Using different sized secondaries

[Hans Tucker]

Apologies in advanced for the stupid question. Can you use different size secondary mirrors on a Newtonian? e.g. Smaller secondary for Planets and larger for DSO's. Is there any advantage to this?

[Stonius]

The light cone is the light cone. The only advantage would be if you were shortening the scope tube to get more backfocus, eg for astrophotography. In that case, the light cone will be wider (because you are intercepting it earlier) and a larger secondary will be required. You give up some contrast though.


Markus

[ausastronomer]

Definitely not a stupid question. A really good one in fact. This is actually pretty complex with a number of influencing factors to consider and weigh up. I'll reply in detail tomorrow when I have more time, but in some cases and depending on current setup, for visual astronomy there can be some significant gains with a downsized secondary.

Cheers
John B

[mura_gadi]

Hello,

The secondary is very important, but I'll leave John to explain in an English version.

Try a program call "Newt for the web", it will allow you to change the secondary and get a visual on how the light cone behaves when the secondary is changed.

All the section have help and Stellafane is a good site in general, it also has examples of wide and narrow field scope designs.

[astro744]

I always use Mel Bartels web site for diagonal sizing. Lots of other info there too on short focal ratio telescopes.

See https://www.bbastrodesigns.com/NewtDesigner.html#optimizeDiagonalS ize

And also link to http://www.damianpeach.com/simulation.htm

[Rainmaker]

I tested my 18” F3.5 scope extensively with my 2 secondaries, first with a 4” 1/10th wave and then a 4.5” 1/12th wave.

After several nights with each secondary on different targets I decided to keep the 4.5” (25% obstruction) ahead of the 4” (22.2%) as the difference visually was not really noticeable as far as contrast and illumination goes and using the larger mirror means the flatness of very edges of the secondary and the actual placement isn’t as critical.

[astro744]

I did the same with a 10.1” f6.4 (Suchting) with 2.1” and 1.83” secondary. Installed the 2.1” for the same reasons but also I prefer less magnitude drop at the edges in my wider field low power eyepieces. I can interchange them fairly easily.

I got the best view ever of Saturn in a 16” f5 (Suchting) but with a 2.6” secondary. Almost like having a 16” refractor. However I felt the low power wide field views were noticeably dimmer at the edges. If it were mine I’d probably go for a 3.1” secondary at least maybe 3.5”. Note other factors such as tube diameter, tube thickness and focuser height come into play, (also use of Paracorr).

[Rainmaker]

This was one of the differences I noticed when switching from my 18" F5.6 to my 18" F3.5 scope, the illumination drop off in the slower scope is significantly more acute.

[ausastronomer]

Hi Hans,

You’ve already been given some good references with “Newt”, Mel Bartels website and Damian Peach’s website. Just remember that a lot of Damian Peach’s comments relate to imaging and not visual astronomy. The effects of central obstruction on imaging and visual astronomy are significantly different, although Damian Peach’s comments on the effects of central obstruction on the Modulation Transfer Function (MTF) curves and spatial frequency, is relevant and accurate. This is also covered very well on pages 49 to 62 of Suiter’s book “Star Testing Astronomical Telescopes”. It’s also covered in somewhat less detail in Chapters 18.6 and 18.7 of Telescope Optics by Rutten and Van Venrooij.

The secondary size of a Newtonian inter alia influences 2 very critical factors:-

1) The size of the Fully Illuminated Field. (FIF)
2) The effect on the MTF curves caused by diffraction.

Unfortunately, the 2 work against each other. As you go to a progressively larger secondary mirror to increase the size of the fully illuminated field, you significantly increase the effects on the MTF curves, which causes image quality degradation through loss of contrast, caused by diffraction. It gets further complicated because as Newtonian telescopes get faster in F-Ratio, they require a progressively larger secondary mirror to achieve the same sized FIF for a given aperture.

When a telescope designer / builder sets their design parameters, they are in most cases assuming that the telescope will be used for a mixture of deep sky observing and planetary observing, so they try to build an “allrounder”. I won’t go into 100% illumination zones and 75% illumination zones, which are covered in “newt” and elsewhere, as they overcomplicate this post. A good general rule of thumb in terms of the size of the FIF is that you should aim for a FIF size of 50% of the longest focal length eyepiece, you are likely to use frequently. In most cases with modern Newtonians generally being between F3 and F7, this means people are looking to obtain a FIF of between 10mm and 15mm.

The other side of the equation is the effect on the MTF curves. For some reason the “magical” number is 20%. When the Central Obstruction (CO) goes under 20% the views will closely approximate the views of an unobstructed telescope. When the CO goes over 20% the effect on the MTF curves becomes progressively greater. In Suiters Book he shows examples of the effect of a 20% CO on the MTF curves, the effects of a 32.5% CO and the effects of ¼ wave of spherical aberration on the MTF curves, which is spherical aberration on a “diffraction limited” optic. The example shows that a 20% CO has minimal effect, ¼ wave of spherical aberration has a slightly greater effect than the 20% CO, but still fairly minimal and the 32.5% CO has a somewhat greater effect than both of those. Of course then you start to “add” the cumulative effects of things like the size of the CO, spherical aberration, astigmatism, seeing, etc etc; iot all goes pear-shaped pretty fast. The “Aberrator Software” is pretty dated now, but you can still use it to view some decent simulations of the effect on image quality of a whole range of things. It’s a free download from here.

http://aberrator.astronomy.net/

With F4 and faster telescopes its difficult to get the CO under 20% due to the steep light cone. With a specialist planetary instrument, which would invariably be slower than F5 and usually F6 to F8, that won’t see much use for deep sky, you can intentionally downsize the secondary to give say a 5mm or 6mm FIF and end up with a 12% to 16% CO. This will make a great lunar / planetary instrument, but the downside will be a clearly visible fall off in image brightness towards the edge of the FOV of longer focal length eyepieces. The other -ve effect is that when you downsize the secondary to this degree you bring the edges of the secondary mirror into play, which is where most secondary mirror aberrations are likely to be.

