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Help Me with Refractor Apertures


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After reading much here, I had about ruled out refractor scopes. Various considerations led me to more or less decide on an 8" dobsonian.

The problem is that there are no many dobs for sale within reasonable geographic distances. I am, however, seeing refractors and some catadioptric scopes in the local market. For example, I have a line on a Celestron Nexstar 80GT. Is this aperture too small to last me for very long? By that I mean, will I find its limits to be too confining to hold my interests.

By way of explanation, when I settled on a dob, it seemed that the best advice I read said that an 8" was that much better than a 6" and that the cost of a 10" would prove beyond my budget. I'm looking for similar advice about the refractors. Is the 80mm aperture too small for the money and I should try to go above 100mm?

TIA

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IMHO you should aim for 100mm at least. There is a bigger difference inbetween a 80mm refractor and a 100mm than there is between a 100mm and a 120mm refractor. Its very much like the 6"/ 8"/ 10" situation with Dobs.

That said, I'd try and stick it out for the 8" Dob if at all possible.

Edited by GazOC
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I think you would exhaust the possibilities of an 80mm aperture refractor quite quickly. A 100mm is much better or even a 120mm, which is better still. More aperture is almost always better and a decent 8" dobsonian will deliver better views than all of these on a good night.

Personally I think your original plan was the best option and the one that will deliver the most "bang for your

buck".

Edit: great minds think alike Gaz !

Edited by John
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I don't have any personal experience regarding this particular scope but I would suggest you read a review on it by Wayne Eidelbus on the 'Cloudy Nights' Forum.

In summary he reckons that the scope is a very good performer. The red dot finder isn't great and the manual carries several amendments which is not surprising given that the main points of reference regarding the instructions, feature only the 60 and 114 models. He actually went on to buy better eyepieces and there was an issue about adequate power supply. In short it seems ok and my hunch is that it is a product aimed at the beginner market and for terrestial viewing. The plus side is that it has GOTO if that appeals but of course the 'cake has to be cut' and spending on this instead of the glass (aperture) isn't necessarily money best spent, especially as he had to effectively replace the eyepieces tat came with the box.

In my opinion, I would at least aim at a 4" scope but be careful about the focal length. If the scope is an achromat (cheaper with a doublet aperture lens assembly) then make sure it is an F8 or above to avoid chromatic aberration or colour fringing which will be noticeable round the edges of bright objects. Some people can live with it but you mind find it distracting. If it is an aprochromat (dearer with a triple aperture lens assembly) then be careful that the focal length is not to short because the wider light cone is brought to a steep focusing point within the tube assembly which will warrant dearer eyepieces that have to work harder to straighten this light out in preparation for your eyepiece.

This will help explain the difference in prices and the tolerance that eyepieces will be subjected to. Remember also that it is better to buy the scope, the finder, eyepieces separately, which appear a dear way buying gear but at least you get to choose what you want to do with the scope and the accessories that will help get you there.

Please come back with any questions.

James

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Thank you, all, for the responses.

This was exactly the type of information I was seeking.

As it happens, I had just found and read the cloudy nights review by Mr. Eidelbus before returning here to see responses.

I'm glad to to know that 100mm is the rough equivalent to the 8" in my quandary of figuring out which sizes can be even roughly equated.

Ideally, I'd just buy a new Orion 8" Intellisense and be done with it, but money is a major issue (retired) so I'm shopping for used gear. With dobs, where you are matters a great deal because of shipping costs. It seems that here in Texas there are a great many more refractors within driving distance than reflectors - almost no dobs. I've even seen several Maks - but beyond my spending limit.

I'm open to either a refractor or a reflector in the right configuration and at the right price. I just needed to get my head around the relative utility sizes in each type.

Thank you again for the responses. As usual, you guys nail it quick!!

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What do you mean by relative utility sizes? If you are referring to the maxim that states an objective lens equates to a mirror twice its size then I would argue that this is not strictly true. I believe its around about 65% so that a 4" lens is equivalent to about 6-7" mirror. Of course you have to subtract from the mirror the size of the secondary obstruction which reduces the available light that can reach the mirror in the first place.

I noticed tonight what I would call, a 'polite argument' being negotiated whilst examining the posts within the refractor section on Cloudy Nights Forum. Here the topic being 'discussed' was whether in fact a 6" long achromat refractor could out perform an 8" Cassegrain. The usual exchange of caveats ensued with real world experiences being traded in an attempt to rationalise the science behind it all. I would invite you to read some of it to reveal that differences found in the quality of both lenses and mirrors make this area a tricky one to analyse and seems to come down to subjective preference.

