planetary eyepiece

[mullac2001]

i have a skywatcher explorer 130/900 scope and im looking at getting an evostar 90 for taking on hols and for my kids (6 and 11) to use at home.

what would be the max mag eyeiece i could realistically use in both? my budget is of about £50.

im thinking 8mm or maybe 6 tmb"s?

all suggestions welcome and thanks in advance

[Stargazer_00]

I'd say 8mm is probably a good choice. 6mm being the lowest id go in those scopes.

the 8mm in the 130 will give just over a 1mm exit pupil @ 112x, the evostar will also give 112x mag but due to the smaller aperture it will provide a smaller exit pupil of 0.8mm. this means the image will be dimmer and less resolved.

If you move this further to a 6mm you get 150x in both scopes but only 0.86mm exit pupil in the 130mm and a very low 0.6mm in the evostar. This would be dim, grainy, you;d be seeing floaters in your eye tissue.

My personal limit is a 1mm exit pupil, I dont much enjoy the views below this limit. Both my scopes are F5 and the smallest eyepiece I have is a 5mm which gives 1mm exit pupil.

[alan potts]

Normal max rule is 2X dia in mm but I don't agree with it,, I think it is too much.

I thought it was an Evo Star 80mm, I didn't know they did a 90mm., still. BST make a nice 8mm which gets high praise on site, in fact there was one in classified ads. I think 6mm will be as far as I dare go on the smaller scope, here your suggestion would be fine. I would settle for the 8mm though myself.

Alan.

[Stargazer_00]

My personal rule is diameter of aperture in mm = max magnification for scope (as that gives a 1mm exit pupil)

[Stu]

It's not a bad rule as a start, but is not always accurate, particularly for apo refractors but for other scopes too. It does depend on the scope type and quality of optics.

I use my 106mm refractor up to x230 and beyond at times. The exit pupil is very small and I do suffer with a floater in my observing eye but find ways around that. I'm using a Binoviewer more to try to avoid this issue, or I would if the sky ever cleared....

Stu

[Stargazer_00]

Oh for sure it's not in any way scientific or accurate. Just when I'm window shopping its an easy to remember rule of thumb

I would expect the mroe you pay for a scope the less accurate this rule becomes. All my scopes to date are F5 and relatively inexpensive. I'm sure by the time im paying more for a scope than a small car I will expect more than aperture = max mag

[Naemeth]

Oh for sure it's not in any way scientific or accurate. Just when I'm window shopping its an easy to remember rule of thumb

I would expect the mroe you pay for a scope the less accurate this rule becomes. All my scopes to date are F5 and relatively inexpensive. I'm sure by the time im paying more for a scope than a small car I will expect more than aperture = max mag

I've no problems with a 4mm on my scope (0.8mm exit pupil), the 2.5mm is usable on a rare occasion for seconds out of each minute .

Back to the OP, I'd personally go for a 5mm and a 8mm BST ED (Starguider ED), both at £47 each.

[Moonshane]

if you have a 15mm eyepiece then a barlow might be a good buy (or vice versa). this would provide a decent mag with the barlow (effectively 7.5mm) and a mid range field in the 15mm.

[Stu]

Good thing is they both have the same focal length so mags will be the same. The Evostar 90 is f10 so should be quite easy on ep's

The BST Explorers get good reviews, the 8mm would be good, my only concern would be how much use the 5mm would get. x180 might be pushing it a little so a 6mm might be a better option at x150.

Maybe the Vixen NPL 6mm would be worth considering?

Stu

[Stargazer_00]

For sure it will work, and I do crank mine up with barlows occasionally to WAY beyond what they say you should - you just have to do it once in a while for the fun of it. I just find extended observations of a target, say for 5 minutes or more on the same thing, i most comfortable at no less than 1mm exit pupil.

The 5mm and 8mm BST Starguiders are both good options, the 8mm would likely get more overall use.

[Stu]

Of course, any ep will work, it's just a question of getting the OP ones which will get good use and sharp views.

The 8mm will definitely be fine, so maybe start with that and look at other, shorter f/l options once you have more experience with the scopes.

Cheers

Stu

[Qualia]

I wrote this the other day to a similar enquiry, so I hope I'm not overtly repeating myself:

When it comes to planetary viewing, I've noticed that slight changes in the focal length of eyepieces makes a difference to what can be tweaked from the given object. In terms of general useful magnification, I have a little collection of EPs which run the following parameters:

• f/10: 200x, 166x, 142x, 133x, 111x, 100x, 83x.
  
• f/5: 250x, 208x, 178x, 166x, 138x, 125x, 104x.
  

For Lunar work, all these magnifications offer up something to be amazed by. The low power help frame the Moon, the mid-range frames craters and seas and mountain ranges and the high-power tweak out subtle features like terraces, tiny craters, ray grazes etc.

For Jupiter, I have used mags between 140x to about 180x.

