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Old 05-06-2014, 09:39 AM
209herschel (Herschel)
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8" or 10" Dob - eyepieces, collimation

Hi everyone,

I've read quite a few posts on people's advice on buying an 8" or 10" dob. I'm in the same boat. I just had a few questions I haven't been able to answer. Thanks in advance for your time.

1. I can't afford great eyepieces. So I'll be relying on the GSO 30mm super for quite a while and the standard plossls that come with the GSO dobs from Andrews. Will the 8" be far more forgiving than the 10" with these eyepieces? And is this the case with eyepieces generally? I'm also looking to buy a Baader hyperion 5mm because I'm keen to get great views of the moon and planets (will hopefully dazzle my son!). But I'm reading that it will again be better on the 8" than 10". Is aberration significantly different on the f5 10" versus the f6 8"?

2. Is the extra aperture so significant that I'd take on board the more sensitive collimation requirements and the more sensitive aberrations in eyepieces in the 10"? Sorry if I'm getting some of the terms wrong - quite new to this.

3. I'm aiming to make it out to dark sites occasionally but most of my viewing will be in my backyard, inner west suburb of Sydney. I can see the little star in the southern cross pretty easily without any adjustment time for my eyes so I'm hoping light pollution isn't too bad. Any advice on whether you'd go for the 10" over the 8" given these concerns would be greatly appreciated. The 8" from Andrews is $449 (seems an amazing deal) and the 10" is $649. I wish is was $600! Both come with the 30mm super, 9mm, 15mm and 24mm plossls.

Thanks again.
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Old 05-06-2014, 10:24 AM
astro744
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You're going to get off axis coma in both telescopes but it will be more obvious at f5. Coma is a linear aberration and a function of a paraboloidal mirror and can be eliminated/reduced with a coma corrector. It will be more noticeable with the lower power eyepieces as they have a larger field stop.

However, off axis coma may not bother you and to actually see pure coma and nothing else you need a highly corrected eyepiece such as a Nagler. Other less well corrected eyepieces will likely exhibit coma, astigmatism, field curvature all mixed in together.

Now before others step in and advise against recommending such expensive eyepieces I would like to say that I am merely letting you know what a Nagler will do for you, (and not do i.e. eliminate primary mirror coma).

You are likely not going to be bothered by the off axis coma at f5 and if you can afford the larger aperture then go for it. You can always add a coma corrector later if it really bothers you.

I actually like seeing pure coma through a Nagler at the edge of field and there are no other aberrations and no to very minimal field curvature depending on the telescope used. What I don't like is significant field curvature in the outer 30% of field and I hear it said so many times about low cost eyepieces that "this eyepiece is great but the outer 30% is out of focus". You may as well buy a Plossl with a smaller apparent field than have a wide angle with 30% out of focus. If stars are blurry due to field curvature you have no chance of spotting a galaxy off axis.

Collimation will be a little more sensitive at f5 but once you learn the right technique it is easy to do.

Last edited by astro744; 05-06-2014 at 11:47 AM. Reason: Added 'primary mirror' to i.e. do not eliminate coma
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Old 05-06-2014, 10:37 AM
209herschel (Herschel)
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Originally Posted by astro744 View Post
You're going to get off axis coma in both telescopes but it will be more obvious at f5. Coma is a linear aberration and a function of a paraboloidal mirror and can be eliminated/reduced with a coma corrector. It will be more noticeable with the lower power eyepieces as they have a larger field stop.

However, off axis coma may not bother you and to actually see pure coma and nothing else you need a highly corrected eyepiece such as a Nagler. Other less well corrected eyepieces will likely exhibit coma, astigmatism, field curvature all mixed in together.

Now before others step in and advise against recommending such expensive eyepieces I would like to say that I am merely letting you know what a Nagler will do for you, (and not do i.e. eliminate coma).

You are likely not going to be bothered by the off axis coma at f5 and if you can afford the larger aperture then go for it. You can always add a coma corrector later if it really bothers you.

