Reverse Engineering the Skymax 180

[alex_stars]

On 06/07/2020 at 19:03, yporti said:

I'm not sure if it is oversized since the meniscus would diverge the light and require a bigger mirror. My Mak180 have 200mm primary but still operates at 170-171mm, so I don't think the cause is the primary mirror size.

Hi, how did you estimate the effective aperture your SkyMax is operating on?

BTW, I agree with you, it is certainly the meniscus lens that brings the effective aperture down to about 170mm.

Edited August 4, 2020 by alex_stars

[alex_stars]

I had a chance to image Jupiter yesterday with Ganymede casting a shadow (31.07.2020 at 21:38:36 UTC)

The apparent size was 47.2 arc-seconds and I imaged with my ASI 224 at a back focus of 82.5 mm (original visual back + 2 to 1.25" adapter and ASI 224 to sensor). Using that info and some error estimates, I get a EFL = 2417 +/- 23 mm at that back focus and a f2 = 126.9 +/- 1.5 mm. Translating that to the back focus of the original setup (original visual back + 2" diagonal at 164 mm back focus) results in a stock EFL = 2720+/-27 mm. So for most practical purposes I'd say I can confirm the f2 estimate of Magnus and we can assume that SkyMax 180 has indeed a stock EFL of 2700 with the shipped VB and 2" diagonal.

Cheers and clear skies,

Alex

[yporti]

On 01/08/2020 at 13:42, alex_stars said:

Hi, how did you estimate the effective aperture your SkyMax is operating on?

BTW, I agree with you, it is certainly the meniscus lens that brings the effective aperture down to about 170mm.

I did the test using a flashlight in the eyepiece (focused on infinity) e measuring the iluminated circle in front of scope.

[Captain Scarlet]

If you remove and examine the secondary baffle, you will understand why the meniscus might be responsible. The baffle is certainly not a thin-walled cone. At the point where it's glued to the meniscus it is a rather thick-walled tube, 2mm thick in fact, and the silvering itself shows outside the baffle. If the full diameter of the unobscured meniscus is, say, 40mm, at least 1mm is OUTSIDE the bottom of the baffle, losing 2mm diameter, and the baffle itself consumes a further 4mm, leaving only 34mm doing any work. Add to that, that the baffle itself is likely not centered (look at my photos, mine was MILES off) and it'll be worse.

A good upgrade would be to replace the stock baffle with a thin-walled one, properly centered.

[Highburymark]

Fascinating and beautifully written report Magnus. Pleasure to read.

[alex_stars]

On 05/08/2020 at 06:18, yporti said:

I did the test using a flashlight in the eyepiece (focused on infinity) e measuring the iluminated circle in front of scope.

Thanks. I will have to try that. However I read that one can get quite some inconsistent results depending on the flashlight position, eyepiece used and distance to wall combinations.

[DainiusU]

Great post, thanks! I have a question here - are these two boots holding something or I can safely remove them and attach bracket using them later? I was afraid the hold focuser or something else and in doubt can I remove them both without the consequences?

 

[Captain Scarlet]

9 minutes ago, DainiusU said:

Great post, thanks! I have a question here - are these two boots holding something or I can safely remove them and attach bracket using them later? I was afraid the hold focuser or something else and in doubt can I remove them both without the consequences?

 

You can safely remove them, they are for a finder bracket, allowing the finder to be “above” the dovetail whichever side of a mount you set the scope.

[Captain Scarlet]

To round this off, now that I’ve made all the measurements necessary to eliminate a key (and it turned out, wrong) assumption I made in my original post.

To recap: in my original post I didn’t have the wherewithal to actually measure the focal length of the secondary mirror. So I assumed the EFL of the scope was as advertised, 2700mm, with the supplied accessories and associated back-focus value, and from that I "backed-out" the implied FL of the secondary.

