Quote:
Originally Posted by Lux
I am seeing advertised Newtonians OTAs that are "imaging optimised".
Could someone tell me exactly what has been "optimised" and any subsequent pros (apart from imaging) and cons?
What appeals to me about these scopes is their short length. An f4 200mm scope is only 80 cm long, which makes it extremely portable (IMHO).
Would it be possible to mount one of these on an alt-az mount?
Would anyone recommend a set-up like this as a first scope?
I know I could get a 200mm Dobsonian but these are larger, and as such not as portable. I'm not particularly interested in astrophotography, so an EQ mount is not that high a priority, plus I don't necessarily want the hassle of an EQ as a beginner.
Just trying to weigh things up here - any advice appreciated.
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Hi Justin,
I am going to throw a dissenting opinion in here. To be truthful I think buying one of these scopes is a bad way to get into visual astronomy. As a beginner and probably only wanting to spend a limited amount of money to “dip your toes in the water” as far as visual astronomy goes, I think there are far better options available to you. Your first telescope is always going to be a compromise between transportability, storability and what you can see with it. More aperture is better from a “what you can see” point of view. Unfortunately, the bigger they get the harder they are to transport and store. As a generalisation I would advise you to avoid an equatorial mount if you end up going with a newtonian telescope. A newtonian requires a solid mount and in equatorial format these are expensive. On top of that because the eyepiece is at the top of the telescope you can get your head and body into some extremely difficult, awkward and uncomfortable positions when trying to view targets in certain parts of the sky. A dobsonian mount is cheap, very strong and stable and very user friendly, particularly for someone just getting started. Most importantly, you don’t need to be a contortionist to view some targets, like you do when using a newtonian with an equatorial mount.
The main things that change with a scope optimised for imaging are:-
1) Faster F-Ratio
2) Much larger Secondary Mirror
3) The focal plane is pushed further out to reach focus on an imaging chip.
4) Generally they have thicker spider vanes than on a visually optimised newtonian.
There are reasons why each of these 4 things detracts from these telescopes ability to perform as a visual instrument. Also keep in mind that some of these scopes are “more” imaging optimised than others. Some scopes for example like the 8"/F4 GSO newtonian are "partly" optimised for imaging and at a pinch do a “fair” job as a visual telescope, when used with good eyepieces. They certainly aren’t first choice as a visual telescope. Some others for example like the Vixen R200SS which is an 8”/F4 imaging newtonian, are fully optimised for imaging and make pretty poor visual telescopes.
Disadvantages with faster F-Ratio telescopes for visual use:-
1) At F4 a paracorr becomes a necessity to get good views off axis. In other words at and near the edge of the field of view (EOF). A paracorr alone costs about $570.
2) As the F-Ratio of the telescope gets faster the need to use “premium” eyepieces to get good views off axis increases. Cheaper and medium grade eyepieces will provide pretty poor performance in an F4 newtonian due to coma from the telescope, off axis astigmatism in the eyepiece and field curvature of the eyepiece. With cheaper and medium grade eyepieces you will not see stars as points, off axis. Off axis stars will look like the Starship Enterprise travelling at warp 9.
3) As primary mirrors get larger and faster it becomes increasingly more difficult to make them well. While an 8” mirror is small and easy to make, it is infinitely more difficult to make an 8”/F4 mirror well as compared to making an 8”/F6 mirror well. This is particularly so when talking about mass produced optics from the far east. Hence there is the likelihood that with the cheaper mass produced telescopes, you may well get a mirror that isn’t great. With mass produced optics you have a much greater chance of getting a good 8”/f6 mirror than you do of getting a good 8”/F4 mirror.
4) Whilst it is no harder IMO to collimate an F4 scope than it is to collimate an F6 scope, collimation at F4 becomes much more critical to give good views. At F6 the views will not deteriorate much at all, if at all, with a telescope that is “slightly” off in collimation. With an F4 telescope the views will be noticeably affected as soon as the collimation drifts away from perfection.
