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Telescope speed and image scale question

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#1 davesastrophotos

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Posted 06 August 2020 - 07:59 PM

Hi All,
I’ve been doing astrophotography for 5 years now and have gradually expanded my kit over that time as my skill level slowly improves.  Currently my kit includes a Celestron 6se, Skywatcher Esprit 80, Skywatcher AZEQ6GT, ZWO ASI 1600mm pro, and ASI 120mm.  I’ve been doing a lot of nebula imaging over the past year and am looking to do more galaxy and potentially planetary imaging.  As a result I'm looking to purchase a new telescope that fits my current mount which will improve my imaging in these two areas.  It’s also worth noting that I don’t have an observatory and need to setup and tear down for each imaging session.

 

My first thought was the Celestron Edge series, specifically the 9.25.  However I understand that at f/10 the Edge would be the same speed as my 6se (with ff/reducers it brings this down to f/7 and f/6.3 respectively).  This brings me to my question: how much of an improvement could I realistically expect to see by stepping up from the 6se to the Edge?  As the two imaging systems are roughly the same speed I understand that my imaging time would be roughly the same (to capture the same brightness of the target galaxy), have I understood this correctly?  I know there would be an improvement in image scale between the two (with ff/reducers its a difference between 945mm focal length and 1645mm) but I'm wondering if this would lead to a significant improvement in resolution between the two systems?

 

The Edge 9.25 is a significant investment so I'd like to be confident of the improvement I can expect and I feel like I've researched this to the point where I'm going in circles and would love to hear the thoughts of others on this forum.  If there is another scope that would fit my Skywatcher AZEQ6GT (40lb capacity) and better meet my needs I'd be interested to hear those suggestions as well.  Thank you!!!!!



#2 Midnight Dan

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Posted 06 August 2020 - 08:38 PM

For astrophotography, the improvement will be significant.  

 

The Edge series has a 43mm image circle, which is flat and sharp across the whole thing.  A standard SCT has significant coma around the edge.  Using a focal reducer shrinks the image circle, so it brings those aberrations in the outer edges in towards the middle.  You may not see a lot of difference with your 1600 camera because it has a 17mm diagonal.  But if you move to an ASP-C sized sensor, or full frame, you'll see a huge difference between the images from an Edge vs a non-edge.

 

When it comes to speed, you need to compare at the same effective focal length, or pixel scale.  Your 6" SCT has a focal length of 1500mm.  The Edge 9.25 with the focal reducer has a focal length of 1625mm.  So, at roughly the same image scale, the 9.25 will be able to image twice as fast as the 6" (f/7 vs f/10).

 

-Dan


Edited by Midnight Dan, 06 August 2020 - 08:39 PM.


#3 bignerdguy

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Posted 07 August 2020 - 08:55 AM

The 9.25 has a bigger mirror and as a result with the above in mind will also pull in a lot more light and thus be slightly faster for the same targets.  Also with the focal length being longer it will have a higher effective power so objects will be closer than before.  This scope is excellent for planetary as with a Barlow you can get quite high of a power and thus magnify the planets that much more while also seeing a lot more details. DSO's will be brighter but because of the longer focal length will appear bigger even with the FR/FF and thus some of the more spread out objects may require you to do mosaics to capture them all.  However if you want good images this is one way to do it and for planetary it is the way i would most often recommend. If on the other hand you WANT a wider field of view for some DSO's then a APO refractor would be the way to go, however these don't do planetary as well since they cant go quite as high of  a power as a longer focal length scope can.  Its a trade off either way.



#4 Grimstod

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Posted 07 August 2020 - 09:01 AM

For astrophotography, the improvement will be significant.  

 

The Edge series has a 43mm image circle, which is flat and sharp across the whole thing.  A standard SCT has significant coma around the edge.  Using a focal reducer shrinks the image circle, so it brings those aberrations in the outer edges in towards the middle.  You may not see a lot of difference with your 1600 camera because it has a 17mm diagonal.  But if you move to an ASP-C sized sensor, or full frame, you'll see a huge difference between the images from an Edge vs a non-edge.

 

When it comes to speed, you need to compare at the same effective focal length, or pixel scale.  Your 6" SCT has a focal length of 1500mm.  The Edge 9.25 with the focal reducer has a focal length of 1625mm.  So, at roughly the same image scale, the 9.25 will be able to image twice as fast as the 6" (f/7 vs f/10).

 

-Dan

1600 has 22mm diagonal. The sensor is 17wide. 



#5 Midnight Dan

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Posted 07 August 2020 - 09:40 AM

1600 has 22mm diagonal. The sensor is 17wide. 

