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R/C or SCT?

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#1 bloom691

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Posted 25 February 2021 - 10:53 PM

So I am looking at upgrading my Meade 8" LX90. I am mainly interested in imaging galaxies, but my location limits my total exposure-time in a single evening to about 4-5 hours.  Based on my mount (iOptron CEM60EC) I can certainly manage 10", probably 12", of aperture. I am looking for something which I can use for the next 3-5 years, and would consider a larger mount and scope after that - maybe a nice 12.5-14" CDK ;) I generally have good seeing (certainly not excellent) but with limited/low light-pollution. 

 

Anyways... should I go for a SCT, or an R/C? Since this is unlikely to be a "forever" scope, I am thinking a Meade/Celestron 10-12" SCT (with mirror-locked and external focuser) is generally a little cheaper than a similar-sized R/C, and probably going to yield similar results - unless someone knows otherwise?! 

 

Thoughts?



#2 TxStars

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Posted 25 February 2021 - 11:06 PM

How much astrophotography have you done/ been doing?

If you are just getting started a short apo refractor on your mount would be better to learn..

If you have to have longer FL you already have the 8"


Edited by TxStars, 25 February 2021 - 11:07 PM.

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#3 Dan Crowson

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Posted 25 February 2021 - 11:52 PM

Would your seeing support those larger scopes? I have a lot of New Mexico nights where the seeing there doesn't. I never had a night that supported it in Missouri when I was testing.
 
I would also think a 12" RC/CDK would be too heavy with all the add-ons on a CEM60. I think my cheap AT12RCT is over 65 lbs with the imaging train. Also remember if you have to setup every night, there's not a lot of fun in moving around 200+ lbs of equipment.
 
If still thinking the SCT or RC, other things to consider are:
 
1. Dew - if that could be an issue, the RC might not even need anything but the SCT would require a lot...
2. SCTs are a real challenge for imaging. If you have temp swings, the focal length can even change over a short period of time. Even with locks, you could have movement yielding flexure if not using an off or on axis guider.
3. The long focal lengths will want larger pixels or binning.
 
Not trying to push not moving to a long focal length - just pointing out the challenges.
 
Dan


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#4 bloom691

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Posted 26 February 2021 - 06:14 PM

How much astrophotography have you done/ been doing?

If you are just getting started a short apo refractor on your mount would be better to learn..

If you have to have longer FL you already have the 8"

Been imaging "seriously" for about 6 months now.

 

My 8" SCT has FL 2000mm, and I am using an OAG with great results - my guiding is very precise. I am using a ZWO ASI174 for guiding and a OSC ASI294MC-Pro as my imaging camera. I like the long FL since most of the galaxies are pretty small.

 

 

Would your seeing support those larger scopes? I have a lot of New Mexico nights where the seeing there doesn't. I never had a night that supported it in Missouri when I was testing.
 
I would also think a 12" RC/CDK would be too heavy with all the add-ons on a CEM60. I think my cheap AT12RCT is over 65 lbs with the imaging train. Also remember if you have to setup every night, there's not a lot of fun in moving around 200+ lbs of equipment.
 
If still thinking the SCT or RC, other things to consider are:
 
1. Dew - if that could be an issue, the RC might not even need anything but the SCT would require a lot...
2. SCTs are a real challenge for imaging. If you have temp swings, the focal length can even change over a short period of time. Even with locks, you could have movement yielding flexure if not using an off or on axis guider.
3. The long focal lengths will want larger pixels or binning.
 
Not trying to push not moving to a long focal length - just pointing out the challenges.
 
Dan

I think my mount will handle a 10" R/C or a 12" SCT (at least Meade's) without a problem. I will be pushing close to the maximum rated capacity - I will see how it handles it and can always upgrade the mount if I am wrong and it cannot give me the accuracy I need ;) 

 

I have Dew heaters on my 8" SCT and it is coping well here in North Carolina. So I am using an OAG, so hopefully flexure should be okay in regards to guiding. My ZWO ASI294MC has 4.63um pixels - I could probably start binning since with 8" at FL=2000mm I am oversampling based on my seeing conditions. 

 

Looking online I see pretty good images from 12" SCT's. But I am going to be disappointed ? Hard to tell without trying both side-by-side. Now there's an idea...! ;) 



#5 TxStars

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Posted 26 February 2021 - 06:51 PM

@bloom691   Was just making sure you had some idea of what you were getting into..