I actually like seeing pure coma through a Nagler at the edge of field and there are no other aberrations and no to very minimal field curvature depending on the telescope used. What I don't like is significant field curvature in the outer 30% of field and I hear it said so many times about low cost eyepieces that "this eyepiece is great but the outer 30% is out of focus". You may as well buy a Plossl with a smaller apparent field than have a wide angle with 30% out of focus. If stars are blurry due to field curvature you have no chance of spotting a galaxy off axis.

Collimation will be a little more sensitive at f5 but once you learn the right technique it is easy to do.
Thanks very much for this advice. So you think there won't be that much difference between the 8" and 10" when it comes to using the eyepieces that come with it, the GSO plossls? I'm actually looking forward to the challenge of collimation, having read a lot of stuff. As a side question, I've been thinking a cheshire collimator is all I need? Some people say the laser collimator often causes more trouble? I guess the extra 2" aperture is valuable in the long term - I just wish it wasn't an extra $200. Thanks again.
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Old 05-06-2014, 11:03 AM
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Camelopardalis (Dunk)
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Regarding the aperture...larger aperture is usually a good thing, but in the inner west of Sydney I'm not convinced you'd see much difference however, at a dark site the difference would be more noticeable on the fainter objects. Whether it is worth the extra $200 is something only you can decide.

But keep your eye out in the Star parties section as a group of us regularly meet up around New Moon in the Blue Mountains, and there are other meets in and around the Sydney area. Depending on who/what scopes are present, you may be able to try before you buy...
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Old 05-06-2014, 11:37 AM
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mental4astro (Alexander)
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As astro744 says, you will see coma through both those scopes. BUT, coma is the least of the problematic aberrations that will be seen. Coma is actually a very minimal aberration in terms of observing. The ultimate quality of the eyepieces you use will ultimately dictate just how much coma you see. And as astro744 also says, coma is not a big deal. It also does not bother me, and my newtonians go down to f/4 where coma is much more prominent than at f/5 or f/6. With the eyepieces I have, all the other aberrations are well controlled, so a little coma is insignificant for me. There are many people who use 'coma correctors', which is fine too.

Eyepiece quality, like I said is more important. All too little is ever said that not all eyepieces work equally well in all scopes. While this discussion could get very technical, it is enough to say that an eyepiece that gives a cracker jack image in one scope, say a Newtonian, the same eyepiece will quite literally be unusable in an SCT, and vise versa. I'll mention more about eyepieces a little further down.

In astronomy, the whole thing is about gathering as much light as possible. So bigger is better in terms of aperture. While there may not seem too much difference between 8" and 10", it is a function of area, and the difference between the two is significant. Yes, collimation is more critical in faster scopes than slower, f/5 compared to f/6 respectively, but collimation is NOT complicated. It only sounds that way because it is an unfamiliar word. It is only a process of aligning the optics, and once you get your head around the idea and process, it is a task you actually end up doing automatically as the image quality benefit is well worth the 2min it takes!

So, an 8" or 10"? In an ideal world I would say the 10". The difference size wise between the two is the 10" is only a little 'fatter'. Both are not difficult scopes to move around and transport. The 10" costs $200 more - but you are forgetting it costs more to produce. It might be made in China or Taiwan, but the material costs are not insignificant, even to the Chinese. Don't forget, these are also the cheap end of the 'good' quality market. An excellent quality instrument will cost a whole lot more.

There is the adage "you get what you pay for". But, I am also a believer that there some real gems to be had in the less expensive end of equipment. Not perfect, but cracker jack good for their price. The 30mm Superview is one! I still have mine. It was my first 2" eyepiece, and it is an excellent eyepiece to use first up. While it is not my first preference eyepiece any more (it was for 3 years), I keep it as it is an excellent and very easy eyepiece for novices to use. Yes, there are eyepeices that are easier to use than others. While not perfect, for some people they only have the 30mm Superview as their low power eyepiece.