Eventually I was able to make measurements to accurately establish the true value of the secondary's focal length. I didn’t measure the mirror’s FL directly, that’s still too difficult, but I did measure the whole system’s focal length very accurately, by photographing star-fields, and was thereby able to back out the secondary's true FL.

The upshot is that with the supplied accessories – a 52mm-long visual back and a 2” diagonal – the focal length of the system is 2883mm (+/- 16mm). Or F/16 assuming 180mm aperture. This certainly helps explain why I was seeing smaller fields-of-view than I was expecting, which started the whole process off for me.

In other words, as supplied this scope is NOT the 2700mm specified, it’s nearly 200mm longer.

Also in the process I've been able to home in far more accuyrately on the FL of not just the secondary, but the primary too. They are:

Primary FL: 463.3mm +/- 1.6mm; Secondary FL: 115.7mm +/- 0.7mm

The back focus required to achieve the 2700mm FL is 118mm, which may well be what the Visual Back + Diagonal configuration was when it was originally released with a 1.25” back.

I have also reformulated the chart of EFL-vs-BF to replace the one in the original post.

Cheers, Magnus

Edited January 1, 2021 by Captain Magenta

[M40]

Brilliant Magnus, dread to think how many hours you have spent sorting that lot out for all. If I have the math right, F2 is 118mm co-incidentally the same as the back focus at 2700 FL; and the curvature of the primary mirror is 944mm radius. Hopefully someone will correct me if I am wrong 🤪

All the best. Les

Edited January 1, 2021 by M40

[Captain Scarlet]

I should have added those other details that have changed from the original post too, thanks for reminding me. In fact my new more-accurate method caused me to change both the FL of the primary mirror as well as that of the secondary. I've added these details to my "recap" post just above.

Basically, I've added the following: "Primary FL: 463.3mm +/- 1.6mm; Secondary FL: 115.7mm +/- 0.7mm"

Cheers, Magnus

Edited January 1, 2021 by Captain Magenta

[Nik271]

Thank you Magnus, this is very useful information! I tend to use mine with a 1.25 inch diagonal so pleased to see that the focal length in that configuration is close to the advertised 2700mm.

Incidentally I realised the effective central obstruction is not 20% because it's not  coming from the secondary. In fact it's over 30%: As your very helpful measurements show it comes from the retaining ring of the primary which is 63mm. So the central obstruction linear ratio is 63/200 =31.5%. Still small but not even close to the 20% advertised.

 

Nikolay

[Captain Scarlet]

Yes it’s rather disingenuous to claim the central obstruction at 23% ... skywatcher themselves make that claim!

[alex_stars]

23 hours ago, Captain Magenta said:

Basically, I've added the following: "Primary FL: 463.3mm +/- 1.6mm; Secondary FL: 115.7mm +/- 0.7mm"

Hi Magnus,

I see you have revised your primary FL (F1) also down to 463.3 from the initial 472 mm. When I run my estimates with this F1, I get a secondary FL (F2) of 114.6+/-0.7mm, so we agree within error bars. 😀. May I ask what caused the revision of the primary FL?

Clear skies,

Alex

[alex_stars]

22 hours ago, Nik271 said:

Incidentally I realised the effective central obstruction is not 20% because it's not  coming from the secondary. In fact it's over 30%: As your very helpful measurements show it comes from the retaining ring of the primary which is 63mm. So the central obstruction linear ratio is 63/200 =31.5%. Still small but not even close to the 20% advertised.

Nikolay

Hi @Nik271,

I agree this is rather upsetting. However it gets even worse. Given that the primary mirror is oversized (200 mm instead of required 180 mm) to presumably avoid edge effects of a non-perfect mirror, we should probably calculate with 63/180, resulting in a 35 % linear central obstruction. If this is indeed the case it would be on a similar visual contrast level as the typical SCs, say a C8 (33.8 %) or a C9.25 (36.2 %). Both would have more aperture and thus more resolution, especially for imaging.

This will keep me thinking for a while now.

