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Messing Around with a C8 HD at F/7

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#1 Ethan Chappel

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Posted 05 February 2021 - 04:55 AM

Hi all,

 

Some of you may be familiar with my planetary work in the Solar System Imaging & Processing Major and Minor Planetary Imaging forum, but I have also done occasional deep sky imaging. My father and I have been doing some through our AT115EDT for a few years now, but this post is about my own messing around with a C8 EdgeHD. I hope to do some more serious DSO work with it in the future, but the focus right now is on getting a feel for how equipment like this should be operated.

 

Right now, I am doing tests with a Starlight Xpress SXVR-H674C and Celestron 0.7x focal reducer, which are currently joined together with the visual back that came with the C8. I doubt the camera is at the proper back focus position of 105mm with the reducer, but this is something I plan to address once I have a concrete vision for what the final configuration will be.

 

I find it nearly impossible to achieve good color balance with this camera, especially on galaxies. A few workflows have been attempted in PixInsight over the past few days, but no luck so far. I suspect this is more likely due to improper processing on my part, but I have ran out of ideas on how to solve this issue. Perhaps I can put out a sample of the data if someone else want too take a look.

 

Another sticking point in my experimentation has been the Advanced VX I use with the C8. This mount has been great for planetary imaging the past 7 years, but the same can't be said when it comes to deep sky imaging at F/7. I ended up discarding 40 out of 120 frames because of tracking issues.

 

Celestron C8 EdgeHD, 0.7x Focal Reducer, Advanced VX, Starlight Xpress SXVR-H674C,

2020-02-01: 80x60"

 

Ethan

 

integration_DBE.jpg

 

 


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#2 the Elf

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Posted 05 February 2021 - 02:50 PM

Don't get me wrong, you have taken a great image but it shows what almost everyone will tell you. The AVX is not a good match to your long focal length telescope. You loose a lot of detail because of mount problems that your scope could easily deliver. I aligned my version exactly to your image. Look at the brown dust. That is the amount of detail you see at only 1100mm focal length on an EQ6-R. Obviously I added Ha data and you could do this, too. It is the red flares that make the object so special.

 

M82_2020_Full_registered.jpg


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#3 RonaldNC

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Posted 05 February 2021 - 06:40 PM

My next major step in imaging is mounting my C8 Edge HD on my EG6-R.  I would love your opinion of the Celestron reducer and OAG... and how difficult of a transition this is for me from my current 80mm refractor.  I also plan on using the ZWO ASI 183MC Pro camera.

 

Good systemic fit???

 

BTW... Beautiful images!

Ron



#4 the Elf

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Posted 06 February 2021 - 11:33 AM

A reducer is the right thing for imaging DSOs with a long focal length. Astrophotography needs signal and signal comes for a fast scope. OAG is the way to go for pinpoint stars from instruments that are know for mirror flop or internal flex.

The systemic question pretty much boils down to the imaging scale. For the average person the seeing is 2 arcseconds in good nights, often 3 or 4. For imaging the sampling theorem applies, spacial sampling here. Nyquist and Shannon tell us sampling must be at least 2 times the highest relevant (spatial) frequency, in practical application a bit more. So for a mono camera you want something like 0.7 to 1 arcsecond per pixel. As color cameras have less resolution depending on the camera as a rule of thumb you use 2/3 of a mone, that is roughly 0.5 to 0.7 arcsec per pixel.

Please note: this is for the long exposure times for DSO imaging like 1 - 15 minutes. In planetary you can go for far more detail using very short exposure times like 50 frames per second and get occasional clear images even at 0.2 arcsec per pixel. For DSO such a scale only cuts down the signal.

So, insert your pixel pitch and focal length in a calculator and check it out!



#5 RonaldNC

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Posted 06 February 2021 - 05:19 PM

Thanks!

 

My current setup (80mm APO refractor) yields a 1.03" per pixel FOV.  My current guiding is around .5 to .7 RMS arcsecs... so I usually have nice round stars.

 

With the C8 Edge HD and .7 reducer, I will have a 0.35" per pixel FOV... which doesn't sound good with my current guiding.  So... should I further reduce, convert to a Faster configuration, or figure out how to improve guiding?

 

I really appreciate the help... want to spend my money wisely.

 

Thanks!