A 5mm Hyperion is another excellent eyepiece. The Hyperion range is primarily designed for use in Schmidt Cassegrain Telescopes (SCT), and are a poor optical match to Newtonians. BUT, the exception is the 5mm Hyperion! I too have this eyepiece, and it lives permanently in my eyepiece case, I use it routinely for my high power lunar and planetary sketches. In my fast f/4 8" Newt dobbie, I pushed it to 400X on a night of very good conditions, and it was very sharp across the whole field. I used it to do my best sketch of Saturn that night. If you have an SCT, the entire range of Hyperions will be excellent. In anything else, like a Newtonian, then only the 5mm will work well.

Another thing about eyepieces, eyepiece design as come a long, long way in 30 years. What you will find with plossls, while they are a good optical match with Newtonians, because they are old technology (over 50years actually), as the focal length shortens, the eye lens (the lens you look through) becomes smaller and smaller, and your eye needs to get closer and closer to see into them. Plossls have become very cheap to produce today as manufacturing processes have improved. But new eyepiece designs make use of more exotic glass types that allow for much bigger eye lenses, and for more comfortable use. There is a world of difference between a 6mm plossl and a 5mm Hyperion.

Collimation: While I've been using Newtonians for over 20 years, the collimation process had been poorly explained to me up until a year ago. Laser collimation tools only do part of the work involved with properly collimating optics. The are good only for the primary mirror. For the secondary, they do only part of the job, and certainly not the 80% that they miss. A Cheshire eyepiece will do the entire collimation job of a Newtonian, but a Cheshire alone is a real pain to use. Now, using both a Cheshire and a laser, then we really begin to cruise. Remember, collimating optics is not a big hairy monster. It is not a difficult thing to do, and will become second nature. Do I collimate all the time? No. At home because the scopes are not being transported very far, I might do a laser check every second time, and a full check every 6 or 8th time. But, when I go bush, yes I check every time when I set up. Things do move, and the image quality that one gets is well worth the effort,

As you are in Sydney, I'd be happy to show you how to make the most of your scope, be it an 8" or 10" you get. It makes things a whole lot easier if someone can show you than the confusion that can happen otherwise. Just let me know. I'm in Maroubra.

Mental.
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Old 05-06-2014, 11:37 AM
N1 (Mirko)
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Herschel,

Good decision to get a dob. I got the GSO 10" not too long ago and I'm loving it. I can't really comment on how that compares with the 8", but it's the sweet spot of aperture vs portability at the larger end, at least for me. I do not put aperture above all else, so I could see the 8" working for somebody else who was in a different situation to me. Advice I had sought on this forum beforehand was spot-on. I also got this tool from Bintel to collimate it:

http://www.bintel.com.au/Accessories...oductview.aspx

Once you figure out what's what when you look through it, it's actually straight forward. The device itself is remarkably simple and built like a tank. I would recommend getting the knurled collimating knobs for the secondary. They were the only mod I felt was needed. Bintel sells those as well. The philips head screws were just a pain. The primary knobs & locking screws are sufficient IMO and do not need replacing. I understand they were a recent addition by GSO.

In terms of eye pieces, I was fortunate enough to already own a few premium ones, and these give unbelievable views in the dob, despite a bit of coma (which I find perfectly acceptable). However, I think the GSO 30, 24, and maybe even the 15 will keep you happy for a while. Personally I did not like the 9mm, it was crap TBH and I have no use for it. Could be a bad sample though.

I intend to fill the gaps in my eyepiece lineup with some ES samples later on. I have looked through quite a few and found them to be excellent value.

Anyway enjoy your scope once you get it out under the stars. Inner West sounds worse than it is. I know I'm a bit further out, but depending on where you are, you may actually get quite reasonable views.

OK back to work, lunch break nearly over..

Edit: Alex, interesting read as always. 5mm Hyperion has been added to my wish list

Last edited by N1; 05-06-2014 at 11:53 AM.
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Old 05-06-2014, 12:05 PM
209herschel (Herschel)
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Originally Posted by mental4astro View Post
As astro744 says, you will see coma through both those scopes. BUT, coma is the least of the problematic aberrations that will be seen. Coma is actually a very minimal aberration in terms of observing. The ultimate quality of the eyepieces you use will ultimately dictate just how much coma you see. And as astro744 also says, coma is not a big deal. It also does not bother me, and my newtonians go down to f/4 where coma is much more prominent than at f/5 or f/6. With the eyepieces I have, all the other aberrations are well controlled, so a little coma is insignificant for me. There are many people who use 'coma correctors', which is fine too.

Eyepiece quality, like I said is more important. All too little is ever said that not all eyepieces work equally well in all scopes. While this discussion could get very technical, it is enough to say that an eyepiece that gives a cracker jack image in one scope, say a Newtonian, the same eyepiece will quite literally be unusable in an SCT, and vise versa. I'll mention more about eyepieces a little further down.

In astronomy, the whole thing is about gathering as much light as possible. So bigger is better in terms of aperture. While there may not seem too much difference between 8" and 10", it is a function of area, and the difference between the two is significant. Yes, collimation is more critical in faster scopes than slower, f/5 compared to f/6 respectively, but collimation is NOT complicated. It only sounds that way because it is an unfamiliar word. It is only a process of aligning the optics, and once you get your head around the idea and process, it is a task you actually end up doing automatically as the image quality benefit is well worth the 2min it takes!

So, an 8" or 10"? In an ideal world I would say the 10". The difference size wise between the two is the 10" is only a little 'fatter'. Both are not difficult scopes to move around and transport. The 10" costs $200 more - but you are forgetting it costs more to produce. It might be made in China or Taiwan, but the material costs are not insignificant, even to the Chinese. Don't forget, these are also the cheap end of the 'good' quality market. An excellent quality instrument will cost a whole lot more.

There is the adage "you get what you pay for". But, I am also a believer that there some real gems to be had in the less expensive end of equipment. Not perfect, but cracker jack good for their price. The 30mm Superview is one! I still have mine. It was my first 2" eyepiece, and it is an excellent eyepiece to use first up. While it is not my first preference eyepiece any more (it was for 3 years), I keep it as it is an excellent and very easy eyepiece for novices to use. Yes, there are eyepeices that are easier to use than others. While not perfect, for some people they only have the 30mm Superview as their low power eyepiece.

A 5mm Hyperion is another excellent eyepiece. The Hyperion range is primarily designed for use in Schmidt Cassegrain Telescopes (SCT), and are a poor optical match to Newtonians. BUT, the exception is the 5mm Hyperion! I too have this eyepiece, and it lives permanently in my eyepiece case, I use it routinely for my high power lunar and planetary sketches. In my fast f/4 8" Newt dobbie, I pushed it to 400X on a night of very good conditions, and it was very sharp across the whole field. I used it to do my best sketch of Saturn that night. If you have an SCT, the entire range of Hyperions will be excellent. In anything else, like a Newtonian, then only the 5mm will work well.

Another thing about eyepieces, eyepiece design as come a long, long way in 30 years. What you will find with plossls, while they are a good optical match with Newtonians, because they are old technology (over 50years actually), as the focal length shortens, the eye lens (the lens you look through) becomes smaller and smaller, and your eye needs to get closer and closer to see into them. Plossls have become very cheap to produce today as manufacturing processes have improved. But new eyepiece designs make use of more exotic glass types that allow for much bigger eye lenses, and for more comfortable use. There is a world of difference between a 6mm plossl and a 5mm Hyperion.

