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which is A wiser choice for Astrophotography-SCT or RC?

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#1 Rags Jr

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Posted 05 January 2020 - 09:25 PM

I am currently imaging with an Heq-5 pro and A 130SF reflector. I utilize A DSLR and get roughly 2 minute un-guided subs, my targets of choice are nebula, and potentially galaxies. I would like to purchase the most telescope that my mount can reasonably handle, that will give me the nicest quality images. my thoughts are an Astro Tech 6" RC or perhaps the 8" version if not overly heavy for the mount. I was wondering if an SCT would be A wiser choice, say A C6, or an Edge 8, though they are quite costly. Does the central obstruction of these telescopes effectively reduce the diameter by that given amount? Are the Astro Tech RC's of good quality, and reasonable to collimate? What factors would help someone choose between an SCT or RC? Am I correct in thinking that going from F5 to F10 along with A larger aperture will increase the size/detail of A given object? I'm sure the image will be A bit darker as well, which could become an issue due to my 2 minute max exposures. What I want are the largest, clearest images I can get with this mount. 

 

Rick



#2 Jim Waters

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Posted 05 January 2020 - 09:32 PM

I don't own a RC scope and never used one but I have been told that they are VERY hard to collimate.



#3 Dan Crowson

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Posted 05 January 2020 - 09:42 PM

I would definitely say RC. SCT's are a real challenge because of the moving mirror. The tubes can also expand and contract (the focal length changes). The RC you mention also more or less acts like a dew shield. I could image with my AT6RC without any kind of dew prevention up until around 90% humidity with no issues.
 
I've had several of the Astro-Tech RCs. I don't recommend the focusers but I'm guessing it would depend on what you use to image. I think the newer models also have plates so the image train doesn't connect directly to the mirror which helps.
 
Collimation is not much of an issue with the right tools - hopefully a fellow club members because tend to only do it once. I never had to collimate my AT6RC in the several years and trips I used it. I've collimated my AT12RCT once.
 
Dan


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#4 mikefulb

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Posted 05 January 2020 - 10:32 PM

A 6 inch f/5 Newtonian might be a consideration also.  Tracking at 1000+ mm with your current mount and a 8 inch OTA could be challenging and depending on your camera pixel size just lead to oversampling.


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#5 Rustler46

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Posted 05 January 2020 - 11:11 PM

The Edge HD SCTs are highly respected for imaging.  The short length of the optical tube versus focal length places less demands on the mount. All telescope designs have issues. With knowledge these can be successfully addressed.


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#6 Benni123456

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Posted 06 January 2020 - 07:02 AM

the edge hds do not produce spikes.

Problem is they are rather dark, f10.

Do not waste money buying a celestron 0.7 reducer for these edges because for some reason celestron saved money and they used glass with chromatic aberrations.

 

the rc, in contrast, is natively at f8.  This means shorter exposures.

 

Problem is they produce spikes. Because of the massive assembly with the secondary, some stars look a bit like squares in gso rcs due to diffraction effects.

 

In theory, the edge is a bit easier to collimate since you rarely have to move the corrector plate and usually only move the secondary screws.

 

For an rc, you need to align the oaz, primary and secondary.

 

In my view, Celestron should make a  good f7 or f8 edge hd scope. Or sell a better reducer. Then it would be perfect.

 

Or gso could use glass plate to hold the secondary of the rc. Then the inner tube would be protected and it would not produce spikes.



#7 carolinaskies

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Posted 06 January 2020 - 12:01 PM

I am currently imaging with an Heq-5 pro and A 130SF reflector. I utilize A DSLR and get roughly 2 minute un-guided subs, my targets of choice are nebula, and potentially galaxies. I would like to purchase the most telescope that my mount can reasonably handle, that will give me the nicest quality images. my thoughts are an Astro Tech 6" RC or perhaps the 8" version if not overly heavy for the mount. I was wondering if an SCT would be A wiser choice, say A C6, or an Edge 8, though they are quite costly. Does the central obstruction of these telescopes effectively reduce the diameter by that given amount? Are the Astro Tech RC's of good quality, and reasonable to collimate? What factors would help someone choose between an SCT or RC? Am I correct in thinking that going from F5 to F10 along with A larger aperture will increase the size/detail of A given object? I'm sure the image will be A bit darker as well, which could become an issue due to my 2 minute max exposures. What I want are the largest, clearest images I can get with this mount. 

 

Rick

Realisticly your mount limitations in weight capacity narrow your choices of reasonable OTAs.   Central obstructions are of little consequence to imaging in real life.  Refractor owners love to point out the obstruction, but the loss of light gathering power of the center tends to be minimal, due to the offset by the larger apertures available.  

