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Question regarding F numbers and Meade telescopes

bl9au

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bl9au

bl9au

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Hi all,

Hope everyone is keeping well.

I have been progressing with knowledge for the past year and Ive grasped the basics of my telescope but am getting to the point now where resolution is starting to annoy me. I have a celestron EQ114 scope aperture4.5", F/1, 1000m FL. I understand that magnification is the focal length divided by the EP no. but what does the F/1 mean? is it EP max aperture? so the bigger the better FOV at distance? please help me on this one, as I am thinking of upgrading to a daddy scope, maybe the Meade ETX 105EC range? also views on the ETX scope would be appreciated....

Also steve have you got special discounted rates for SGL members? :)

Lee :venus:

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FLO

FLO

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Hi Lee,

Yes, magnification is the telescope focal length / eyepiece focal length.

The f ratio is the telescope focal length / aperture

If the telescope has a low f ratio such as f4 or f5, it is considered 'fast'. Fast telescopes typically deliver lower magnification, wider field-of-view and shorter exposure times when imaging.

If the f ratio is high such as f8, f10, f15, etc, it is considered 'slow'. Slow telescopes typically offer greater magnifications, a narrower field of view and longer exposure times when imaging.

Generally, fast scopes are short and fat and preferred for low-med magnification wide-field views of deep-sky objects. Whereas slow scopes are long and thin and preferred for high magnification views of planets and the Moon.

Also steve have you got special discounted rates for SGL members? :)

SGL members can claim 10% discount on over 90% of the products listed on FLO's website 8)

Details here: http://tinyurl.com/yxzq8b

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bl9au

bl9au

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Steve,

Thanks for the instant feedback on that fellow devonshire hobbit. Yeah I want to look at deep sky objects and get involved with the astroimagery side of deepsky objects too. Thanks for the info on the F no.'s. So high F numbers would normally have catadioptrics?

Thanks for the website ill have a browse.

Cheers

Exeter Lee!

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John

John

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Hi,

I don't think the scope your link leads to would be particularly good for deep sky objects as it has a long focal length (as Steve says it's a "slow" scope) which would give a relatively narrow field of view. Also it's mirror is smaller than that of the scope you currently own so it would not be able to see any fainter objects.

It's actually quite difficult to find a scope which is good at both deep sky objects and high resolution planetary type work. A reasonable compromise, and a good step up from your current scope in terms of capability would be to look for an 8 inch newtonian reflector with a focal length of around 1200mm - an F6. That would make a decent job of showing you deep sky objects and details on the planets. If it is mounted on a motorised equatorial mount then it could be used for photography as well.

Many members of SGL use this type of scope and they are not too expensive - around £350 or so.

Hope that helps,

John

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FLO

FLO

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Celestron's new SE Series are looking good and proving popular (particular the 6SE). But as John says, the 4SE's slow/narrow Maksutov design lends itself more to planets than deep-sky observing.

Some deep-sky objects are quite large; the Andromeda galaxy appears several times larger than the Moon.

The chart below shows the difference in the FOV (area of sky covered) when using a 10mm Plossl in two refractors with the same aperture, but different f ratios - f10 for the Evostar and f5 for the Startravel.

Hope that helps.

post-12699-133877326658_thumb.jpg

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FLO

FLO

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The Celestron 6SE has been winking at me for a while now.

I haven't had a chance to look through one (they are very thin on the ground) but I made a point of checking one over at Astrofest and was impressed at how sturdy the set-up felt and at the overall construction. Most of my observing is done either under Greg's dark sky or at the NLO but I'd like something for the back garden and the 6SE looks to be just the ticket!

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John

John

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I was looking at the 6SE's myself but I have concerns about the mount stability at high power. Have you used one of these yet Steve?

I used to have a Nexstar 5 which had a similar (but probably not identical) mount to the 6SE and that was pretty steady even at high powers. The single fork arm was much more solid in real life than it looked in pictures.

John

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FLO

FLO

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I used to have a Nexstar 5 which had a similar (but probably not identical) mount to the 6SE and that was pretty steady even at high powers. The single fork arm was much more solid in real life than it looked in pictures.

The Celestron Nexstar 5 SE does have a good mount but the Nexstar 6 SE is more sturdy again - the same mount as supplied with the Nexstar 8 SE.

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bl9au

bl9au

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Hi,

I don't think the scope your link leads to would be particularly good for deep sky objects as it has a long focal length (as Steve says it's a "slow" scope) which would give a relatively narrow field of view. Also it's mirror is smaller than that of the scope you currently own so it would not be able to see any fainter objects.

It's actually quite difficult to find a scope which is good at both deep sky objects and high resolution planetary type work. A reasonable compromise, and a good step up from your current scope in terms of capability would be to look for an 8 inch newtonian reflector with a focal length of around 1200mm - an F6. That would make a decent job of showing you deep sky objects and details on the planets. If it is mounted on a motorised equatorial mount then it could be used for photography as well.

Many members of SGL use this type of scope and they are not too expensive - around £350 or so.

Hope that helps,

John

Thanks for the posts John and Steve. The 8 inch newtonian with F6 sounds good, but I really would like the autostar feature, which I believe will increase my knowledge of the sky more rapidy. For this reason I was looking at the celestron SE range because ive heard good write ups about its allignment system. Meade autostar seems to have a lot of allignment problems...?

So am I wright in saying mirror size increases resolution? what about the 5SE?

Steve do you do discount on the SE range?

Cheers

Lee

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FLO

FLO

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A reasonable compromise, and a good step up from your current scope in terms of capability would be to look for an 8 inch newtonian reflector with a focal length of around 1200mm - an F6. That would make a decent job of showing you deep sky objects and details on the planets. If it is mounted on a motorised equatorial mount then it could be used for photography as well.

The 8 inch newtonian with F6 sounds good, but I really would like the autostar feature, which I believe will increase my knowledge of the sky more rapidy.

Have you considered the Celestron C8-N GT? Whilst it is less portable than the SE series, its eight inch aperture has considerably more light grasp than the Celestron 5 SE. Also, at f5 (compared to the 5 SE's f10) it has a wider angle of view.

The 5 SE would make a great portable back-yard scope. Its just that it isn't as well suited to deep-sky observing as a larger 8" f5 Newtonian.

Be aware that whilst GOTO is great, being able to see a lot in a short space of time doesn't increase your knowledge of the sky. My first scope (ETX105) had GOTO. After a year I realised that I'd seen loads but still had no real idea where everything was. I moth-balled the ETX and bought a 10" Dobsonian - I learnt more about star-hopping in one month with the Dobsonian than I had in one year of using the ETX!

After saying that, I have gone full-circle - my latest mount has GOTO :)

So am I wright in saying mirror size increases resolution? what about the 5SE?

Yes, you are right. An 8" aperture has more resolution than a 5" aperture.

Steve do you do discount on the SE range?

Yes, SGL members can claim 10% discount off the entire Celestron range :)

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