Collimation: While I've been using Newtonians for over 20 years, the collimation process had been poorly explained to me up until a year ago. Laser collimation tools only do part of the work involved with properly collimating optics. The are good only for the primary mirror. For the secondary, they do only part of the job, and certainly not the 80% that they miss. A Cheshire eyepiece will do the entire collimation job of a Newtonian, but a Cheshire alone is a real pain to use. Now, using both a Cheshire and a laser, then we really begin to cruise. Remember, collimating optics is not a big hairy monster. It is not a difficult thing to do, and will become second nature. Do I collimate all the time? No. At home because the scopes are not being transported very far, I might do a laser check every second time, and a full check every 6 or 8th time. But, when I go bush, yes I check every time when I set up. Things do move, and the image quality that one gets is well worth the effort,

As you are in Sydney, I'd be happy to show you how to make the most of your scope, be it an 8" or 10" you get. It makes things a whole lot easier if someone can show you than the confusion that can happen otherwise. Just let me know. I'm in Maroubra.

Mental.
That is incredible information and I really want to thank you for all of this information. I've learned more from the few replies this morning than from my varied online reading!

It seems the 10" is the one. I'm also concerned that if I get the 8", I'll always be left wondering... I'll aim to get the Hyperion and make use of the eyepieces that come with the scope. I wish cost wasn't an issue because I'd pick up a laser and cheshire collimator at the same time! But if I had to choose, which would you recommend? I was going to pick one up when I buy the scope.

I'll definitely take you up on that offer once I've got the scope and I've learned my way around it a little bit! It's very kind of you. On another note, are there places in Sydney to go that are a little darker but not too far away? I've looked at a couple of astronomy club sites and it looks like their dark sites are about 90min away on average. I've been googling to find if there are places one can just drive to just privately and use a scope outside of the city areas? Eg, is a place like Maroubra, say near the beach where there may be less light, definitely better than a Marrickville backyard?

One last question! What brands are really good dobsonians? Just so I can take a look at what's actually out there.

Thanks so much!
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Old 05-06-2014, 01:57 PM
209herschel (Herschel)
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Herschel,

Good decision to get a dob. I got the GSO 10" not too long ago and I'm loving it. I can't really comment on how that compares with the 8", but it's the sweet spot of aperture vs portability at the larger end, at least for me. I do not put aperture above all else, so I could see the 8" working for somebody else who was in a different situation to me. Advice I had sought on this forum beforehand was spot-on. I also got this tool from Bintel to collimate it:

http://www.bintel.com.au/Accessories...oductview.aspx

Once you figure out what's what when you look through it, it's actually straight forward. The device itself is remarkably simple and built like a tank. I would recommend getting the knurled collimating knobs for the secondary. They were the only mod I felt was needed. Bintel sells those as well. The philips head screws were just a pain. The primary knobs & locking screws are sufficient IMO and do not need replacing. I understand they were a recent addition by GSO.

In terms of eye pieces, I was fortunate enough to already own a few premium ones, and these give unbelievable views in the dob, despite a bit of coma (which I find perfectly acceptable). However, I think the GSO 30, 24, and maybe even the 15 will keep you happy for a while. Personally I did not like the 9mm, it was crap TBH and I have no use for it. Could be a bad sample though.

I intend to fill the gaps in my eyepiece lineup with some ES samples later on. I have looked through quite a few and found them to be excellent value.

Anyway enjoy your scope once you get it out under the stars. Inner West sounds worse than it is. I know I'm a bit further out, but depending on where you are, you may actually get quite reasonable views.

OK back to work, lunch break nearly over..

Edit: Alex, interesting read as always. 5mm Hyperion has been added to my wish list

