Wider FOV eyepiece for Mak

[Sunshine]

I was wondering if Mak users can help out with my next eyepiece purchase for my Mak 150.

Mak’s with their enormous focal lengths are notorious for narrow fov’s, I find that my 24mm is not enough to squeeze in the Pleiades, and barely enough for the double cluster. 

The question is, how wide can I go before I reach the limit,35mm maybe?  With my 24mm, looking at the double cluster, each cluster is right at the field stop, I would be happy with a little more FOV. 

If not wider a wider eyepiece then maybe a broader remedy which will increase every eyepieces FOV, focal reducer?

Thanks!

[RT65CB-SWL]

Morning @Sunshine.

You may have mentioned your 24mm e/p elsewhere, but if not, can you remind me what type it is? - sorry, but I am in the "I'm with stupid camp!" this morning.

I did try my 13mm Nagler/Type 1 a few months ago in my 're-modded' ETX105, (due to my work commitments, I have yet to try my 13mm/Type 6 in the same 'scope). Overall the Type 1 does give a wider field of view when compared with a 13mm Plossl... (82deg. vs 50deg.), and I did not notice any vignetting, though averted vision may induce it. In my opinion, a focal reducer/field flattener may be or will be the cheaper option.

[AstroNowt]

Until yesterday I was looking for a Celestron f6.3 reducer to attach to my Mak127 for AP use. I missed out on one but found a new scope instead!

I had done a fair bit of reading and felt confident that this would bring the scope down to around f7.5 and any vignetting would be outside of my sensor anyway. I guess for visual you may see something but if it was me I would buy used and flip it if not happy. The Celestron's seem to be around £60ish used.

[Orac]

I am also curious about this. Because I have the same need. I want a bigger TFOV on my 127 Mak. 

As far as I understand it goes like this. The maximum TFOV is a combination of scope (the focal length) and the fieldstop in the eyepiece and the focal length in the eyepiece. So there is a technical maximum for all eyepieces based on the fieldstop. The maximum fieldstop is the width of the eyepiece barrel.  So if you maxed out on what is possible with the eyepiece (I own a BST Starguider 25mm 60 degree AFOV) and you want a bigger TFOV a reduction of the focal length of the scope should do the trick. 

At least this is what I was thinking about yesterday and wanted to ask but you beat me to it. But I wanted some expert advice before I order a focal reducer. 

[John]

The field stop of the eyepiece limits the apparent field of view that you can get with a 1.25" barrel format as follows:

24mm = 68 degrees

32mm = 52 degrees

40mm = 43 degrees

If you do the maths you  find that all the above show pretty much the same true field of view so going for the longer focal length alters the exit pupil size but does not show more sky. Personally I prefer the 24 / 68 approach because of the darker background sky and wider apparent view are more pleasing to my eye.

It might be possible to use a focal reducer but I suspect that vignetting (loss of light) at the field edges due to the internal diameter of the rear port of the scope might limit the gains made from that approach.

When all is said and done, the mak-cassegrain is not a wide field design. Sometimes one has to "bite the bullet" and get a faster focal ratio scope, such as one of the F/5 refractors or a newtonian, to get the wide fields of view.

This is why many folks end up with more than one scope

 

[Orac]

33 minutes ago, John said:

It might be possible to use a focal reducer but I suspect that vignetting (loss of light) at the field edges due to the internal diameter of the rear port of the scope might limit the gains made from that approach.

When all is said and done, the mak-cassegrain is not a wide field design. Sometimes one has to "bite the bullet" and get a faster focal ratio scope, such as one of the F/5 refractors or a newtonian, to get the wide fields of view.

This is why many folks end up with more than one scope

 

I do have an older scope with a much shorter fl. But it only has an aperture of 70mm. And yes there the Pleiades are gorgeous to behold. 

My bigest problem is finding stuff (nebulas and galaxies) with my Mak. So I was thinking to use a focal reducer to find what I am looking for, center the DSO and use eyepieces without the focal reducer to see and study. 

[AstroNowt]

Have you considered getting a goto mount? Two star alignment and away you go. It will point at anything your heart desires.

Changing out a reducer mid session will be a PITA.  

[Orac]

I already have a Goto mount.  So I get in the right neighbourhood but not always spot on. Stars are not a problem but it’s more the fuzzies that are the issue. 

[AstroNowt]

I used to have that problem then I discovered TENA Lady an illuminated reticule https://goo.gl/y6Ah9e

If you get your alignment spot on (3 star?) you should not have any issue. I would guess the middle and always be a bit off until I had a cross to aim for.

[Orac]

I will start a new thread regarding my issues with the alignment. So we can focus on the question regarding a focal reducer. ?

And thank you for the tip. 

