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#26 Dave Mitsky

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Posted 16 October 2018 - 12:19 PM

i was reading smewhere that once you get to 30mm and above you should buy 2" eye pieces , i can accomodate 2" eyepiece as i have a baader 2 " click lock with a 1.25 reducer.

I always recommend maximizing the true field of view of a telescope, which is approximated by dividing the apparent field of view that is inherent to the eyepiece by the magnification the eyepiece produces in a particular telescope, to those who are seeking advice on purchasing eyepieces. Your Schmidt-Cassegrain telescope has a "slow" focal ratio of f/10 which is very forgiving of the wide-field eyepiece aberration known as astigmatism so relatively inexpensive Erfle-type eyepieces will perform very well with it.

 

Here are some 2" wide-field eyepieces that you may want to consider.  I'm not sure what the availability will be in United Kingdom.

https://explorescien...egree-eyepeices

 

https://agenaastro.c...a-eyepiece.html

 

https://agenaastro.c...epiece-set.html

 

https://www.telescop...e/66.uts#tabs-2

 

https://optcorp.com/...pheric-eyepiece

 

https://optcorp.com/...pheric-eyepiece

 

Dave Mitsky



#27 aeajr

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Posted 16 October 2018 - 12:58 PM

The only reason to go to 2" is for wide field of view.  Typically this is 65 to 82 degree AFOV in 18 mm and longer FL, so you want to look at eyepieces with at least 65 degree FOV or there is no real reason to go 2".   

 

There are 2" that go down to as short as 4 mm focal length so you can achieve FOV of over 100 degrees, but they are VERY expensive.    Many people only have one 2" eyepiece.  I have two.   And I would say most people go to 1.25" below 20 mm, but there are those who are 2" all the way to 4 mm. 

 

 

My 2" are 70 degree (38 mm) and 82 degree FOV (20 mm).  After that I am all 1.25"

 

If you have a preference for Celestron brand eyepieces, the Luminos line has 31 and 23 mm 2" 82 degree eyepieces

https://www.firstlig...-eyepieces.html

 

 

Also consider the Explore Scientific 68 or 82 degree line.  I have the 82s in 1.25"

https://www.firstlig...-eyepieces.html

 

 

Going back to my earlier post (#14), even if you are not going to spend a lot of money on eyepieces now, plan out your eyepieces.  Set magnification targets.   And consider AFOV requirements.  Finally your budget for eyepieces.   You can easily spend more on eyepieces than you spent on the scope. 

 

This will also help you decide what barlow you will want, 1.5, 2X, 2.5X or 3X.    For your scope I would think 1.5 or 2X and no higher.   There are some that will give you 1.5 and 2X.  I have one like that. 

 

 

Then you can decide if you are going to keep using the 25 mm you have now and add something lower or higher power now with a plan to replace the 25 mm later, or if you want to go all wide now and retire/sell that 25 mm. 

 

If people are on a tight budget, I tend to recommend that people keep using what they have now and fill in the set with a barlow planned in for economy purposes. Then replace what you have once you have the rest of the set filled out or when you feel the need to replace. At some point you might retire the barlow too. 

 

Right now you have a good 25 mm.   So, is it really cost effective to replace it with a 24 mm?   Only if you feel you need a wider FOV right now.    That is up to you.


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#28 aeajr

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Posted 16 October 2018 - 01:01 PM

If you are trying to hold down the cost for our 1.25", the BST StarGuider is the same as the AT Paradigm sold in the USA.  These have a a very good reputation.  If 60 degree AFOV is sufficient, these are a good choice at a moderate price. 

https://www.firstlig...-eyepieces.html

 

Discussion about Paradigm eyepieces
https://www.cloudyni...s/#entry8229760
https://www.cloudyni...s/?hl=+paradigm



#29 pete123

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Posted 16 October 2018 - 01:26 PM

im at work at the momoment as sosoon as i geget home ill try to ananswer some of the quesquestion about the scope and me , in the meantime thanks everyone , ill answer in an hour or so


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#30 jallbery

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Posted 16 October 2018 - 01:37 PM

The only reason to go to 2" is for wide field of view.  Typically this is 65 to 82 degree AFOV in 18 mm and longer FL, so you want to look at eyepieces with at least 65 degree FOV or there is no real reason to go 2".   

