6" or 8" Dob

[Benboy (Ben)]

Hi, I just registered for this forum.

I am new to astronomy and looking at getting my first scope.

I'm in New Zealand and the options here are pretty limited and expensive. I was considering getting an 8" skywatcher Dob but have noticed that the bigger you go with apperature on a Dob - the small the focal length ratio. For example the 6" Dob is f/8 whereas the 8" is f/6 and so forth. I know mostly people say apperature is king but I was wondering if I would get better looks at planets etc with the 6" dob because of the greater focal length ratio? I'm not really wanting to have to buy another scope down the track if I can help it and I am sure I will be interested in both DSO and planets.

The other thing is that I can probably get a good deal on 6" near new skywatcher dob at the moment - but if I go for the 8" I will probably have to pay close to double what I can get the 6" for.

So am I better to pay more and go for an 8" Dob (will that give just as good images of the planets with less focal length ratio?) - or go for the cheaper 6" Dob - (or will I regret not having more aperature for DSO viewing later?)

Also if anyone is in NZ and knows anyone with a telescope for sale (even other types) let me know.

Thanks for your help

Cheers
Ben

[erick (Eric)]

Hi Ben and welcome!

Re "I'm not really wanting to have to buy another scope down the track if I can help it"

I suggest that you console yourself to the fact that you will buy and sell more scopes in the future as your interests and desires change. It seems to me that 99% of amateurs on this site do that. Don't panic - in Australia, resale value is fairly good, so it is not money wasted. In NZ??

For planets, a quality, smaller aperture refractor, would do fine - but the $s will be more than you seem to have budgeted, if you are looking at 6" and 8" reflectors on dobsonian mounts. For observing planets, f6 or f8 is much of a muchness I expect and the somewhat better resolving power of the 8" will be worthwhile. But is it worth twice the price - I don't think so.

The moment you mention observing DSOs, aperture becomes a prime consideration. In that case, you should definitely hold out for an 8" or even 10" reflector that you can afford.

Keep asking questions. Have you found anyone local with scopes that you can look through so that you can decide?

BTW, the changing focal ratios are probably just a way of keeping the size of the scopes manageable - not too long, nor too short for a dobsonian mount. It's not something intrinisic to the mirror diameter.

Eric

[niko]

Welcome Ben,

Damn - I just deleted yesterday the contacts for scopes in NZ I searched for for a mate in Whakatane.

I did a search on the NZ telescopes site and $900 for an 8" dob seems excessive but in my opinion I would get an 8" over a 6" - it will keep you happier longer; but Eric is right - I started with an 8" dob and now own a 10" dob and an ED80 for photography (and a celestron 130SLT for playing with!)

I would put a WANTED ad here on this site and see what turns up.

I think I bought my 10" for about $450 here and we seem to have more and more members from across the ditch!

Good luck with the hunt and welcome aboard!

niko

[dannat (Daniel)]

the size is a factor..i prefer smaller scopes - the longer focal length will be kinder to your eyepieces & planets. i am probably in the exception but i would take the smalller & make sure it was collimated to perfection (te longer f/l will also help this)

[astro744]

A 6" f8 is an excellent telescope that will last you a lifetime. The longer focal ratio does mean that you can use less expensive eyepieces not specifically designed for shorter focal ratios since as off axis abberations will not be as pronounced. However premium eyepieces have other attributes than just sharp stars to the edge and would be worthwhile additions.

An 8" mirror has almost twice the light gathering capability as a 6", (8x8=64, 6x6=36, 64/36=1.78). Many DSO's are faint and every bit of light is welcomed. An 8" f6 will likely have a 2" focuser whereas a 6" f8 may only have 1.25" (2" can be added though not always easily). A 2" focuser will allow much wider true fields with long focal length 2" eyepieces, but with an f6 telescope these eyepieces will need to be highly corrected ones such as Tele Vue or you will see abberations towards the edge of field.

The maximum true field for a 1200mm telescope with 1.25" focuser is 1.3deg whereas for a 2" focuser is 2.2deg.

A 6" will show you all the planets in the Solar System (even Pluto is within reach), all the Messier objects and many galaxies, globular clusters, open clusters, planetary nebula and even one Quasar, 3C 273. An 8" will show the same a little brighter and a few fainter galaxies not within reach of the 6". Note galaxies not within reach with the 6" but visible in the 8" are going to be very faint.

If you buy a 6" now your next telescope if any should be a 10" for 3x the brightness increase. If you get an 8" now then to see a significant increase in brightness your next telescope should be at least a 12".

[Benboy (Ben)]

Thanks guys for the responses. I have to make a decision on the 6" within a few hours, still undecided yet. A good point was brought up about size. Are the 8 and 10" versions easy enough to transport to darker skies?

[niko]

some good comments here - if it's a good price buy it - you won't regret it.

the 8" I had I could move myself as one unit the 10" I now have I have to break doen into its components - much more combuersome because the tube is so much bigger

I've never handed a 6" but I'm sure it's a breeze to move around

n

[astro744]

My first telescope was a 6" and I still have it and use it.. The brighter views in an 8" are noticeable and if I had the money I would have bought an 8" back then. However, I have been very happy with what the 6" telescope has shown me over the years and I think you will be too.

A 6" telescope was once considered the smallest serious telescope for the budding astronomer and you can certainly do some serious observing with it.

[Benboy (Ben)]

Cheers guys. I'll see how good a deal I can get on the 6" and if its too good to pass up I'll take it but otherwise I think I will wait a bit more (I am still learning the sky anyway) and get an 8" which by the sounds if it will be a better all rounder and keep me happier longer. I took the advice and put a wanted ad in. Hopefully someone in NZ has an 8" they want to get rid of and will see the ad.

[Suzy]

[QUOTE=astro744;583603]. An 8" f6 will likely have a 2" focuser whereas a 6" f8 may only have 1.25" (2" can be added though not always easily). A 2" focuser will allow much wider true fields with long focal length 2" eyepieces, but with an f6 telescope these eyepieces will need to be highly corrected ones such as Tele Vue or you will see abberations towards the edge of field.

Ben!
Just a correction to the above quote: The 6" does have the 2" crayford focuser, and I do use 2" eyepieces on it.

I have a 6" Saxon Dob and I love it to pieces! I'm a bit of a fly weight, but I can just carry this unit in whole for a tiny bit (though I put wheels on it). The 6" in itself does take up a lot of room in the car. I have a big boot and the base does not fit into it.

From much research I did not so long ago, I agree with others here... you would upgrade from a 6" to a 10", or an 8" to a 12". I love the fact with my 6" I can take it out on a whim whenever I want as it's fairly easy to move around. So, I use it lots! You will be amazed at what you see out of the 6". The larger you go though, the more the atmosphere mucks up your seeing; bigger appertures can show this up much more easily.. I barely comes across bad seeing on planets when I look thru my 6".

[The3rdKind (John)]

Gidday Ben,

I back Suzy in her comments 100%. We had an offline discussion about aperture, light gathering capability, and field of view and then did a rough comparison between Suzy' 6" and my 10". Sure I get a lot more faint fuzzies but you will find that you'll go mad on the planets before anything else. Just wait till August mate!!

The point I want to make is that at some point in your viewing you're going to want to get a Barlow lens if you haven't already. As you may know this will increase whatever EP you happen to be using at the time double!! (e.g 10mm with 2x Barlow = 5mm)

Now, bearing this in mind the closer you get to a planet the more atmospheric abberation you are going to get (hazy, fuzzy, wobbly, limited contrast etc.) In a scope like a 6" this is kept to a minimum. Once you talk 8" and above these factors start to compound until the stage where you need an exceptional night to see planets nice and close. Of course maybe some of this can be overcome with additional filters and better EP's but not drastically so. With a 10" like mine you need to go to 2" Barlows and Ep's which start getting expensive for good quality viewing.

My advice stay with the 6" as long as you can; Invest in a GOOD quality barlow (2x) and some decent EP's (say 7.5, 10, 12, 15, 18, up to 25). Planned and staged as your viewing confidence grows wont break the bank and give you a lifetime of rewarding viewing. I gawked thru Suzy's scope a couple of weeks ago at a highly light polluted site and I was getting more than enough DSO's, more than I could deal with.

As your viewing likes and dislikes change, grow and mature you'll have a better idea of how big you really need to go. Good things come in small packages mate!!

That's my 2bobs worth anyhow

Cheers,
John

[Benboy (Ben)]

Thanks for all the responses guys. Well I decided to go for the 6" mainly because I knew I wouldn't be able to get a better deal in NZ. I paid $330 NZD (about $250 AUD) - it is only 3 months old and I would have been paying $700 NZD (about $535 AUD) + postage to get it new or about $900 to get an 8" so I'm pretty happy with the deal and I'm sure I will get good resale on it if I decide I want to go bigger later.

John you mentioned getting a good barlow lens. The scope comes with a super 10mm and super 25mm plus a 2" adaptor. What barlow lens would be good for this scope? - how can I tell it is a good one or will suit the eyepieces?

Cheers again everyone for the advice.

[renormalised (Carl)]

Ben, with barlows, as you might know, the barlow increases the magnification of whatever eyepiece you use by the rated amount inscribed on the lens (x2, x3 etc). This is all well and good, but what you have to take into account is just how much magnification can your scope actually handle. There are a couple of quick and dirty equations you can use to determine this...diameter of mirror/lens in mm x f ratio / ep' lens diameter, or, diameter of mirror/lens in inches x 60 (which gives the max mag your scope can handle). Since you've bought a 6" f8 scope, using the smallest ep you have (10mm), the max mag for your scope is ((150 x 8)/10=) x120. Now, the highest mag your scope can handle is around x360, however for various reasons, for a 6" scope this is impractical. Mainly to do with the conditions you observe in and quality of the optics of your scope. For the most part, with a 6" scope I would be reluctant to push the scope beyond about x180, and even then I would be hesitant to use such a high power. Even in the best of conditions. So, if you have, or are going to purchase, a barlow, don't go beyond a x2 barlow and don't go less than a 10mm ep for your setup. A 10mm, 15mm, 25mm and 32mm set of ep's would suffice and give a good spread of mags.

Hope that helps

[Benboy (Ben)]

Thanks renormalised that does help. But are there recommended brands etc of barlows or ones i should avoid?

[renormalised (Carl)]

I'd stick with the meade, celestron, pentax, televue, vixen and the like. You pay a bit more for them than some you'll see on the various websites, but you will be getting what you pay for.

[The3rdKind (John)]

Gidday Ben,

I am really happy you chose the 6". Get ready for some exciting views :ey epop::eyepo p:: eyepop:

By the sounds of things your EP's are 1.25". You may already know that the 2" adapter is there when you feel ready to move to larger size EP's. You can get a 2" Barlow but this is fairly redundant in a 6" scope. Here's why.

A 2" Barlow will give you the ability to get your 2" EP's up close and personal with planets, but also give you a wider field of view. This is especially good for DSO's. The redundancy comes from having a much wider FOV, in any case.

By this I mean that you can get really amazing EP's in this size that can do the work of the barlow for you any way (eg a 2" 30mm EP Barlowed down (2x) is the same as using a 15mm 2" ep. You're getting as close as you need to without forking out for a 2" Barlow. GOOD Barlows in this size range are very expensive. GOOD 2" EP's in this size are expensive too, but as a long term investment and when conditions are just right! (When the stars don't twinkle mate, just fine pinpoints of light). 2" EP's are good for say Andromeda, or Orion to name just two. You get the closeness but you also get the SPACE!!

However, with 1.25" EP's you can utilise a Barlow to it's full benefit as an Astonomical aid. Get into the moon when you get a clear night. It's the next best thing to actually walking around on it. Furthermore you ain't gonna max out the credit card

Start your serious collection with a GOOD 1.25" 2 x Barlow when finances permit . This will get you close to the planets and you wont suffer from atmospheric abberation too much. Mate, I think you can go right down to a 4mm EP and up to a 32, if you like, which will effectively give you 2mm and up to 16 WOW!!! Remember the tube is a 6" (refer my previous post).

I have evidence to back these comments up as in my previous post I said I gawked through Suzy's scope with said EP (4mm) and Barlow. Remember also that this scope is fine for DSO's too, just maybe not 14 magnitude or higher. It does not matter mate!! with your 30 and 32mm you get great viewing.

My Barlow and my EP's are all 1.25". The only time I have been satisfied with my 2x barlow and 7.5mm is looking at the moon. Sharp and crystal clear!! Mate I was walking on the Sea of Tranquility!! But in this size scope (10"), looking at Saturn say, I'm going to have to wait for that rare evening. You don't have this worry my friend.

Good Luck and Happy Viewing, Cheers,

John

[Suzy]

Your theoretical power for the 6" is 360, your practical power is 240, which is a 5mm ep. I encounter no probs on my 6" dob with a 5mm ep and a 4mm (300x) ep unless the atmospheric conditions are really bad, then detail suffers. As an example, with poor "seeing", the planet looks like your looking through boiling water. On typical nights the 7mm, 5mm or 4mm is what I use on planets. The bigger the aperture, the more you magnify poor atmospheric condtions. The 6" is considered to be the entry level "serious scope". On most nights, it will not disappoint you. Star clusters I find on mine are best viewed with a 25mm

Here's how you do the math for mag. on ep's:
focal length of your scope (this is yours) 1200
Divide by focal length of eye piece, e.g. 10mm = 240 mag.

To work out the max. power for your scope, multiply 60 per inch of aperture (60 x 6) this is 360 for your scope.

Here is a link that explains a bit about aperture and magnification:
http://www.seqas.org/

Regarding mid range eyepieces, I like the Celestron and Orion Expanse for their contrast and sharpness.

[Suzy]

Originally Posted by The3rdKind

I gawked through Suzy's scope with said EP (4mm) and Barlow.

John

Just a correction Johnno, it was a 4mm straight on my 6" - no barlow.

Ben, depending on the weight of the barlow you get, you may have to counterweight the tube.

[The3rdKind (John)]

Originally Posted by Suzy

Just a correction Johnno, it was a 4mm straight on my 6" - no barlow.

Ben, depending on the weight of the barlow you get, you may have to counterweight the tube.

Suzy,

I stand naked (frightening!!) in the face of truth. Sorry Benboy
Cheers,

John

[renormalised (Carl)]

Mate I was walking on the Sea of Tranquility!!

John, you didn't trample all over Neil's and Buzz's footprints, did you??!!