Narrowband DSO imaging: Doublet or triplet refractor?

[Brian]

Hello stargazers,

I currently have a pretty ad-hoc AP rig. I use an SCT atop an AVX mount and a DSLR camera. Learning with this setup has been a good experience. I now definitely know that I enjoy imaging nebulas over planets, globs, and galaxies.

I am planning out a dedicated AP rig for this purpose which will use a cooled cmos camera and a shortish focal length refractor. I plan to get a new mount, camera, and refractor for this with approximately a $5k budget.

Here’s the question: Since I will use this for narrowband DSO imaging, do I really need a triplet that brings a broad range of wavelengths into focus? Would it be better to get a high quality doublet? What other considerations are there between doublets and triplets besides color quality?

Any suggestions on a good combination of components is welcome.

Thanks

[yobbo89]

I use a cheap refractor with narrow band with good results and it really tones down the bloating from the unfocused wavelengths, I wouldn't use this with an osc camera.

I can't really comment too much on each other all I can say is that you can achieve reasonably good results with mono imaging on cheaper scopes

Personally since I image in narrowband I would be looking for best quality in my price range and the scopes specs f ratio ect and how well it is corrected.a good doublet would be more suited for me if I was in the shop for a new scope for narrowband.

Sorry I can't awnser your question in full, someone else will have to pick up from me, never owend a triplet and I'm more. Of a mirror astronomer

[Aidi]

Quite alot of the narrowband objects in the sky are quite large. I tired with my Esprit 120, but that was overkill.. So, i started to look at scopes in the 70mm region. I ended up with a 70mm Quadruplet. I would do a bit of reading and see if you can find a quad. The main advantage being, to get a flat field all you need to achieve is focus. No flatterners or such like.

The one i got was about $1400 and i am very impressed with the results. To give you a rough idea of its FOV, i can get the whole of M31 in frame!

[Pauls72]

You are better off with a Triplet and Field Flattener or a Quad.
Visible light is made up of 3 colors: Red, Green and Blue light.
A Triplet or Quad gets all three colors of light in focus at the same point.
A doublet only gets 2 of those colors in focus at the same point. So with a mono camera when you focus the third filter the scale of your image on the sensor will be slightly different. This makes the object slightly larger or smaller in the one color. It also causes the distances between stars to be slightly different when using that third filter.
Using a doublet with a OSC (One Shot Color) camera you get CA (Chromatic Aberration), which usually shows up as a blue or purple halo around bright stars.

A well corrected Doublet will do a decent job and you can produce nice AP images. Just not quite as good as a well corrected Triplet. It really all boils down to what amount of imperfection are you willing to accept and how much money are you willing to spend.

The type of glass used makes a difference too. The better the glass, the more crisp and pinpoint the stars will be. Abbe is a measure of the material's transparency or dispersion, higher is better.
Here are the Abbe numbers (Vd) of some commonly used ED glasses:
Crown & Flint (Schott): 60
FPL-51 (Ohara): 81.54
H-FK61 (CDGM): 81.61
FCD1 (Hoya): 81.61
OK-4 (LZOS): 92.1
FCD100 (Hoya): 94.66
FPL-53 (Ohara): 94.93
CaF2: 94.99 (Fluorite)

Refractors using better glass are more expensive too.

If getting a Doublet or Triplet, you want to get a Field Flattener. These make the stars round all the way out to the edge of the FOV (Field Of View). In most cases you want to get a Flattener from the same manufacturer that is matched to their scope.

The quality of your focuser matters too. You don't want a focuser that will sag or slip. Ideally you want one that can be motorized late on.

FYI: Unfortunately in the astronomy world APO (apochromatic) has become a marketing term and different manufactures apply the term differently. There are no standards that a scope has to comply with to be called an APO.
https://lenspire.zeiss.com/photo/en/art ... ifference/

You would not go wrong with the mid-tier scopes of any of these brands:
Stellavue
Sky Watcher
Willam Optics
Tele Vue
Takahashi
TS Optics

[Jennifer Christine]

If you're taking narrowband one filter at a time, you can use an achro.