 

Just to clarify a few things...

 

The reason to go to 2" is for a wide TRUE field of view.   At F/10, a wide AFOV is not a requirement to necessitate a move to the 2" format.   A 55mm plossl will maximize the useful field of a C8, and it is only a 50-52-degree AFOV eyepiece.   68-degree eyepieces are typically in 1.25" format if they are at 24mm or shorter.

 

A C8 will be limited to just under 0.8 degrees of true field unless you either move to 2" format or use a focal reducer.   The Celestron F/6.3 Reducer/Corrector will allow you to use 1.25" eyepieces to achieve similar true fields and magnification without the expense and weight of 2" eyepieces (the R/C costs about the same as a good 2" SCT diagonal).

 

With either the R/C or a 2" diagonal, you can get 1.2-1.3 degrees out of a C8.


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#31 aeajr

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Posted 16 October 2018 - 02:41 PM

I have 1 lens at the moment which is a celestron e-lux 25mm plossl 1.25  , so u rcomend getting a 12mm x-cell and a 2x barlow lens , and at a later date i could get a 30mm x-cel ,  i was reading smewhere that once you get to 30mm and above you should buy 2" eye pieces , i can accomodate 2" eyepiece as i have a baader 2 " click lock with a 1.25 reducer.

 

 

Just to clarify a few things...

 

The reason to go to 2" is for a wide TRUE field of view.   At F/10, a wide AFOV is not a requirement to necessitate a move to the 2" format.   A 55mm plossl will maximize the useful field of a C8, and it is only a 50-52-degree AFOV eyepiece.   68-degree eyepieces are typically in 1.25" format if they are at 24mm or shorter.

 

A C8 will be limited to just under 0.8 degrees of true field unless you either move to 2" format or use a focal reducer.   The Celestron F/6.3 Reducer/Corrector will allow you to use 1.25" eyepieces to achieve similar true fields and magnification without the expense and weight of 2" eyepieces (the R/C costs about the same as a good 2" SCT diagonal).

 

With either the R/C or a 2" diagonal, you can get 1.2-1.3 degrees out of a C8.

Jallberry, 

 

As always your posts are great and hit the mark.   You are certainly right about the goal being True FOV.  And just to clarify, pete123 says he already has the ability to take 2" eyepieces.  See his post quoted above.

 

pete123,

 

There is always a compromise and balance between magnification and true FOV and budget.   A 56 mm Plossl will maximize FOV, assuming no vignetting, but will do it with reduced magnification.   One has the option to go to wider AFOV eyepieces to achieve a wide true FOV while going to a shorter FL to increasing magnification.

 

Let's illustrate.  I will reference specific examples for each one but there are others that would fit.   

 

Meade 56 mm 52 degree AFOV = 36.2X and 1.43 degree True FOV   using the AFOV/Mag formula, assuming no vignetting.  Others can comment on whether you can actually get this wide on a Celestron nexstar 8 gps xlt.  

https://agenaastro.c...l-eyepiece.html

 

 

40 mm 68 degree = 51X and 1.33 degree FOV

https://agenaastro.c...piece-40mm.html

 

 

31 mm 82 degree =  65.5X and 1.25 degree FOV

https://agenaastro.c...iece-93435.html

 

As we go up in mag, using these eyepieces we lose a little true FOV.  Is a .1 degree difference worth the drop from 51X to 36X?    Is the .08 degree difference between 51X and 65.5X important?  That is up to the individual.   