All of my scopes have been carefully designed and components selected to give the right balance. My 10” SDM has a 1.83” secondary for an 18.3% CO. My 14”/F4.5 SDM has a 2.6” secondary for a 18.5% CO and my 18” Obsession has a 3.1” secondary for a 17.2% CO. My longest focal length eyepiece is a 31mm Nagler and none of the scopes show a detectable drop in illumination near the EOF in that eyepiece, but with all of them having CO’s under 20%, they all perform exceptionally well as lunar planetary scopes. This wouldn’t be achievable with a sub F4 scope, but they have their own inherent advantages in regards to portability and eyepiece height.

In Matt’s case in his 18”/F3.5 he tried a 4.5” (25% CO) and a 4” (22.2% CO) and didn’t see any improvement in contrast. If he had been able to try a 3.1” secondary for a 17.2% CO, I have no doubt there would have been some contrast gains, had seeing and thermal equilibrium all co- operated. The downside would have been a noticeable drop off in image brightness at the EOF in long focal length eyepieces. With the scope being fast at F3.5 there is just no way around this.

The mass produced Asian dobs usually have slightly oversized secondaries at around ~25%. Those with good optics still deliver excellent planetary images. I still have a 10”/F5 GSO that I leave at a friends property to use when I go there and save transporting a scope. It has a 2.6” secondary for a 26% CO and delivers excellent lunar planetary images, considering what it is and what it cost. It takes pretty good seeing before my 10” SDM with Suchting mirror and 18% CO pulls ahead and not be a huge amount. I have no doubt it would be slightly improved by a smaller (sub 20%) high quality secondary mirror.

What are the parameters on the scope you are looking to downsize the secondary mirror on ?

Cheers
John B

[Hans Tucker]

Hi John. There has been some excellent advice, information and references given in this thread and I thank all those that have provided input. The Newtonian I have is an Orion Optics (UK) VX10 f4, with 1/10 WPV Primary, which comes with a 63mm (2.48") secondary. I intend to replace the Spider and Secondary holder with one from Protostar, when they decide to ship to Australia again. I also intend to replace the secondary with a Antares 1/30 although their sizing differ from the native 50mm, 63mm, 75mm etc.

[astro744]

Antares secondaries are based on sizes that have been in use since the beginning of time. These are typically mounted in secondary holders that have a lip around the edge of the secondary serving three purposes: first to hold the secondary in place, second to mask any edge surface errors on the secondary and third to allow the secondary to float in its housing with no thermal stress from any directly applied glue. Antares uses what used to be industry standard sizes and are recommended if you have the correct holder.

The secondaries used today in most mass produced telescopes are based on metric dimensions likely sized to rounded numbers in mm but not in line with the previous industry standard sizes. Most telescopes too these days have their secondaries glued with no lip around the edge. I’m not sure that I’d want to glue a 1/20 or 1/30 secondary (for fear of glue not holding) and whether I’d even still get 1/20 or 1/30 performance from it should there be any thermal stress between the back of the secondary, the glue and the plate/stalk the glue adheres to. I’d be changing the secondary holder too which has its own challenges, one being the diameter of the centre bolt needs to fit the spider.

[mura_gadi]

Thanks for the post, I got a new link to try and I'm off to read the suggested pages in D Souter's Star testing. I have spent ages deciding on my secondary size.

Good luck with your secondary.


PS -
Looking at the ad pictures, you appear to have chrome components within the OTA optics path, I'd be blackening them out. Reducing stray light will improve the image and there are lots of suggestions for controlling light scatter in newts on the web.

One thing about newts, you can trick them out quite a lot from the standard base units.

[glend]

Speaking specifically about imaging setups, several scope manufacturers have increased secondary sizes to provide "full frame" camera sensors with larger unvignetted fields. In the past APS-C sensor sizes were the norm, and secondaries were designed with that in mind. A good example is what Skywatcher did to the venerable MN190 Mak-Newt. Their version of this very good scope had a 52mm secondary, which was perfectly adequate in terms of APS-C sensor coverage. The secondary on a Mak-Newt cannot be moved (they are mounted directly on the rear of the corrector, and offset is fixed by design), and focal length cannot be adjusted. With the advent of "full frame" cameras vignetting became an issue. Skywatcher solution was to increase the size of the secondary to 63mm, to provide a sensor field adjustment with less Risk of vignetting. So all new MN190s now come with 63mm secondaries. Of course this impacts central obstruction, and does degrade the scope slightly in terms of visual use. However, it was obvious that Skywatcher hoped to sell more of these scopes, to full frame camera owners, and avoid questions about vignetting. As a former owner of a small secondary MN190, I regret selling mine, as they are now collector's items, and imho out perform the larger secondary versions.