Ron 



#6 JamesTX

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Posted 06 February 2021 - 08:11 PM


 

I find it nearly impossible to achieve good color balance with this camera, especially on galaxies. A few workflows have been attempted in PixInsight over the past few days, but no luck so far. I suspect this is more likely due to improper processing on my part, but I have ran out of ideas on how to solve this issue. Perhaps I can put out a sample of the data if someone else want too take a look.

 

What are you using for color calibration?

 

In pixinsight I will either use PCC (PhotometicColorCalibration) or Backgroundnutralization and then color calibration. 

 

For color calibration, use a reference box around your galaxy core for the white reference and for the background reference a clean spot in your background.



#7 JamesTX

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Posted 06 February 2021 - 08:14 PM

Thanks!

 

My current setup (80mm APO refractor) yields a 1.03" per pixel FOV.  My current guiding is around .5 to .7 RMS arcsecs... so I usually have nice round stars.

 

With the C8 Edge HD and .7 reducer, I will have a 0.35" per pixel FOV... which doesn't sound good with my current guiding.  So... should I further reduce, convert to a Faster configuration, or figure out how to improve guiding?

 

I really appreciate the help... want to spend my money wisely.

 

Thanks!

Ron 

Another option is to change out the camera for one with larger pixels.  The little asi533 will get your image scale up to .53".  I'm using this exact setup (rq6r+edge8+.7R+asi533), an RMS of .7 to .5 works fine.  The chip is small but I find that the fov works pretty good for targets such as galaxies and planetary nebula.
 



#8 the Elf

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Posted 07 February 2021 - 04:20 AM

RonaldNC:

You can use the small pixels and scale the image down in processing to reduce the noise. The oversampling is not a problem on it's own, it does not harm the image. The point is your scope could be faster without loosing any real life resolution. So you are not getting enough signal and have to compensate by far more integration time. If spending the money wisely is in terms of image quality for the money there is no point in investing in the C8. First of all you have to decide which objects you want to image. If they are large the focal length is determined by the field of view you need to frame the object. In general a faster scope is always better for AP but there comes a point when things get difficult. I would not recommend a scope faster than f/5 because focus becomes more and more difficult and you have to refocus more often due to temperature change. Next all scopes have pros and cons and the cons can be solved by more money. So the basic decision is what to image and how much to invest. If you are interested in galaxies a long focal length may make sense, many emission objects are best framed at about 400 mm focal length. You might want to reduce your 80mm for DSOs.



#9 RonaldNC

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Posted 07 February 2021 - 08:18 AM

Thanks James and Elf for the advice.  It seems that my current OTAs are near the ends of the useable spectrum.

 

I really like the 80mm for DSO images (galaxy and nebula) and it handles about 60-70% of desired targets.  I like the idea of reducing it a bit for the occasions when I need a wider FOV.

 

What I'm missing is a solution for smaller objects (5-15 arcmins).  I was hoping to leverage my 8 HD Edge for this, but it sounds like the cost of making it useable may be high (camera, reducer, OAG, Fastar, etc.).  Perhaps I need to reconsider and look to buy another refractor.... maybe a 120mm?

 

So many options... so little money!  LOL!

 

Thanks again,

Ron



#10 the Elf

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Posted 08 February 2021 - 09:16 AM

 

smaller objects (5-15 arcmins)

The GSO RC8 / RC8 carbon is 1600mm focal length. I'm using mine with an OAG and the TS-CCD47 reducer which is a clone of the astrophysics. These are non flattening reducers so you can dial in the factor in quite a wide range according to your needs. I happened to land at 1100 mm focal length. That gives me a 30x90 arcmon FOV at 0.73 arcsep/pix with a 22MPix DSLR.

The RCs field is curved but not so much so I'm using it for objects smaller than 2/3 of my FOV height. An object smaller than 20 arcsec is pretty much in focus without correcting the curvature. Some people complain an RC is difficult to collimate. I never run into any trouble. Might be luck, don't know.

A reasonable alternative is the Skywatcher 190 MN, a Maksutov Newton with a flat field. Image quality is level with the RC.