For Saturn - I have found 140x to 200x useful mags to play around with. I'm sure one can go higher here, but a lot will depend on the night's atmospheric conditions.

Mars. The way I look at is that Jupiter is about 3x the size of Mars and to see Jupiter nicely is, say, at around 150x which means that for me to get a 'similar' view of Mars I need 450x. Well, I'm just not going to get that kind of viewing quality. So, I figure a compromise is needed, say, around 250x which still means seeing conditions will have to be very good. As such, I haven't found Mars a particularly giving planet.

Other than that, in general, try to view the given object as close to the zenith as possible and bear in mind that as a general rule of thumb the brightness of an object will decline as you up the magnification. If I up the mag twofold, say, I'm reducing the image brightness by a factor of four. If I keep on doing this eventually details just disappear.

On the other hand, increasing the mag does make detail more apparent, so, as you can appreciate, we're now at a trade-off: will increasing magnification gain more detail even though I'm making the object fainter?

I've found that playing around with this trade-off - dependent on the evening's seeing (LP doesn't really affect planets) - does make a difference. Even as little as 1mm increase or decrease in the mag - about 10% to 15% difference of magnification - can be quite surprising which is the main reason why most planetary observers will have quite a run of high-mag EPs.

For planetary work I use Orthos and in particular those that were made by Baader - the Baader GOs which are no longer in production but regularly crop up on the secondhand market from anything between £50 to £100. The quality of image in these EPs is as good as it gets. If you want a similar quality image EP but with a wider field of view and perhaps a more comforting eye-relief (I haven't had any problems with the BGOs) you're going to have to spend a fair bit more.

Final thought, whenever possible try to sit with your given object for a peaceful twenty to sixty minutes or so and you'll find that they'll be moments of great clarity and seeing. By practicing this attentive sitting you come to notice more and more detail from the given object.

I hope this helped a little

[mullac2001]

if you have a 15mm eyepiece then a barlow might be a good buy (or vice versa). this would provide a decent mag with the barlow (effectively 7.5mm) and a mid range field in the 15mm.

i have a 15mm vixen npl and a 2.5 revelation barlow . would i be better off saving my money do you think.?

the only 2 times ive managed to use both so far ive been unsatisfied but this may have been due more to conditions

[Moonshane]

I think it's likely to be conditions on that basis. I'm assuming that you are letting the scope cool for maybe 30 minutes or so before using it (outside)? this makes a big difference too.

it may be that the 2.5x barlow is too much. maybe you'd be better with a 2x barlow which is what I envisaged in my earlier post.

[mullac2001]

yes scope is left out for an hour usually. so you think a 2.5 barlow is too much for my 15mm possibly ?

i just didnt seem to be able to get a nice crisp image of jupiter and i had hoped for more given the good reviews of the vixen npl"s.

using it with and without the barlow made no difference but as i say i have had few chances 2 use them really so may be its the conditions and my impatience that are really the issues

[Stu]

Would expect it to be sharp without the Barlow. Have you checked collimation?

The 2.5 Barlow will give the equivalent of a 6mm which is towards the top end so cooling and collimation will need to be right.

I would tend to stick with my original suggestion which was to get an 8mm BST Explorer and see how you get on with that. It will be a safe bet and you will know if the views are sharp then it is either cooling, collimation or seeing conditions

Stu

[mullac2001]

i have wondered about collimation but as a newbie am rather wary of making it even worse. i think i might just get the evostar 90 for now see how the vixen performs in that and compare and go from there.

thanks so much everyone for the help and suggestions. im thinking an 8mm explorer will be the next purchase after the evostar though!

[mullac2001]

well thats the evostar ordered from flo.

sorry about the clouds !!!!!!

[Stu]

Good stuff, hope you enjoy it.

I have so many new toys which have yet to see the light of day ( or rather night!) it's getting a bit ridiculous.

I need a clear night or seven to stop me constantly thinking about what to change with my kit next

Enjoy

Stu

[Stargazer_00]

Good luck with the new scope

[mullac2001]

just noticed the bst is still for sale in the classifieds hhhmmmm.................. will i get away with it ?

[mullac2001]

Good stuff, hope you enjoy it.

I have so many new toys which have yet to see the light of day ( or rather night!) it's getting a bit ridiculous.

I need a clear night or seven to stop me constantly thinking about what to change with my kit next

Enjoy

Stu

say how do you guys know its really for me ?

its for the kids honest !

[Stu]

That's what we all say!! :-)

[Ohmless]

General rules of thumb for eyepiece powers are as follows. For deep sky, you want to use the highest power that still frames the object so that you get a darker background. For planetary, you want to use the lowest power that still shows the details you are trying to achieve.

My preference is for 0.6-0.7 exit pupil for the gas giants and 0.5 for Mars. Exit pupil is objective diameter divided by magnification.