Skymax 90 maximising FOV

[Mr niall]

Hello there!

Maths isn’t my thing... I’ve just bought a Skywatcher Skymax 90 - and I’m very much a non goto observer, so I need to maximise my FOV at all magnifications if poss! But I’m aware with long focal length scopes like the Mak that the very very wide angle eyepieces will become kind of pointless because of the vignetting of the OTA.

So I was wondering if anyone knows the maths I need to use to see if an eyepiece will be too wide at a given focal length for my scope? I know there’s no such thing (technically) as too wide but I don’t want to pay a premium for a widefield eyepiece only to lose the edges through vignetting.

many thanks, sorry if that doesn’t make a whole lot of sense!

[davyludo]

Can't help, but I'm interested to learn this one as well! 

Did you swap your 127 for a 90?  

[Mr niall]

Just now, davyludo said:

Can't help, but I'm interested to learn this one as well! 

Did you swap your 127 for a 90?  

Yeah had to downsize to cover unbelievably priced electronic gifts for the munchkins. Managed to get hold of a very cheap second hand 90 so absolutely over the moon.

[davyludo]

Ach that's a shame. Was nice of you to sell your toys so that the wee ones could get unbelievably priced electronic gifts! 

Glad you managed to pick up a 90 for yourself though. Interested to hear how you get on with it. 

[Louis D]

You need to update your signature.

Since the 90mm Maks have even smaller rear ports than 127 Maks, I'd guess trying to use 2" eyepieces with it would be pointless.  Try a 32mm plossl to maximize your TFOV.  They're not that expensive and work very well in slower scopes.

[John]

As Louis says, a 32mm plossl or a 24mm 68 degree super wide will show you as much sky as the 90mm mak-cass can. In both cases thats around 1.3 degrees true field.

 

[davyludo]

So does the 90 mak actually have a slightly larger fov compared to the 127? 

If I'm remembering correctly, I think I read somewhere that the 127 is around 1.1 degree. 

Niall, the following equations will give you tfov for an eyepiece:

True field of view = apparent field of view / magnification 

Or substituting magnification:

Tfov = afov / (scope focal lengh / eyepiece focal length)

[Geoff Lister]

I have both the 90 & 127 Maks, and I use both with my Celestron 32mm plossl, for initial alignment and the wide-angle views.

[Cosmic Geoff]

If you search diligently, you should be able to find the post I saw a while ago which listed the actual size of the rear hole for various sizes of Maksutov and SCT telescopes.  If vignetting occurs, it will, I assume, not prevent the outer part of the field being used for finding or alignment.

Since the 90mm Mak is a relatively small instrument, one would expect the rear hole to be correspondingly small.  I concluded that getting wide field eyepieces for my 127mm Mak was not worth the bother.

If you really want wide field, you could probably pick up a used 102 or 130mm Newtonian OTA for some silly price, cheaper than a new eyepiece. For example, Astroboot have a 114mm f8.7 Newtonian OTA for £22.50.  A Startravel short focal length refractor might be better, but beyond your present budget.

[Geoff Lister]

My second 'scope was an Astromaster 130EQ-MD. I found the EQ mount fiddly to set up, could never get the tracking speed right, and regularly hit the end stops on the two manual adjusting knobs. However, the 650mm OTA fits nicely on the Skymax mount for a wider FOV. 

[Mr niall]

20 hours ago, Cosmic Geoff said:

If you search diligently, you should be able to find the post I saw a while ago which listed the actual size of the rear hole for various sizes of Maksutov and SCT telescopes.  If vignetting occurs, it will, I assume, not prevent the outer part of the field being used for finding or alignment.

Since the 90mm Mak is a relatively small instrument, one would expect the rear hole to be correspondingly small.  I concluded that getting wide field eyepieces for my 127mm Mak was not worth the bother.

If you really want wide field, you could probably pick up a used 102 or 130mm Newtonian OTA for some silly price, cheaper than a new eyepiece. For example, Astroboot have a 114mm f8.7 Newtonian OTA for £22.50.  A Startravel short focal length refractor might be better, but beyond your present budget.

Thanks Geoff but no I don't want widefield specifically - I've already got a pair of 15x70s. I just want to maximise the Maks FOV

[ronin]

I would say that the "normal" 30/32mm plossl would be the best option. You might find that a 40mm plossl will give a fraction more, but it will be so small as to be kind of insignificant. When I got my 105 Mak I recall that I worked out that the 40mm managed a fraction more. I bought the 40 therefore. I suspect that a 2" eyepiece is not an option of the Skymax.

Problem is some say 30mm some 32mm and then comes the FoV which can be 50 or 52 degrees according to whichever manufacturer. And to be honest I am not sure how true/accurate each claim actually is. I have seen identical looking plossl's with fractionally different claims.

If you consider say a 30mm and 50 degree plossl (both nice round values) then the numbers work out an identical field of view to you as the BST Starguider. It all can become a bit involved for what could be little return.

Looking at TV plossl's the 32mm will give a field of 1.28 deg, the 40mm will give 1.376 deg. Sort of 0.1 of a degree, which was about what I found the first time round. Didn't realise TV Plossl at 32 and 40 were that costly now.

 

[djhartist]

Have a Revelation Astro 32mm plossl. This has very slight vignetting in the skymax but is clear and bright, and is a larger fov than the 25mm plossl that was supplied. You could use a 24mm 68degree or 25mm bst but I prefer the 32mm plossl as it has a slightly larger exit pupil which gives a brighter image, which I find preferable for this small aperture scope. Personally wouldn't bother with a 40mm plossl as it would only have a narrow fov. Great scopes though.