r.a. axes that an autoguider cannot calibrate out/ account for. I tried autoguiding a couple times and it was just an exercise in frustration, I had less trouble just using short exposures and "lucky imaging", throwing out the bad frames and keeping the 30% or so that were acceptable.

I've attached probably the best shot I managed with the 8SE on wedge, M17. 40, ten second shots (out of probably 150) taken with a Nikon D3100 and stacked.

So it's possible to get some results, but it will not be easier than using your EQ-6, unless you're just talking in terms of carrying the EQ-6 vs. the Nexstar SE mount.

Attached Thumbnails

  • gallery_210606_321_261323.jpg

Edited by Justin Fuller, 18 April 2021 - 09:08 PM.


#8 gatehealing

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Posted 18 April 2021 - 09:09 PM

First of all, the 8SE is not a reflector, it is a Schmidt-Cassegrain Telescope, or SCT for short. It has a really long focal length, making it suitable only for smaller DSOs.

 

Contrary to popular beliefs held on this site, SCTs are NOT bad scopes for imaging, they are bad to LEARN with. You will find that reflectors are MUCH easier to learn with, and you will get better quality images as a result.

 

Now, let's just say you decide to continue with the C8. If you are serious about doing any sort of DSO AP, you'll need a lot of equipment, and the cost can get really high really fast. Be sure you are making informed purchases! I'm not the kind of guy to go around recommending a bunch of stuff because it costs money, so take what I'm about to say with a grain of salt. 

 

If you are using the mount that came out of the box, you need to change that ASAP. With imaging, the single most important piece of equipment in your toolkit is the mount. If you plan on doing any serious deep sky imaging, you need an equatorial mount. A regular Alt-Az mount will limit you to extremely short exposures (usually 30 seconds or less), which is no good for some of the fainter DSOs. By the looks of things, you already have an EQ6-R Pro. This is the exact mount that I have, and it is great!

 

Now that's settled, next up is your imaging train. So far it looks OK, F/6.3 reducer is definitely a priority, obviously something to fit your camera, etc. You'll need an autoguiding solution to go with all this as well. I like OAGs, they fit nicely onto the C8 and pair well with the ASI174MM-Mini. Highly recommend. You'll need to bring your PC along with you and hook it up to PHD2 or some other guiding software. 

 

--------

 

OK, now after that rant, I'll get to your main question: the extenders. Let's assume for a moment that you're going with the f/6.3 reducer, OAG, and ASI224MC. The thing you need to look out for is backfocus, which is the distance between the camera sensor and the reducer. There is no formula for this, It's a process that takes time to learn. However, you can get a good estimate. Generally speaking, the backfocus distance for the f/6.3 reducer is 105mm. The OAG's body length is 29mm, and the distance between the sensor and the lens of the ASI224MC is 12.5mm, meaning that somehow you're going to have to come up with 63.5mm of spacing. Celestron's OAG comes with some spacers so you can play around with those and the extension tubes to get yourself to 105mm. 

 

Clear skies and good luck!

Thanks! I do have another setup for actual imaging: WO Star 71 5-element apo (the asi224mc cool goes on this) with a 60mm Orion guide scope with an asi290mm mini, all mounted on an EQ6R Pro (I got something that big to grow into), and an asiair pro, and my laptops (plus the external battery pack to power it all). I have a handful of software programs including sharpcap, phd2, the asi suite, etc. etc. to figure out what I like best. For now, I just want to get the hang of what that 224mc cool is doing and how it plays with the software.

 

This one is just going to be easier to get the hang of software with since it's so much lighter to take down from the 3rd floor frequently. .. should've been clearer about that. My jacked-up back makes lugging a 50+pound (plus counterweight) mount down 2 flights of stairs a bit of a challenge till I'm all straightened out.



#9 gatehealing

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Posted 18 April 2021 - 09:10 PM

You are looking for about 105mm from reducer to the sensor. You won't notice it if the actual spacing is a bit less or more. You should measure the T adapter but I think it is about 45mm. So you want 105 - 45- 17.5 (sensor recess) =42.5. So you need from 40 to 45mm in additional T rings. You have 37mm (16.5 + 21 = 37). That will is close enough for use on a C-8 SCT. All that it means is that  your won't have exactly .63 reduction.

 

If you plan to use a filter holder subtract that thickness.

Thanks! Its actually a Nexstar 8se SCT, not a C8 (unless those are different names for the same thing?). Does that change any of this?
 



#10 gatehealing

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Posted 18 April 2021 - 09:12 PM

Thank you for the correction, jupiter122! Of course, it is a type of reflector, but that's kind of the point. Since the SCT is a type of reflector that uses a combination of curved mirrors that you mentioned, it has a really large focal length, making it ideal for smaller DSOs, but would not be suitable for imaging larger objects such as M31 or the Veil Nebula. 

This hereby marks one of the first times I completely understand what somebody has said about why something works for something! lol.gif
 



#11 gatehealing

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Posted 18 April 2021 - 09:13 PM

I too started with an 8SE.  It’s a good scope for moon and planetary images.  For DSO things, not so much, especially with the Alt/Az mount.  Combine that with the ASI224MC camera, which is higher mag, makes it even harder.

 

Doing DSO requires longer exposures.  You will need a GEM mount of some sort to get that, as well as a guide camera.  It would also be better to have a wider field camera, like the ASI294MC, for DSO.

 

What I have today, for the 8SE, is a Celestron AVX mount, ASI120mm-S guide camera, and the ASI295MC Pro camera..  I also have an iPolar scope from iOptron which makes getting a good PA much faster and easier.  SharpCap can also do this, which I also have.  Using the reducer also helps when starting out as the wider FOV is more forgiving.

Yeah, I'll probably sell the 224 at some point, but it'll be good for the moment since I've spent so much already (it came with the WO rig that I'll actually be using for EAA/imaging once I learn more).
 



#12 gatehealing

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Posted 18 April 2021 - 09:16 PM

What brand is the f/6.3 reducer? These can have different working distances for optimal correction.

As mentioned by Mr. Pepap, you'll want to use your EQ-6 for imaging. Imaging with the smaller Nexstar SE mount will in fact making things harder for you. I tried my hand at astrophotography with the Nexstar SE mount on a wedge, and it's definitely just luck of the draw if you get good results, no matter how well you polar align, the mount is just not a precision piece of gear and autoguiding (despite having an autoguide port) will not help you much, as there seems to be a lot of hysteresis in both dec. and r.a. axes that an autoguider cannot calibrate out/ account for. I tried autoguiding a couple times and it was just an exercise in frustration, I had less trouble just using short exposures and "lucky imaging", throwing out the bad frames and keeping the 30% or so that were acceptable.

I've attached probably the best shot I managed with the 8SE on wedge, M17. 40, ten second shots (out of probably 150) taken with a Nikon D3100 and stacked.

So it's possible to get some results, but it will not be easier than using your EQ-6, unless you're just talking in terms of carrying the EQ-6 vs. the Nexstar SE mount.

Yup. Partly just talking lugging the EQ6R around. Partly talking about figuring out how to use the software with an actual scope looking at something vs reading about it or watching youtube.

 

Now what exactly does that wedge do other than put the mount at a 45 degree angle?
 



#13 Justin Fuller

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Posted 18 April 2021 - 09:36 PM

Yup. Partly just talking lugging the EQ6R around. Partly talking about figuring out how to use the software with an actual scope looking at something vs reading about it or watching youtube.

Now what exactly does that wedge do other than put the mount at a 45 degree angle?


https://www.celestro...nt-with-a-wedge

This will spell it out as well as I can. Though I'll add it only places the mount at a 45 degree angle if you happen to live at 45° N or S latitude.

#14 Mr. Pepap

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Posted 19 April 2021 - 06:46 AM

Thanks! Its actually a Nexstar 8se SCT, not a C8 (unless those are different names for the same thing?). Does that change any of this?

Yes, people often shorten the names of things on this website, and no, it doesn't really affect anything. C8 is for the NexStar 8SE (or the Edge HD 8"), z12 for the Zhumell 12" Dob, WO81 for the William Optics ZenithStar 81 APO, etc. 



#15 rj144

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Posted 19 April 2021 - 09:25 AM

First of all, the 8SE is not a reflector, it is a Schmidt-Cassegrain Telescope, or SCT for short. It has a really long focal length, making it suitable only for smaller DSOs.

