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Refractors... is size important?


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Hi All,

I'm planning to get a fairly fast refractor as a main scope with an aim to using it for imaging (Planets & some DSO), but haven't a clue whether, as a total newbie, it's worth getting as large an aperture as possible within my budget, or settling for something mid-range.

What is the difference between an 80mm and say a 127mm? and as a newbie am I really going to be able to tell the difference?

Will a cheap-ish 127mm be "better" than a high quality 80mm

Confused!

Any help appreciated :)

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Are you referring to Apochromats (I'll include ED doublets in that) or Achromats or aren't you differentiating?

If the price difference is because you are comparing a 127mm achromat and a 80mm Apo there is a world of difference. the main thing you'll notice is the absence of colour fringing on the Apo. Apart from that the lens in the Apo will probably have a better figure. The apo will probably come with a better focusser and have a better build quality generally.

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Hi Haitch,

Thanks for the reply.

I think my question was a bit misleading... what I'm trying to get at is if there is any major discernible difference between the quality of scopes in the same range as you move up the aperture sizes.

e.g. Using SW Evostar ED range example

Evostar 80ED £375

Evostar 100ED £670

Evostar 120ED £1095

Will I notice a major difference between the 100ED & 120ED as a newbie, other than the difference in the weight of my wallet?

Many Thanks.

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I am not an imager but based on general comments of others, in terms of image quality and assuming the same optical quality in the comparison, there will be no difference. a wider aperture for the same focal ratio means more focal length which means it's harder to guide/track accurately and it is more affected by wind etc. also, more focal length means a stronger/heavier/more expensive mount than is required for a smaller aperture / shorter focal length.

furthermore, the available field will either fill the chip in your camera or expand beyond it and certain targets will either fit in view or not depending on the field of view.

many people use an 80mm refractor as a good start and never move on from what I can see. aperture means not a lot in terms of AP, it's all about the mount. if you have a good mount and guiding system then I think you'd get great shots with a 20mm refractor, it would just take a little longer.

visually it's a whole different world and aperture then rules to a large extent, depending upon the application.

hope this helps and I am sure others will chip in eventually.

Edited by Moonshane
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Hi All,

I'm planning to get a fairly fast refractor as a main scope with an aim to using it for imaging (Planets & some DSO), but haven't a clue whether, as a total newbie, it's worth getting as large an aperture as possible within my budget, or settling for something mid-range.

What is the difference between an 80mm and say a 127mm? and as a newbie am I really going to be able to tell the difference?

Will a cheap-ish 127mm be "better" than a high quality 80mm

Confused!

Any help appreciated :)

I have both an ED80 (f/7.5) "apo" and a ST120 (f/5) "achro".

I very rarely get the 120 out, because it is bigger, heavier and less versatlile.

The one area it shades the ED80 is on feint DSOs, for everything elseI use the ED80.

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For imaging you do need an apo. The camera will be very sensitive to the unfocused colours of an achromat, notably in picking up huge blue fringes on stars. I absolutely would not consider an achro for DS imaging.

A large apo is not better than a small one. It is different. The small one will give you a wide field (from short focal length), the larger one a smaller field (from a longer focal length), so you choose according to your target.

Here are a couple of examples. First the Saggittarius Triplet in an 85mm/328mm FL apo. You fit them all in. Then the Trifid in a 140 apo with FL of 980mm. Only the Trifid fits. (Actually in the widefied I added the Trifid and the Lagoon from the big scope to improve resolution but that's by the by.)

935757461_PCNBS-S.jpg

928841121_dugLY-S.jpg

Since making fast apo refractors becomes less and less possible as aperture increases, the reality is that the small ones on the market are the faster. Our 85mm Tak is F3.9 or 5.3 while the big TEC is only F7. This argues in favour of the small ones. So does the ease of guiding at short FL. If you are going to use a DSLR than faster is 'even more better' because an uncooled camera cannot take extra long subs to compensate for the slow F ratio.

Olly

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Hi Haitch,

Thanks for the reply.

I think my question was a bit misleading... what I'm trying to get at is if there is any major discernible difference between the quality of scopes in the same range as you move up the aperture sizes.

e.g. Using SW Evostar ED range example

Evostar 80ED £375

Evostar 100ED £670

Evostar 120ED £1095

Will I notice a major difference between the 100ED & 120ED as a newbie, other than the difference in the weight of my wallet?