[Panda Alvin]

Throwing in my 2 cents wall of text here… I have the 1.25” Antares 0.5x focal reducer, which fits on to the end of the Baader MK4 zoom.

As for me, I only started in late July this year so I am still very new to this. To start with, I struggled to find Andromeda. I know roughly where it is and how it should look like in a 5” scope, but just I couldn’t find it on my Mak. Star finding is also difficult with the limited FOV (even with the go-to mount, it doesn’t always point accurately) and because of that I really wanted to increase it. I thought about upgrading, via a Mak - SCT adaptor, then either slot a 2” visual back to accept bigger eps, or buy the 0.63x SCT focal reducer (true be told… I did also get the 2” visual back).

Before I went about it, I did looked at existing topics and posts regarding Mak upgrades. The comments were overall negative (not in a nasty way), ranging from potential coma at such focal reduction, to significant vignetting with the 27mm internal baffle tube of the 127 Mak. The general consensus is to mainly “stick at small objects where it is good at”. This advice did put me off doing so for a while until a cheap second hand Antares 0.5x came along for £9 which I was willing to take a gamble at that price. Despite still having major reservations, I was happy to treat it as an experiment, and probably consign it to my reserve astro goodies box should it not work out.

When the FR came, I did a quick test on a terrestrial object. Long answer short, yes there was vignetting. However, it wasn’t a deal breaker for me because it was only very slightly noticeable in the corner of my eye; this was with the zoom set on 24mm (well 48mm) at 45°. Slowly zooming in, the vignetting disappeared at 20mm(40mm) at 50° for me. I am happy with this because I am still technically gaining FOV at this rate. I also guess the few comments around different forum, mentioning the human eye don’t notice vignetting less than 15% were also correct.

Then two weeks ago, I went to an field meet for the first time. I was looking at planets until somebody pointed out that Pleiades had just risen up above the tree line. Mind you, I never seen Pleiades before (let alone know its Messier number) and I was trying to find it after slewing to the general direction. This took a while but eventually I think I found it, just by its 4 main centre stars that was brighter than the rest. At first sight, it really wasn’t that impressive as it was just some stars. This was until I added the reducer which showed more, not significantly but enough for me to instantly understand why people like it. You get a better sense of scale and pick out it's nebulosity. Such as an example from the simulation below.

In reality the reduced 24mm it looks slightly tighter than the simulation, this might have been the “vignetting” at play, which I now can’t see at all in the dark.

The two other people with me at the time, having 3-4 years’ experience each also had a look through my scope. I asked if there were any issues with the view, but neither could find anything wrong with it. Anyway, after that session, Pleiades also became one of my favourite and I can’t wait to look at it again properly when the next opportunity arises.

Overall personally, the reducer is one of the best astro experience for money I have brought to date (I guess due to low price and low expectations). For me it just happens to fit in really well, but I suppose the still quite mediocre F ratio of 5.9 and FOV limited to 45° I had may have played big a part in quality. I suppose your mileage may vary depending on your existing experience, and the EP and scope you are using (especially OPs 6 inch probably have bigger internal baffle but longer focal length).

 

Oh and as for the Andromeda I talked at the beginning, I definitely needed that reducer to find it. In a way, it helped defined between the dark sky and the grey fuzzy cloud. The object just stood out a tiny tad more.

 

[Louis D]

I've used my 40mm Meade 5000 SWA to good effect in my 127 Mak for widest field views of about 1.7 degrees by adding a Mak-to-SCT thread adapter, a 2" visual back, and a 2" diagonal.  I do get some some odd artifacts in the FOV when bright stars pass the edge of the Mak's rear port, but the wide views are fantastic.  I really don't notice the vignetting that must be present.

[John]

13 minutes ago, Louis D said:

I've used my 40mm Meade 5000 SWA to good effect in my 127 Mak for widest field views of about 1.7 degrees by adding a Mak-to-SCT thread adapter, a 2" visual back, and a 2" diagonal.  I do get some some odd artifacts in the FOV when bright stars pass the edge of the Mak's rear port, but the wide views are fantastic.  I really don't notice the vignetting that must be present.

I'd love to see a photo of that setup - the 40mm 5K Meade is a huge eyepiece !

(you have de-cloaked yours though I seem to recall ?)

 

 

[Orac]

4 hours ago, Panda Alvin said:

Throwing in my 2 cents wall of text here… 

 

Well worth the read. And worth more then two cents. 

It describes what I am expecting.  Just a bigger view so I can better home in. So vignetting is not a bother. I’ll search around until I can find one for reasonable price. 

[Red Dwarfer]

As an experiment recently I tried a 32 and 40mm Plossl in my SW127 Mak on the Eastern Veil nebula and , though tight , it was definitely visible - had to pan up and down to see it all ... it was almost directly overhead at the time which no doubt helped .

