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Celestron Astrofi 6 Maximum useful magnification

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I live on Crete and, although there is a magnificent observatory here, there aren't any shops specializing in telescopes. That's why I'm asking for your adviceūüôā. The skies are mostly clear and there isn't any light pollution to speak of.

I'd be very grateful for any help.

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Depends what you are observing. A 5mm eyepiece will give 300x, but achieving that regularly on anything other than the Moon would probably be difficult. A zoom eyepiece might be a good idea to experiment with. The Orion 7~21mm impressed me for its retail price.


Also, bear in mind a 6" catadioptric can take up to 90 mins to an hour at least to acclimatise or cool down outside. Maybe longer. SCT's have a microclimate inside the OTA and need to reach a thermal equilibrium where the outside and inside temperatures are the same. Until they are, warm air currents inside the tube will prohibit achieving a sharp image. 



Edited by Nightspore
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Hi and welcome to The Yard.


It wasn‚Äôt that long ago that I was in your position (venturing into astronomy) and questions re eyepieces as useful magnification were at the forefront of my mind. The question re ‚Äėuseful magnification‚Äô has recently been discussed at some length in a thread headed ‚Äúeyepiece for glasses wearer‚ÄĚ so it may be worthwhile for you to read it as much of it will apply to your situation. A number of eyepiece (EP) types are also discussed in the same thread.

If you look in the equipment review section you will also find some good reviews on different EP types.

Youre quite right to mention Clarity of image, as it’s easy to get an EP that gives too high a magnification for your scope and you end up with a highly magnified blurred image.


It looks like you have a nice scope by the way.ūüôā



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2 hours ago, badbishop said:

Many thanks for your advice. I think first of all I'll invest in a barlow lense.



Personally I doubt a Barlow will be necessary with a scope of 1500mm focal length. It's more likely that a focal reducer would be more useful. 


Introducing a *negative lens group into the light chain can sometimes cause issues on slow catadioptrics in my experience. If the Barlow and eyepiece aren't matched well it can cause glaring and/or ghosting effects.


SCT's and MCT's are high power instruments with limited fields of view. They are not ideal rich field (low magnification/wide angle) scopes.


Open clusters, many nebulae and large but dim deep sky objects don't require large magnifications as much as they need more light and therefore a greater exit pupil. Calculating exit pupils for SCT's is easy as you just divide the eyepiece focal length by 10.


For lunar/planetary targets and maybe some DSO's (globular clusters, distant galaxies) a spread of between approximately  1.5mm, 1mm, 0.7mm and 0.5mm is probably a good start. Bearing in mind the human eye is not very efficient with exit pupils narrower than 0.5mm. 


High magnifications are also dependent on atmospheric seeing and overall transparency (humidity/cloud). Furthermore the target itself and its position in the sky has to be taken into consideration. On a good night the Moon, when high, can take up to 60x or 75x per inch of aperture without much loss of detail. The higher the target's altitude the less atmosphere there is to look through.


Unfortunately, the planets are low in the northern hemisphere at the moment. Jupiter is notoriously difficult primarily as it is a high contrast target. Getting well defined views of a low Jupiter (even at transit) is not easy at very high magnifications. Even with a quality refractor. Refractors don't reflect light unlike reflecting telescopes and have no secondary obstructions. Therefore they will always have more visual acuity and contrast than reflecting scopes.


So, my advice is just to buy better quality eyepieces than the ones bundled with your scope. I'd start off with two or three inexpensive Plossls (maybe 12.5mm, 10mm and 7.5mm) to begin with. Plossls have a 50 degree FOV with good acuity and contrast which is why they became popular with amateur astronomers to begin with.  






*A Barlow works by essentially artificially increasing the focal length.

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3 hours ago, badbishop said:

Hi Nightspore,


Thanks for this. I'll get the 12.5 and 7.5 ploessls and take it from there. Surprised at how cheap they are from FLO.


You're welcome. I'm not sure who make these Plossls but they're as good as the Barsta-made ones that were usually marketed as Orion, Celestron and Sky-Watcher. Plus the 'Astro Essentials' have chromed brass barrels, which are an improvement on the aluminium barrels IMO. I have the 17mm and the only difference I can see with the Barsta equivalent is that the field stop is fractionally smaller. 




Celestron are owned by Synta and Sky-Watcher is the Synta house name. 

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4 hours ago, badbishop said:

The eye pieces with the scope are Kellner. Are they any good? By the way, do you think that here on Crete viewing the sky will be better than, say, from the UK?


Reversed Kellners are OK in slow scopes but the bundled 'giveaways' are usually just to get you started. 





This Japanese 'volcano top' Kellner (above) is quite good quality but in any scope faster than f/8 it will display lateral astigmatism. This is where stars near the field stop appear stretched out or look like tiny 'seagulls'.




These 2" GSO reversed Kellners (above) will show a flock of seagulls in most of my refractors, although I can just about get away with it in my f/7.5 80ED (below).




Kellners are often maligned but they are simple (three element) with good transmission. They can produce the famous 'floating in space' effect where images in the eyepiece appear almost three dimensional. I've seen this with open clusters at low magnifications.




Most large Celestron SCT's are usually bundled with 40mm and 13mm Plossls (my 235mm Celestron SCT and 40mm Plossl above). It might be worth getting a 40mm Plossl as well. They have a limited field but are the only way to get a decent exit pupil on a big cat' without using a focal reducer. It would give your scope 37.5x with a 4mm exit pupil. This would enable you to use some UHC or OIII filters to help see faint nebulae. 


I'm guessing the skies in Crete are better than mine in Worcestershire lol.

Edited by Nightspore
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