Thinking of a second scope

[Jimtheslim]

Morning all, it's been a while.

Now my garden is starting to dry out and I'm thinking of getting back out in the evenings for a look around the night sky I was considering upgrading from my Evostar 90 refractor. Question is would something like a 120 refractor or a 250 reflector make a really noticeable difference? I've been having a look at a couple of Skywatcher 120 refractor OTA's which come with rings and the dovetail mounting rail. Would this fit on to my EQ2 mount that came with my evostar or would it require a more substantial mount (I note that these scopes, when sold with a mount are usually much larger)?

Does anyone have any thoughts/opinions on the Skywatcher Evostar 120 refractor or the Skyliner 250p rigid tube dob reflector?

I don't take photos and have come to realise that an EQ mount isn't entirely necessary for 'having a look around'

Thanks.

[Peter Drew]

A 250mm Dobsonian would make more than a noticeable difference to the performance of your 90mm Evostar and would be a good partner to it. A 120mm refractor would be better than a 90mm but would not represent such an upgrade.

[Fozzie]

And you would defo need a new mount with the Evostar 120, the EQ2 wouldn't cut it at all..

+1 for the 250 Dob, but go and have a look at one first.. fairly big units if your used to an EVO 90, which would make a great grab and go scope if you decided to keep it!

Ta

Fozzie

[Neil27]

Hi Jim,

If your purely visual and not interested in imaging at this point, then I would echo Peters sentiments and go for a 250mm dobsonian  - it would be very quick to set up and also get you 'having a look around' without too much hassle.

The suggestion of upgrade from your 90mm to 120mm refractor aperture although noticeable would not be as obvious I think.

The dobsonians are great as you just nudge them from object to object, pound for pound they are great value.

Hope that helps,

[Stu]

250mm dob would be very nice, but large. A 120ED would be a decent step up in quality of views but obviously would not have the ultimate resolution of the dob. It would also be nice for solar with a wedge. A more stable mount would be needed so the dob is probably the cheaper option.

What sort of budget do you have?

 

[Knighty2112]

I have a Starwatcher Startravel ST120 refractor on an AZ3 mount, which makes it still portable enough to pick it up and take it outside in one go. Get some excellent views from it. Saw the GRS on Jupiter last night for the first time since clear skies and seeing allowed me to observe Jupiter well since I got the scope about 6 months ago now (viewed with a 4.5mm Planetary EP at x133).

Hoping to get to a darker site at some point in the next few months, but even from my light polluted backyard I get some great views of the sky through it. In my personal case I wouldn't go with the 250 don for size & storage reasons, plus I really don't like to have to collimate a reflector at all. Can have my scope up and running in about a minute from picking it up inside the house & then placing it down to use in my dark corner of the garden.

[Jimtheslim]

Thanks all, 

I was leaning toward the dob, just for something a bit different (I like a challenge) and ease of use once it's up (ie being able to just nudge it around rather than faff on with alignment and micro controls). I guess it would also allow me to keep the 90 frac for travel. Problem is I have a fair bit of LP when viewing in my garden, especially toward the East and would love to be able to take something a bit bigger to a darker sky site. Seems to be that the difference would be far more noticeable with a 250dob rather than a 120 frac. 

I read somewhere that a frac is about equivalent to a reflector of twice the aperture, I'm guessing there are a few more factors that come into this but if it's generally true then I'm guessing a 200 reflector isn't going to be a noticeable difference? I recall ages ago there was a thread where someone had a link to a site which offered examples of how objects look through various scopes. Anyone know where this is?

[Jimtheslim]

The other thing that drives me crazy is not being able to look straight above with the frac, would the dob allow much easier viewing of objects overhead where they are less tarnished by LP?

[Jimtheslim]

1 hour ago, Stu said:

250mm dob would be very nice, but large. A 120ED would be a decent step up in quality of views but obviously would not have the ultimate resolution of the dob. It would also be nice for solar with a wedge. A more stable mount would be needed so the dob is probably the cheaper option.

