Help me select my proper eye piece lengths

[THL22]

Hello Gazers!

I need some help ones again. 
Lets say I have a scope with a 150 diameter and a 1800 mm focal length.(f/12) I'll supplied with an 28mm eyepiece.
I would like to know which lengths I should get? I'm in the market for PST pieces.
These are the lengths they offer:

2x barlow
12mm
15mm
18mm
25mm
8mm
5mm
3,2mm

All have a field of view of 60 degrees. In which order should I buy them? 

Clear skies:.)

 

[rwilkey]

Hi THL22, low power first, so the 25mm, then medium power 12mm, then the 8mm for the Moon.  That will get you started. 

[THL22]

1 minute ago, rwilkey said:

Hi THL22, low power first, so the 25mm, then medium power 12mm, then the 8mm for the Moon.  That will get you started. 

Awesome! But I got a 28 Apex 2" skymaster piece. Won't that overlap with the 25? 

[rwilkey]

The 25mm will darken the background sky more, I find the 28mm Apex a bit clumsy to be honest and the quality is not up to the 25mm, you will notice quite a difference.

[THL22]

1 minute ago, rwilkey said:

The 25mm will darken the background sky more, I find the 28mm Apex a bit clumsy to be honest and the quality is not up to the 25mm, you will notice quite a difference.

Ah very good to know! Thank you so much:)

[Charic]

rwilkey advised me way back, and no regrets so far.
Having all the BST's I would start with the 12mm first - high power (matches your scopes focal ratio, then work from there ) the 8mm next to give a squeeze more power, but the scopes aperture and conditions at this power will diminish what you will see, then whatever you fancy next. The supplied 28mm will be ok, but the BST may have a better field of view with its 60° afov, though you don't mention the brand of your 28mm.

Which ever order you buy, its probable, you'l buy more!

[THL22]

2 minutes ago, Charic said:

rwilkey advised me way back, and no regrets so far.
Having all the BST's I would start with the 12mm first - high power (matches your scopes focal ratio, then work from there ) the 8mm next to give a squeeze more power, but the scopes aperture and conditions at this power will diminish what you will see, then whatever you fancy next. The supplied 28mm will be ok, but the BST may have a better field of view with its 60° afov, though you don't mention the brand of your 28mm.

Which ever order you buy, its probable, you'l buy more!

haha yes I'm quite sure of that! 

[25585]

I like to go for relative magnifications. So have 5, 10, 20, 32, 55 with a low power barlow for any other.

Have added models for better eye relief but keep to the five focal lengths above.

 

[THL22]

3 minutes ago, 25585 said:

I like to go for relative magnifications. So have 5, 10, 20, 32, 55 with a low power barlow for any other.

Have added models for better eye relief but keep to the five focal lengths above.

 

Can you explain when I should pull out the barlow? From "playing" with terrarium i''ve found that I like to take it out when for instance I want to look at saturn and even a 3,2mm eyepiece doesn't show much more than a "ball" 

[rockystar]

a 3.2mm ep is going to give you x563 magnification, which is too much for your scope give the rule of thumb of 2x aperture.

not seen where in the world you are, but here in the UK, days of good enough seeing for that kind of mag are very rare.

what you gain in size, you will lose in detail.

you are also starting to get a very small exit pupil (0.3mm) so you will start to see floaters.

8 & 12 I think are good starters, as already mentioned, and then decide if you want to replace your 28

 

[THL22]

3 minutes ago, rockystar said:

a 3.2mm ep is going to give you x375 magnification, which is too much for your scope give the rule of thumb of 2x aperture.

not seen where in the world you are, but here in the UK, days of good enough seeing for that kind of mag are very rare.

what you gain in size, you will lose in detail.

you are also starting to get a very small exit pupil (0.5mm) so you will start to see floaters.

8 & 12 I think are good starters, as already mentioned, and then decide if you want to replace your 28

 

Good advice! Stellarium ofcourse doesn't show if the magnification is too much. Can you explain this rule?
For example with an 150 mak

[rockystar]

5 minutes ago, THL22 said:

Good advice! Stellarium ofcourse doesn't show if the magnification is too much. Can you explain this rule?
For example with an 150 mak

indeed, stellarium has some very nice zoomable pictures:)

I've update my calculations by the way, they were a bit off.