[Sunshine]

14 hours ago, Philip R said:

You may have mentioned your 24mm e/p elsewhere, but if not, can you remind me what type it is?

Hello! i have a 25mm Celestron Xcell LX, I was wondering if maybe a 30-35mm would allow me to squeeze a bit more FOV out and maybe see the double cluster without swinging my eyeball from field stop to field stop lol.

[John]

24 minutes ago, Sunshine said:

Hello! i have a 25mm Celestron Xcell LX, I was wondering if maybe a 30-35mm would allow me to squeeze a bit more FOV out and maybe see the double cluster without swinging my eyeball from field stop to field stop lol.

You would get a slightly wider true field from a 32mm plossl or a 24mm 68 degree eyepiece (ie: max field stop for 1.25" format). With your 150 mak-cassegrain:

25mm X-Cell LX = .83 degree true field

24mm 68 (eg: ES 24 / 68) = .91 degree TF

32mm plossl = .92 degree TF

Thats about as much sky as a 1.25" eyepiece will show with the scope. Longer focal length eyepieces in that format don't deliver a wide true field.

 

[cloudsweeper]

Interesting points about an issue that lots of us go through!  Most objects don't need a wide TFOV, but it can be generally more pleasing to frame objects nicely.  I have WA 2 inch EPs to optimise the TFOV in my SCT, and I also tried a low-cost x0.5 1.25" reducer.

With that reducer, tests showed that the TFOV increased only with shorter FL EPs, but was still no better than when using longer FLs and no reducer.  So the reducer was redundant.  If I ever see a 2" reducer at a knock-down price, I'll experiment with that!

As John says, if you really want wider views, you'll need another 'scope.  I love the views through the ST120 frac - well over 4 deg with large exit pupil, and around 3 deg with a better exit pupil.  A slower 80mm frac can deliver around 5 deg at a reasonable exit pupil - and that's my next purchase!

Doug.

 

[Paz]

My own experiences around this are I use a 32mm plossl for wide fields in my 127mm maksutov and 200mm SCT. I have a 32mm plossl because it has more eyerelief than the 24mm/68 degree options I've looked at and it gives a darker background than a 40mm plossl. If it wasn't for the eyerelief issue I'd have a 24mm.

I've got an 0.63 focal reducer for the SCT but I admit I don't use it, it's more glass in the light path and although it's been a while since I tried it I think I recall seeing the secondary or a shadow from it if trying to go really low in magnification.

For finding things I use a 9*50 finder and this makes it possible to leave the main scope on very small fields of view and still find things ok.

If you want to stick to one scope and make it as flexible as possible a focal reducer will expand its capabilities a bit, and is a lot smaller, cheaper, and simpler than getting another scope, but I would recommend considering  just getting a 32mm plossl or a 24mm wider field eyepiece.

 

[Louis D]

On 19/10/2018 at 10:59, John said:

I'd love to see a photo of that setup - the 40mm 5K Meade is a huge eyepiece !

(you have de-cloaked yours though I seem to recall ?)

 

 

Here's some photos I took of my dual scope setup with a large 1.25" vs large 2" eyepieces, swapping sides for reference.  I don't think any of them look all that out of place.

Getting back to the OP's original request for a wider FOV eyepiece for this Mak, I also did a quick measurement in the 127 Mak with a yard stick at about 35 feet to compare the maximum field in a 1.25" eyepiece (with the 24mm APM UFF which shows slightly more field than a 32mm Plossl, though the edge is a bit soft despite the sharp field stop) versus the maximum field in a 2" eyepiece (with the 40mm Meade 5000 SWA) and got 5-7/8" for the 1.25" and 10" for the 2".  Thus, the 2" shows 1.7x more linear field and 2.9x more area than the 1.25".  This agrees completely with the 46mm maximum field stop for a 2" eyepiece versus a 27mm maximum field stop for a 1.25" eyepiece.  If the OP wants the widest FOV possible in the 127 Mak, this is definitely the way to go.

40mm Meade 5000 SWA on 72ED and 24mm APM UFF on 127 Mak:

40mm Meade 5000 SWA on 72ED and 24mm APM UFF on 127 Mak:

17mm ES-92 on 72ED and 24mm APM UFF on 127 Mak:

24mm APM UFF on 72ED and 17mm ES-92 on 127 Mak:

24mm APM UFF on 72ED and 40mm Meade 5000 SWA on 127 Mak:

24mm APM UFF on 72ED and 40mm Meade 5000 SWA on 127 Mak:

24mm APM UFF on 72ED and 35mm Baader Scopos Extreme on 127 Mak: