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Light Collection with Q3.5, 5 and 7

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#1 Kevin Barker

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Posted 22 May 2017 - 11:38 PM

Like others I am intrigued about the possibility of the Q 5. Obviously thankful for the military reasons for potential production as pointed out by a couple of gentlemen in another thread!!

I own a Q 3.5 fieldscope which has broadband coatings and have a good feel for what this 1/3 obstructed optically excellent wee scope can deliver. Crunching numbers tells me it has a similar light collection ability of a top notch 83 mm refractor (assuming BB coatings are 97% efficient) and allowing for the CO. I note my Q3.5 seems to perform a fair bit worse than a 80 mm refractor on low contrast planetary perhaps similar to a top notch 60 mm refractor. I am using the assumption that a good approximation is that the aperture minus the central obstruction. 89-29 for a Q 3.5.

 

I do not own a top notch 60 mm refractor currently.

 

Running similar comparisons for a Q5 I calculate the Q 5 should perform similarly on light collection to a 118 mm refractor for light collection and about 85 mm for low contrast performance.

 

Similarly one can extend this to the Q 7 and we get a light collection similar to a top notch 165 mm refractor and about 119 mm for low contrast performance.

 

I have a quality IM 703 which is 180 mm and 1/3 obstructed, I suspect these numbers are quite good approximations.

 

The aesthetic qualities and portability etc of the Q scopes are not considered here.

 

I am assuming the refractors have very close to 100% light throughput.

 

Your thoughts on this would be appreciated. 

 



#2 ehallspqr

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Posted 23 May 2017 - 03:50 AM

I'd agree with your approximations, Maks vs refractors. I always thought that a Q3.5" is equivalent to a 70-75mm, my 5" Meade Mak to a 90-100mm and the Q7 to a 125-150mm refractor. The Maks offer noticable light gathering advantages vs their smaller refractor counterparts. I have done direct comparisons between a 7" Questar Broadband and top of the line 5" Apochromatic refractors such as Astrophysics, Takahashi and found the acuity to be just about equal. On any given night the Q7 can hold its own and even surpass an excellent performing 5" refractor and the math says it should but....Premium Apo refractors such as AP and Tak offer such razor sharp crisp views that its hard for any obstructed design to compete. Side by side comparison this manafests itself in a slightly, and I mean slightly softer image for the Q7. Comparisons in light gathering, the Q7 wins hands down in that department producing noticably brighter images. This is very apparent on dim objects. Looking through a decent quality 6" ED refractor, I thought I might have seen better views through the Q7 although I did not compare them side by side. Mentioning contrast. Both designs offer excellent contrast that in some ways make up for smaller aperture. Good contrast makes dim objects stand out and allows more structure detail to be teased out.

 

Where I get excited is comparing my Q3.5" to my Meade ETX 125. They are both so similar in many ways but of course the optical performance in every catagory goes to the ETX 125, which is to be expected. If Questar scales up the 3.5" and retains the basic form/function, the 5" Questar may just be the perfect size Questar ever, in terms of performance vs convenience and portability. I loved that Q7 but theres no getting around the fact its quite a big telescope and your losing allot of what you love about your little Questar 3.5". I don't feel your giving up much when comparing the fork mounted 5" ETX. Someone mentioned in another thread the 5" Questar could be the perfect size "Goldilocks" Questar. What a perfect analogy. I'm saving my money.....


Edited by ehallspqr, 23 May 2017 - 10:44 AM.


#3 Codbear

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Posted 23 May 2017 - 12:16 PM

I concur with ehallspqr...it's the contrast that really makes up for the obstruction and smaller aperture. My views of M42 in the Q3.5, while dimmer than that of my TV Genesis sdf, showed much more extensive filament structure and overall was much more pleasing to me than through the sdf.

 

I have not, however, noticed any difference in the pinpoint view of stars between my Q7 and my AP130GT. Both views are "stellar". As expected, the extra light gathering power of the Q7 beats the AP130 every which way except for the reason I bought the AP130...the amazing wide-field view! The Q7 can't break a degree FOV, much less the 3 degrees or so I can get through the AP130.

