What’s a MAKs Focal Length?

[PeterC65]

I use the Stellarium Ocular Plugin to check what I should be seeing through the eyepiece of my Skymax 127. I’ve often thought I was seeing less of the sky than predicted and when I started using a Binoviewer the difference seemed dramatic. So over the last few days I’ve done some daylight tests to try to measure the True Field of View.

I put the scope on a GOTO mount and measured the TFoV by recording the difference between the azimuth or altitude reading as I slew an object from one side of the eyepiece to the other. I used an Explore Scientific 24mm 68° eyepiece which I’m fairly confident meets its spec then from the measured TFoV I calculated the scopes focal length (telescope FL = eyepiece FoV x eyepiece FL / TFoV).

I did the same thing with the Binoviewer (the eyepieces for this are 20mm 66°) but it’s much harder to see when an object enters and leaves the FoV so the results are less accurate.

I knew that adding kit between the MAK and the eyepiece would affect the focal length but I didn’t realise how much. I’ve swapped the visual back of my MAK for a 2” Baader Clicklock, swapped the diagonal for a 32mm Baader prism, added two ZWO filter wheels, and a Baader 1¼” Clicklock eyepiece clamp (see photo, fitted to my refractor).

All of this, plus the washers and adapters, comes to 167mm compared to 105mm for the original visual back and diagonal. I didn’t think the additional 62mm would make much difference but my tests suggested that the focal length with all this kit at the backend is 1809mm rather than the 1500mm at which the original scope is specified!

After some online research I discovered that for a MAK, adding length to the optical pathway adds x3.75 that amount to the focal length. So the extra 62mm becomes 232.5mm and the scopes focal length becomes 1732.5mm. This is less that I measured so I’ve done the same test with my 432mm refractor which came out at 462mm, so I’m assuming that my measurement is a bit on the high side in both cases.

The Binoviewer adds another 100mm to the optical pathway, so 375mm to the focal length, and that explains why I’m seeing much less sky through the Binoviewer.

There is a similar multiplying factor for any Catadioptric telescope it seems. This is because this design of scope is focused by moving the mirror and that changes the focal length. So the stated focal length of any Catadioptric telescope is only a nominal value. Reflector and Refractor telescopes on the other hand have a fixed focal length as I understand it, with the eyepiece having to be moved (by the focuser) to sit on the fixed position focal plane.

Obvious when you know it, but I hadn’t realised before.

I was wondering whether others have carried out this sort of test on the Skymax 127? I’ve been assuming that the focal length with the original visual back and diagonal is indeed the nominal 1500mm but my measurements suggest that this may not be quite right?

 

[vlaiv]

Yes, focal length depends on primary to secondary distance.

I've seen people measure their scopes before and it is linear relationship.

By the way - spherical correction also depends on this distance, so there is only really one focus point where scope works at optimum.

Another thing to keep in mind - if you did measurements during daytime on target that is not at infinity - focus position changes for objects that are near by - adding to the problem as focus point moves further out - thus prolonging focal length as well (maybe that is why you got higher measurements then expected?).

[Louis D]

You have to find out the design back focus relative to the rear port; and then focus something like the moon, which is effectively at infinite distance, on something like Scotch tape at the back focus distance.  Now your mirror is at the design distance from the secondary and you can start to take measurements.  The problem is, has Synta ever published the design back focus for their 127 Maks?  Celestron and Meade generally published this information with their SCTs which have the same issue.

[PeterC65]

3 hours ago, vlaiv said:

Another thing to keep in mind - if you did measurements during daytime on target that is not at infinity - focus position changes for objects that are near by - adding to the problem as focus point moves further out - thus prolonging focal length as well (maybe that is why you got higher measurements then expected?).

The objects I used to carry out the measurements are about a mile away, so probably close enough to infinity to be lost in the noise of measurement inaccuracy. I think the main source of inaccuracy is probably locating the object at either side of the FoV (not just outside or just inside it).

3 hours ago, Louis D said:

You have to find out the design back focus relative to the rear port; and then focus something like the moon, which is effectively at infinite distance, on something like Scotch tape at the back focus distance.  Now your mirror is at the design distance from the secondary and you can start to take measurements.  The problem is, has Synta ever published the design back focus for their 127 Maks?  Celestron and Meade generally published this information with their SCTs which have the same issue.

From photographs of the different configurations, it looks like the Skymax 127 is always sold with the same standard visual back and diagonal. The visual back has the female "MAK" thread at one end and a two thumbscrew, no compression ring clamp at the other. I've measured its optical length at 35mm (by ruler). The diagonal is 1¼” with a barrel on the scope side and a single thumbscrew, no compression ring clamp on the eyepiece side. I've measured its optical length at 70mm (by ruler). So my assumption is that with 105mm of back focus the Skymax 127 has a focal length of 1500mm. But as you mention, there seem to be no published figures for any of this.

With the updated focal length figures plugged in to the Stellarium Ocular Plugin (1732.5mm for a single eyepiece, 2107.5mm for the Binoviewer) I do now get a much better approximation to what I actually see through the scope.

[vlaiv]

2 minutes ago, PeterC65 said:

The objects I used to carry out the measurements are about a mile away, so probably close enough to infinity to be lost in the noise of measurement inaccuracy. I think the main source of inaccuracy is probably locating the object at either side of the FoV (not just outside or just inside it).

Say they were 1500000mm away (1500m = 150000cm = 1500000mm), focus position shift goes like this:

1/focal_length = 1/object_distance + 1/image_distance (image_distance being effective focal length in our case).

1/1500 = 1/1500000 + 1/X

1/X = 1/1500 - 1/1500000 = 1000 / 1500000 - 1 / 1500000 = 999 / 1500000

X = 1500000 / 999 = 1501.5

So yes, change in focus position for something 1500m away is very small - only 1.5mm of out focus needed compared to infinity focus - this translates to about 5mm change in focal length. Not much, but still there.

As a comparison - testing on target 100m away would yield focus position at 1522.84 or about 22.84 out focus or about 85.7mm of focal length change.

 

[Louis D]

Speaking of close focusing, when taking test photos inside my house through the 127 Mak with an eyepiece, I can't rack the mirror enough to bring an object at 35 feet into focus.  It's not a problem at all with either the 72ED or 90MM APO to reach focus, though.  So, I add about 50mm of extension to the rear port and leave the mirror roughly at its infinity focus position.  As a bonus, it is still operating close to its design focal length.

Apparently, close focusing is not a strong suit of Maks.

[vlaiv]

13 minutes ago, Louis D said:

Apparently, close focusing is not a strong suit of Maks.

I think that it's the focal length of instrument that is important here.

You can easily see this if you examine what will happen if object and image distances are the same. This happens at twice focal length - regardless of the scope.

In another words - for Mak127 - if you place object at 10 feet (twice the focal length if foot is ~300mm) focal plane will also be 10 feet to other side.

For both 72ED and 90mm APO - this is closer to 3 feet to either side.

Larger the ratio between object position and focal length of scope - less out focus is needed (if we extend object to infinity - zero out focus is needed - regardless of focal length of scope as focal length of the scope is always finite value).

[Avocette]

I have the SkyMax150. The focal length quoted for this scope is 1800mm. In use for planetary astrophotography I attach an ASI533 as closely as possible to the visual back and measure the FL (by platesolving) as longer than 1800mm, so I presume that the specified focal length is that which would be measured (theoretically) at the end of the Maksutov baffle tube, ie certainly not that measured in the practical application with a star diagonal and eyepiece.