Clear skies

James

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Hi James!

As usual, your answers home right in on the issues and shed light. Thank you!

My position is that I'm not finding many dobs offered within any reasonable radius of my location; however, I see refractors and the occasional Cassegrain or Mak on offer.

Knowing full well that within 30 seconds of purchasing anything else an 8" dob will immediately come on offer that I can afford, I'm simply trying to find relative value among different types. Although I'd like to buy an 8" dob, I might be buried before one comes available in my area. I've determined that an 8" is that much 'better' than a 6" in newtonians. From the responses thus far I've learned that a 100mm is that much 'better' than an 80mm. From the prices at which I've seen the gear offered, I'm making a rough equivalence - only in my mind's eye - of an 8" reflector with a 100 mm refractor. I'm making these judgments ignoring many other factors that would determine the purchase of an 'ideal' scope for me at this time in this place. Availability and price are likely to be my biggest determinants.

That is the best I can do to explain the context of my questions and the less-than-completely-informed judgments I've expressed. As I've said, I'm convinced that an 8" dob - especially the Orion intellisense - would be the best for me right now; but I'm trying to figure out what I might settle for. If I get a newt, I don't want less than 8". I think, that I don't want to get less than 100 mm refractor. Where would I place myself with a Cassegrain? I don't know yet because the choice has not made me try to learn where I would buy into the continuum. I've read about the relative merits of refractors vs newtonians vs cassegrains vs Maks for various uses. Given that I'm a tyro (and poor), I don't know that AP will be my ultimate passion - or even if any aspect of the hobby will keep me enthused. But I do know that I'd like to be able to see the moons of jupiter again and the mountains of the moon. I last saw them through a very cheap 3" refractor more than 30 years ago.

Sorry to ramble, just trying to expose the paucity of my probity I suppose.

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You can see the moons of Jupiter in a pair of binoculars. If you aren't going the AP route then I'd suggest a small apo might not be really what you want. Budget sounds like it precludes Mak's/SCT's, so really you're choice is newtonians.

To keep it cheap get a dob - but if you need accurate tracking then an eq mount is more appropriate.

You can of course get a tracking dob - good compromise - makes a world of difference not to have to nudge the scope to follow an object. Go for the biggest aperture within budget and leave some for ep's, collimator, power etc. 8" is a great size to start with - keep you busy for years.

All the best :)

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I have plenty of experience with refractors between 80 and 140mm and for me they become staisfying visually at around 100mm. These will give planetary detail approaching what is really possible in anything, though deep sky is not their strongest suite. However, there is, for many people, something special about the view in refractors and this makes me like them a lot.

I'm very surprized that you are having trouble finding something second hand in Texas. Have you tried a 'wanted' ad?

Olly

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I wouldn't apologise for your 'ramblings' as I believe you are going about the purchase of future kit in the right way. Trying to understand all the terminology and the many variations that make up different gear is not any easy trick to juggle, especially as you naturally want to get in there and start observing as soon as possible. There are many that I have known who have 'dived in there' so to speak, who later realise that the next model up was probably the better option and so will then have to pay the price for that impatience.

You are also demonstrating a good deal of commonsense in that you are thinking ahead of time to ensure that you will still remain 'focused' (couldn't resist it - sorry:D) on this great subject in years to come. I bought an NEQ6 mount not because at the time I needed it but as a relative cheap way of future proofing my kit should I later need or want a bigger scope.

Keep asking all the questions you need to help you make the right decision for you. Thinking back, I must have been quite a pain asking questions, looking through people's scopes, eyepieces - you name it I had a question about it (...still do :)) as long as you get to that point of knowledge where you can make up your mind, then its all worthwhile. Given that you have made your mind up, how you get your hands on 'that kit' is an entirely different question especially in the absence of a large wallet. Here I have few answers and would probably rely on a bit of patience and a whole lot of luck It can happen!

Clear skies for now

James

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Thank you again, folks!

My problem is one of availability of offers. I have seen one dob on offer in the last month and it was over-priced for used gear. He wanted near new prices which I wouldn't pay.

I have seen several refractors nearby with prices I could reach. In order to see something before the undertaker comes, I was considering getting a decent refractor from among those on offer.

I don't know why it is that refractors seem to be more plentiful in the used equipment market. Perhaps those with dobs are more satisfied with their purchases and less willing to part with them than are those who bought refractors then repented.