The HEQ5 realistic weight limit of OTA/imaging package well balanced is just over 22lbs unless you're exceptionally gifted at balancing weight then some individuals have gone for the full 30lbs, not recommended for anything but observatory conditions.  

Drawbacks of the 6" RC.  Star spikes, though I'm sure you've lived with that on your 130SF,  twitchy collimation, F/8 exposure requirements vs your current F/5,  @130mm and RC @150mm you are not gaining substantial aperture.  Colimation and focuser issues and often need for correctors. An 8" RC is similar in weight to an 8" SCT

Drawbacks of an 8" SCT.  F/10 vs F/5 so effective light gathering is less in your 2 minute exposures. Aperture will help somewhat as you are jumping to 200mm.  Using F6.3 on a standard (non-edge) SCT will help.  Edge is far more pricey and accessories more pricey.  Long focal length 2000mm will require very precise polar alignment for unguided imaging. W/F6.3 reducer 1280 FL is more manageable.

Optional OTAs -
RASA 8, F/2 400mm focal length 200mm aperture.  Exposure times can be 30sec with results similar to 2min on your current 130mm.  17lb weight... within capabilities of mount without issue.  Durable design once dialed in

RASA drawbacks - imaging only no visual use, No DSLR, Mirrorless cameras and dedicated cameras only, somewhat pricey.  Diffraction spike possible if not careful how cables are run.  

127mm refractor - smaller aperture but more useable vs obstructed 130sr, F/7.5 (18lbs & 34 inches)  ES ED127 version.  

 



#8 Rags Jr

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Posted 06 January 2020 - 12:41 PM

Thank You So much Paul, and everyone else who has replied! Maybe the best thing I can do is use my current setup for A bit. I have been using it for less than A year, and made A big jump from an Eq-1 and St80! I’m not sure how much the New England weather will let me get out there either. It sounds like this scope and mount are reasonably matched, and future changes should perhaps be in autoguiding or cameras. You get in A situation where you want to optimize what you have, but I guess every component will effect the others in some way, and you need to keep things balanced. 



#9 carolinaskies

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Posted 06 January 2020 - 03:40 PM

Thank You So much Paul, and everyone else who has replied! Maybe the best thing I can do is use my current setup for A bit. I have been using it for less than A year, and made A big jump from an Eq-1 and St80! I’m not sure how much the New England weather will let me get out there either. It sounds like this scope and mount are reasonably matched, and future changes should perhaps be in autoguiding or cameras. You get in A situation where you want to optimize what you have, but I guess every component will effect the others in some way, and you need to keep things balanced. 

If you haven't shot with your ST80 on your HEQ5 I'd do some of that too.  BTW, you could do a side-by side 130/80mm using the 80 as a guide camera and only have to buy a guide camera.   

What DSLR camera are you using btw?   You might consider upgrading your camera to a mod-DSLR or even an older dedicated astro camera.  I actually picked up some older Orion cameras for this reason and was able to get a manual filter wheel w/filters to boot!   

Rather than trying to buy new, I've scoured used listings at different sources and have picked up most of the telescopes you'll see listed in my signature (The 16" LX200 being my biggest score).   



#10 Rags Jr

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Posted 06 January 2020 - 06:57 PM

I am currently using A Sony A57, as it was A camera that my Girlfriend had. It is not modified, and I'm not even sure if it can be. It does not seem to be compatible with any software, so computer control is out of the question I guess, though I have an intervalometer and would rather not drag out A laptop anyway! It is actually A nice camera, but I know most everything is geared towards Canon. The mount and both telescopes are all used, either from friends or Cloudy Nights, money is an object after all! I like the DSLR because of its simplicity, and the size of its sensor, it is A crop but I believe comparable sized CCD or Cmos cameras are still quite expensive. A friend of mine uses A stand alone guider with good success, but that is A bit expensive too. I would consider that avenue, or perhaps the Asair, and just use my Ipad. I will probably try the ST80, I just need to pick up A mounting rail. BTW, what is it that causes tracking to be more critical as focal length increases? Is it the decrease in field size, the increase in magnification, or both?