 

Just showing how we can balance specs to achieve specific goals.  Am I trying to maximize FOV at any mag, or am I trying to balance FOV with magnification?  And what about budget?   There is no right answer, only options to be considered. 



#32 Dave Mitsky

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Posted 16 October 2018 - 02:44 PM

Eyepieces shorter in focal length than about 17 millimeters that have 2" barrels, as opposed to the 1.25/2" barrels on some of the Tele Vue eyepieces, are not true 2" eyepieces.  Just look at the field stops.  There's no optical advantage to having a 2" barrel below that focal length.

 

I've used a number of long-focal-length telescopes, one being 6477mm, over the years and greatly prefer a 2" 40mm 70-degree eyepiece to a 2" 55 or 56mm Plössl.  A 40mm 70-degree eyepiece will produce almost the same true field of view at higher magnification and with less background skyglow.  

 

Most observers will need only one 2" eyepiece.

 

Dave Mitsky


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#33 aeajr

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Posted 16 October 2018 - 03:07 PM

Pete123,

 

is this your scope?   What do you have it mounted on?

https://www.amazon.c...y/dp/B00006371U

 

The NexStar GPS Series 8GPS Specifications:
8" (203.2mm) Schmidt-Cassegrain Optical Design
2032mm Focal Length, f/10
StarBright Multi-Coatings
Dual Fork Arm
Carbon Fiber Optical Tube
Fastar Compatible for f/2 CCD Imaging
Double line, 16 character Liquid Crystal Display; 19 fiber optic backlit Computer Hand Control
RS-232 communication port on hand control
Auxiliary Port and Autoguide Port on Drive Base
40,000+ object database
16 channel GPS Receiver
Fork Tine and Optical Tube Weight 42 lb.

 

The NexStar GPS Series 8GPS Standard Accessories:
1-1/4" 40mm Eyepieces (51x)
1-1/4" Star Diagonal
9x50 Finderscope and Mounting Bracket
1-1/4" Visual Back
AC Adapter (car battery adapter is included with some models)
NexRemote Telescope Control Software


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#34 Dave Mitsky

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Posted 16 October 2018 - 03:53 PM

Here's a shot of my "2-inch" 9mm Explore Scientific 100-degree eyepiece.  Notice how small the field lens is compared to the outer barrel.  The field stop diameter stated at https://explorescien...roducts/100-9mm is 15.7mm.

Browse https://www.astronom...ield-ethos.html for a photo of a 8mm Tele Vue Ethos with a dual barrel.

 

Dave Mitsky

Attached Thumbnails

  • 9mm ES IMG_4891 CN.JPG

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#35 pete123

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Posted 16 October 2018 - 04:28 PM

Just want to fill in the blanks here.

 

You say you have a Celestron nexstar 8 gps xlt.   I don't see that listed as one of Celestron's scopes.   There are 2 families of NexStar scopes that have an 8" OTA, both of which I believe, have a GPS option. 

 

On this page you can select the NexStar  scopes that Celestron offers.  Which one do you have?

https://www.celestro...ons/telescopes#

 

 

And BTW, your eyepieces don't have to be Celestron.  You can use any brand as long as they fit  your 2"/1.25" inch set-up.  

its an older version , has the carbon fibre tube and starbright xlt coating



#36 pete123

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Posted 16 October 2018 - 04:29 PM

Pete123,

 

is this your scope?   What do you have it mounted on?

https://www.amazon.c...y/dp/B00006371U

 

The NexStar GPS Series 8GPS Specifications:
8" (203.2mm) Schmidt-Cassegrain Optical Design
2032mm Focal Length, f/10
StarBright Multi-Coatings
Dual Fork Arm
Carbon Fiber Optical Tube
Fastar Compatible for f/2 CCD Imaging
Double line, 16 character Liquid Crystal Display; 19 fiber optic backlit Computer Hand Control
RS-232 communication port on hand control
Auxiliary Port and Autoguide Port on Drive Base
40,000+ object database
16 channel GPS Receiver
Fork Tine and Optical Tube Weight 42 lb.

