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6" vs 8"


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Hi all, as the title suggest im just stuck deciding whether or not its worth getting a 8 inch. Im looking at the sw explorer range. Gonna try and get a second hand eq5. So the 150 would be pretty good on that mount. Mainly interested in DSO, clusters etc But im a bit worried that 6 months down the line i'll be pretty dissapointed with the 6 inch. Do those 2 inches really make that much of a difference?

Comments/thoughts welcome :)

Many thanks

Drew

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Two inchs is a noticeable change but people do tend to suggest when upgrading to jump up two places. i.e with your 6 inch jump to a 10 inch, for a 8 inch jump to a 12 inch.

This will give a much more noticeable improvement :)

From what I have read eight inches and upwards is when DSO's really start to shine :)

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If your interests are with DSO's and stay that way then I reckon you will contract "aperture" fever quite quickly as there is simply no substitute for it when it comes to viewing galaxies, nebulae etc.

But then again you could use the 6" to "hone your skills" while you save for more aperture. A 6" scope can show you a lot if you can get it under dark skies as we found at the SGL6 star party earlier this year :)

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> Do those 2 inches really make that much of a difference?

Yes.

You will see all kinds of silly marketing nonsense about "Gathering 78 percent more light" and so-on, and whilst this is technically true - and you can have fun juggling the numbers about - it's actually not very helpful in the practical sense.

So, to cut to the chase, here's the simplest way of looking at it. Just use the scopes' apertures in millimetres (150mm and 200mm) and you can confidently state that an 8" scope magnifies things at 200x with the same quality and brightness that a 6" shows things at 150x

- it's really that simple.

(well - "all things being equal" as they say... :) )

Edited by great_bear
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Oh - I would concur with the others above however, inasmuch as for Deep Sky Observing (nebulae, galaxies etc.) an owner of a 6" scope should probably make a much bigger jump (to 10" as suggested) to make an "upgrade" worthwhile.

But in this case, I gather that you're not upgrading - you're picking what to start with?

If so, I'd say that the sole reason for picking a 6" over an 8" would be because you thought the 8" too physically big (or expensive) for you to cope with. As is always the case in such matters, it's all about how far and how often you'll be lugging it around under awkward conditions.

If it's a stay-at-home scope that you're simply wheeling into position each night then - well - get the biggest thing you can push about! :)

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The best thing is to get the biggest scope you can afford, move around and store, ie that's practical for your circumstances. There is a difference between the two but anybody new to observing wouldn't really notice it without a direct comparison. That is, your first impressions wouldn't be all that different between the two. I used a 6" for a couple of years and was still finding different things.

I suppose what I'm trying to say is that yes the 8" is better but if the 6" is more practical then you will end up seeing alot more with a 6". As stated above the extra light gathering power stated by manufacturers is very misleading. Hope this makes sense!

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wow! Awesome responses already! Thanks all! Pretty much what I expected. I think I might just take the plunge and get the 8 inch not till later in the year though so plenty of time to change my mind!!!

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Hi Drew

It's true that for DS observing that aperture is king.

It is also true that an 8" is a more powerful scope than a 6".

However if DS observing is your thing you will want a larger scope after a year or so regardless of which you pick.

DS observing and aperture fever are closely linked:D

For those reasons either is a good call. I personally would go with the 6" as then my next upgrade would see a bigger improvement:)

Good luck with your choice.

Regards Steve

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Also, a 6" scope will out perform an 8" if the 6" is under dark clear skies and the 8" is in a town suburb, it would probably outperform a 10" if the 10" was in a large city (unless filtered of course). So location is also a factor, but under the same location an 8" should give you brighter images.

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I started with a 6", which was nice, then borrowed an 8", which was very nice, and now have a 10", which is better again and is the biggest scope I can practically store and use.

The jump between 6" and 8" is the biggest jump between any two standard aperture sizes but 6" to 10" is a real wow jump.

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had a 150p, now have a 200p. i couldnt possibly comment on viewing differences as its been so bleedin cloudy lately and i didnt get a chance to use the 150 much, tho i used a 130 before and going from 5 to 8 made a big difference for saturn :)

I will say tho that the size difference between a 6 and an 8 is quite a bit bigger than you might expect. the 200 is certainly a beast after being used to 5/6 inches!

dont let this put you off tho :) if you can afford it and manage to lug the ****** around, go for the 200, you will definately wish you had 2 months down the line if you dont :headbang:

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i upgraded from 6" to 8" and the difference was quite noticeable,i also upgraded last year from 8" to 10" and i did not think the difference was that great to be honest.

i am now going to drop back to a superfast 8" and concentrate more on imaging simply because as has been stated many times on here the ccd sees more than the human eye ever can.

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I can't agree more with the advice above.

Years ago, when I first started, I picked a 6" dob because it was lighter and more managable than the 8" I lifted. I had a long walk from storage to garden and I didn't fancy lugging something heavy, specially as Dobsonians are easy to use, but awkward to carry.

I planned to swap it with an 8" after a few years, till the unpleasant realisation dawned on me that there is no jack of all trades. I moved up to a 10" newtonian on an equatorial mount which is great for planned sessions. However, I still go back to the 6" for quick grab and go: to the garden, or to just throw in the car en route to a lesser light polluted area than mine.

