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10" Dob vs 8" SCT views of a Globular Cluster?

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#26 coopman

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Posted 24 May 2019 - 11:26 AM

In my experience, the glob. clusters resolve much deeper with 10" vs 8". In fact, this is the main type of object where the aperture increase made a big difference. M13 and M3 became incredible with the 10" Dob.
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#27 REC

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Posted 24 May 2019 - 01:20 PM

In my experience, the glob. clusters resolve much deeper with 10" vs 8". In fact, this is the main type of object where the aperture increase made a big difference. M13 and M3 became incredible with the 10" Dob.

I did use my binoviewers in the 10" Dob on M13 and it was great!



#28 dave253

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Posted 25 May 2019 - 02:55 AM

Hi Bob,

 

Before you can compare them you need to be comparing apples with apples, to do that you must make a number of assumptions.

 

1) Both scopes are of equal optical quality.   (ie lets assume diffraction limited, which is reasonable).

2) Both scopes are properly collimated

3) Both scopes are properly cooled.

 

Given the above 3 prevailing conditions there isn't anything the 8" SCT will do optically better than the 10" dob.

 

The laws of physics see to it that this is so. Each optical surface introduces aberrations and light loss, albeit small in each case. The SCT has a lot more reflective and refractive surfaces than a newtonian

 

SCT = Corrector Plate, Primary Mirror, Secondary mirror, Star Diagonal.  4  surfaces

Newtonian = Primary Mirror and Secondary mirror only = 2 surfaces

 

The 8" SCT will also have a much larger secondary than the 10" Newtonian (about 35% compared to 25% in the 10% dob).  The larger secondary causes additional diffraction and affects the MTF curves.  This only applies to visual not imaging.  A premium 10" Newtonian will have a further smaller secondary than a 10" mass produced dob.  Usually a premium 10"/ ~F5 Newtonian will use a 1.83" secondary for an 18% CO whereas the 10"/F5 Asian made dobs use about a 63mm secondary.

 

The 10" dob will give much better wide field low power views.  You can still wind the power up with a 10" dob by using shorter focal length eyepieces but you can't widen the FOV and reduce the power in a SCT beyond a certain point due to the long focal length and slow F-ratio.

 

While I have only ever owned newtonians and a couple  of refractors over 47 years as a visual observer, I have spent a lot of time looking through colleagues SCT's at every type of target available.  Given equal aperture I am yet to come across any SCT that can come remotely close to the views through a decent well set up and tuned newtonian.  Giving 2" of aperture away is like taking a knife to a gun fight.  The SCT will have  a greater "depth of focus" but at F5 the depth of focus is reasonable on the Newtonian. The SCT will have a far greater tendency to "dew up" than the Newtonian.  Corrector plates are renowned dew magnets.

 

Cheers

Excellent synopsis mate. Another thing to consider is that a quality Barlow effectively turns the big F/4.5 into a narrower F/9 light cone or more. Yeah, collimation is critical, but we know how to do that.



#29 WyattDavis

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Posted 25 May 2019 - 04:11 AM

I just bid adieu to my C8 and now have 10" dobs as my "big" scopes. I always thought the C8 was attractive because of the easy form factor, but the extra aperture and - for me - relative ease of cooling and collimation on the dob overcame the fact that the dob is definitely bigger and a bit more of a full-contact sport to use. It does seem to me that everything on the SCT has to be much closer to spot-on to get great optical performance, and then there is the aperture difference.

 

I enjoyed this thread on translating reflectors to equivalent unobstructed/refractor aperture:  https://www.cloudyni...three-7s/page-2

 

Using these ideas (and setting aside impact on contrast from obstructions), it looks to me like a C8 translates to roughly 6.6" of equivalent unobstructed aperture, while the 10" dob is about 8.9" - that is around 2/3 magnitudes deeper which is significant. By the same approach, a C11 comes in at around 9" - a virtual tie with the 10" dob. 


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#30 BillP

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Posted 30 May 2019 - 07:27 PM

All else being equal the 10 inch will show about a 56% brighter image.  That is fairly significant.   Put another way the aperture of the 8 inch lost to the 10 inch is equal to the light grasp of a 6 inch.  So the light of an 8 plus a 6 would equal that of a 10..

 

If you factor in a 34% CO for the SCT (CN and Company 7 refs) and a 25% CO for the Dob (Orion XT10 spec), an overall system efficiency of 83.5% for the SCT (per Celestron StarBright XLT) with a 99% diagonal and a 92% transmission primary and secondary (GSO standard) for the Dob, then the 10" will show about 70% more light gathering.  Sizable and easily visible.


Edited by BillP, 30 May 2019 - 07:42 PM.

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#31 REC

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Posted 31 May 2019 - 10:15 AM

If you factor in a 34% CO for the SCT (CN and Company 7 refs) and a 25% CO for the Dob (Orion XT10 spec), an overall system efficiency of 83.5% for the SCT (per Celestron StarBright XLT) with a 99% diagonal and a 92% transmission primary and secondary (GSO standard) for the Dob, then the 10" will show about 70% more light gathering.  Sizable and easily visible.

Wow, I forgot that the CO in the SCT is that much bigger than the 10" is. Makes it pretty bright in the end. Thanks!



#32 Tony Flanders

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Posted 31 May 2019 - 02:28 PM

Wow, I forgot that the CO in the SCT is that much bigger than the 10" is. Makes it pretty bright in the end. Thanks!

Although the big secondary mirror in an SCT hurts its contrast significantly, it has very little effect on the light-gathering. A 34% CO allows 88% of the incoming light through, compared to 94% for a 25% CO. Just a 6% difference, compared to the 56% increase in area moving from an 8-inch to a 10-inch mirror.


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#33 edwincjones

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Posted 31 May 2019 - 02:45 PM

GCs are where a zoom ep really comes in handy.

 

edj


Edited by edwincjones, 31 May 2019 - 02:45 PM.

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#34 REC

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Posted 01 June 2019 - 09:01 AM

GCs are where a zoom ep really comes in handy.

 

edj

Yup, that's what I'm going to use if we ever get a clear night! Have  BHZ 8-24mm that I'm waiting to use it on a glob. Have not done so since I've had it.




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