Clear skies,

Alex

[Nik271]

54 minutes ago, alex_stars said:

Hi @Nik271,

I agree this is rather upsetting. However it gets even worse. Given that the primary mirror is oversized (200 mm instead of required 180 mm) to presumably avoid edge effects of a non-perfect mirror, we should probably calculate with 63/180, resulting in a 35 % linear central obstruction. If this is indeed the case it would be on a similar visual contrast level as the typical SCs, say a C8 (33.8 %) or a C9.25 (36.2 %). Both would have more aperture and thus more resolution, especially for imaging.

This will keep me thinking for a while now.

Clear skies,

Alex

It's probably a bit better than that because the front meniscus is a negative lens, i.e. the 180mm front aperture gets 'dispersed' into wider beam, I presume between 190 and 200mm which is captured by the primary. This image is than reflected by the secondary into the rear opening, so the size of the image of the central obstruction will change in the final image circle.

When I did the flashlight test with my Skymax 180 I measured the full aperture 180mm with a central shadow of 55mm, giving 55/180 =30.5% but this is highly dependent on the distance of my light source at the back focus. I didn't position that very carefully so can't be sure about the final result. 

 

 

[Captain Scarlet]

1 hour ago, alex_stars said:

Hi Magnus,

I see you have revised your primary FL (F1) also down to 463.3 from the initial 472 mm. When I run my estimates with this F1, I get a secondary FL (F2) of 114.6+/-0.7mm, so we agree within error bars. 😀. May I ask what caused the revision of the primary FL?

Clear skies,

Alex

I measured the whole-system EFL - i.e. took photos of a star-field - at two widely-spaced values of back-focus. I have an 80mm extension tube. When I ran the numbers of resulting EFLs with my existing values and equations, I couldn't get them consistent with properly-measured mirror-separations or back-focus values. One or other value of back-focus, but not both. Somehow I needed an extra degree of freedom. I suddenly realized that one measurement I had made was a weak point: that of the FL of the primary. It was the shakiest measurement of all those I'd made, furthermore I had done it "naked", i.e. without the corrector plate in front of it. So I re-ran my maths ignoring the measured FL of the primary as well and solved for two unknowns: the FLs of both the primary and the secondary. All my other measurements I was happy with, had small errors and those fed through into the small errors I have for the FLs and the system EFL.

It's still not perfect, the effect of the corrector plate is not mathematically accounted for. Rather I've just let it become a "property of the system" inherent/embedded in the implied results, but my back-focus <=> EFL and mirror-separation <=> EFL all now give correct enough results over the focus-range.

Cheers, Magnus

PS I had the scope out last night, I really do like it...

Edited January 2, 2021 by Captain Magenta

[alex_stars]

1 hour ago, Nik271 said:

It's probably a bit better than that because the front meniscus is a negative lens, i.e. the 180mm front aperture gets 'dispersed' into wider beam, I presume between 190 and 200mm which is captured by the primary. This image is than reflected by the secondary into the rear opening, so the size of the image of the central obstruction will change in the final image circle.

Right. However I was wondering what aperture vs obstruction values actually define the resolution of the telescope. Is it

• the 37/180 = 20% at the meniscus lens?
• or is it 63/180 = 35% (taking the primary mirror hole and the front aperture)?
• or is it 63/190 = 33% (taking the primary mirror hole and the dispersed beam)?

I don't know the answer on top of my head. Probably should research that, or measure the modulation transfer function (MTF) at one point, that would describe the whole system.

@Captain Magenta, thanks for the details.

Cheers,

Alex

[Nik271]

11 hours ago, alex_stars said:

Right. However I was wondering what aperture vs obstruction values actually define the resolution of the telescope. Is it

• the 37/180 = 20% at the meniscus lens?
• or is it 63/180 = 35% (taking the primary mirror hole and the front aperture)?
• or is it 63/190 = 33% (taking the primary mirror hole and the dispersed beam)?