A 10" RC is the same focal length as the C-8 so you are just gaining image brightness, but also more scope weight.

If you can keep your sub length short then you might be able to use your current mount.

I think the 12" Cas extra Fl is going to push you over what your mount can do. YMMV



#6 BQ Octantis

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Posted 26 February 2021 - 06:59 PM

With limited imaging time and a 60-lb mount, why not go with a RASA?

 

https://www.astronom...-tube-3421.html

 

At f/2.2, you could capture 3 targets per night (vice 3 targets per season with an f/7).

 

Another fast option would be the SCT with an f/2 Hyperstar conversion…

 

BQ



#7 TxStars

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Posted 26 February 2021 - 07:26 PM

The OP said they are wanting image scale for galaxies, so the RASA is out..



#8 BQ Octantis

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Posted 26 February 2021 - 07:37 PM

The OP said they are wanting image scale for galaxies, so the RASA is out..

 

How strange. I've shot about 1000 galaxies at 200mm. And i can't even fit the LMC into a 200mm—I need a 50mm to get the full extent. I figure I could get a lot more at the RASA's 650mm…and at f/2.2, way faster than one of those short apos suggested! (What are their focal lengths…?)

 

So now I'm wondering…what targets?


Edited by BQ Octantis, 26 February 2021 - 08:00 PM.


#9 TxStars

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Posted 26 February 2021 - 08:12 PM

How strange. I've shot about 1000 galaxies at 200mm. And i can't even fit the LMC into a 200mm—I need a 50mm to get the full extent. I figure I could get a lot more at the RASA's 650mm…and at f/2.2, way faster than one of those short apos suggested! (What are their focal lengths…?)

 

So now I'm wondering…what targets?

Umm Are You ok ?



#10 BQ Octantis

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Posted 26 February 2021 - 10:27 PM

Umm Are You ok ?

Why, is my apo aversion showing? wink.gif



#11 BQ Octantis

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Posted 26 February 2021 - 10:42 PM

On a more analytical front, here's a look at the various options on the table. There was no specific apo recommended, so I went with the largest I could find at Agena Astro: the Skywatcher Esprit 150 f/7 apo. Skywatcher also makes a 0.77 focal reducer for it.

 

comparison.png

 

My thoughts:

 

  1. Line 13: with the ASI294MC, you're oversampled with all but the C11 Hyperstar, the RASA 11, and the Espirit 150 with a 0.77 focal reducer. This is based on the 1-2"/pixel rule-of-thumb, which says any sampling smaller than 1" is lost to seeing (so wasted flux and bloated stars).
  2. Line 19: compared to your current imaging time at f/6.3 (assuming you're using a focal reducer), you get a massive (91%) integration time reduction with the C11 Hyperstar, followed by the RASA 11 (88%). The apo with a focal reducer reduces the imaging time by 25%; the RC without a focal reducer increases imaging time by 61%.
  3. Lines 28 & 29: These are how much of the sensor is taken up by Bode's Nebula (M81), which I take to be a representative size for a galaxy. Without a focal reducer, the RC makes it barely fit the sensor (and with dithering, it may very well not fit at all). I don't know what is ideal, but the RC with a 0.75 focal reducer obviously puts the most undistorted pixels across the galaxy. But it comes at the expense of long integrations and bloated stars from oversampling.

 

If it were my money, I'd go with the C11 and a Hyperstar. If I got bored (or found a really interesting target to spend more time on), I'd switch to the focal reducer. Plus I could rotate between planetary and DSOs with just a single aperture.

 

BQ


Edited by BQ Octantis, 27 February 2021 - 04:16 AM.


#12 Rasfahan

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Posted 27 February 2021 - 04:57 AM

I cannot talk about the C11 but I think the RASA is missing resolution for smaller galaxies. I am not talking about M31/33/LMc/SMC, of course, and I think M81 is very large still, but those smaller fuzzies from the NGC. Its spot size as published is 4.6 microns which limits resolution to 1.6“. To realize this you also have to put in considerable work on each setup to get spacing, tilt and collimation perfect. It is a wonderful widefield instrument, though, and I have great fun using mine.

 

I looked at a lot of APO images of smaller galaxies and found that all taken with apertures smaller than 6“ are lacking in detail as compared to larger optics. So I think for small, detailed galaxy images you will need 6“ unobstructed apertures minimum. 