What sort of budget do you have?

 

As for budget, I could probably stretch to the £400 or so for a 250p dob, I might have to look at selling my 90 frac in order to budget and make physical space for something as big as a 250

[nightfisher]

Go for the 200 dob, in budget and still portable in a car

 

[Ricochet]

1 hour ago, Jimtheslim said:

I read somewhere that a frac is about equivalent to a reflector of twice the aperture, I'm guessing there are a few more factors that come into this but if it's generally true then I'm guessing a 200 reflector isn't going to be a noticeable difference? I recall ages ago there was a thread where someone had a link to a site which offered examples of how objects look through various scopes. Anyone know where this is?

I would suggest that whoever wrote that was either trying to excuse buying an expensive apochromat or got their calculations wrong. If you assume a significant secondary obstruction and 90% reflectivity mirrors for a 200mm Newt but 100% light throughput for the frac and its diagonal you need a 170mm frac. Assume the frac mirror is also 90% and you need 180mm which is 2X the aperture and hence 4X the light gathering potential of your current 90mm frac so I think you will see a difference.

As for the website did you mean this? http://astronomy.tools/calculators/field_of_view/

[mikeDnight]

1 hour ago, Jimtheslim said:

Thanks all, 

I was leaning toward the dob, just for something a bit different (I like a challenge) and ease of use once it's up (ie being able to just nudge it around rather than faff on with alignment and micro controls). I guess it would also allow me to keep the 90 frac for travel. Problem is I have a fair bit of LP when viewing in my garden, especially toward the East and would love to be able to take something a bit bigger to a darker sky site. Seems to be that the difference would be far more noticeable with a 250dob rather than a 120 frac. 

I read somewhere that a frac is about equivalent to a reflector of twice the aperture, I'm guessing there are a few more factors that come into this but if it's generally true then I'm guessing a 200 reflector isn't going to be a noticeable difference? I recall ages ago there was a thread where someone had a link to a site which offered examples of how objects look through various scopes. Anyone know where this is?

It is often claimed that a 3" refractor is the equivalent of a 6" reflector, but it's not really so. You'll often read such statements as that in books by Patrick Moore. It's good to remember that many reflectors back then we're home made using silvered mirrors, or made of rapidly tarnishing speculum metal. Most reflectors today however are of a reasonably high optical standard by comparison to many of the past, and that's one reason reflectors have acquired a reputation for being second rate. In reality a good modern reflector of 6" or above will keep an intelligent person occupied for a lifetime.

However, telescopes of different designs have different qualities, each a specialist in its own right. The real beauty of a Newtonian for example is its ability to offer significant light grasp, drawing in those dim DSO's that even the best apo refractors would struggle to detect visually. The Newtonian will also give excellent lunar and planetary views, but most modern Newts are short and this affects their off axis performance. A long focal length Newtonian of around F10 is a formidable planetary performer. They are big though, and this is where a good apo refractor can come into its own.

As a true jack of all trades, a good ED apochromat will give a sharper wider field of view than a Newt and because of its ability to offer high contrast  views, it gives wonderful views of brighter DSO's. As a lunar and planetary scope I would back a good 4" apo against any off the shelf Newtonian upto 10" aperture because of its superior definition. I know many would disagree with my view but that's likely because the greatest variant in any optical system is the observer himself. It's best to kit yourself out with what suits you best and not what suits another person, or to go with popular opinion.

Mike

[LukeSkywatcher]

200mm from 90mm is a lovely big jump with huge difference noticable. 120mm from 90mm not that much of a difference really.

[Jimtheslim]

1 hour ago, Ricochet said:

I would suggest that whoever wrote that was either trying to excuse buying an expensive apochromat or got their calculations wrong. If you assume a significant secondary obstruction and 90% reflectivity mirrors for a 200mm Newt but 100% light throughput for the frac and its diagonal you need a 170mm frac. Assume the frac mirror is also 90% and you need 180mm which is 2X the aperture and hence 4X the light gathering potential of your current 90mm frac so I think you will see a difference.

