What does doublet mean?

[guv]

Looking at refractor telescopes, I've notice some are shown as doublet or triplet.

What does that mean?

Example. http://www.williamoptics.com/telescopes/zenithstar71_features.php

[IanL]

It refers to the number of glass elements in the primary lens:

Chromatic aberration occurs because different wavelengths of light refract (bend) by different amounts when they pass through a lens. This leads to colour fringes around bright objects (such as stars or the edge of planets/ the moon), although actually it affects the entire image making features less distinct.

In a doublet the lens is made of two different types of glass with different refractive indices, which corrects chromatic aberration for two wavelengths of light, and in a triplet you have three elements which corrects another wavelength. That isn't all there is to it, as they types of glass, quality of the lens manufacture, etc. also play a part but generally speaking a triplet gives best colour correction, and unsurprisingly tend to be (a lot) more expensive.

[Physicist13]

The number of lenses used in the objective. Usually sandwiched together with small airgap. They are less probe to chromatic aberration but price increases. Depends what you intend to use the scope for..

[ronin]

A simple lens is a single piece of glass ground to whatever radius is required for the focal length and the glass refractive index. With a single lens all the wavelengths fall at slightly different distances from each other.

A doublet is simply 2 lens together.

They are usually of different glass types = Crown and Flint glass.

If designed right, considering the glass refractive index's you can get 2 different wavelengths to fall at the same plane.

So the the colours are closer and it makes a better image.

Doublets can be cemented together if the appropriate 2 faces are the same radius, and if the design is correct for this.

They can be seperated by a small amount or a relatively large amount. In which case the 4 faces can all have different radaii,

A triplet is simply 3 lenses together, glued or seperated. With 3 lens you can get 3 wavelengths at the same distance/plane so even better colout correction. But more factors to tke into account.

[guv]

Ok thanks guys. I guess that unless it specifies, it would definitely mean its not got 2 or 3 lens?

For DSO will it make a big difference?

[Physicist13]

Tht depends.. Chromatic err shows up visually esp around bright stars (blue halo usually) if u intend astrophotog with narrow band filters then in fact its less critical, but refocus for each filter may be needed.

[guv]

Tht depends.. Chromatic err shows up visually esp around bright stars (blue halo usually) if u intend astrophotog with narrow band filters then in fact its less critical, but refocus for each filter may be needed.

Hmm. This stuff gets more and more complicated! So using filters affect the physical focus?

[IanL]

Two things:

- Yes filters can themselves alter the focus point since they are an additional glass element which further refracts the light. In theory you can obtain filters in a set which are parfocal (i.e. all have the same refractive index and thickness), but in practice people seem to refocus even with these.

- The main reason you re-focus is that even in a high-end triplet, there will be some chromatic aberration remaining, so the focus point is different for each wavelength of light. Because the filter only lets through light in a band of selected wavelengths, you can adjust the focus to the right point for that band (since most of the other wavelengths are blocked). So it is less critical to have a high end triplet for imaging if you are using filters, especially narrowband filters which only let through a small band of wavelengths. (Not to say that your images wouldn't benefit from the better optics in other regards, as CA is not the only problem you have to deal with).

Would be easier to point you in the right direction if we knew what your rough objectives were.

[guv]

Thanks again Ian. To be fair, I am just attempting to learn. I don't think I can say I have any real objectives in terms of "this is exactly what I want to do and achieve". I know that isn't really going to help is it - but as a newbie, I'm just unable to answer.

I do have a neximage 5 CCD for use on solar objects with an SCT, so it will be purely for DSO. At this stage, I would be using a DSLR, but would be looking at a modded DSLR eventually. And I am looking at a 66 -80 ED APO. Shout if there s anything specific I'm still unsure you might want info for!

[Physicist13]

66-80 apo will be v good! Cost is everything. Get the best you can afford knowing that youll always want better at sone point! Mainly though , enjoy!!

[IanL]

Okay that helps a lot.