I remain convinced that the BEST scope for me would be an 8" dob; however the scope that is most AVAILABLE to me might be a refractor. The dilemma is in the choice. I've only been trying to educate myself about the relative merits of each size within each type.

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Rabbithutch, you've got your head screwed on right.. and you know patience will be its own reward. That refractor NOW will offer some pleasure and experience under your belt, but you'll want that Dob all the more! :) (..just admit it, you'll want both whichever way you go :):p )

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I guess this really depends on how much patience you have. However, you won't see anything through an 8inch dob that you don't have! The advice regarding "aperture rules" is true, but you can see an awful lot with a 100 to 120mm refractor if your skies are not too light polluted.

As someone who traded a 10 inch dob for an 80mm refractor, I can say that I love the view, and rarely miss the aperture- not because I can see as much on a given object, but because the smaller scope is much easier to handle, has a wider field of view and makes finding stuff easier. The view is not degraded by coma and diffraction spikes and just pleases my eye.

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I guess this really depends on how much patience you have. However, you won't see anything through an 8inch dob that you don't have! The advice regarding "aperture rules" is true, but you can see an awful lot with a 100 to 120mm refractor if your skies are not too light polluted.

As someone who traded a 10 inch dob for an 80mm refractor, I can say that I love the view, and rarely miss the aperture- not because I can see as much on a given object, but because the smaller scope is much easier to handle, has a wider field of view and makes finding stuff easier. The view is not degraded by coma and diffraction spikes and just pleases my eye.

Yup, I can sympathise with that view. Because of my work I have a good few scopes available from 20 inch down to 70mm.

The other night we had the big dob and the 85 Tak (with eyepiece in the end for once) out on Comet Hartley and everyone liked the little one best on that night. However, swing over to M51 and it's another story. Horses for courses.

Olly

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I think that you are giving up way too soon about finding an 8 inch Dob. In the USA, the shipping charges are not excessive unless you select next day or two day delivery especially if you purchase direct. In fact you can get free delivery for Zhmell Dobson telescopes.

For a Zhmell Z8 which has free shipping within the USA, excluding Alaska and Hawaii, go to: http://www.telescopes.com/telescopes/dobsonian-telescopes/zhumellz8deluxedobsonianreflectortelescope.cfm

For an orion DOB goto http://www.telescope.com/control/telescopes/dobsonian-telescopes/orion-skyquest-xt8-classic-dobsonian-telescope Shipping is not free but reasonable unless you elect one, two, or three day delivery.

Which is best an Orion or Zhmel? ... that's a Chevy vs Ford discussion. Both are excellent telescopes.

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Depending on your skies you could well exhaust the abilities of an 80mm. One point I will make is that there are many instances of people selling their 80 ED scopes and regretting it.

They seem to fill a niche'. Just big enough to be useful, usually good images, easy to take and set up. However an 8" Dob isn't difficult either.

A 100mm would of course be better however bigger and a greater expense, and refractors aren't cheap.

I presume that both, Dob and refractor, are not an option?

My thought is that I would buy an 80mm for the small size and all round use, I wouldn't go up to 100mm, so if it is something bigger that is wanted wait for a Dob to amble by.

An 80mm refractor would compliment a Dob but is not an alternative to one.

Reading CN a night or two back and someone was saying that Dobs are not holding a decent price on the used market. Made me think that there was therefore a few around for sale.

Any clubs around you that may have members selling a Dob?

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Hi Rabbithutch - good to have you on board. The scope that I use most is a fast achro refractor - nothing very flash. It's a 120mm (5") f5 scope mounted on a go-to equatorial platform.

The reason is use it most frequently is that:

- it shows most of the deep sky stuff that is readily visible (given light pollution, poor seeing etc)

- it has a pretty good aperture and shows much more than my 80mm refractor

- it's easy to handle (weight and size wise)

- it's ready to go almost immediately (no cool down)

- it's robust (strongly built)

- it doesn't need collimating

I have to trade off some false colour on brighter objects (e.g. on the Moon, Jupiter, v. bright stars), but I can live with this for the reasons above. It was also pretty cheap!

There are plenty of good reasons to go with a largish refractor. Please bear in mind that I've never owned a Dob mounted scope, but one thing that I've learned in this hobby is that the practicalities of scope ownership can quickly outweigh the absolutes of big aperture etc.

Good luck with your search for a scope.

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