Meade 90mm refractor question

[Kirkus]

Hi everyone!

I have the opportunity to buy a used Meade 90mm DS-2090 (Refractor) for $200 U.S. It's been discontinued, but was definitely an entry-level scope.

I would want it for planetary imaging using my webcam, etc.

I currently have a 6" Meade Reflector (FL 762 - f/5). The DS2090 is FL 800 - f/10.

In your opinions, would I get that much better planetary images from this refractor than I do with my reflector?

As always your insight is very much appreciated.

[Kal]

I would say a resounding no, the refractor will definately be a big step backwards.

- You are losing 2.5" of aperture, and aperture is important
- The refractor will be an achromat, with chromatic abberation, especially at the long focal lengths and high resolution that you will need to work at. Reflectors have no chromatic abberation.

[renormalised]

I agree with Andrew.....the refractor will be a big disappointment. It just doesn't match up to the reflector you have.

[Kirkus]

I had no idea. I knew I'd find the answer here. Thank you so much for the answers.

[renormalised]

How much money would you be prepared to spend to get yourself a good refractor??

[Kirkus]

Actually I've been saving for a larger reflector with a fast focal ratio. Somewhere around 8". I just happened upon this refractor ad locally and the price seemed reasonable.

[renormalised]

Well, in that case get yourself the lightbucket and forget about the refractor....although an ED100 on a good mount (say an EQ6Pro) would also make an excellent scope. Not quite the lightbucket the 8" would be but the planetary images will be crisper and your skyglow much reduced in both eyepiece and CCD. Yet you'll still get fabulous piccies of nebs, galaxies etc. Just have a look at some of the piccies here.

[Kirkus]

Thanks, Carl.

So it looks like I could pick up a new Orion ED100 for around $700 (US). That's a little more than what I was going to pay for an 8" reflector. But is there really that much difference between my 6" SN and an 8"?

It's almost making more sense to keep my 6" for my attempts at DSOs (a much bigger challenge) and spend my money on an ED100 for use with planetary. Does that sound about right?

[renormalised]

Yes, the 8" will gather 177.78% more light than the 6", so you're going to be able to see much fainter objects. The ED100 will be great for planetary work, but it'll also be good for DSO's as well. The big advantage with the ED is that it'll be a lot more portable than the 8". Your 6" with a good CCD or DSLR will do DSO's a lot of justice, and being a SN you should have fairly well corrected shots across a rather wide FoV.

Another thing to consider is you will need a guidescope and guider for when you start to take piccies. You don't want something terribly big for that, a 60-80mm refractor will suffice. It could be a cheap achro' refractor as you probably won't use it to take piccies...say like an Orion 80mm short tube and their guider. That would make a good combo for guiding.

However, if you got a Meade 8" SN and the guidescope combo, you'd have a really sweet setup to take piccies with. It would be really good for DSO's (especially large fuzzies at its native FL/f ratio) and with a powermate in the image train it would be good with planets as well. Though, I would replace the Meade mount with an EQ6 or similar, as the Meade mount won't handle the weight you'd have setup on it.

[Kal]

The 6" reflector will be better than the ED100 for planetary, and for the same points as mentioned in my first reply - bigger aperture, and the reflector has zero chromattic abberation.

The ED100 will have much improved chromatic abberation compared to the 90mm achromat, but the reduction in chromatic abberation is suitable for visual work, where the eye is less sensitive than any CCD, and low power deep sky astrophotography, where the chromatic abberation that is present will be confined to only a pixel or two because of the short focal length work. To get any image scale for planetary imaging, you will need to run that ED refractor at maybe F40+, and this immediately means that those 1-2 pixels of chromattic abberation become 5-10 pixels, destroying any contrast on the features.

Stick with a reflector type scope for planetary (wether it be a reflector, a schmidt cassegrain, or a makstutov cassegrain), and use the refractor for deep sky astrophotography. If you want an idea of what type of scope is suited to what type of astrophotography spend half an hour in the "images" section of these forums and look at the types of scopes used for planetary and deep sky work respectively.

