My 4" refractor is seeing deeper than my 5.1" reflector!

[pez_espada]

As the title says.  My  102mm F11 achromat is showing fainter stars  at 46.7X than my new  130mm F5 Newtonian does at 40.6X.   I' m aware the effect magnification has on seing fantier point light sources, but according my math 6X more should not overcome 3cm of larger apertura of the reflector. On the contrary, my math tell me that the reflector should see around  0.1 magnitude deeper than the frac does...

I have check mi collimation to the best of my current abilities. 

Clear skies

C

[joe aguiar]

  That's norn for 1 inch smaller refractor to equal a inch bigger reflector. 

Take into account the slight light lose due co and a mirror doesnt reflect 100% of starlight it lose about 10 percent per bounce.

Joejaguar 

Edited January 27, 2020 by joe aguiar

[vlaiv]

Indeed - here is quick calculation:

102mm vs 130mm

102mm at about 98.5% transmission will be equal to 51^2 * pi * 0.985 = 8048.71mm2

130mm with 32% Central obstruction and 91% reflectivity on two mirrors will be equal to (65^2 - (65 * 0.32)^2) * pi * 0.91^2 = 9866mm2

Still larger and therefore more light gathering, but not 3cm larger in diameter, more like only 1cm larger as 9866mm2 is equal to about 112 - 113mm of clear aperture (sqrt(9866 / pi) * 2).

Add to this that F/5 scope has much tighter critical focus zone - harder to obtain proper focus, especially if scope has single speed 1.25" focuser that is not very precise, and the fact that larger aperture will suffer more from seeing effect and also that strehl numbers of two scopes can differ considerably. If 5.1" newton is mass produced it will probably have system sthrel of about 0.8 and F/11 refractor will probably have over 0.9.

Put everything together and you get result that you have.

[pez_espada]

So all that said I can confirm that a reflector to be able to compete with a refractor, the former has to be at least 27-30% larger in apertura. 

The myth becomes truth. 

Edited January 27, 2020 by pez_espada

[vlaiv]

5 minutes ago, pez_espada said:

So all that said I can confirm that a reflector to be a le to compete with a refractor, the former has to be at least 27-30% larger in apertura. 

The myth becomes truth. 

Might be so for small apertures, but as aperture gets larger - differences get smaller.

For example - we have seen that 130mm is equivalent to 112mm of clear aperture, so that is reduction of about 13.8% by diameter.

If you do the same with for example 8" F/5 newtonian, you will get different result. It has 26% CO, so math will be sqrt((101^2 - (101*0.26)^2) * pi * 0.91^2 / pi) *2 =  ~183.5mm of clear aperture. That is difference of 9% by diameter,

[pez_espada]

19 minutes ago, vlaiv said:

Might be so for small apertures, but as aperture gets larger - differences get smaller.

For example - we have seen that 130mm is equivalent to 112mm of clear aperture, so that is reduction of about 13.8% by diameter.

If you do the same with for example 8" F/5 newtonian, you will get different result. It has 26% CO, so math will be sqrt((101^2 - (101*0.26)^2) * pi * 0.91^2 / pi) *2 =  ~183.5mm of clear aperture. That is difference of 9% by diameter,

For an 8" @ F5 to have a fully illuminated field one is looking more to 30-32% of CO, i.e a secondary of 60-63mm.

Edited January 27, 2020 by pez_espada

[CraigT82]

1 hour ago, pez_espada said:

The myth becomes truth

I wouldn't say that there's ever been much myth about it. I think the vast majority of amateur astronomers with a bit of experience would agree that you need a newt at least one size up from any given frac if you want to compete with it in terms of light gathering (4" frac vs 6" newt, 6" frac vs 8" newt etc etc...)

[vlaiv]

36 minutes ago, pez_espada said:

For an 8" @ F5 to have a fully illuminated field one is looking more to 30-32% of CO, i.e a secondary of 60-63mm.

Ah sorry - my bad, I meant F/6 - that one has about 26% CO (I've got one - dob mounted 8" F/6), but you are right larger secondary is required for F/5, and even larger for imaging OTA.

[newbie alert]

It's focal length.. your f11 scope has a focal length of 1122mm... your 130 f5 has a fl of 650.. aperture is for brightness..fl for depth..