My mistake.  Thanks for the correction! :-)

 

-Dan



#6 Grimstod

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Posted 07 August 2020 - 11:34 AM

My mistake.  Thanks for the correction! :-)

 

-Dan

At least you did not call me short wink.gif  I made the same mistake with this format to. 



#7 davesastrophotos

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Posted 07 August 2020 - 01:48 PM

Thanks for the replies all!

 

Dan, you raised a good point that the Edge is significantly faster at roughly the same focal length. At that focal length I'm slightly oversampled at .48" per pixel with my 1600MM for average seeing conditions which is probably the case more often than not here in Michigan.  How problematic is this amount of oversampling generally speaking?  I'm wondering if this would then necessitate a different camera like the ASI174 which has a larger pixel size?  I do have a Nikon D750 which fits the bill but given its lack of internal cooling I rarely use it for astrophotography.

 

You also raise a good point regarding the imaging circle which I hadn't considered.  So with my 1600 I wouldn't really be taking advantage of the full benefit of the flat field the Edge offers.  Again my Nikon D750 would do this so perhaps I just need to do a bit more research on imaging with a DSLR to see if that's an option I should consider.  


Edited by davesastrophotos, 07 August 2020 - 01:54 PM.


#8 bobzeq25

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Posted 07 August 2020 - 02:49 PM

Thanks for the replies all!

 

Dan, you raised a good point that the Edge is significantly faster at roughly the same focal length. At that focal length I'm slightly oversampled at .48" per pixel with my 1600MM for average seeing conditions which is probably the case more often than not here in Michigan.  How problematic is this amount of oversampling generally speaking?  I'm wondering if this would then necessitate a different camera like the ASI174 which has a larger pixel size?  I do have a Nikon D750 which fits the bill but given its lack of internal cooling I rarely use it for astrophotography.

 

You also raise a good point regarding the imaging circle which I hadn't considered.  So with my 1600 I wouldn't really be taking advantage of the full benefit of the flat field the Edge offers.  Again my Nikon D750 would do this so perhaps I just need to do a bit more research on imaging with a DSLR to see if that's an option I should consider.  

Oversampling lowers signal to noise ratio.  You can compensate for doing more total imaging time, ie shooting more subs.  Note that, in average skies or worse, oversampling gets you very little.  Seeing blurs the results, anyway.

 

In my mediocre skies, I've pretty much given up on image scales numerically lower than 1.0.  I tried 0.5 once, was not happy with the results.  As it is my light pollution needs a lot of subs (often hundreds, depending on the scope) and a good processing computer.


Edited by bobzeq25, 07 August 2020 - 02:50 PM.


#9 Midnight Dan

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Posted 07 August 2020 - 02:55 PM

Personally, I'd stick with the ASI1600 Pro.  Cooling is a HUGE factor in astrophotography.  Noise is the enemy and cooling reduces it dramatically.  

 

At 0.48"/pixel I agree that you're a bit too oversampled.  You'd probably want to be in the 0.65 to 0.7" range.  But oversampling isn't the end of the world.  You can just downsample in software.  

 

Note that, while the EdgeHD 8 has a 43mm image circle at native focal length, when you add the 0.7x FR, that shrinks to about 30mm.  So if you use a FR, you're using a larger percentage of the image circle.  I don't think the ASI174 comes in a cooled model so I would avoid that for DSOs.

 

If you're looking for a camera that is a better match for the scope, and you plan to use it with a FR, I would consider the ASI2600.  It's an APS-C sized sensor with a 28mm diagonal size.  The pixels are smaller than your 1600, but at 3.76microns, you can do a 2x bin in software and you'll be at around 7" per binned pixel.  Binning also improves your SNR.

 

** Edit **

 

Never mind. Lol :-) Looks like you're doing mono imaging and the ASI2600 does not come in a mono model.

 

-Dan


Edited by Midnight Dan, 07 August 2020 - 02:57 PM.


#10 davesastrophotos

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Posted 07 August 2020 - 10:34 PM

Regarding the binning option it seems 2x2 would indeed put me nearly at 1"/pixel with the ASI1600.  This comes at the cost of resolution but if I've understood correctly with average seeing conditions I wouldn't be benefiting from the higher resolution anyways, have I understood this correctly? 

 

Does the loss in resolution from binning negate any advantage of the Edge 9.25 vs. the 6se in terms of resolution alone?  If so this would seem to suggest that around 1000mm focal length is the best I can do and I should focus on getting the fastest system at around that focal length... just want to be sure i'm understanding this correctly...

 

Thanks again for the input!!




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