What difference would a triplet make?

The test of a doublet is shooting RGB or in your case with a UV/IR filter. The area most affected by a doublet is purposefully deep into the blue spectrum, usually below 420nm. If you have any fringing then you can use slightly narrower filters to kill most of it. My FPL51 doublet responds well to using an Astronomik L3 UV/IR filter and in the case of mono imaging, using a slightly narrower blue filter like the Antlia of Astronomik sets. Most of the light pollution filters like the L-Pro would also kill the deep blues and reduce any fringing. 

In short, test with a super wide UV/IR to see if you need to do anything, then try an L3 or L-Pro and see if you are happy. I love my doublet and have no urge to upgrade. The L3 filter was far less expensive than the cost difference of an equivalent triplet and I rarely image RGB.

Piers Palmer:
Ah ok; but I think the 2600mc pro has a cut filter built-in?  I’m shot the linked image with a NBZ dual band it as I’m in a Bortle 3 zone (my sqm reading last night was 21.36) so maybe I could simply shoot without one with decent results. That’s a different question…just rambling!

Correct. The protect window on the camera is itself a UVIR filter:

https://astronomy-imaging-camera.com/manuals/ASI2600_Manual_EN.pdf

This happened to me. I had a WO Zenithstar 73 which is a doublet with FPL-53. I became convinced that a WO GT81 which is a triplet with FPL-53 would be better. Your Elephant Trunk Nebula looks pretty good to me. However, I'm not a big pixel peeper. I bought the GT81 and sold my Zenithstar. Granted there's a tiny bit more aperture. I saw a little bloating in the stars of my doublet that I definetly don't see now in the triplet. Overall was it worth it? Probably not. But I'm personally more satisfied with the GT81. I think the stars are tighter and overall the objects are sharper. I admit this sharper thing could be my imagination. The increase in aperture and the triplet made me happier in thinking I'm getting the best data possible. It's a mental thing sometimes It's the illusive "being satisfied" with what you have. That's why they call astrophotograhy a very deep rabbit hole. 


Mike

Thanks both. I did have an FRA 400 once and that have pinpoint stars but I could never get any colour in them so maybe the “bloat” helps a bit with that. 

I struggle a bit with integration time at the moment as I’m quite far north and we only have 4 hours of darkness at the moment and very few clear nights, but I’ll definitely keep going this autumn/winter and get to grips with multiple sessions. 

The thing going through my mind is an Esprit 100 would give me more aperture and be a bit faster…but my Vixen is paid for and seems decent.

I’ll get drunk one night and buy something!

I think you have a great telescope.

And then you’ll run into tilt, or collimation issues lol !

I’ve already got an 8” newt for that. Shudder!

The important thing to understand about optical design is that there are a lot of things that you need  to control to produce a high quality image.  In general, it involves solving simultaneous equations to find a solution and in most cases, there isn't a single "perfect" solution.  In general, optical design codes work to minimize the rms errors of a specific merit function that defines what the designer is trying to control.  And here's where the difference between a doublet and a triplet come in.  The more variables that you have in the optical components, the more things that you can control.  Variables consist of radius of curvature, thickness, refractive index, dispersion, and spacings.   The things that you are trying to control are the focal length along with the on-axis optical aberrations such as spherical aberration and  longitudinal color correction (i.e. change of focal length with color).  In the field, you need to control coma and astigmatism, and lateral color (change in magnification with color) along with (to a lesser extent) distortion.  Field curvature just happens as a result of optical power…and you you need some field optics further downstream to fix that problem.  The reason that you have air-spaced doublets is that you get additional variable to work with with respect to a cemented doublet.  That's why there are virtually no high-end cemented refractor objectives.  With a doublet, you are generally only able to achieve near perfect color correction at two wavelengths.  With an air-spaced triplet, you can get nearly perfect correct at three wavelengths–and have more variable to work with to control field aberrations.  A triplet (or more) also allows correcting all this stuff at faster optical speeds.  The downside is that the more elements you have, the more it's going to cost to make the components, which is why high-speed, high quality refractor objectives can be so expensive.  On the other hand, slower, less expensive doublets can produce very nice results–as you've discovered with your scope.

John

As long as you're satisfied with the results I wouldn't look into a triplet.
Your pic looks pretty good, nice round stars, the stars are a little green to me which can be easily battled with scnr.

I have a doublet as well and other than a little more post work I don't see it perform worse than some triplets.
There are even Takahashi doublets, can't be so bad.

If you want to go triplet you can with a completely new type of scope.
I went from the 81 doublet to a 127.

I like your image and I think you'd be disappointed going to a gt, the difference might be far less noticeable than you'd expect.

Piers Palmer:
There's no perfect scope is there!

Never will be, but there are a few coming close if you got the necessary coins.
I went with the (in my opinion) good price/value option.
Explore scientific seems to have a sub par quality control, but if you're lucky and get a good one I think it's a nice purchase.

The vixen looks nice and if you got the reducer/flattener already it could be a nice addition.
Just note (might be only true for me), but with a bigger scope comes much more struggle.

I'm not sure my climate warrants one telescope, let alone more, but normally my 8" Newt behaves itself and the results are decent, so I know if I did ever try and replicate the focal length with a refractor it would be a mistake, but I'm good at making mistakes!

In my opinion, stars. 

here my doublet vs triplet of same target.

doublet-
https://www.astrobin.com/uo58t1/B/

triplet-
https://www.astrobin.com/d3futo/

The only difference in the image is stars. I added stars from my triplet to the image captured with doublet telescope. 

The trade off for the cost/weight and stars quality is something that you’ll have to decide based on your preferences and taste. 

i like my stars tight, round and without CA. So I went with triplet

hope it helps.

thanks,
deepan