 

RC8:

+ no front element, no dew heater needed (mine never fogged)

+ camera at the rear/long backfocus

+ focal length can be varied in a range

+ short tube

- must be cropped even at APS-C sensors, field is not flat

- possible hard to collimate

+/- spikes

full res example images:

https://www.elf-of-l...l_2019_full.jpg

https://www.elf-of-l...1_2020_Full.jpg

 

 

190 MN:

+ flat field to the corners

+ easy collimation

+/- no spikes

- needs dew strap, big glass element in front

- little back focus, no reducer possible

- long tube

- camera position

 



#11 RonaldNC

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Posted 08 February 2021 - 02:30 PM

These are a couple of really good suggestions!

 

I'll need to take a deeper look into both, but the 190MN looks very appealing.  However, it weighs about 30 lbs, which is a bit much with my EQ6-R's 44 lbs capacity.  I also need to research the ability to mount a electronic focuser on it.

 

Your GSO RC8 setup also looks great, but looks challenging for a rookie like me.  However, the weight is much more manageable.

 

You gave some things to think about!

Thanks,

Ron



#12 the Elf

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Posted 08 February 2021 - 06:05 PM

Here is the RC8 on the EQ6-R.

 

Berlebach_low.jpg

 

The full setup (scope, focuser, reducer, OAG, DSLR, guide cam) is 8.8 kg.

The tripod is Berlebach Planet small.


Edited by the Elf, 08 February 2021 - 06:09 PM.

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#13 JamesTX

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Posted 09 February 2021 - 01:24 AM

Thanks James and Elf for the advice.  It seems that my current OTAs are near the ends of the useable spectrum.

 

I really like the 80mm for DSO images (galaxy and nebula) and it handles about 60-70% of desired targets.  I like the idea of reducing it a bit for the occasions when I need a wider FOV.

 

What I'm missing is a solution for smaller objects (5-15 arcmins).  I was hoping to leverage my 8 HD Edge for this, but it sounds like the cost of making it useable may be high (camera, reducer, OAG, Fastar, etc.).  Perhaps I need to reconsider and look to buy another refractor.... maybe a 120mm?

 

So many options... so little money!  LOL!

 

Thanks again,

Ron

Buying an asi533 + zwo OAG is cheaper than buying a MN190.  The 8" RCs are cool scopes.. but I don't see it as an upgrade to your Edge 8HD, more of a lateral move.. maybe even a half step back.  You'd still need an OAG and your going to be at similar image scales.  You can certainly run with your 183.. i was just throwing it out there as an option.  The Edge 8 is a great imaging scope.. you already have the .7 reducer.  You're almost there..

 

A couple of my recent Edge 8 images:

 

get.jpg?insecure

 

get.jpg?insecure

 

 

Attached are couple targets that are still a work in progress for me.. I'm still collecting data on them (weather permitting!) and thus these are just quickly processed. 

There's also folks imaging EdgeHD8's on atlas class mounts (like the eq6r) that are way better than me. 

 

If you are really worried about image scale and you want to stick with your 183.. I'd run with a 70-80mm triplet.  Once you feel comfortable with the whole process.. then get a camera for that Edge and have fun with it.

Attached Thumbnails

  • WIP-M1_M82.JPG

Edited by JamesTX, 09 February 2021 - 02:05 AM.


#14 the Elf

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Posted 09 February 2021 - 02:38 AM

The RC8 with reducer in my configuration is f/5.7

The Edge is f/10, with the 0.7x reducer it is f/7

 

(7/5.7)^2 = 1.5

 

so the RC is a 50% signal boost.That is now overwhelming but it is an improvement. I would not swap the Edge for an RC8 only for the bit more fastness. If someone is close to buying a long focal length reflector I'd put the Edge 8, the 190 MN and the RC8 on the list. I don't think any of the three is standing out. They all have pros and cons.

 

I like the RC's weight distribution. As you can see on the photo it is mounted at the rear end of the dovetail where the heavy primary sits. With the reducer in place the camera is close to the end so I can track several hours over the meridian without a leg crash for most objects.


Edited by the Elf, 09 February 2021 - 02:41 AM.


#15 JamesTX

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Posted 09 February 2021 - 02:59 AM

The RC8 with reducer in my configuration is f/5.7

The Edge is f/10, with the 0.7x reducer it is f/7

 

(7/5.7)^2 = 1.5

 

so the RC is a 50% signal boost.That is now overwhelming but it is an improvement. I would not swap the Edge for an RC8 only for the bit more fastness. If someone is close to buying a long focal length reflector I'd put the Edge 8, the 190 MN and the RC8 on the list. I don't think any of the three is standing out. They all have pros and cons.