[renormalised]

Yeah, that's true....no CA, but the longer the FL the less CA you have. Around f12-18 it's pretty much non existent and in any case, you can get rid of any CA by post processing of the images. However, the big advantages a short FL/f ratio refractor has over a reflector in any sort of imaging, planetary and DSO's is that it's far more portable, has far less loss of contrast hence much sharper and darker images (less skyglow and loss of light in optical path), far less need for collimation of the optics and they're far less affected by tube currents and cool down a lot quicker than a reflector. The only real advantage a reflector has, is its light gathering capacity and no CA. The best scopes for planetary work are long focal length refractors, but an ED with a powermate might help. The refractors are far more tolerant of not so good seeing than the reflectors are. Have a slightly humid night with temperature fluctuations and a slight breeze and any image you take with a reflector will look like it's been taken through a fishtank!!!. That's why long focal length refractors make much such good planetary scopes, but they're expensive. If you're interested in good, long focal length refractors, here's a site to goto... http://www.dgoptical.com/
(http://www.dgoptical.com/)
The reason they made apo refractors with three element lenses and fluorite is that it was the only way they could create short, fast portable refractors that had little to acceptable CA (or nearly none at all).

If you go for a mirror telescope, get an SCT. Being enclosed helps, but they still suffer from tube currents and such. Their big advantage is their focal lengths and f ratios are fairly large....f10-f12 for most models, which is far better for planetary viewing and small faint fuzzies.

[Kal]

You can't barlow out CA in a refractor, so any CA present at it's native focal length will simply be magnified as you increase focal length through barlows and powermates. Since you are going to need long focal lengths for planetary imaging this is not a good thing. The amount of CA present is dependant just as much on aperture as focal length, so you can't accurately state any focal length range where CA is non existant in a refractor.



True, but I would add that it's not just light gathering advantage for the reflector, but also resolution advantage with the larger aperture. Also contrast between a planet and skyglow is a non consideration with planetary imaging. As long as it is night time you can image a planet from the middle of a bright city and still get great results.



Simply not true. All of the worls best planetary imagers use reflectors or CATS. A quick browse of the images section of this forum will confirm this.



Tolerance to seeing is a function of aperture, not design of the scope. Typical atmospheric cells that affect seeing are around 6" - 12" in size, so going above this aperture range you will need to be more considerate of seeing conditions when imaging.
(http://www.dgoptical.com/)

[jjjnettie]

Another thing to take into account.
Refractors fog up at the drop of the hat. You NEED to have some sort of dew control. Which means spending more $$$ and having more cables and leads draped around your mount.
Reflectors are relatively immune to dew.

[renormalised]

CA is entirely due to the degree to which the lenses of the scope bring the various wavelengths of light to focus at the focal point. It has nothing to do with aperture and everything to do with how the lenses are figured and the refractive indices of the elements of the lens assembly. The reason why they use fluorite is that it is optically more transparent than normal Schott glass and has a low RI which means there is little or no subsequent deviation of the light path through the lenses and hence little or no misalignment in the focusing the various wavelengths of light...they are brought into focus as close to the desired focal point as you can get.



I forgot the resolution (oops!!), but you are correct there. Yes it is...any amount of light which effects the contrast of the features on a surface will effect the quality of the image. What looks brighter, a white circle against a dark blue background or a white circle against a nearly black background. Of course you'll get a good image with a CCD or even an DSLR against skyglow for the simple fact that you can post process out a lot of skyglow. Plus you have a bright target.


All of the world's best planetary images are taken with reflectors or cats because they cheaper than an equivalent sized, long FL refractor (or even a short FL one, at that) and they're using their large light gathering capacity to overcome the difficulties I mentioned earlier. Just because those piccies taken here and elsewhere by amatures of planets, are done with reflectors and/or cats is neither here nor there. Would anyone here be able to afford the $90000 to $200000 (or more) it would cost to buy an 8-10" f10-f15 APO??. Or even shell out $8000-$10000 to buy a very good 10-12" F12-15 Achro??. Most wouldn't consider the achro on the basis of how smaller and faster ones perform and the apo would be way out of range of most people. If you've ever looked at a planet, or double star or the moon through a good, long FL apo or achro of a reasonable size, you'll know what I mean when I say that there's no comparison. The refractor blows the reflectors away for clarity, crispness and resolution of a target.