As for the website did you mean this? http://astronomy.tools/calculators/field_of_view/

it wasn't this specific site but very similar. Looks like there isn't a lot of difference between 200 and 250 when viewing planets, these and the moon would be what I would spend most my time on

[brantuk]

You may have been told about this site for fov calculations: http://www.12dstring.me.uk/fovcalc.php

I find it's great for a general idea of how stuff looks with different eyepieces, scopes, and cameras. Just remember you won't see colour in the eyepiece - just black, white, and shades of grey.

If you're going for a dob - get the largest within budget that is manageable and portable for you. I'd go with a 250P - but that's just me lol

[Jimtheslim]

Yes it's a good site that one, again it's not the one I saw originally but offers a seemingly more realistic reoresentation

[John]

I have a 102mm ED refractor, a 120mm ED refractor and a 12" dob. On deep space there is obsolutely no contest - the 12" dob wins hands down entirely as you would expect (of course !). For the largest deep sky objects though, the 102mm ED refractor can show 3.8 degrees of sky so it can show views that either of the other scopes just can't manage.

On the moon and planets the 12" dob does show more detail and allows higher magnifications to be used but it's not as far ahead of the 120mm ED refractor as you might expect given the pretty massive aperture difference. Under mediocre conditions the refractors probably give more pleasing views of planetary targets in some ways, even though their ultimate resolution is lower.

The 12" dob is a decent one by the way with F/5.3 Orion Optics primary of around 1/9th wave PV and .987 strehl. The refractors do seem to "punch above their weight" so to speak. The ED120 was showing Jupiter as well as a number of 8" SCT's at a star party last night although I can't vouch for the state of collimation of the SCT's !

Although the 12" dob will out resolve the refractors on double stars, the presentation of the star images with the refractors in terms of the text book definition of the airy disk and diffraction ring is preferrable to my eyes so I use those scopes for that purpose.

It's quite complicated isn't it !

 

[Jimtheslim]

2 hours ago, John said:

 

It's quite complicated isn't it !

 

I'll say. Every time I think I have made up my mind I seem to change it depending on what I happen to read. 

One hesitancy I have about these dobs is that my garden isn't 100% flat, the best views being from my lawn which is very soft and in places uneven. No problem for my current tripod but I worry about there being too much tilt with the weight of a massive dob on a flat base 

[DRT]

I have spent the last four years buying and selling (perhaps more buying than selling!) telescopes of different types and sizes to find out what I like. I have a garden lawn that is best described as "lumpy" with a flat patio at either end. I have no view to the north, very limited view to the east and reasonable views to the south and west over roof tops (so nothing close to the horizon).

I have owned the Skywatcher 200P, 300P Flextube and (currently) the 400P Flextube. The 200 and 300 were fantastic scopes but I sold them simply because I found it difficult to find enough sky for them to look at because they could only really be used on the patios at the east and west ends of the garden. Anything below about 50 degrees from the horizon just could not be seen. The 400P has similar issues but I do not use it as my main scope and can live with that limitation to have fabulous views of limited targets or use it at a star party.

I have probably owned around 15 refractors, including the Evostar 120 that you mentioned which would definitely be too big for your EQ2. I had it on an EQ5 and the mount was barely up to the job. The best refractor I have owned is the Skywatcher 120 ED, but that is way out of your budget even if you find one second hand. You could buy a very good second hand 100 ED for £400 or slightly less. It is a fantastic scope and might even sit on your EQ2 until you can afford to upgrade that too.

As John says this is a complicated thing and one scope is unlikely to fill all your needs. On balance I would suggest that you pick up a second hand 200P Dob for around £200 to see how you get on with it. You will easily get your money back on the second hand market as these scopes are very popular.