Many Thanks.

Ok a bit clearer - I think Olly has picked up on this in part. Essentially if you had 2 scopes with the same focal length, thereby giving the same magnification or field of view, but the 2 of them had different apertures then the one with the larger aperture would gather more photons per degree of sky and give a brighter image.

If both scopes had the same focal ratio then the larger aperture one would have a longer focal length giving higher magnification and smaller field of view but still gathering the same amount of photons per degree of sky as the smaller aperture scope.

The 100Ed and the 120ed both have a 900mm f/l giving the same magnification/fov with the same EP. The f/l of the 120 is 7.5x its aperture as opposed to 9x with the 100. The 120 collects more photons per degree of sky giving it the brighter image. How noticeable the difference is visually is arguable given how poor our eyes are at photon collection in low light situations but it will certainly make a difference in how long an exposure you will need in photography. The shorter exposure will be more forgiving of your mount and alignment.

Do I make sense? (the wife tells me not)

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I think one area has been missed, if all the scopes have similar F ratios and optics quality the larger aperture will have a greater resolving capability assuming everything else (mount, guiding etc) is equal or appropriate for the weight and size of scope.

A 150mm F6 apo should deliver greater resolution than a 66mm f6 apo (if its been made to the same tolerances). Being able to see the difference is another matter; I view some images zoomed in on a high resolution projector typically at 2.4-3.2 metre diagonal sizes and I can see the difference on some images delivered for example by the AG12 scope and even larger amateur scopes in the US.

Edited by nightvision
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I think one area has been missed, if all the scopes have similar F ratios and optics quality the larger aperture will have a greater resolving capability assuming everything else (mount, guiding etc) is equal or appropriate for the weight and size of scope.

A 150mm F6 apo should deliver greater resolution than a 66mm f6 apo (if its been made to the same tolerances). Being able to see the difference is another matter; I view some images zoomed in on a high resolution projector typically at 2.4-3.2 metre diagonal sizes and I can see the difference on some images delivered for example by the AG12 scope and even larger amateur scopes in the US.

But does anyone make an F6 150mm Apo? I dare say it could be done but it would cost a fortune. The TECs are F7, the Takahashi FOAs are f7.3...

It is quite true that aperture brings resolution but with refractors aperture also brings problems of CA and so apos get slower with increasing aperture. I wish it were otherwise since imaging with refractors is bliss.

Ollyu

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I think one area has been missed, if all the scopes have similar F ratios and optics quality the larger aperture will have a greater resolving capability assuming everything else (mount, guiding etc) is equal or appropriate for the weight and size of scope.

A 150mm F6 apo should deliver greater resolution than a 66mm f6 apo (if its been made to the same tolerances). Being able to see the difference is another matter; I view some images zoomed in on a high resolution projector typically at 2.4-3.2 metre diagonal sizes and I can see the difference on some images delivered for example by the AG12 scope and even larger amateur scopes in the US.

But does anyone make an F6 150mm Apo? I dare say it could be done but it would cost a fortune. The TECs are F7, the Takahashi TOAs are f7.3...

It is quite true that aperture brings resolution but with refractors aperture also brings problems of CA and so apos get slower with increasing aperture. I wish it were otherwise since imaging with refractors is bliss.

Olly

Edited by ollypenrice
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Point taken, I guess the whole experience includes how easy and quick it is get quality data regardless of conditions. I have no idea how one would go about collimating a huge Newt or RC scope or ensuring it was temperature stabilised.

I have yet to do any real DSO imaging but I can see that there exists a sweet spot in terms of equipment and opportunity to use it. I am holding off buying anything else until I move to better skies far away from the UK.

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Something to consider as well... a short tube refractor, which is good for deep sky, is not great for planetary/lunar.. having said that the 80ED will work very nicely, if you are prepared to push it really hard.

This is an image of Jupiter I produced with my 80ED and a 5x Barlow (the exposure levels were extremely pushed, causing more noise, but it worked) with 2 cameras, QHY5v for detail, spc900 for colour.

jupitercopy.jpg

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