 

The NexStar GPS Series 8GPS Standard Accessories:
1-1/4" 40mm Eyepieces (51x)
1-1/4" Star Diagonal
9x50 Finderscope and Mounting Bracket
1-1/4" Visual Back
AC Adapter (car battery adapter is included with some models)
NexRemote Telescope Control Software

yes that is my scope with the same mounting



#37 pete123

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Posted 16 October 2018 - 04:32 PM

Eyepieces shorter in focal length than about 17 millimeters that have 2" barrels, as opposed to the 1.25/2" barrels on some of the Tele Vue eyepieces, are not true 2" eyepieces.  Just look at the field stops.  There's no optical advantage to having a 2" barrel below that focal length.

 

I've used a number of long-focal-length telescopes, one being 6477mm, over the years and greatly prefer a 2" 40mm 70-degree eyepiece to a 2" 55 or 56mm Plössl.  A 40mm 70-degree eyepiece will produce almost the same true field of view at higher magnification and with less background skyglow.  

 

Most observers will need only one 2" eyepiece.

 

Dave Mitsky

so the 2" only comes in to play once you reach 30mm or above which im sure must be a very large view of the sky



#38 Jond105

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Posted 16 October 2018 - 04:39 PM

No. Two inch eyepiece come into play when the field stop cannot produce the degree field in a 1.25" eyepiece. I think I said that right.
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#39 pete123

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Posted 16 October 2018 - 04:48 PM

No. Two inch eyepiece come into play when the field stop cannot produce the degree field in a 1.25" eyepiece. I think I said that right.

 

No. Two inch eyepiece come into play when the field stop cannot produce the degree field in a 1.25" eyepiece. I think I said that right.

field stop as in the F.10  of my scope , can you give an eg im baffled by thsi , its all new to me



#40 pete123

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Posted 16 October 2018 - 04:55 PM

No. Two inch eyepiece come into play when the field stop cannot produce the degree field in a 1.25" eyepiece. I think I said that right.

ok so i had a little read and you meant the field stop of the eyepiece , so it a preference of whether your happy with the field of view , is that what your saying


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#41 pete123

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Posted 16 October 2018 - 05:04 PM

ok so heres some prices of some eyepieces that are of interest , i dont mind paying £140 for an eyepice if its going to be worth it against somthing thats half the price , dearer the eyepiece the longer itll take to build up a collection , but i dont mind that if there is a noticable difference in the viewing .

 

https://www.rotherva...pieces-125.html

 

https://www.rotherva...pieces-125.html

 

https://www.rotherva...r-eyepiece.html

 

what are your thoughts



#42 Jond105

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Posted 16 October 2018 - 05:13 PM

Kind of. Yes. Let's take a 68 degree eyepiece. Field stop seems to always end at 24mm eyepiece. If you want a lower power eyepiece, but still want to see a 68* AFOV through an eyepiece, you'd then need to purchase a 2"

82* generally runs 17-18 last of the 1.25 before you'd have to use a 2"
Plossls the lowest is a 32mm in 1.25.
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#43 Dave Mitsky

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Posted 16 October 2018 - 06:08 PM

what are your thoughts

The Hyperions will produce a more spacious apparent field of view of 68 degrees and have excellent eye relief so they may be the better, albeit more expensive, option.

 

Here's an excerpt from a review posted at https://www.firstlig...ion-review.html

 

Baader Hyperion

A thoughtfully designed and well made range, I felt that the Hyperions were very nice eyepieces to own and use. They are a significant step up from the eyepieces supplied with scopes and standard quality plossls and, with their carefully thought out packaging, seem to me to deliver good value for their approx £90 retail price.

