Sky watcher Evostar 120 & Celestron Omni xlt 120 - what is the difference

[newman]

I'd look into something that will give you around 2mm exit pupil for your DSO low power workhorse. That means the 16-17mm range for the 120mm Evostar. I really think 17-10-6mm eyepiece set would work great for your core collection with this scope. With wide fields, I'd say it's enough. With narrower fields you'd probably want to add a lower power ep, 21-25mm range, for your finder eyepiece. The great thing about the Ethos is, the huge AFOV means I actually need less eyepieces. But everyone has their own approach, it's a personal thing.

[spaceboy]

[quote=casemonster;2252294

I really think the idea should be for me to Join our local astronomical society and have alook through thier scopes and see if my choice of the 5" frac as my next scope is correct. I still have a few months!

I'm sure everyone will agree that's the best advice. Just make sure you find a club that meets up more than once a month as you can almost guarantee club nights will be cloudy Even with cloud someone there could go over collimation with you and you'll be wondering what all the fuss was about. If I'm honest I haven't collimatedmy 200P in several months and even though it could maybe do with a tweak the views are nothing to complain about. The poor seeing I have had lately has had more of an impact on the views than slightly misaligned optics. Despite what everyone says, refractors are not with out there misalignments. If the focuser dosen't run true for example or if the diagonal isn't collimated. Heavy EP's can also introduce error. As with newts you'd be hard pushed to notice any real effect on the image unless the collimation was miles out.

I'm sure what ever decision you make you will be happy with the views you get. As long as you go with a respected brand and not a Seben that is

[spaceboy]

This may be of interest Chris http://stargazerslounge.com/member-equipment-reviews/185144-helios-evostar-120mm-f8-3-first-light-quick-review.html

[casemonster]

Hi Spaceboy,

Excellent spot of the thread.

Obliged as always,

regards

Chris

[catburglar]

I'm late into this thread,but Iwould suspect the difference in image brightness between a 4 inch and a 5 inch scope will not be huge. I know the maths says it willbe 44% brighter, but your eye does not respond in a linear manner. I think the diifference in limiting magnitude will only be around 0.5.

That's not to say that the 120xlt is not a nice scope, but it may not be a big improvement over what you currently have.

I'm not a great fan of newtonians, but collimation is not the reason, and there's no doubt an 8 inch reflector will be a lot brighter than your currentscope (at the same magnification). with an 8 inch F6 dob, along as you don't drop it, onceit's collimated it shouldn't need much more than an occasional tweak.

[spaceboy]

I have always wondered what the big fuss was about collimation then I found out for myself yesterday. I have never had a problem before but I was out of my depth trying to collimate my newly acquired 150PDS. The oversized secondary made it ever so difficult i felt. I don't know if anyone else has found it so with the PDS but it took me at least 10 minutes to figure it out. I was kind of lost to how your supposed to ensure primary clips are equal in the secondary when basically the entire cell is visible. Got there in the end but these scopes must be a nightmare for beginners who have never collimated anything before. I'm just glad you don't have to collimate the secondary very often in newts as for me this would not have been one of my better buys.

[casemonster]

I'm late into this thread,but Iwould suspect the difference in image brightness between a 4 inch and a 5 inch scope will not be huge. I know the maths says it willbe 44% brighter, but your eye does not respond in a linear manner. I think the diifference in limiting magnitude will only be around 0.5.

That's not to say that the 120xlt is not a nice scope, but it may not be a big improvement over what you currently have.

.

Hi Catburglar,

My current scope is a 4" 1650mm focal length f16 frac. I think a 5" 1000mm focal length and an f8.3 will be quite an improvement on what it presently can (or can't see due to being to slow) see in the heavens.

Cheers,

Chris

[casemonster]

I have always wondered what the big fuss was about collimation then I found out for myself yesterday. I have never had a problem before but I was out of my depth trying to collimate my newly acquired 150PDS. The oversized secondary made it ever so difficult i felt. I don't know if anyone else has found it so with the PDS but it took me at least 10 minutes to figure it out. I was kind of lost to how your supposed to ensure primary clips are equal in the secondary when basically the entire cell is visible. Got there in the end but these scopes must be a nightmare for beginners who have never collimated anything before. I'm just glad you don't have to collimate the secondary very often in newts as for me this would not have been one of my better buys.

........told you they more hassle than a refrac!!

[spaceboy]

And so you did :D:D

TBH it was a learning experience and it should take no longer than my other newts next time it needs doing. "He says that now":rolleyes:

May I ask why you feel the shorter FL will improve the views ??

[casemonster]

Sorry, couldn't resist the "told you so" dig. Nothing personal!!

Ok, The more light I pull in, the more I can see and resolve, even though the magnification will be less, the views should be better?

