Lens for Celestron Astromaster90

[chris0]

Hi!!

This is my first post so please bare with me!!

Ive just bought a 2nd hand Celestron Astromaster 90, i know its not the best but i was amazed when i was able to see the rings of Saturn through it!

So im after some advice as ive been reading into it a bit more and the general consensus is that getting better eye pieces will help.

I did look at the Celestron accessory kit but it has mixed reviews

What would be the best step to make next (new eye piece, filters etc)

Thanks

Chris

[cloudsweeper]

Don't know what eyepieces you have (10 and 20mm?), but a 32mm would give a wider field of view (eg a Revelation), and a shorter focal length would give more magnification, eg a 6mm for x167.  Take a look at what's available at sensible prices - again, a 6mm Revelation Plossl is a good, economical choice.  Fancier, costlier EPs become more attractive with similarly costly telescopes!

Great that you saw Saturn's rings - an unforgettable experience.  You can see a lot with a 90mm aperture on an easy altaz mount.  Have fun!

Doug.

[PEMS]

Specification seems to be 90mm and a focal length of 1000mm at f/11. Nice and slow and to a good extent easy on eyepieces. I would say that the previously mentioned 32mm (plossl) would be good for the wider field. For "magnification" it is more complex.

The Starguiders at 8mm and 12mm should be a good option. I would not go less then 8mm, that would give 125x and enough for the vast majority of objects. Greater magnifications will often begin to produce less quality images once you match the aperture in mm. So I suggest that 90x and 100x are a more practical limit. The 8mm for 125x will likely be used less then you think or hope. Seems 60x is a more generally used magnification for observing. That would be an eyepiece of 1000/60 so around the 15mm mark, maybe an 18mm.

Have to remember that something like M33 at 1 degree fits better with an amount of the background "black" sky surrounding it.

Any good with numbers? The Starguiders are a 60 degree eyepiece. So M33 is 1 degree means you need a field of 1 degree so a magnification of 60x. M45 (Pleiades) is 2 degrees (maybe a little more) that means a magnification of 30x - and the low power 25mm Starguider will not allow all of M45 in a single view in the AM90.

Even a 32mm plossl for 31x will just make 1.6 degrees. Life becomes interesting when you start from the object size and work back to "What eyepiece?"

As info and in case: Your field of view is EP filed/Magnification. So to an extent do not get drawn into the desire of large impressive sounding magnifications. They do not usually deliver. And may be best considered as useful for and in specialised situations - double star splitting say.

Should be a nice scope, the lens is as said "slow" but will be easy.

[Ricochet]

I agree with the above post. Starguiders would make a nice match for your scope. Even the long (18/25mm) focal lengths will be well corrected when paired with an f11 scope. 

[chris0]

Thanks for the advice, I have to say a lot of the numbers went over my head!

I do have a 10mm and 20mm Ep at the mo so getting a better 10mm ep would i notice the difference?

also would I was looking at filters, is it better for a filter at the end of the telescope or at the ep end?

Sorry for all the questions, i just dont want to waste money on pieces i dont need

 

 

Edited October 2, 2021 by chris0

[Alan64]

I have the Celestron "AstroMaster" 70EQ...

There are two versions of these kits, one with an equatorial mount, and the other with an alt-azimuth mount.

Your 90mm f/11 achromat falls within the Sidgwick standard for colour-correction...

...so you're good to go there.

I have the same eyepieces...

They're okay.  They're "Kellner" type eyepieces, three-element.  Plossls are four-element eyepieces, and performance-driven for most any telescope, yet economical.

Where in the world are you located?  I could then direct you to listings of these items, and more besides.

Also, for what objects in the sky are you needing a filter?

You can certainly make use of a quality 2x-barlow.  That's all you need, for you can transform it into an approximate 3x mulitplier by simply inserting the barlow into the telescope first...

...and for those occasions when wanting to max out the aperture.

Edited October 2, 2021 by Alan64

[Alan64]

In addition, avoid eyepiece kits like the plague.  Instead, choose eyepieces carefully, one, or two at most, at a time.

[chris0]

Im Based in Jersey Channel Islands UK.

I was only looking at filters as thats whats been mentioned if you want to see certain things stand out like jupiter, saturns rings etc

Im lost with a lot of that chart you put up, also i ve the kit with the eq mount but its not the best of  tripods as it does move on its own slightly

Now just working out what EP to go for!

[Tiny Clanger]

45 minutes ago, chris0 said:

Thanks for the advice, I have to say a lot of the numbers went over my head!

I do have a 10mm and 20mm Ep at the mo so getting a better 10mm ep would i notice the difference?

also would I was looking at filters, is it better for a filter at the end of the telescope or at the ep end?

Sorry for all the questions, i just dont want to waste money on pieces i dont need

 

 

Heh, numbers can be confusing 🙂

First , the stat.s of your 'scope, 90mm is the aperture, i.e. the light gathering 'hole' . A quick search suggests the focal  length is 1000mm (it probably mentions this on a plate fixed to the tube ?) . Those are important numbers to know.

The rule of thumb that gets used oftten is that the maximum magnification you can usefully get out of a telescope is 2x the aperture in mm. So you don't want to exceed  2x90= 180x mag.  That's handy, because (assuming you are in the UK ?) our skies mean more than 200x mag. isn't always possible due to unstable air masses (AKA UK weather )

To work out the magnification of a specific eyepiece with your 'scope, take the focal length of the scope (1000mm) and divide it by the focal length of the eyepiece. So, your 10mm gives you 1000/10=100x mag. , your 20mm gives 50x (1000/20=50)

In theory , you may be able to use something like a 7 or 8mm eyepiece to get more magnification but ( first rule of practical astronomy, 'there is always a 'but' ...) without wanting to add any more maths to this let's just say , there's a thing called f ratio (your 'scope is f11 if I found the right focal length ) and another thing called exit pupil, and if you try a 5mm or 6mm eyepiece you might find that exit pupil becomes a problem , causing you to see 'eye floaters'  Not only that, but the high magnification will magnify any trembling of the 'scope on the mount, or wobbles in the tripod legs just as much as it does the view. So , best to not assume that really high magnification is necessarily a good thing.