As far as I'm aware, It's similar to resolution of a digital photo: low resolution, the less you can zoom in before the image is distorted beyond recognition. So the aperture of the scope is a bit like the resolution.

with a bigger mirror (or lens) you can collect more photons so can "zoom" in more.

[rockystar]

2x the aperture is the often quoted upper limit before the image quality degrades too much

[THL22]

19 minutes ago, rockystar said:

2x the aperture is the often quoted upper limit before the image quality degrades too much

aah ! so I basically cant magnify above 300x. But how do I know what my magnification is? And which eyepiece is too much?

[Stu]

Sorry if this stating the obvious, but the basic equations are:

Magnification = Telescope focal length / Eyepiece focal length

True field of view = Apparent field of view / Magnification

Exit pupil = Eyepiece focal length / Telescope focal ratio

 

So, for example, the 25mm 60 degree afov gives:

1800 / 25 = x72 magnification

60 / 72 = 0.83 degree field of view

25/12 = 2.08 mm exit pupil

[THL22]

24 minutes ago, Stu said:

Sorry if this stating the obvious, but the basic equations are:

Magnification = Telescope focal length / Eyepiece focal length

True field of view = Apparent field of view / Magnification

Exit pupil = Eyepiece focal length / Telescope focal ratio

 

So, for example, the 25mm 60 degree afov gives:

1800 / 25 = x72 magnification

60 / 72 = 0.83 degree field of view

25/12 = 2.08 mm exit pupil

Might be obvious for some but I needed this! Thanks! I'm saving this info for later!

 

[N3ptune]

I am reading this discussion and I wish to interfere with a question.  The telescope has 1800mm FL I think that's way too much for low power observation, 28mm gives 64x and 25mm gives 72x. I think the quality in the views at these powers will be affected by atmospheric conditions all the time while looking at objects requiring low power, other then planets.

I wonder if some kind of reducer of focal length might be a thing to consider from the very beginning for these very slow telescope.

(The question could be useful for the OP too, I never know)

[Stu]

16 minutes ago, N3ptune said:

I am reading this discussion and I wish to interfere with a question.  The telescope has 1800mm FL I think that's way too much for low power observation, 28mm gives 64x and 25mm gives 72x. I think the quality in the views at these powers will be affected by atmospheric conditions all the time while looking at objects requiring low power, other then planets.

I wonder if some kind of reducer of focal length might be a thing to consider from the very beginning for these very slow telescope.

(The question could be useful for the OP too, I never know)

I guess the thing is Maks are just not designed for low power viewing. There no reason not to try a focal reducer but often a longer focal length eyepiece, perhaps even a 55mm Plossl for instance can be just as effective.

[ronin]

At 1800mm focal length you are not really going to get low power. The 25mm BST will still give 72x and that falls very much at the mid range.

Sticking to my genral rule I suggest one at the f number so 12mm, one a bit lower and one a bit more. In this case that would have to be the 8mm and the 15mm. After that the 25mm as that is more medium the low in the scope. Forget the 5mm (360x) and the 3.2mm - 562x is more a joke then anything.

The jump from 12mm to 8mm seems too much on the scope, realistically you are likely to find a 2mm step better. For that reason I wonder if the Vixen SLV's would be a better range as they have a 25mm a 12mm and then a 10mm and a 15mm. Not sure on the performance of the SLV's.

[N3ptune]

2 minutes ago, Stu said:

I guess the thing is Maks are just not designed for low power viewing. There no reason not to try a focal reducer but often a longer focal length eyepiece, perhaps even a 55mm Plossl for instance can be just as effective.

That's a fair point ... thanks Stu.

[25585]

On 30/08/2017 at 21:26, ronin said:

At 1800mm focal length you are not really going to get low power. The 25mm BST will still give 72x and that falls very much at the mid range.

Sticking to my genral rule I suggest one at the f number so 12mm, one a bit lower and one a bit more. In this case that would have to be the 8mm and the 15mm. After that the 25mm as that is more medium the low in the scope. Forget the 5mm (360x) and the 3.2mm - 562x is more a joke then anything.

The jump from 12mm to 8mm seems too much on the scope, realistically you are likely to find a 2mm step better. For that reason I wonder if the Vixen SLV's would be a better range as they have a 25mm a 12mm and then a 10mm and a 15mm. Not sure on the performance of the SLV's.

This is where Barlows come in useful. They can offer a focal length/magnification combined with a prime ep that falls between your prime eps' magnifications.