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Olli

short focal length vs Long focal length

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Hi,

I was wondering if any of you could help me understand the difference in focal length. For example If one scope has a FL of 1325 and a other scope has 650mm of FL but near enough the same size aperture maybe by 1 inch , how much of a difference will there be between the two? This is directly aimed at Lunar viewing.

Many thanks 

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The most obvious difference is in the focal length of he eyepiece needed to reach high magnifications to pull out the best detail. Longer focal length eyepieces often have better eye relief and are more comfortable to use. Not always the case but often is on the lower end and simpler eyepieces where eye relief varies with focal length.

The other factors are normally related to the size of the secondary and the effect this has on contrast and light scatter. The longer the focal ratio of the scope, the smaller the secondary can be (generally) and a smaller secondary does improve high power performance; you sometimes see newtonians optimised for planetary work which have a long focal ratio and with a tiny secondary which will only fully illuminate a 1.25” eyepiece, not 2”. This is to give the best contrast.

Another thing to bear in mind is that faster scopes are more tricky to collimation, and collimation is key to good high power views.

Which scopes are you looking at?

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4 minutes ago, Stu said:

The most obvious difference is in the focal length of he eyepiece needed to reach high magnifications to pull out the best detail. Longer focal length eyepieces often have better eye relief and are more comfortable to use. Not always the case but often is on the lower end and simpler eyepieces where eye relief varies with focal length.

The other factors are normally related to the size of the secondary and the effect this has on contrast and light scatter. The longer the focal ratio of the scope, the smaller the secondary can be (generally) and a smaller secondary does improve high power performance; you sometimes see newtonians optimised for planetary work which have a long focal ratio and with a tiny secondary which will only fully illuminate a 1.25” eyepiece, not 2”. This is to give the best contrast.

Another thing to bear in mind is that faster scopes are more tricky to collimation, and collimation is key to good high power views.

Which scopes are you looking at?

Thanks for the reply I was looking at the 4se mak 127 or 102.  Just wanted to know if FL is usually more important than arpeture when it comes to Lunar viewing but probably is still important.

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5 minutes ago, Olli said:

Thanks for the reply I was looking at the 4se mak 127 or 102.  Just wanted to know if FL is usually more important than arpeture when it comes to Lunar viewing but probably is still important.

As a generalisation aperture is first as it defines resolution and the detail you can potentially see. Focal length determines what focal length eyepiece you need for the appropriate magnification, focal ratio will determine the size of the secondary, and the related contrast, aswell as how hard the scope is to collimate and the exit pupil for specific eyepieces.

The Maks have longer focal ratios than the SCT so in theory should give better contrast. Collimation probably more stable in the Maks too. 

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Posted (edited)
1 hour ago, Olli said:

I was looking at the 4se mak 127 or 102. Just wanted to know if FL is usually more important than arpeture when it comes to Lunar viewing but probably is still important.

You don’t need to worry about collimating a Mak. The 127 will give better  lunar views (and other planets) over the 102. You need to think about if that’s all you want though. I have a 150 Mak (fl 1800) and it has its limitations when it comes to DSO and AP etc... moon views are outstanding though...

I use reasonable priced bst eps.

Hope that helps 😀

Edited by Dazzyt66

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37 minutes ago, Dazzyt66 said:

You don’t need to worry about collimating a Mak.

Unless it needs collimating ;) 

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Posted (edited)
52 minutes ago, Dazzyt66 said:

You don’t need to worry about collimating a Mak. The 127 will give better  lunar views (and other planets) over the 102. You need to think about if that’s all you want though. I have a 150 Mak (fl 1800) and it has its limitations when it comes to DSO and AP etc... moon views are outstanding though...

I use reasonable priced bst eps.

Hope that helps 😀

Well I have a 130p and was looking for a dedicated lunar scope to go with it and use the 130p for other objects as I asked in a other post.  Just wanted to know what the difference in focal length would be like b:)

But yes Maks seem like  good choices.