The eyepieces you got are probably the ones that came with the 'scope when it was new, these are never very good . What you buy depends on how much you have to spend, something better starts at £20 and goes up to as much as you like ! Tell us how much you can spend , and suggestions will roll in.

Oh, and unlike with cameras filters, which go on the front of the lens, so are in many sizes, astronomy filters come in 1.25" or 2" sizes, because they screw on the bottom of the eyepiece (apart from solar ones , which are a special case) . So assuming your eyepieces are 1.25" diameter, that's the filter size.

Heather

 

 

[Tiny Clanger]

7 minutes ago, chris0 said:

Im Based in Jersey Channel Islands UK.

I was only looking at filters as thats whats been mentioned if you want to see certain things stand out like jupiter, saturns rings etc

Im lost with a lot of that chart you put up, also i ve the kit with the eq mount but its not the best of  tripods as it does move on its own slightly

Now just working out what EP to go for!

Photographer's trick for steadying a wafty tripod : hang a weight off the middle of it (or on the shelf if it has one) , a bottle of water and some string will add mass and reduce wobbles a bit !

[Alan64]

24 minutes ago, chris0 said:

Im Based in Jersey Channel Islands UK.

I was only looking at filters as thats whats been mentioned if you want to see certain things stand out like jupiter, saturns rings etc

Im lost with a lot of that chart you put up, also i ve the kit with the eq mount but its not the best of  tripods as it does move on its own slightly

Now just working out what EP to go for!

The planets become quite interesting to view at 150x, and upwards of course.  The focal-length of your refractor, 1000mm, is used to determine which powers you want, and from this eyepiece and that...

1000mm ÷ 150x = 6.7mm eyepiece; that is, either a 6mm(167x) or 7mm(143x).

This is the diagonal that came with the kit...

The manufacturers often include that type of diagonal within entry-level kits for use during the day, and at night.  It is a correct-image diagonal.  But there are diagonals specifically for use during the day, like this 45° correct-image...

...for birds in trees, ships at sea, that sort of thing.  Then, there is a diagonal specifically for use at night, a star-diagonal; for example...

https://www.harrisontelescopes.co.uk/acatalog/celestron-star-diagonal.html

I have that same star-diagonal myself.  Here it is, on the right, and compared to our correct-image diagonals...

Note the apertures of the light-ports.  A star-diagonal will open that telescope up, and with a 32mm Plossl; for example...

https://www.firstlightoptics.com/skywatcher-eyepieces/skywatcher-sp-plossl-eyepieces.html

The 32mm would offer the widest view, and the lowest power(31x).  It would assist the finder, and in locating objects to observe.  Once you've located an object that you would like to see up close, you then insert a 12.5mm Plossl or a 12mm wide-angle type; for examples...

Again, and from this listing... https://www.firstlightoptics.com/skywatcher-eyepieces/skywatcher-sp-plossl-eyepieces.html (80x)

https://www.firstlightoptics.com/bst-starguider-eyepieces/bst-starguider-60-12mm-ed-eyepiece.html (83x)

With a 2x-barlow, a 12mm can be converted into a simulated 6mm(2x-167x); or a 4mm(3x-250x) even, if you insert the 2x-barlow into the telescope first, as described and illustrated previously.

A 2x-barlow, for example... https://www.rothervalleyoptics.co.uk/antares-x2-achromat-fmc-barlow-lens-125.html

You also have the option of a zoom-eyepiece...

https://www.ebay.co.uk/itm/SVBONY-8-24mm-1-25-Zoom-Eyepiece-Multi-Coated-Lens-Astronomy-Telescope-Parts-/283934574144

This is the chart again, and tailored specifically for your telescope; a 90mm f/11 achromat...

Your 90mm f/11 achromat sits at 3.11 on the scale, and lands on the first green block denoting the Sidgwick standard for colour-correction.  It simply means that when viewing brighter objects -- the Moon, the planets, the brighter stars -- you will see only a little false-colour, if any at all.

This is false-colour, and seen through my 80mm f/6 achromat...

Note the blue-violet rim of the Moon there.  That is because the 80mm f/6 achromat is physically shorter in length...

  

The perception of false-colour varies from individual to individual.  Where some see a bit of false-colour, others may see very little or none at all.  As we age, the perception of same diminishes.

[Alan64]

Filters for the planets are coloured...

The only one that I would suggest, and with reservations still, is the light-blue, #82A.  For example...

https://www.firstlightoptics.com/visual-oberving-rgb-filter-sets/astro-essentials-125-wratten-82a-light-blue-filter.html

The rest are only to look pretty, whilst simply standing over them arranged like that. 

[Tiny Clanger]

14 hours ago, chris0 said:

Im Based in Jersey Channel Islands UK.

I was only looking at filters as thats whats been mentioned if you want to see certain things stand out like jupiter, saturns rings etc

Im lost with a lot of that chart you put up, also i ve the kit with the eq mount but its not the best of  tripods as it does move on its own slightly

Now just working out what EP to go for!

Apart from how much you want to spend, it is important to know if you will want to wear glasses while observing or not, some eyepieces do not play well  with spec.s