Thanks very much for this information. I found some ES eyepieces at this site: http://www.vtioptics.com.au/ They've got a 6.7mm for $189 delivered. Is that the sought of eyepiece that would be great for moon and planetary viewing? One other question I had was about buying second hand eyepieces. I've read a couple of things saying that eye lash oil, smudges on the lenses need to be professionally cleaned. Is that true - you can't just clean an eyepiece easily? Thanks again. I asked in another post if anyone knows of places close to the inner west where one can take a scope for a sky that's a bit darker? A place not far away (say within 30min) but that's pretty dark - at least better than an inner west back yard! Thanks again.
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Old 05-06-2014, 04:46 PM
N1 (Mirko)
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I use a 7mm in my 10" GSO for high-ish power. Most nights, this either works or is very close to being acceptable. I also have a 3.5mm, which on most nights does not work. So an EP that gives you about 150-180x isn't a bad choice. I have heard that the eye relief on the 82° ES series can be a bit tight, at least when you want to take in the whole AFOV. Should be OK though if no glasses are involved. Perhaps someone who owns one can say whether this applies to the 6.7 and how "bad" it really is. Also seeing the whole AFOV becomes less important as the exit pupil shrinks, since the darker image prevents the "immersion experience" from happening, IMHO. But a large AFOV is handy at high mag because you have to nudge the tube less often.

Eyepiece cleaning - there are some good threads on this forum on the subject. First time I've heard that they need to be cleaned professionally . You will find that simple and readily-available cleaning products are just fine. Think camera lens cleaning. I use isopropyl alcohol on mine, no probs so far.

Observing site. The inner parts of town are pretty evenly polluted, so to speak. I would think it makes little difference travelling 30mins in any direction. In saying that, Sydney is actually pretty good compared to some other cities I've observed from. Alex might know more on this. The man knows his stuff. When observing from Dunedin, I find the "Town Belt" (a park) to be acceptable if I don't want to travel. So a park or some other green patch near where you are might give a slight improvement. It helps with the seeing too.

The MacArthur Astro society has an observing site available for members and their guests at The Oaks, about 1hr from where you are. No sure about their policies & conditions of use. Check

http://www.macastro.org.au/mas/

Cheers

Edit: The ES that interest me most are the 68°. The one that impressed me most was the 30mm 82° though. 100° Series is great, but probably need a coma corrector in an f/5 scope. The shorter 82° I have not looked through yet.

Had a look around. Adrian (AG Hybrid) might be the man to talk to re the 6.7 ES

Last edited by N1; 05-06-2014 at 06:24 PM.
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Old 06-06-2014, 07:24 PM
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mental4astro (Alexander)
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Lens coatings these days are very tough. It takes a lot to damage them. But, it is never a good idea to be always cleaning them as the cleaning process can ultimately see damage happen. To clean them, isopropyl alcohol is excellent. Not the 'rubbing alcohol' as it is denatured and will leave residue when it dries. Neat isopropyl can be found in hardware stores in little spray bottles where you find the metho. On a slight tangent, I keep a set of eyepieces just for public star viewings for this very purpose. I really don't want mascara, make-up, lolly juice, chip fat, finger prints, and God knows what else on my good eyepieces.

I could jabber on a whole lot more on eyepieces, but all that I'll add is use the eyepieces that come with the scope. While plossls these days are cheap, and they have some short comings, they are still a good optical match with Newtonians. You will learn a lot about how to handle your new instrument, probably more than if you had other eyepieces.

Dark sites around Sydney - now that's a tough one. Trick is to find a place that has minimal affects from dew, safe, and protected from wind. There are a few of us that have been looking around for such places that are not private land for a very long time, all within 2 hours. Not easy. Yes there are clubs that have properties. The Macarthur Astronomical Society is a great mob to join - I'm a member there too. They have two sites, one just outside Campbelltown, and one further out that is darker. The ASNSW has two properties too, one closer at Mt Bowen, and their dark site at Ilford 3.5hrs away. There is also a club Turramarra way. Informal gatherings happen at Mangrove Mountain's Pony Club. We also do informal sessions at Katoomba Airfield. The Airfield is also at 1000m above sea level and we've never had dew! We haven't had a session at either one of the latter two for some time due to the constant crap weather each and every New Moon period for months now. The reason for the two hours drive is it gives really dark conditions that is still relatively close. Other sites we use when the Airfield isn't available is Hargraves Lookout and Mt Blackheath Lookout. Both places are open to the public and you should always go with a mate.
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