Aperture steps and their real world impact on visual performance

[Deadlake]

I'm surprised the MTF of a scope and it's effect on looking at extending objects has not been brought up, aka why can you see the Cassini division with a 3" scope where the diffraction limit would require an 8 or 9" scope.

However that might be the subject of another thread, back to this thread.

Since the above is a little light on numbers here is the effect of aperture and scope type with a change in SQM (Blue rows).

I did this to get an estimate of what more I could observe by limiting magnitude when travelling to dark sites and it's a little old as I've moved house since.

It's a little darker at SQM 21, and yes I notice the difference.

The physical scopes I own on the below chart are LZOS 130mm/F6 and C11. Yes the C11 is full of stars, the extra stars seen is really deep compared to the 130 mm however the lack of sharpness compared with an APO is very noticeable and even if the C11 had many hours to acclimate it's not going to get anywhere close. 

Also compare the scopes at different sites, in purple. A gain of half a magnitude just by being at a relatively dark polluted site to a dark site is quite large.

Site	Wimbledon	Home	Iping Common (45 mins)	Garth ( 5 hours)	Aperture (mm)	Focal Ratio	Weight (kg)	Focal Length (mm)	Eye Piece (mm)	Magnification 26 mm
Telescope\SQM	18.43	20.67	21.2	21.78
FSQ-85	10.6	12.1	12.3	12.7	85	5.3	5	450	20	17.30769231
TS CF-APO 90 mm	10.9	12.2	12.4	12.8	90	6	4	540	20	20.76923077
SharpStar 94EDPH	10.9	12.3	12.6	12.9	94	5.5	4.2	517	20	19.88461538
SharpStar 100QII	11.1	12.5	12.7	13.1	100	5.8	5.2	580	20	22.30769231
Askar FRA600	11.2	12.6	12.8	13.2	108	5.6	6.5	600	20	23.07692308
SharpStar 121DQ	11.5	12.8	13	13.4	121	5.5	8.92	678	20	26.07692308
LZOS 130/780 (F6)	11.8	13.1	13.3	13.7	130	6	12	780	20	30
LZOS 130/780 (4.5)	11.8	13.1	13.3	13.7	130	4.5	12	585	20	15
FSQ-130	11.5	12.9	13.2	13.5	130	5	13.7	650	20	25
Mewlon 210	13.6	14.5	14.7	15	210	11.5	10	2415	20	92.88461538
GSO 10” RC	13.5	14.7	14.8	15.2	250	8	15	2000	20	76.92307692
BS 10” F2.8	13.5	14.7	14.8	15.2	250	2	12	718	20	27.61538462
BS 10” F4	13.5	14.7	14.8	15.2	250	4	12	1000	20	38.46153846
Mewlon 250	13.7	14.7	14.8	15.3	250	10	16	2500	20	96.15384615
Mewlon 250 Reduced	13.7	14.7	14.8	15.3	250	7	16	1750	20	67.30769231
C11 EdgeHD	14.1	15.1	15.2	15.5	280	10	12.7	2800	20	107.6923077
C11 EdgeHD Reduced	14.1	15.1	15.2	15.5	280	7	12.7	1960	20	75.38461538
12" F4 Dobson	13.2	14.7	15	15.3	300	4	32	1200	20	60
GSO 12” RC	13.9	15	15.1	15.5	304	8	24	2432	20	93.53846154
GSO 14” RC	14.1	15.1	15.2	15.6	355	8	30	2854	20	109.7692308
C14 EdgeHD	14.7	15.7	15.9	16.2	355	11	21	3910	20	150.3846154
GSO 16” RC	14.5	15.6	15.8	16.1	406	8	36	3250	20	125
16” Dob	14.7	15.8	15.9	16.3	406	4	40	1600	20	61.53846154
20” Dob	14.9	16.1	16.3	16.7	500	3.3	65	1650	20	63.46153846
24” Dob	15.2	16.4	16.2	17		4		1700		65.38461538
Key		SQM

[vlaiv]

30 minutes ago, Deadlake said:

I'm surprised the MTF of a scope and it's effect on looking at extending objects has not been brought up, aka why can you see the Cassini division with a 3" scope where the diffraction limit would require an 8 or 9" scope.

That is because MTF has minimal effect when observing extended objects.

MTF shows what sort of sharpness in transition you can expect when going from very bright to very dark and vice verse - think stars (how large airy disk is - transition from bright core to black background of space, or planetary detail - again transition from bright details to dark details - regardless what you are observing - craters or festoons). It is important at high magnification and most observing of extended objects (by that I think of DSOs that have surface brightness) is done on low power where differences in MTF between scopes are negligible.

To address the second part - we can see Cassini division in 3" scope because it has nothing to do with diffraction limit. Diffraction limit is measure of how much we can resolve - not how much we can see. It is akin to expecting not to be able to see the stars - because stars have much smaller angular size than Cassini division, in 3" scope.

We see stars that are tiny - just fine. What we can't do in small scope is resolve close binaries. We see them still - there is bright spot, but it is not quite clear if that is one star or two (or perhaps something totally different - a Sponge Bob shaped bright object in the sky ).

Same thing happens with Cassini division - we can see that there is some sort of dark feature on bright background - but we don't have clue what it is - is it just one line or several lines close together or row of dancing monkeys that hold hands.

Resolving power of the scope is about ability to resolve - and more aperture is needed to resolve smaller things - but to see contrast (and this is partly related to MTF as MTF dictates how abrupt that contrast change is) - even small aperture is enough.

[Elp]

10 minutes ago, vlaiv said:

a Sponge Bob shaped bright object in the sky

This is a much better way of defining undersampled.

[vlaiv]

7 minutes ago, Elp said:

This is a much better way of defining undersampled.

Just remember that pixels are not square pants

[Ags]

On 10/12/2023 at 12:07, John said:

4 inch step up in aperture eg: 6 inches to 10 inches, 8 inches to 12 inches etc, etc.

I don't think the 4-inch rule is correct, it approximates another rule, which is the 50% rule. The next scope up needs to be 50% larger in aperture. So the step up from a 6-inch newt would be 9-10", and the next step up would be 14-16". For a refractor, the logical step up from my ZS66 was a 90mm refractor, and the next step would be 120-140mm.

[John]

4 minutes ago, Ags said:

I don't think the 4-inch rule is correct, it approximates another rule, which is the 50% rule. The next scope up needs to be 50% larger in aperture. So the step up from a 6-inch newt would be 9-10", and the next step up would be 14-16". For a refractor, the logical step up from my ZS66 was a 90mm refractor, and the next step would be 120-140mm.

It's not really a rule, it's a guideline that I have often seen repeated online and in print applied to the newtonian design. My personal experience of owning newtonian scopes up to 12 inches in aperture suggests that there is some merit in it but maybe it's time for us to come up with something better 🙂

Logic has not always played a strong part in my selection of scopes though, I have to confess 🙄

 

Edited December 11, 2023 by John

[Ags]

8 minutes ago, John said:

Logic has not always played a strong part in my selection of scopes though, I have to confess

On the planet Vulcan EVERYONE has an 8-inch goto dob. On Earth things are more interesting.