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Refractor Recommendation

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#1 Mmarett

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Posted 17 November 2019 - 09:56 AM

I have a ZWO ASI 1600mm-pro. Assuming my mount can handle anything.

I’d like to make an edition to my wife-field DSO scope with a scope that can capture more detail and a more narrow field. I assume this means something around 1000focal length?

I’m a nutshell what would be a good refractor to capture high quality subs of small DSOs

I was thinking the skywatcher 120?
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#2 SeaBee1

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Posted 17 November 2019 - 10:17 AM

I have a ZWO ASI 1600mm-pro. Assuming my mount can handle anything.

I’d like to make an edition to my wife-field DSO scope with a scope that can capture more detail and a more narrow field. I assume this means something around 1000focal length?

I’m a nutshell what would be a good refractor to capture high quality subs of small DSOs

I was thinking the skywatcher 120?

 

Well... I am not into the AP thing... but I can tell you that the SW 120ED is one fine instrument for the money. One thing to note... I have read that AP with this scope will be a challenge due to the focal length... tracking errors become much more noticeable. What will you mount it on?

 

Good hunting!

 

CB


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#3 Mmarett

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Posted 17 November 2019 - 11:26 AM

Thanks for the words. I will be putting it on a skywatcher EQ6-r unless people feel that isn’t ideal.
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#4 MrRoberts

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Posted 17 November 2019 - 11:52 AM

I can tell you I am liking my new Esprit 80 here in AZ (just enjoying visual here while visiting others). But when I return home next week (Chicago area) there is a new Esprit 120 waiting for me to unbox. It is 840mm FL. The EVO120 is I believe 900mm fl. But I'm not sure FL is the primary answer to you quest for better DSO imaging. In ap a good refractor is a great start but a better mount is  more important. Although I have pondered putting my 120 on a CEM40, I suspect I am going to buy the CEM60/ipolar (non-ec) for it, and a CEM40 for the 80 and C-8/E. These days I rarely do subs greater than 3min's, and usually just use SC's "live stack" program. It's not often we get a night of good atmosphere here in NE Illinois to warrant longer subs. Plus I always have my MOD 3 for just visual enjoyment (prime and afocal). My plan is to take my 120, mount and 294 to AZ in the spring and bring the other 2 back to Illinois.

What area do you live?

Depending on average conditions....How about the Esprit 100 on an Ioptron CEM40/GEM45 or SW EQ6-R?

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#5 Mmarett

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Posted 17 November 2019 - 01:05 PM

I live in Houston, Texas. I have a dark(ish) sky spot I will travel to about 2 hours away that is about bortle 3-4. I like the Esprit 120 idea. I think a skywatcher EQ-6 will be fine. I calculate the whole payload at around 55% which should have no issues. 


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#6 SeaBee1

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Posted 18 November 2019 - 11:19 AM

Thanks for the words. I will be putting it on a skywatcher EQ6-r unless people feel that isn’t ideal.

 

I live in Houston, Texas. I have a dark(ish) sky spot I will travel to about 2 hours away that is about bortle 3-4. I like the Esprit 120 idea. I think a skywatcher EQ-6 will be fine. I calculate the whole payload at around 55% which should have no issues. 

 

I have read many good things about the EQ6-R. I have an AVX, which is good for visual use... if I was to ever try AP, I would consider the EQ6-R to be the minimal requirement. It should handle the SW120 ED fine. However, I still believe the best AP 'scope would likely be a fast 80mm APO. I see too many recommendations to argue against it. So, if I were you, I would consider the Esprit 80mm on the EQ6-R if AP is where you want to land.

 

Good hunting!

 

CB



#7 Jeff Struve

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Posted 17 December 2019 - 11:01 AM

Small DSO's?

 

I'm thinking you need more FL... I use an 80mm f6 for wide, a 152mm f 4.8 and a 152mm f9 for menium wide, an 11" f10 for medium narrow, and a 14" f11 for narrow.

 

My 11" f10 SCT barely works on an Atlas mount.... about 1 minute unguided subs... your EQ6r, IMHO is a notch up, so that may work better than a 120 for 'small' DSO's...

 

Many times I had wished that I had gotten the 9.25" Edge rather than the 11"...



#8 SonnyE

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Posted 17 December 2019 - 07:14 PM

"my wife-field DSO scope with a scope"

 

Poor thing. confused1.gif

 

"I’m a nutshell"

 

waytogo.gif  I tend to agree.

 

You remind me of my wife. She sends texts to the wrong person, calls family by the wrong names, insists she didn't do something then realizes she did. Comical! Although, I am a bit leary of what her future might bring. crazy.gif

 

In my experience, the camera's tend to determine the FOV.

My DSLR takes very wide field images. My G3 use to take very narrow FOV images. And my Atik Infinity takes almost the best FOV. (No, this is not Goldilocks and the 3 Bears.)