#11 carolinaskies

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Posted 06 January 2020 - 07:42 PM

I am currently using A Sony A57, as it was A camera that my Girlfriend had. It is not modified, and I'm not even sure if it can be. It does not seem to be compatible with any software, so computer control is out of the question I guess, though I have an intervalometer and would rather not drag out A laptop anyway! It is actually A nice camera, but I know most everything is geared towards Canon. The mount and both telescopes are all used, either from friends or Cloudy Nights, money is an object after all! I like the DSLR because of its simplicity, and the size of its sensor, it is A crop but I believe comparable sized CCD or Cmos cameras are still quite expensive. A friend of mine uses A stand alone guider with good success, but that is A bit expensive too. I would consider that avenue, or perhaps the Asair, and just use my Ipad. I will probably try the ST80, I just need to pick up A mounting rail. BTW, what is it that causes tracking to be more critical as focal length increases? Is it the decrease in field size, the increase in magnification, or both?

An increase in focal length reduces the field size (magnification is a byproduct).  If you use the same sensor, the sensor area represents a smaller slice of the sky and each pixel on the sensor represents a smaller area of that sky.  A star is represented by more pixels and any tracking error is magnified because the star light moves to adjacent pixels faster.  Proper alignment and precision tracking are essential to keep the stars nice and round instead of elongated as focal length increases.



   



#12 DuncanM

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Posted 07 January 2020 - 12:45 PM

Thank You So much Paul, and everyone else who has replied! Maybe the best thing I can do is use my current setup for A bit. I have been using it for less than A year, and made A big jump from an Eq-1 and St80! I’m not sure how much the New England weather will let me get out there either. It sounds like this scope and mount are reasonably matched, and future changes should perhaps be in autoguiding or cameras. You get in A situation where you want to optimize what you have, but I guess every component will effect the others in some way, and you need to keep things balanced. 

A 'classic' SCT works very well with the Starizona SCT reducers which will give a well corrected APSC field @ F6.3 for 9.25/11in Celestron non-Edge SCTs and ~F7 for Meade non-ACF SCTs;

 

https://starizona.co...-coma-corrector



#13 Astrojedi

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Posted 07 January 2020 - 03:50 PM

Both designs have pros and cons. Like everything else in life you will need to decide what you can live with.

I prefer the EdgeHD vs. the RC for a few reasons. The moving mirror makes it much more flexible and usable with a variety of accessories. The SCT collimation is significantly easier than a RC. The SCT also does not produce diffraction spikes. The EdgeHD also doubles as a superb visual instrument with a flat field.

Another benefit of the Celestron SCTs is that you can use a Hyperstar for F2 imaging. For nebulae fov matters.

With newer low read noise sensors focal ratio is somewhat irrelevant but if you are planning to use a DSLR you can use a 0.7x reducer.

Edited by Astrojedi, 07 January 2020 - 04:03 PM.


#14 Astrojedi

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Posted 07 January 2020 - 04:01 PM

For the second part of your question... the right way to think about this is that for imaging you need a long FL scope for imaging small objects and short FL scope for imaging large objects. For example I pair my EdgeHD 8 & 9.25 scopes with a Redcat 51 for wider fields.

Both RCs and SCTs are long FL instruments and should be treated as such i.e. don’t waste time trying to maximize fov. The whole purpose of a long FL scope is to maximize image scale which it will.

Another point is that with longer FLs your sampling (arc second per pixel) will become finer, so mount accuracy will matter much more. If you were doing 2 minute exposures with short FL scopes, you may not be able to do that with a RC or SCT.

#15 Rags Jr

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Posted 08 January 2020 - 05:21 AM

Thank you very much, I have A better understanding now of focal length’s effect on the image, and the need for more precise guiding, polar alignment, and star alignment to avoid frustration. I still may end up purchasing one to image galaxies and other small targets. I see that there are several 6” RC’s; Orion, Gso, Astro Tech, Ioptron, TPO, and perhaps others. Have you found one to be better than the others in terms of quality and performance? I have read about flexure issues with the AT scope due to the focus tube mirror connection, is this true off all these? Is this an older issue that has been corrected? 



#16 RJF-Astro

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Posted 08 January 2020 - 01:35 PM

As far as I know those are all the same RCs, aside from cosmetic differences. Someone mentioned the 6RC is F8, but it is actually F9 without reducer. You certainly want that 0.67x reducer, because it will both speed up the scope and reduce the scale so your mount can handle it better.

The issue with the focuser connecting to the mirror is still there I presume. I have not noticed it yet. The field looks flat enough to me and I see no changes during imaging sessions. A change would indicate a sagging focuser which also tilts the mirror.

Collimation requires effort. I use the DSI method (you can find the pdf if you google it). This is a star test for RCs, works great and you do not need expensive tools.

#17 Benni123456

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Posted 09 January 2020 - 08:33 AM

regarding guiding performance:

 

i would not use rc or sct scopes without polar alignment by some electronic tool, e.g polemaster. With that, you should be able to use a good h-eq5 mount at 2000mm...




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