 

I like the RC's weight distribution. As you can see on the photo it is mounted at the rear end of the dovetail where the heavy primary sits. With the reducer in place the camera is close to the end so I can track several hours over the meridian without a leg crash for most objects.

Yep, I agree with this.  IMHO sacrificing a flat field for a faster reducer, thus shrinking the usable field due to cropping offsets the increased signal boost advantage somewhat (depending on target of coarse). 

 

I looked at all three of these scopes before pulling the trigger in the Edge 8. 

 

I can appreciate the weight distribution.. I had to add 5lbs of counter weight to the front, but its balanced.. tracking is no issue. I'm usually limited by seeing. 
 

 

Here's a not very good picture of my setup:

 

**edited* I should mention.. thats a 178mm cool thats being used as a guide cam... because thats what I had laying around that would get the job done.  The cooler remains off.  Not my first choice for a guide cam.. although a non-cooled 178 would be excellent.  smile.gif

Attached Thumbnails

  • eq6r_small.jpg

Edited by JamesTX, 09 February 2021 - 03:01 AM.


#16 RonaldNC

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Posted 09 February 2021 - 07:58 AM

I currently have an 80mm APO triplet on my EQ6-R that I use with the ASI183MC Pro and it works great.  I'm currently building an observatory and my setup will be mounted on the new pier within a few days.  So I feel that I have a good wide field solution.

 

Having an observatory/pier is what made me think about leveraging my Edge 8 HD... not having to carry/setup rigs back and forth from the house.  I'm leaning towards adding the reducer and OAG, then experimenting with cameras.  I will try the ASI183MC... but unless my guiding improves appreciatively, I may not like the results.  I've even thought about trying my ASI224MC... however its a small chip.  However, it may provide some feedback on the suitability of the image scale.  Then I can make a decision about buying a ASI533, ASI294 or ASI071.

 

Thanks for all the input!

 

Ron



#17 the Elf

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Posted 09 February 2021 - 03:18 PM

Ron,

 

guiding the EQ6-R is not a problem imho. I trained PEC to mine and have PEC active while guiding. PEC is correcting in advance, guiding only reacts to what has already started to go wrong. In nights with good seeing the guider is really not doing a lot. Often there are no guide signals for 20+ seconds because the mount is just doing the right thing. I have no real proof (like a side by side comparison) but I see that my FWHM is about 0.5 arcsecs lower since I have the mount on the ash wood tripod (Berlebach Planet small). It is damping vibrations what the stock steel tripod cannot do. I have below 2.5 arcsec FWHM on 15 min Ha subs no matter if I image at 0.7 or 1.8 arcsec/pixel. Both scopes are guided by OAG. Being seeing limited is the best you can achieve. If you have better seeing you might be limited by the mount. I am not in the luxurious position to drop all the bad subs with an FWHM above 2.0 arcsecs like some guys from desert states do. Probably most of us aren't.



#18 RonaldNC

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Posted 09 February 2021 - 03:51 PM

Yes, you are right.  I live in Bortle 5 zone... so sometimes it's good... sometimes not.  I have not had a good seeing night since owning the EQ6-R.  I'm anxious to see how good the guiding is on a clear night.

 

The last couple of nights I have been using PHD2's Predictive PEC for RA algorithm on top of multi-star guiding.  Even in below average seeing, I'm getting around 0.8 to 1.1 RMS.  My expectations are high with good seeing.

 

This is where I see a lot of the "fun" in AP.  I'm looking forward to operating from my observatory and pier... and for experimenting with different equipment.

 

Your (James and Elf) guidance has been stellar!

Ron



#19 Moontan13

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Posted 09 February 2021 - 08:31 PM

Here is the RC8 on the EQ6-R.

 

attachicon.gifBerlebach_low.jpg

 

The full setup (scope, focuser, reducer, OAG, DSLR, guide cam) is 8.8 kg.

The tripod is Berlebach Planet small.

That's a very nice tripod. I prefer wood and have 2 scopes in need of wood legs.



#20 the Elf

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Posted 10 February 2021 - 02:33 AM

They ship worldwide afaik.

https://www.berlebach.de/en/




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