Edited by Olli
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2 hours ago, Dazzyt66 said:

You don’t need to worry about collimating a Mak. The 127 will give better  lunar views (and other planets) over the 102. You need to think about if that’s all you want though. I have a 150 Mak (fl 1800) and it has its limitations when it comes to DSO and AP etc... moon views are outstanding though...

You got that right! Lunar and Planetary views in my SW 150 Mak are amazing, lunar especially!

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9 hours ago, Stu said:

Unless it needs collimating ;) 

Ha ha ha true! I think you have to drop one to need it though! 🤔 Mine is fine... but then again I’m not even sure how to go about it on a Mak!

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57 minutes ago, Dazzyt66 said:

Ha ha ha true! I think you have to drop one to need it though! 🤔 Mine is fine... but then again I’m not even sure how to go about it on a Mak!

:) To be fair, the designs do vary a little so I was teasing a little. There are several variants which have Maksutov in their names but are slightly different. 

I’ve had Orion Optics 140 and 200mm Maks. On the 140 the primary is collimateable with the secondary fixed, on the 200 it is the other way around. Not sure about the Skywatcher Maks and you are right, it is not always a straightforward process. If it’s right, don’t mess with it!! It does tend to hold well.

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1 hour ago, Stu said:

:) To be fair, the designs do vary a little so I was teasing a little. There are several variants which have Maksutov in their names but are slightly different. 

I’ve had Orion Optics 140 and 200mm Maks. On the 140 the primary is collimateable with the secondary fixed, on the 200 it is the other way around. Not sure about the Skywatcher Maks and you are right, it is not always a straightforward process. If it’s right, don’t mess with it!! It does tend to hold well.

That’s why I love my Mak 😝

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I pretty much agree with the comments above but would like to add that focal ratio applies to all scopes not just those with larger apertures and focal lengths such as SCT’s and Maks... these type of scopes just have much longer focal lengths and can achive higher magnifications. However, you can’t use a 4mm eyepice with a scope with a focal length of 2400mm and expect to see anything, thus a trade off is required. The relationship between telescope focal ratio, eyepiece focal length and what targets you want to view / image should be the greater consideration.

Telescopes with a higher focal ratio are more forgiving with a given eyepiece and eyepieces that perform better with lower focal ratio telescopes (F6 and below) tend to be more expensive. 

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I have two 127mm Skywatcher Skymax Maks, 1 in the UK, & 1 in France, and a 90mm on my Heritage 90P Virtuoso. None of these have needed collimation; the 90 does not seem to have any external adjustment screws. The one in France was transported there by various modes of land and air, public and private transport. Unlike a Newtonian, most of the collimation work is fixed in the initial manufacture.

The 127 is my weapon of choice for Lunar, Jupiter and Saturn, because its 1500mm focal length means it works well with most of my eyepieces 8mm and above. It also has the long focus range, so I do not need to use a Barlow with my Binoviewer or cameras (I just slide the OTA forwards in the dovetail clamp to compensate for the additional weight at the rear). On dark, clear, nights it is also good for observing the "faint fuzzies", and will show the difference between Uranus, Neptune and their adjacent stars.

Another advantage of the short Mak. design, is that, for the same magnification, a refractor or Newtonian OTA is going to be longer, and thus more likely to be affected by wind gusts.

Geoff

 

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Posted (edited)

I wouldn’t say a Mak or SCT designs deliver better Lunar views than a refractor 🙈 having owned all 3 types the refactor delivers a much nicer view..... there is some merit in the wind comment but don’t forget that by the time your dew shield parameters are factored into the equation, plus the vibrations associated with mounting long focal length short tube designs, wind is not that much of a contributor in my opinion.

Edited by Pig
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3 hours ago, Geoff Lister said:

I have two 127mm Skywatcher Skymax Maks, 1 in the UK, & 1 in France, and a 90mm on my Heritage 90P Virtuoso. None of these have needed collimation; the 90 does not seem to have any external adjustment screws. The one in France was transported there by various modes of land and air, public and private transport. Unlike a Newtonian, most of the collimation work is fixed in the initial manufacture.