 

The modular options such as the fine tuning rings really do work and add flexibility and further value to the range, with the exception of the 24mm which cannot be used with the FT rings. They (like the Vixen’s) are large eyepieces but their extensive use of rubber makes their handling secure and reassuring.

 

Optically they are very comfortable to use and seem able to deliver excellent quality views of a wide range of astronomical objects. In scopes slower than f6 or so I think they perform at their best. In faster scopes than this the edge of field astigmatism could prove distracting when viewing extended objects but this will not bother everybody by any means. While their control of light scatter around bright objects does not appear to be quite as good as the Vixens, by others standards, they are still pretty decent in this respect.
 

Dave Mitsky



#44 pete123

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Posted 16 October 2018 - 06:12 PM

The Hyperions will produce a more spacious apparent field of view of 68 degrees and have excellent eye relief so those may be the better if albeit more expensive option.

 

Here's an excerpt from a review posted at https://www.firstlig...ion-review.html

 

Baader Hyperion

A thoughtfully designed and well made range, I felt that the Hyperions were very nice eyepieces to own and use. They are a significant step up from the eyepieces supplied with scopes and standard quality plossls and, with their carefully thought out packaging, seem to me to deliver good value for their approx £90 retail price.
The modular options such as the fine tuning rings really do work and add flexibility and further value to the range, with the exception of the 24mm which cannot be used with the FT rings. They (like the Vixen’s) are large eyepieces but their extensive use of rubber makes their handling secure and reassuring.
Optically they are very comfortable to use and seem able to deliver excellent quality views of a wide range of astronomical objects. In scopes slower than f6 or so I think they perform at their best. In faster scopes than this the edge of field astigmatism could prove distracting when viewing extended objects but this will not bother everybody by any means. While their control of light scatter around bright objects does not appear to be quite as good as the Vixens, by others standards, they are still pretty decent in this respect.

 

Dave Mitsky

but are they worth the xtra money over the others



#45 pete123

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Posted 16 October 2018 - 06:14 PM

but are they worth the xtra money over the others

sorry i mist that in your orignal post , thanks



#46 Dave Mitsky

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Posted 16 October 2018 - 06:16 PM

Plossls the lowest is a 32mm in 1.25.

Well, there are 40mm 1.25" Plössls but in most cases a 32mm is preferred, since it produces a larger apparent field of view (50 degrees versus 43 or so) and has less excessive eye relief.  The quoted field stop diameters for the Tele Vue 32 and 40mm Plössls are the same, namely 27mm.

 

The maximum field stop diameter for a 1.25" eyepiece is around 27mm.  The maximum for a 2" is 46mm.

The Field Stop and Apparent Field of View

The field stop is the metal ring inside the eyepiece barrel that limits the field size. It's projected by the eyepiece so that it appears as a circle out in space when you look through the eyepiece. The angular diameter of this circle is called the apparent field of view (AFOV) and is a fixed property for each eyepiece design. For example, Plössl eyepieces have an AFOV of 50°, DeLites have 62°, Panoptics have 68°, Delos have 72°, Naglers have 82° and Ethos eyepieces have 100° or 110°.

http://www.televue.c...rn=Advice&id=79

 

Dave Mitsky


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#47 jallbery

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Posted 16 October 2018 - 10:15 PM

Jallberry, 

 

As always your posts are great and hit the mark.   You are certainly right about the goal being True FOV.  And just to clarify, pete123 says he already has the ability to take 2" eyepieces.  See his post quoted above.

 

pete123,

 

There is always a compromise and balance between magnification and true FOV and budget.   A 56 mm Plossl will maximize FOV, assuming no vignetting, but will do it with reduced magnification.   One has the option to go to wider AFOV eyepieces to achieve a wide true FOV while going to a shorter FL to increasing magnification.

 

Let's illustrate.  I will reference specific examples for each one but there are others that would fit.   