- If you're using a one-shot-colour (OSC) camera like a DSLR then you wouldn't typically use RGB filters since each pixel already has a red, green or blue filter applied to it. You'd end up imaging with only a quarter (red, blue) or half (green) of the pixels per filtered frame which creates more problems that it solves.

- Not that you can't use filters. Provided you mod the DSLR filter as you plan, you can use a Ha filter, but it will never be as good as a mono CCD as only a quarter of the DSLR pixels (red) are going to respond and not as efficiently as a mono CCD. That's not meant to put you off, a modded DSLR is a much cheaper option than a CCD which is why they are so popular.

- Having a scope with good correction for chromatic aberration would be a benefit when shooting OSC as you want to get as many wavelengths as possible to the same focus. That should end up with a sharper image, smaller stars, etc.

- On the other hand would it make sense to spend a lot of money on a high-end triplet if you are just dipping your toes in the water with a DSLR? Personally I would buy a decent doublet or a cheaper triplet and keep some cash back for a CCD, better mount, guiding rig, etc. (There is always more kit to buy in imaging). You can always trade up later if you get the bug.

I use an unmodded Canon 500D and a Skywatcher 80ED DS Pro (can't run to the £££ for a CCD at the moment), and here are a few attempts through that rig. The 80ED is available as an OTA only for about £350 to give you an idea. You can definitely see a fair bit of bloat around the larger stars (though it could be improved with more processing effort) but I am pretty happy with the results.

[guv]

66-80 apo will be v good! Cost is everything. Get the best you can afford knowing that youll always want better at sone point! Mainly though , enjoy!!

Cheers. I'm currently being told that I'm better going off going smaller so I'm not setting my sights too high and get frustrated.

[guv]

Okay that helps a lot.

- If you're using a one-shot-colour (OSC) camera like a DSLR then you wouldn't typically use RGB filters since each pixel already has a red, green or blue filter applied to it. You'd end up imaging with only a quarter (red, blue) or half (green) of the pixels per filtered frame which creates more problems that it solves.

- Not that you can't use filters. Provided you mod the DSLR filter as you plan, you can use a Ha filter, but it will never be as good as a mono CCD as only a quarter of the DSLR pixels (red) are going to respond and not as efficiently as a mono CCD. That's not meant to put you off, a modded DSLR is a much cheaper option than a CCD which is why they are so popular.

- Having a scope with good correction for chromatic aberration would be a benefit when shooting OSC as you want to get as many wavelengths as possible to the same focus. That should end up with a sharper image, smaller stars, etc.

- On the other hand would it make sense to spend a lot of money on a high-end triplet if you are just dipping your toes in the water with a DSLR? Personally I would buy a decent doublet or a cheaper triplet and keep some cash back for a CCD, better mount, guiding rig, etc. (There is always more kit to buy in imaging). You can always trade up later if you get the bug.

I use an unmodded Canon 500D and a Skywatcher 80ED DS Pro (can't run to the £££ for a CCD at the moment), and here are a few attempts through that rig. The 80ED is available as an OTA only for about £350 to give you an idea. You can definitely see a fair bit of bloat around the larger stars (though it could be improved with more processing effort) but I am pretty happy with the results.

Thanks Ian. I have seen HA mentioned and read what it does and the results look incredibly good. The problem I have is I'm itching to get something (secondhand) and have to make sure I buy and at a price I'm happy to pay, the telescope I really want. Always too easy to get impatient!

[Physicist13]

very true!...good idea always is to go to local astro club and get to talk to members and even play/use what they have before you decide!

[guv]

very true!...good idea always is to go to local astro club and get to talk to members and even play/use what they have before you decide!

Couldnt agree more. Retailers are in the business of selling. They often will sell you whatever they can without telling you there might be cheaper / better alternatives.

With a bit of like I am visiting someone shortly who is going to let me have a play in their observatory and also run through my own equipment. I expect I will learn far more from this than weeks of reading.