(he sits back in his chair and waits to be shot down in flames)

[spaceboy]

I am assuming you are comparing the f ratio to that of a camera lens ??

[casemonster]

Indeed......the optics are the same are they not. eg I have a 18 -200 mm zoom lens that has a f of 6.3 at 200mm I have a70-200 mm f2.8 across the whole scale. I use the 2.8 every time. There is not enough light hitting the sensor at 6.3 to use it for sports photograhy!!

Thats my take on it.....but I am open to wisdom

[spaceboy]

To a degree a slower scope will offer a "slightly" duller image as yes the light has a longer path to your eye. But as a rule of thumb at any given magnification the exit pupil is the same so in theory there should be no noticeable difference to the brightness of image. So there really would not be a great deal in it between the f/16 and f/8.3 regards the brightness of the object you are viewing. The only difference being the FOV will be larger in the shorter FL EVO. Only increasing the aperture will increase the brightness.

[casemonster]

Ok,

Could you explain exit pupil?

As you used to have one of these you would know that a 5" 1000m evo would have more aperture than 4" 1000mm It must make some appreciative difference to the view otherwise they would not sell any of the 120 model?

regards

Chris

[spaceboy]

Exit Pupil Technical Guide Article by Eagle Optics

EVO 120 = 120mm = 4.75" is the aperture

That is a gain of 20mm .75" over the 100mm 4" aperture scope you already have. Not a lot!

This amounts to the faintest steller magnitude being visible in the 120mm as 13.1 over the 12.7 you should already get.

[catburglar]

For visual use, F ratio does not impact image brightness as it does in photography, it's magnification or exit pupil (the two are related) that makes a difference.

The exit pupil is the disc of light that enters your eye after passing through your telescope optics. If you have a 100mm objective, and you use an eyepiece that gives a magnification of 50x, the exit pupil is 2mm. If you use an eyepiece that gives a magnification of 100x the exit pupil will be 1mm and the image will be dimmer.

What this means in practice is that the same magnification in a larger scope gives a larger exit pupil, however, as noted by myself and space boy, the 0.75 inch increase, results in a very slight increase in the faintest stars visible because the response of the eye (and the stellar magnitude scale) is logarithmic.

I appreciate that it's difficult to get a low magnification and therefore a bright image with a 1600mm focal length, but I didn't want you to think the view through a 120mm will be drastically brighter than what you have already got. If you have or can get a 40mm eyepiece, you will get a magnification of around 40x and an exit pupil of about 2.5mm. For me, this is about the sweet spot of brightness for many DSO's.

If you want to see DSO's better, I would advise a larger scope, and an eyepiece that gives the same exit pupil. In my experience, the change from 80mm to 100 or from 100 to 120 is marginal.

As for the point above about collimation- it's a bit difficult getting it sorted first time, but you can get it pretty close in the daylight, so you are not wasting observing time, and as I said, a solid tube F6 dobsonian should be pretty robust.

If you still want a refractor then the 120 is a decent example, and will give you some nice images. The quality of lenses and the shorter focal length may make it easier to get sharp views than with your current scope,but they just may not be a lot brighter.

[casemonster]

Thanks to you both for the replies and I appreciate your honesty but....Arghhh now I am really confused....So little point in buying an evostar 120 or a Omni XLT 120 as there is nothing in it for an extra £400. I might as well invest in a larger diameter eyepiece for the 4" Irving frac?

Catburglar, what would you suggest as an upgade to what I have, bearing in mind a budget of £400?

Ta, Chris

[catburglar]

Chris,

I'd consider a 6 or 8 inch scope as a minimum, anything less probably won't be much of an upgrade in terms of viewing DSO's, although a smaller scope of higher quality than your current one might improve views of the planets.

As Olly said earlier, an 8 inch dobsonian like this is a great scope. Only problem at present is that they're in short supply. However, there is a Celestron badged 8 inch dob on UK Astrobuysell at the minute which might be worth a look. You could also get a similar scope on an equatorial mount here

If you really want a refractor, then a used one of these will be good on DSO's but less so on planets because of chromatic aberration. It may also be a bit easier to mount than the F8 version.

If it were my £400 and I was looking for a good all round setup, it would be the dob plus a decent 30mm wide angle 2 inch eyepiece like this. This will give you a low power (40x) 5mm exit pupil and about 1.75 degree FoV which will be good for finding stuff. I think the included eyepieces are Plossl design, and should be better than your existing 20 and 12mm Kellner eyepieces. You're 6mm Omni Plossl will be a good high mag eyepiece for planets.

Just to let you know, I've had a couple of newtonians in the past, prefer refractors just like you, and now have a Mak because for me it's the best compromise between convenience and light gathering. But that doesn't alter the fact that the Newtonian is the CHEAPEST way to get MOST light into your eye.

Sorry if it's not what you wanted to hear.