 

Meade 56 mm 52 degree AFOV = 36.2X and 1.43 degree True FOV   using the AFOV/Mag formula, assuming no vignetting.  Others can comment on whether you can actually get this wide on a Celestron nexstar 8 gps xlt.  

https://agenaastro.c...l-eyepiece.html

 

 

40 mm 68 degree = 51X and 1.33 degree FOV

https://agenaastro.c...piece-40mm.html

 

 

31 mm 82 degree =  65.5X and 1.25 degree FOV

https://agenaastro.c...iece-93435.html

 

As we go up in mag, using these eyepieces we lose a little true FOV.  Is a .1 degree difference worth the drop from 51X to 36X?    Is the .08 degree difference between 51X and 65.5X important?  That is up to the individual.   

 

Just showing how we can balance specs to achieve specific goals.  Am I trying to maximize FOV at any mag, or am I trying to balance FOV with magnification?  And what about budget?   There is no right answer, only options to be considered. 

Ed...

 

I did read his post.  He said he had a 2" baader clicklock with a 1.25" adapter.   I interpreted this as saying he had the 2" Clicklock visual back with a 1.25" adapter, meaning he had visual back that could accommodate a 2" diagonal, but at present, he was using a 1.25" diagonal.  If I misinterpreted him, and he already has a 2" diagonal. I apologize for the rat-hole.

 

The AFOV/magnification calculation is not accurate enough to provide anything more than a rough estimate of true field.   In the case of that 56mm Plossl, I can guarantee you that you won't get 1.43 degrees, and it has nothing to do with vignetting:

  • Few of Meade's 52-degree plossls actually give a 52-degree field.
  • Distortion further inflates the field.
  • A 2" SCT diagonal will lengthen the focal length of a C8 to about 2100mm (or more).

Assuming a 56mm Plossl, and a 41mm 68-ish (or 38mm 70-ish) both have the same 46mm fieldstop that should be possible for a max-view 2" eyepiece, you'll get exactly the same field of view: a bit under 1.3 degrees (46*57.3/2100 = 1.26 degrees).

 

A 56mm plossl is useful with an F/10 scope because of the bigger fieldstop.  Under dark skies or with a nebula filter, the brighter view can be helpful.   Other times, a 38-41mm superwide might be preferable.  They both have their place.  On a traditional SCT, the lower power also may better mask coma and field curvature.


Edited by jallbery, 16 October 2018 - 10:18 PM.

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#48 aeajr

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Posted 16 October 2018 - 11:04 PM

Ed...

 

I did read his post.  He said he had a 2" baader clicklock with a 1.25" adapter.   I interpreted this as saying he had the 2" Clicklock visual back with a 1.25" adapter, meaning he had visual back that could accommodate a 2" diagonal, but at present, he was using a 1.25" diagonal.  If I misinterpreted him, and he already has a 2" diagonal. I apologize for the rat-hole.

 

snip...

You may be correct.  I don't have this scope.  



#49 pete123

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Posted 17 October 2018 - 02:47 AM

okok i would like to ssay thanks to everyone who offerd advice im pleased i decided to join this forum , ive decided i will go with the celestron x-cel lx eyepieces , and i think i will go for an 18mm and a 2x barlow which i can then use with the 25mm plosll i already have to make a 12.5mm  and then the 18mm to make a 9mm , then i can add some more at a later date , the only thing  thats troubling me now is the ammount of talk on eyepieces that are being discussed from 30mm to 58mm , im finding it hard to imagen what you would actualy see through these as i thought telescopes were for magnification of the sky .


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#50 Jond105

Jond105

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Posted 17 October 2018 - 04:08 AM

Depending on your focal length, the lower power 30-55 still will magnify where you are viewing easily. And the darker the sky, with a low power, will put on quite a show through the eyepiece. Planets you'll want to magnify, certain Messier objects, but like me with my focal length on my major scope, with a low power 82* eyepiece, would fit all of M45 in my eyepiece.

Edited by Jond105, 17 October 2018 - 04:09 AM.

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