5) With an F4 telescope you have a much smaller “depth of focus range” than you do with a slower F-ratio telescope. This means the focusing range that the image will appear to be in perfect focus is much smaller with the faster telescope. It then becomes more important to have a premium focuser fitted like a Feathertouch or a Moonlite to “easily” keep the image in good focus as the seeing fluctuates. With an F4 telescope and a cheap mass produced focuser it can be a bit of a chore continually working at the focuser to keep the image in good focus. With a slower telescope like an F6 telescope the image does not drift in and out of focus anywhere near as much as it does in an F4 telescope, as the seeing fluctuates
Disadvantages of a larger secondary mirror:-
1) It lowers the quality of the image, to a noticeable degree to an experienced observer, particularly as you increase magnification, because of the diffraction effects caused by the larger obstruction.
Here is a link to an excellent article by Damian Peach, who is widely recognised as one of the worlds’ best planetary imagers, if not the best.
http://www.damianpeach.com/simulation.htm
It is well worth reading. Of particular note should be the increase in visible detail as the aperture is increased. This is covered in part 1. In part 2 the effects of the size of the central obstruction are shown. It is worth looking at this keeping in mind that in broad terms a specialist visual newtonian will generally have a CO <20% and an imaging optimised newtonian will have a central obstruction >30%
2) The larger secondary mirror slightly reduces the light gathering ability of the telescope as it blocks out a larger part of the primary mirror than the smaller secondary and holder. This isn’t a great effect due to the fact that the surface area in the centre of the mirror is less than it is at the edges for a given diameter of obstruction.
Disadvantages of pushing the focal plane out to focus on an imaging chip:-
1) By pushing the focal plane further out the side of the telescope it means that a large number of eyepieces will not reach focuser because of insufficient focuser out travel. You can use an extension tube to help these eyepieces reach focus. However, depending on the eyepiece focal length and the field stop diameter, some of these eyepieces will vignette (clip the image) when used in an extension tube.
Disadvantages of oversized spider vanes:-
1) Oversized spider vanes increase diffraction effects on the image in a simlar way to increasing the size of the secondary obstruction. Thick spider vanes in combination with an oversized secondary mirror (>30% obstruction) will contribute to a noticeable level of image degradation, particularly at higher powers under good seeing conditions .
2) Thick spider vanes will increase noticeably the size and brightness of any diffraction spikes when viewing bright targets like the moon, bright stars and the brighter planets.
Where does this leave you in terms of your search for your first telescope for visual astronomy? Well I would be staying well away from those 8”/F4 imaging newtonians and staying away from an equatorial mount. I would be staying away from a small refractor. They just don’t have enough aperture to provide the “wow” factor on most targets, which is something a newcomer needs IMO. Most newcomers expect to see images in the eyepiece like they see in books or taken with CCD cameras on the internet. You will never get those images at the eyepiece, but, the bigger the telescope the greater the “wow” factor and the more likely you are to keep an interest in the hobby. Despite the fact that many people will advise you to the contrary, with the proliferation of decent quality medium aperture telescopes coming out of Asia in recent times I would consider an 8” to be the minimum starting point.
This 8” collapsible tube dob would be a great start. At F6 it is easy to use, collimation isn’t super critical and it is a little more forgiving on cheaper eyepieces.
http://www.astroshop.com.au/products...sp?id=MAS-041E
If you wanted to go a bit bigger and spend more money you could go to a 10” scope. These cost more and will generally be either F4.7 (Skywatcher collapsible) or F5 (GSO solid tube) which means collimation becomes more important and you need to use better quality eyepieces to get good views.
http://www.astroshop.com.au/products...sp?id=MAS-041F
Personally I think the 8” gets you into a good easy to use scope which is fairly portable / transportable, without spending a lot of money. If you enjoy stargazing then upgrade to something bigger and better in a year or two’s time. Your 8” will always be saleable 2nd hand and you would recover some of your initial outlay.
Cheers,
John B