 

Someone please correct me if I'm wrong, but I believe the net light gathering calculations you made on the Q7 Kevin didn't account for the oversized mirror of the Q7 to adjust for the obstruction so as to achieve a net 7" light gathering ability.



#4 Kevin Barker

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Posted 23 May 2017 - 01:57 PM

I concur with ehallspqr...it's the contrast that really makes up for the obstruction and smaller aperture. My views of M42 in the Q3.5, while dimmer than that of my TV Genesis sdf, showed much more extensive filament structure and overall was much more pleasing to me than through the sdf.

 

I have not, however, noticed any difference in the pinpoint view of stars between my Q7 and my AP130GT. Both views are "stellar". As expected, the extra light gathering power of the Q7 beats the AP130 every which way except for the reason I bought the AP130...the amazing wide-field view! The Q7 can't break a degree FOV, much less the 3 degrees or so I can get through the AP130.

 

Someone please correct me if I'm wrong, but I believe the net light gathering calculations you made on the Q7 Kevin didn't account for the oversized mirror of the Q7 to adjust for the obstruction so as to achieve a net 7" light gathering ability.

The oversized mirror is required to allow the full aperture for a Mak. It does not allow the Mak to beat Physics however. A Q 7's aperture is 178 mm due to this being the corrector size and it's obstruction is 33%(due to the spot baffle on the corrector)

If the mirror was only 7 inches yes it would lose out even more.

 

 

 

I have a friend in the South island with one, an old one with standard coatings.



#5 JamesMStephens

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Posted 25 May 2017 - 05:01 PM

Oversized mirrors: In the Q 3.5 the mirror has 3.80" diameter, so the 3.5" entrance pupil is the correct number to condider for discussions like these.



#6 Kevin Barker

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Posted 25 May 2017 - 06:03 PM

Here is a table with calculations for BB(Light transmission 97% or 0.97) and Standard (Light transmission 84% or 0.84

Attached File  ApertureLTCOQuestarCalculations.docx   11.91KB   98 downloads

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#7 Mike Allen

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Posted 11 June 2017 - 05:38 PM

I have a Q 3.5 and a Unitron 3-inch refractor.  I've compared them side by side, and the Questar has a better image in many ways.  Deep sky objects are brighter in the Q.  Planetary features on Jupiter display more vivid colors.  For example, the blue festoons and the red spot are more colorful through the Questar.  Same improvement observing colorful double stars.  On the Moon, the Questar pulls far ahead of the Unitron.  The Q3.5 is also slightly behind my TSA 102 in performance, so comparing the Q to a 80mm class refractor is quite fair, but from my experience, the Questar should never perform worse than a 75mm refractor if the Questar's coatings are in good shape.



#8 Kevin Barker

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Posted 12 June 2017 - 01:58 AM

Mike

I also own a good 80 mm refractor. An old uncoated Zeiss AS80/840 lens. And I have compared this lens several times with my Questar fieldscope (BB and Pyrex mirror). The views are similar in most respects but....

 

My observation has been the Zeiss is superior with respect to planetary detail, especially on Jupiter and it's low contrast belts and garlands etc. The AS80/840 is also better with respect to picking up fainter stars in clusters but not by much it is a close run thing. I have yet to compare them on the moon but suspect the Zeiss would probably keep up with the Q.

 

Having said that last evening just before a band of cloud started to roll in I was using the Q3.5 in near perfect seeing. Jupiter was stunning with a shadow transit occurring. The best view I have had with this tiny scope.  The Galilean Moons were pinpoint airy discs. Saturn did well at up to 175 X with the Cassini division visible all the way around and the polar regions and disc of Saturn really beautiful.

 

I was unable to do a comparison as the window of opportunity closed as cloud rolled in. But I suspect the Zeiss would have been better at showing the fine details on Jupiter.

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