I don't know the answer on top of my head. Probably should research that, or measure the modulation transfer function (MTF) at one point, that would describe the whole system.

@Captain Magenta, thanks for the details.

Cheers,

Alex

I just realized a sure way to measure the real obstruction ratio: on a night of good seeing focus on a bright star. Then defocus a little until you see the shadow of the obstruction. Take a picture and then  measure the ratio from the image.

 

I'm planning to do this the next clear night. Looks like I might have to wait a bit for it according to clear outside ☹️

[Nik271]

I spotted an opening in the clouds this morning and took a quick shot at defocused Polaris. This was taken with the Skymax 180 and a DSLR attached to the 2' visual back (no diagonal).

I should have defocused more because the edges are quite fuzzy with the diffraction rings and its hard to see where the field stop is. Clouds arrived before I can experiment more, but already we can see that the central obstruction is close to 1/3 of the diameter.  

 

As a bonus I shot an image of Polaris and companion, it's easy because they don't move and can use longish exposures without any tracking 

There is some better weather predicted for the weekend, I will experiment more to nail this down. As I was focusing in and out it seemed that inside focus and outside focus diffraction patterns looked slightly different, I don't know if this is normal for Maks or just my scope. 

Nikolay

Edited January 5, 2021 by Nik271
typos corrected

[Captain Scarlet]

9 hours ago, Nik271 said:

 As I was focusing in and out it seemed that inside focus and outside focus diffraction patterns looked slightly different, I don't know if this is normal for Maks or just my scope. 

Nikolay

That reminded me that I thought I'd read in Suiter that Maks necessarily have some detectable SA inherent in the design. I looked it up: In Edition 2, Sections 11.1.1 & 11.1.2 are the requisite bits. He seems also to imply that it doesn't matter much, they can still be very good.

Magnus

[alex_stars]

@Nik271, great work.

My weather conditions did not allow yet for some nighttime testing, but I took this photo just now on my desk.

Its basically a zoomed smartphone image into the EP-holder (no diagonal) and the scope against a white wall. You can see the smartphone lens reflection in the secondary. Also when you look at the supposedly black areas outside the primary, you see that my baffle tube could do with some flocking  

Anyhow if I measure the central obstruction here, I get 34%. So there we go.

@Nik271 great image of Polaris BTW 👍

[Nik271]

14 hours ago, Captain Magenta said:

That reminded me that I thought I'd read in Suiter that Maks necessarily have some detectable SA inherent in the design. I looked it up: In Edition 2, Sections 11.1.1 & 11.1.2 are the requisite bits. He seems also to imply that it doesn't matter much, they can still be very good.

Magnus

yes! Reading up on this I realized that for an accurate result I should not be defocusing by moving the primary but by moving the sensor/eyepiece  by equal amounts in and out of focus to compare.

Some clear spells are forecast tonight, to be continued...

 

Nikolay

[Nik271]

1 minute ago, alex_stars said:

@Nik271, great work.

My weather conditions did not allow yet for some nighttime testing, but I took this photo just now on my desk.

Its basically a zoomed smartphone image into the EP-holder (no diagonal) and the scope against a white wall. You can see the smartphone lens reflection in the secondary. Also when you look at the supposedly black areas outside the primary, you see that my baffle tube could do with some flocking  

Anyhow if I measure the central obstruction here, I get 34%. So there we go.

@Nik271 great image of Polaris BTW 👍

Right, this proves that 34% is the best possible, and in some situations of long back focus distance it may be worse than this because the light cone may be further clipped by the primary or secondary baffles 

34% is basically the same as a C8 SCT, that's disappointing.

 

[alex_stars]

8 hours ago, Nik271 said:

34% is basically the same as a C8 SCT, that's disappointing.

I agree, that is indeed disappointing, given that I got my Mak to have better contrast than a SCT. 😞 Well well, it's still a nice scope...