All of these are through 1 telescope, my ED80T CF.

 

I would recommend you do more research before you start buying telescopes and find you get the same FOV because you are using the same camera.

I would also recommend you download Stellarium, then use the Oculars part to get a sampling of what different combinations would look like.

 

And proof read your dictation before you post. blush.gif


Edited by SonnyE, 17 December 2019 - 07:15 PM.


#9 Jared

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Posted 18 December 2019 - 02:50 PM

Assuming:

- On a night of good seeing and good guiding, your skies will support 2 arc second FWHM stars

- You will be using your 1600mm camera which has 3.8 micron pixels

- Nyquist theory suggests optimum sampling (for 2” skies) of 1 arc second per pixel

 

Then you would want an aperture of 110 mm or greater to ensure resolution, even for long exposures, is not limited by the resolving power of the telescope, and you would want a focal length of 750 or 800 mm.  That would yield “optimum” sampling.  Note that a little over sampling is probably better than a little under sampling when imaging smaller objects, so anything up to 1,000 mm focal length would still be quite reasonable.  Much beyond that, and you won’t actually be capturing any additional details since you’ll already have everything typical good sky conditions would support.

 

Obviously, the more aperture you have the better until you either run out of budget or until your mount can no longer support and accurately track and guide the load.  Something in the 4” to 5” f/6 or f/7 range with flattener and/or reducer would be perfect for smaller targets without being insanely expensive.  If you start looking at faster scopes, you run the risk of losing some resolution, but your signal to noise ratio will pick up and you will need less integration time to “go deep”.  
 

Personally, I’d look at something like the SkyWatcher Esprit 120 if that’s within budget.  Easily enough aperture to deliver all the resolution typical skies will support.  The 840 mm focal length would yield 0.93 arc seconds per pixel with your camera.  No need to buy a flattener.  It’s got a good focuser.  As a compliment for your widefield scope it looks about perfect.  If that’s out of budget, the 100 mm version is also an excellent choice for smaller deep sky objects.  Either is likely to be limited in resolution primarily by your seeing conditions.

 

I’m sure there are other scopes you can look at as well.  I’m trying to give you an idea of what size and focal length is optimal for long exposures on smaller objects given your camera choice. I wouldn’t drop below about 700 mm focal length or so if I were you since this is intended to be your “high resolution/small target” scope.  Anything above about 1,300 mm focal length and you would be so over sampled with the 1600 camera that you would probably want to bin 2x2.  So anything with good color correction, a sturdy two-speed focuser, an appropriate flattener (or reducer/flattener), and a focal length between 700 mm and 100 mm should work really well for your intended use.  


Edited by Jared, 18 December 2019 - 02:56 PM.


#10 Jared

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Posted 18 December 2019 - 03:13 PM

Small DSO's?

 

I'm thinking you need more FL... I use an 80mm f6 for wide, a 152mm f 4.8 and a 152mm f9 for menium wide, an 11" f10 for medium narrow, and a 14" f11 for narrow.

 

My 11" f10 SCT barely works on an Atlas mount.... about 1 minute unguided subs... your EQ6r, IMHO is a notch up, so that may work better than a 120 for 'small' DSO's...

 

Many times I had wished that I had gotten the 9.25" Edge rather than the 11"...

His intended camera has 3.8 micron pixels, so unless he has exceptional skies and/or is planning on planetary imaging, there is no point to a focal length greater than about 1,000 mm.  At that focal length he would be at about 0.75 arc seconds per pixel which would already provide a fair amount of headroom for 2 second skies.  Even your 152 mm f/4.8 would be 1.07 arc seconds per pixel which should yield all the resolution most people could ever get for smaller DSO’s on all but the best nights.  An 11” f/10 scope would need to be binned 3x3 with the OP’s camera.  Not ideal even if the guiding is up to it.  Probably better to use something with a more moderate focal length and crop if necessary.



#11 Im2bent

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Posted 18 December 2019 - 08:32 PM

AT115edt is considered the best bang for the money and its from Astronomics.

https://www.astronom...ractor-ota.html

or if you have the budget

https://www.astronom...riplet-ota.html


Edited by Im2bent, 18 December 2019 - 08:32 PM.

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#12 HydrogenAlpha

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Posted 25 December 2019 - 12:15 AM

I have a ZWO ASI 1600mm-pro. Assuming my mount can handle anything.

I’d like to make an edition to my wife-field DSO scope with a scope that can capture more detail and a more narrow field. I assume this means something around 1000focal length?

I’m a nutshell what would be a good refractor to capture high quality subs of small DSOs

I was thinking the skywatcher 120?

Skywatcher Esprit 120 yes, Skywatcher Evostar 120 no (unless you have a good deal on a used one. You can buy 5" triplets at this price). 

 

But yes, the SW Esprit series are a proven scope for imaging. They are exquisite and will allow you to capture high quality subs. 




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