 

Contrary to popular beliefs held on this site, SCTs are NOT bad scopes for imaging, they are bad to LEARN with. You will find that refractors are MUCH easier to learn with, and you will get better quality images as a result.

 

Now, let's just say you decide to continue with the C8. If you are serious about doing any sort of DSO AP, you'll need a lot of equipment, and the cost can get really high really fast. Be sure you are making informed purchases! I'm not the kind of guy to go around recommending a bunch of stuff because it costs money, so take what I'm about to say with a grain of salt. 

 

If you are using the mount that came out of the box, you need to change that ASAP. With imaging, the single most important piece of equipment in your toolkit is the mount. If you plan on doing any serious deep sky imaging, you need an equatorial mount. A regular Alt-Az mount will limit you to extremely short exposures (usually 30 seconds or less), which is no good for some of the fainter DSOs. By the looks of things, you already have an EQ6-R Pro. This is the exact mount that I have, and it is great!

 

Now that's settled, next up is your imaging train. So far it looks OK, F/6.3 reducer is definitely a priority, obviously something to fit your camera, etc. You'll need an autoguiding solution to go with all this as well. I like OAGs, they fit nicely onto the C8 and pair well with the ASI174MM-Mini. Highly recommend. You'll need to bring your PC along with you and hook it up to PHD2 or some other guiding software. 

 

--------

 

OK, now after that rant, I'll get to your main question: the extenders. Let's assume for a moment that you're going with the f/6.3 reducer, OAG, and ASI224MC. The thing you need to look out for is backfocus, which is the distance between the camera sensor and the reducer. There is no formula for this, It's a process that takes time to learn. However, you can get a good estimate. Generally speaking, the backfocus distance for the f/6.3 reducer is 105mm. The OAG's body length is 29mm, and the distance between the sensor and the lens of the ASI224MC is 12.5mm, meaning that somehow you're going to have to come up with 63.5mm of spacing. Celestron's OAG comes with some spacers so you can play around with those and the extension tubes to get yourself to 105mm. 

 

Clear skies and good luck!

 

Agree with most of this, but other than the SCT not being a reflector, the statement 30 second subs are no good for DSO is not quite accurate I don't think.  All of my shots are 30 seconds or less and they're fine.  As long as the total integration is long, it's fine.   Alt/Az is not great due to the field rotation.


Edited by rj144, 19 April 2021 - 09:25 AM.


#16 EdDixon

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Posted 19 April 2021 - 10:38 AM

Yeah, I'll probably sell the 224 at some point, but it'll be good for the moment since I've spent so much already (it came with the WO rig that I'll actually be using for EAA/imaging once I learn more).
 

I have both the ASI224MC and the ASI294MC Pro cameras.  I still use the 224MC for planetary imaging which works great.  Keeping it is probably a good idea if you can.  Anything like Jupiter or Saturn needs more mag for reasonable images.



#17 Mr. Pepap

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Posted 19 April 2021 - 10:56 AM

Agree with most of this, but other than the SCT not being a reflector, the statement 30 second subs are no good for DSO is not quite accurate I don't think.  All of my shots are 30 seconds or less and they're fine.  As long as the total integration is long, it's fine.   Alt/Az is not great due to the field rotation.

You are right, but I was talking mostly in terms of fainter DSOs. Field rotation is definitely a concern and limits you to <30 sec-ish exposures, which are perfectly fine... if you get enough subs. For a beginner, faster f/stop scopes are best because you can take a handful of longer exposures and process those. For slow scopes with an Alt-Az mount, this would require much more subs and integration time to get the same effect, which can be a hassle. 



#18 gatehealing

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Posted 19 April 2021 - 12:17 PM

You are right, but I was talking mostly in terms of fainter DSOs. Field rotation is definitely a concern and limits you to <30 sec-ish exposures, which are perfectly fine... if you get enough subs. For a beginner, faster f/stop scopes are best because you can take a handful of longer exposures and process those. For slow scopes with an Alt-Az mount, this would require much more subs and integration time to get the same effect, which can be a hassle. 

I have a cgem as well (eq6r pro). . . .it's just heavy as heck so I won't use it as much . . .but for DSO I'll be using it for imaging with my WO Star71 five element apo rig
 




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