The 127 is my weapon of choice for Lunar, Jupiter and Saturn, because its 1500mm focal length means it works well with most of my eyepieces 8mm and above. It also has the long focus range, so I do not need to use a Barlow with my Binoviewer or cameras (I just slide the OTA forwards in the dovetail clamp to compensate for the additional weight at the rear). On dark, clear, nights it is also good for observing the "faint fuzzies", and will show the difference between Uranus, Neptune and their adjacent stars.

Another advantage of the short Mak. design, is that, for the same magnification, a refractor or Newtonian OTA is going to be longer, and thus more likely to be affected by wind gusts.

Geoff

 

#maksrock

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20 hours ago, Olli said:

Hi,

I was wondering if any of you could help me understand the difference in focal length. For example If one scope has a FL of 1325 and a other scope has 650mm of FL but near enough the same size aperture maybe by 1 inch , how much of a difference will there be between the two? This is directly aimed at Lunar viewing.

Many thanks 

I popped your figures into the field of view calculator  (see the resources tab above, tools, FOV Calculator ) and it will clearly show the difference between the field of view when viewing using either scope with a suggested 8" aperture ( you choose your own ) whilst observing the Moon, but using the same EP it would appear that the Moon itself is the same size, I for example used a 8mm EP.

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Posted (edited)

A 127 Mak would be a great addition , Olli

On a Sky-tee 2 , or equivalent mount , it would look great beside the 130P 

I got a used one recently and the views on Jupiter are excellent and very similar - if not identical - to my 200P 

Just as an experiment , I tried the Mak on the Sombrero galaxy - an 8.98 magnitude target - and could see it very well at low power , 25-40mm - but , unsurprisingly , lost it at higher power 

On the Moon , planetary nebula and double stars , the 127 also performs very well too 👍

Edited by Red Dwarfer
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12 hours ago, Red Dwarfer said:

A 127 Mak would be a great addition , Olli

On a Sky-tee 2 , or equivalent mount , it would look great beside the 130P 

I got a used one recently and the views on Jupiter are excellent and very similar - if not identical - to my 200P 

Just as an experiment , I tried the Mak on the Sombrero galaxy - an 8.98 magnitude target - and could see it very well at low power , 25-40mm - but , unsurprisingly , lost it at higher power 

On the Moon , planetary nebula and double stars , the 127 also performs very well too 👍

It is well known that Maks are known as ‘planet killers’ because they perform so well on planetary. Having had one now for a few years I can also confirm, at low power, they will also provide great views of the brighter dso’s - that said, I haven’t really tried to look for dimmer items as I have too much LP.

I have seen most of the easy to spot Messiers with mine. The ring and Dumbell nebulars are two of my favourites and I’m hoping to image some more once the clouds clear as I’ve had some promising quick tries previously but haven’t taken enough to process properly 😀

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Maks are certainly great on Globs and Planetary Nebulae where the field of view is not and issue and you need high mag. Great on planets and the moon too as said, I had great views of Jupiter through a 140mm Mak the other night.

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2 hours ago, Stu said:

Maks are certainly great on Globs and Planetary Nebulae where the field of view is not and issue and you need high mag. Great on planets and the moon too as said, I had great views of Jupiter through a 140mm Mak the other night.

....ditto a 180 Mak - the views of Jupiter were stunning.

Although they are not "DSO scopes", in my experience they function pretty much the same as most scopes of the same aperture* on smaller DSOs (obviously the field of view is more limited than a short-focus scope, so you won't see The Veil in its entirety!) 

Chris

* note that the true aperture of some Maks (eg 127) is less than the stated value. My 127 Mak is nearer to 120mm, although some later designs (eg my 180 Mak from about 2 years ago) are as stated.

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One thing that immediately popped to mind that no one has seemingly addressed is that for refractors, it is easier to achieve better correction with longer focal ratios (longer focal lengths given equal aperture).  The shorter the focal ratio, the more exotic glasses or secondary lenses are needed to bring more wavelengths of light to a common focal point.  Thus, short, large aperture APOs are very expensive.

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