I would suggest you pick an 8" if you can manage it well and plan to stay with one scope, or are prepared to step up to a 12-14" aperture when the bug bites you.

Edited by hamiclar01
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I'm using a 6 inch and have done so for a good couple of years now. I had it at a knock down price from a retailer with a good mount upgrade so it was a no brainer. Its excellent to learn with and feels very manageable.

I've never looked through an 8 inch scope so I can't say how big the difference will be other than it will be better than the 6 inch for the reasons listed above. As a result, if cost is not an issue for you then why not go for the 8 inch - it does seem to be the happy medium.

I know in the back of my mind that if my interest holds then at some point in the future I will fork out for my second scope and it'll be the biggest aperture I can practically transport from house to garden to dark site (10 inch seems about right), and then I can start my Messier and Caldwell lists, etc, all over again :-)

My 6 inch may then develop into an imaging scope. Lottery win comes first of course.

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Thanks all for the comments. Really helpful! im leaning towards the 200 as i wont be lugging it far. But one reason for me to get a 150 over 200 would be so i could dabble in astrophotograpy. if im honest the extent would be sticking a dslr on it. i know i should get a heq5 but if im not planning on guidescopes etc so i might as well get the 200 right?

Comments most welcome

Edited by drewzilla
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I am also seriously thinking about upgrading now and this has been very useful, except i want to go for a 250mm aperture telescope. I was wondering if a skywatcher EQ5 mount would hold a 250mm scope or would it be too heavy? Thanks =)

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250 will be too big for the eq5. It may hold it, but I think it would struggle. Back to the original post, I started with a 6 inch to experiment with imaging. Decided it was too difficult/expensive, so jumped to a 12" dob. Based on you wanting to try a dlsr on it, i would say get the 6".

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I have a 150p on EQ3/2 and 200p dob both are fantastic. I am lucky and live with very little light pollution. Yes the 200 is a little brighter when finding DSO's but to be honest not that much better!

If i had choice again I would have got the 150p on EQ5/6 as would be brilliant for astrophotography on a real budget. I have had (in my opinion) good success with the 150 on the EQ3/2 and taken some good images.

The 200 on an EQ mount might be a hell of a lot better though. Perhaps I just dont like the Dob idea?!

Cheers

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An 8" will show stars 0.6 magnitude fainter than can be seen with a 6". An experienced observer will notice the difference, a less experienced observer probably won't.

With regard to DSOs the most crucial thing is the darkness of the sky. Neither a 6" nor an 8" will show galaxies in daylight, or if the sky is too light polluted. But if the sky is dark enough for the Milky Way to be seen naked eye, binoculars will show all the Messier objects, and a 6" will show hundreds of NGC objects.

So before getting a bigger scope I'd suggest you try and find a darker sky, otherwise you risk further disappointment. The biggest difference you are likely to notice between a 6" and 8" is weight.

My rule of thumb for upgrading is to increase aperture by (at least) 50%, which gives a magnitude difference of nearly 1. But it's the sky that really counts, not the scope.

All of this refers to visual observation rather than imaging. With regard to the latter I can offer no advice.

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I am also seriously thinking about upgrading now and this has been very useful, except i want to go for a 250mm aperture telescope. I was wondering if a skywatcher EQ5 mount would hold a 250mm scope or would it be too heavy? Thanks =)

If you are looking at a 250mm scope then I would suggest nothing more than an EQ6. The EQ5 / 200P combo is OK for visual, but I would suggest a 200P / HEQ5 being the ideal combo, especially if you want to bolt on cameras etc.

A 150P on an EQ5 would also make an excellent combo... especially if you want to start basic imaging with a dSLR web cam etc.

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hmmm so much to think about! You guys make a lot of great points. I need to think whether or not i will get into imaging as that will determine a lot of choices i make :)

questions though what does the heq5 allow u to do when taking photos that the eq5 does not, with a 200p? (not including guidescopes ccd etc) does it just allow u to take longer exposures? Wouldnt the 'brighter' image on the 8 inch make up for the shorter exposure compared to the 6inch? Sorry if my questions are getting stupid. Its getting late....:)

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hmmm so much to think about! You guys make a lot of great points. I need to think whether or not i will get into imaging as that will determine a lot of choices i make :)

questions though what does the heq5 allow u to do when taking photos that the eq5 does not, with a 200p? (not including guidescopes ccd etc) does it just allow u to take longer exposures? Wouldnt the 'brighter' image on the 8 inch make up for the shorter exposure compared to the 6inch? Sorry if my questions are getting stupid. Its getting late....:)

Unfortunately it doesn't work that way. Both the 150P and 200P are f/5 focal ratio so the exposure times are the same. The difference is that the 200P has 1000mm focal length, the 150P has 750mm, so the object you are imaging will appear larger with the 200P.

With imaging it is the focal ratio and focal length that really matter. For visual observation, aperture is the main factor. If you want to observe visually, get the biggest aperture scope you can afford and manage to move about. If you want to take images, get the most stable mount, with the highest precision motors you can, then stick the OTA with the fastest focal ratio (lowest f/ number) on it; aperture almost doesn't matter.

Edited by RikM
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