I'm sure your targets have more reach with the f11

[pez_espada]

4 minutes ago, newbie alert said:

It's focal length.. your f11 scope has a focal length of 1122mm... your 130 f5 has a fl of 650.. aperture is for brightness..fl for depth..

I'm sure your targets have more reach with the f11

I see this in the practice indeed but how's that from the theoretical point of view? What about the "aperture is King"  tenet in astronomy then?

"Aperture is King"  for "seeing finer detail", but also for "seeing deeper". This is what is repeated ad nauseam in every astro forum since I know of.

Edited January 27, 2020 by pez_espada

[vlaiv]

Just now, pez_espada said:

I see this in the practice indeed but how's that from the theoretical point of view? What about the "aperture is King"  tenet in astronomy then?

That is short version of that saying. Longer and proper version of that saying goes like this: Aperture is King everything other being equal

 

[pez_espada]

5 minutes ago, vlaiv said:

That is short version of that saying. Longer and proper version of that saying goes like this: Aperture is King everything other being equal

 

That would mean that a 6" F8 refractor and 6"F8 Newtonian are expected to perform around the same, I guess. To pickup just an example of the top of my head..

Edited January 27, 2020 by pez_espada

[newbie alert]

Ok, put it as a practical lesson and  look at  something like Tycho on the moon,try the 102 first, then the 130 using the same ep or camera..

Aperture is only king if you're not looking for more fl

[newbie alert]

As a example, same camera.. yellow box is the 130, red is the 102

Edited January 27, 2020 by newbie alert
Added info

[vlaiv]

47 minutes ago, pez_espada said:

That would mean that a 6" F8 refractor and 6"F8 Newtonian are expected to perform around the same, I guess. To pickup just an example of the top of my head..

If they have same central obstruction, which is not the case, if they have same transmission, which is not the case, if they have same F/ratio, which is here the case, if they have same surface quality and correction - strehl ratio - which might or might not be the case and are affected the same by atmoshpere - then aperture rules .

I support the notion that aperture rules, and I'm quite familiar with the idea that newtonians and other mirrored systems in general have slightly lower light grasp then their aperture would suggest - like we shown above - that depends on quality of coatings and size of central obstruction.

For what is worth, 6" F/8 Newtonian is going to blow away 6" F/8 refractor of the same price class in almost every aspect. It will also be outclassed in almost every aspect by high quality ED/APO 6" F/8 scope, but on the other hand - if you pay that much for a mirror and general construction of newtonian - gap to ED/APO closes real fast (never going to be equal, but very close).

 

[John]

For a while I owned a Skywatcher ED120 refractor and a Russian 150mm F/6 maksutov-newtonian. The mak-newt has a small central obstruction (below 20%) and no secondary support vanes. I compared the performance of these 2 excellent scopes on a number of occasions. On the moon, planets and double stars they performed pretty much identically. On deep sky objects the 150mm scope showed faint objects just a little bit better than the 120mm. I would expect a good 150mm F/8 newtonian to perform similarly. A 150mm F/5 would have a larger secondary mirror which would pull it back a little on the high resolution observing and perhaps a touch on deep sky reach as well.

 

[pez_espada]

Well It seems that the large CO (35%) of the 130mm is enough to impact the image as to obviously show less stars in a similar FOV, magnification and exit pupil than a 102 mm refractor despite of the 3cm of more aperture of the former. Lesson learnt.

[pez_espada]

On 27/01/2020 at 13:13, vlaiv said:

Indeed - here is quick calculation:

102mm vs 130mm

102mm at about 98.5% transmission will be equal to 51^2 * pi * 0.985 = 8048.71mm2

130mm with 32% Central obstruction and 91% reflectivity on two mirrors will be equal to (65^2 - (65 * 0.32)^2) * pi * 0.91^2 = 9866mm2

Still larger and therefore more light gathering, but not 3cm larger in diameter, more like only 1cm larger as 9866mm2 is equal to about 112 - 113mm of clear aperture (sqrt(9866 / pi) * 2).

Add to this that F/5 scope has much tighter critical focus zone - harder to obtain proper focus, especially if scope has single speed 1.25" focuser that is not very precise, and the fact that larger aperture will suffer more from seeing effect and also that strehl numbers of two scopes can differ considerably. If 5.1" newton is mass produced it will probably have system sthrel of about 0.8 and F/11 refractor will probably have over 0.9.