On the other hand, seeing will probably limit the benefits of aperture for resolution beyond 8“. So something in the 8-10“ range is, I think, the sweet spot, and smaller means a smaller mount can handle it.

 

I think none of the „cheaper“ scopes (RC or SCT) are really great mechanically which is why I opted for a CFF 10“ RC (these are no longer made, but maybe some with (from my experience very good) GSO optics are still available).

 

I you have the space (I do not) I would recommend a nicely made 8 or 10“ imaging newtonian with a high-quality coma corrector. TS ONTC and the Lacerta Carbon Imaging are European ones that look very well made. I saw an advertisement recently for an 8“ American truss design that looked excellent, too. I can only share my research, though, and have no personal imaging newton experience. If I had the space, I would have gotten one instead of the RC.


Edited by Rasfahan, 27 February 2021 - 04:58 AM.


#13 imtl

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Posted 27 February 2021 - 05:24 AM

I think you are over estimating what your CEM60 can handle...



#14 Rasfahan

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Posted 27 February 2021 - 06:21 AM

That is another matter. If you want an increase in aperture for more light gathering you will need a beefier mount by far. A 12“

instrument will overload the Cem60 significantly. I am quite sure a 10“ Newton will be too much, too.



#15 bloom691

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Posted 27 February 2021 - 10:51 PM

Some good feedback here!

 

I think you are over estimating what your CEM60 can handle...

So the Meade SCT 12" F/10 OTA is 35lb. With OAG/external-focuser/camera/etc. that will add 10-15lb. So I will be around 50lb - why would the CEM60 struggle with that? Right now I am keeping my subs in the 5-10 minute range.

 

 

On a more analytical front, here's a look at the various options on the table. There was no specific apo recommended, so I went with the largest I could find at Agena Astro: the Skywatcher Esprit 150 f/7 apo. Skywatcher also makes a 0.77 focal reducer for it.

 

attachicon.gifcomparison.png

 

My thoughts:

 

  1. Line 13: with the ASI294MC, you're oversampled with all but the C11 Hyperstar, the RASA 11, and the Espirit 150 with a 0.77 focal reducer. This is based on the 1-2"/pixel rule-of-thumb, which says any sampling smaller than 1" is lost to seeing (so wasted flux and bloated stars).
  2. Line 19: compared to your current imaging time at f/6.3 (assuming you're using a focal reducer), you get a massive (91%) integration time reduction with the C11 Hyperstar, followed by the RASA 11 (88%). The apo with a focal reducer reduces the imaging time by 25%; the RC without a focal reducer increases imaging time by 61%.
  3. Lines 28 & 29: These are how much of the sensor is taken up by Bode's Nebula (M81), which I take to be a representative size for a galaxy. Without a focal reducer, the RC makes it barely fit the sensor (and with dithering, it may very well not fit at all). I don't know what is ideal, but the RC with a 0.75 focal reducer obviously puts the most undistorted pixels across the galaxy. But it comes at the expense of long integrations and bloated stars from oversampling.

 

If it were my money, I'd go with the C11 and a Hyperstar. If I got bored (or found a really interesting target to spend more time on), I'd switch to the focal reducer. Plus I could rotate between planetary and DSOs with just a single aperture.

 

BQ

Agree with Line-13. But can I not just bin to get things a little better? The camera has plenty of resolution and at 2x2 I should still have plenty of detail, no?



#16 BQ Octantis

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Posted 27 February 2021 - 11:03 PM

Agree with Line-13. But can I not just bin to get things a little better? The camera has plenty of resolution and at 2x2 I should still have plenty of detail, no?

Sure, but it's at the expense of field-of-view. Matching the native pixel pitch to the desired spatial sampling makes better (more complete, anyway) use of the full image spot from the aperture.



#17 BQ Octantis

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Posted 28 February 2021 - 01:18 AM

Check out this post of the Sunflower Galaxy from today with a 400mm RASA 8 (make sure to follow the link to the full size on Astrobin—be sure to login so you can get to it):

 

https://www.cloudyni...alaxy-at-400mm/

 

For what it's worth, 17.5 hours at f/2 with 2.4µm pixels is equivalent to 42.3 hours at f/6 with 4.63µm pixels.


Edited by BQ Octantis, 28 February 2021 - 01:45 AM.



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