Put everything together and you get result that you have.

I see what you mean but still I am surprised that all these factors put together can defeat an advantage of 27% more aperture in the reflector. This is 62% more light grasp, and the smaller 4" scope still not only matches the larger but defeat it. I am surprise that nobody else  seem to be surprised by this observation.

Edited January 28, 2020 by pez_espada

[John]

The 102mm F/11 achromat refractors have very well figured objective lenses I believe. With the exception of a modest amount of CA (very modest at F/11) a high proportion of the light gathered by the objective will be concentrated into the airy disk of a star being observed, with a faint diffraction ring around the brighter ones.

With the 32mm obstruction, secondary support vanes (I assume), the reflectivity of the primary and secondary being 91% and the system strehl (ie: primary and secondary, assuming perfect collimation) being almost certainly lower than the refractor, I reckon a good proportion of light gathered is falling outside the airy disk as diffraction of one sort or another.

 

[vlaiv]

1 hour ago, pez_espada said:

I see what you mean but still I am surprised that all these factors put together can defeat an advantage of 27% more aperture in the reflector. This is 62% more light grasp, and the smaller scopes still not only matches the larger but defeat it. I am surprise that nobody else  seem to be surprised by this observation.

I think that really important part of the puzzle here is precision of the focus. Next time if you feel like experimenting - try not having perfect focus and seeing how it affects threshold visibility of stars.

[pez_espada]

1 hour ago, vlaiv said:

I think that really important part of the puzzle here is precision of the focus. Next time if you feel like experimenting - try not having perfect focus and seeing how it affects threshold visibility of stars.

Will do, and I will keep on going trying to improve my collimation skills, etc. I am pretty new to Newtonians.  Perhaps an autocollimator will do? any experiences? I was looking at one of these https://www.firstlightoptics.com/catseye-collimation-tools/catseye-infinity-xlkp-autocollimator.html

Are those better than the typical Cheshire/SightTube than I am currently using?

[vlaiv]

Just now, pez_espada said:

Will do, and I will keep on going trying to improve my collimation skills, etc. I am pretty new to Newtonians.  Perhaps an autocollimator will do? any experiences? I was looking at one of these https://www.firstlightoptics.com/catseye-collimation-tools/catseye-infinity-xlkp-autocollimator.html

Are those better than the typical Cheshire/SightTube than I am currently using?

Can't really tell - I'm not using anything in terms of collimation aid. I do occasional touch-ups on a star - pop in high mag EP, keep star centered (Polaris does great job with dob mounted manual tracking scopes) and do slight defocus and observe how concentric rings are.

[mikeDnight]

On 28/01/2020 at 13:04, pez_espada said:

Well It seems that the large CO (35%) of the 130mm is enough to impact the image as to obviously show less stars in a similar FOV, magnification and exit pupil than a 102 mm refractor despite of the 3cm of more aperture of the former. Lesson learnt.

There is something else at play here too. The diffraction pattern in both scopes is different. In the refractor the diffraction pattern of a Star is caused by the lens cell, and shows as concentric rings around the Airy disc that get fainter as they move out from the star. In a Newtonian, the edge of the secondary obstruction adds a further diffraction effect, causing the first diffraction ring to be noticeably brighter than that of the refractor. In scopes of equal aperture the Airy disc should be of equal brightness, but because of the first diffraction ring in the reflector being brighter, the Airy disc is fainter. This is so small a difference that its not immediately obvious, but when stellar point sources are at the limit of visibility, it can mean the difference between seeing a faint star or not seeing it. Add to that the diffraction pattern caused by the spider veins and the faintest stars can be lost because of the light spread.

A similar thing can be seen when observing extended objects such as the planet's. If you imagine a planetary disc as seen through a telescope to be constructed of thousands of tiny Airy discs, the smearing effects of diffraction can soften the view in an obstructed telescope and so definition at the limit of visibility can suffer.

Having said all the above, I've seen a 4.5" F11 Newtonian out perform a 4" Vixen Fluorite while viewing Saturn. 

Bottom line is, the better the optics the better the telescope. A reflector mirror has to be at least four times more accurately figured than any one of the, at least four surfaces on a refractor, which makes reflector much more difficult to make to a high standard. So the average off the shelf cheap reflector is highly unlikely to deliver refractor like performance. 

Edited January 29, 2020 by mikeDnight