Tolerance to seeing has a hell of a lot to do with the design of a scope. By their nature alone, mirror based scopes, such as newts and SCT's are plagued by tube currents, cooling difficulties etc etc. Sure, refractors do suffer from tube currents to an extent but the optics are far less prevalent to image distortion etc, caused by tube current variables and the tubes of refractors are essentially closed systems. Only an SCT is closed like a refractor, but they have a greater thermal mass due to the greater volume of air inside the tubes that's closer to the mirror cf to a refractor, therefore are far more prone to image distortion via tube currents. Aperture is also a consideration where external factors of the weather are concerned...and in any case a misty/cloudy/windy or whatever sky will seriously degrade any type of scope's performance. But with SCT's or newts, the smaller the scope the better the performance w.r.t. tube current effects, as you have mentioned.

[renormalised]

True, but SCT reflectors can be just as bad. Even in winter, where I live, I still get plenty of dew on my corrector plate. Newts are less prone to dew, due to their open construction, but when it's humid that's different entirely. It all really depends on location....if you were living in the Atacama at anytime of the year, you'd be lucky to even crack a sweat, it's that dry!!!:eyepop::P:D But here in Oz, you really need to have a dew heater for any sort of scope.

I just had an idea.....if you could do it. It'd probably have to be done at the manufacturer's plant, but what would solve the (internal) dew problems of any closed tube scope would be to purge the tube with pure nitrogen or argon gas. It's dry and would help to drive out any moisture that might be in the tubes.

I wouldn't like to try and do it myself.

[Kal]

As I stated above, CA in a refractor is dependant on aperture as well as focal length. If all things are equal, and you have the same design and the same glasses used, and you went from a 4" F8 refractor to an 8" F8 design, everything will scale, including the spot sizes at the focal plane. To achieve the same amount of correction you therefore need to double the focal length, so a 4" F8 refractor will have the same amount of color correction as an 8" F16 refractor. You can see why refractors are not popular at larger apertures!

The reason why fluorite is used is used has nothing to do with it's transperancy compared to schott glass. For starters, schott is just a company, and they make all kinds of glass, and not just for telescopes. The reason why telescope makers use fluorite and PFL-53 glass is because of their dispersion value off the abbe line. To get better colour correction you need to use at least one glass that deviates from the abbe dispersion line, and fluorite makes a good match to the readily available and cheap BK7.

[renormalised]

I forgot to mention dispersion, thanks.

Unfortunately fluorite it's not cheap, otherwise we'd all have PFL-53 glass in our refractors!!

Anyway....I was comparing like with like, not the ED with the 6" SN. Naturally the 6" would outperform the 4"....on all accounts.

BTW...that glass made by Schott, is called "Schott Glass"....it's just a manufacturing name, as you'd already know. However, the best glass ever made is a "shot" glass....with a wee drop of the good stuff to go with it:P:D:D

Just like my take on the "Galileoscope"....called the "Gallianoscope", and you get to see plenty of "stars" with it as well:P:D:D

[Kirkus]

I love this place!

I only caught about half of all that, but I'm going to reread it. I appreciate the discussion and will learn from it.

I already have a guide scope. It's a cheap 60mm short tube that works really well. I can get the long exposures (not as long as I'd like due to light pollution where I live).

I can't move beyond my LXD75 at the moment, so weight is a huge consideration. Not too bad for planetary since it's only carrying my webcam. But for DSOs the guide scope and DSLR weigh it way down.

Anyway, I'm going to go out and get a glimpse of Jupiter before the fog rolls in (I'm about 1/2 mile from the coast) then come back in and read this thread again.

Thank you all so much! :)

[renormalised]

You want to have a try of my "Gallianoscope" sometime. Doesn't matter how cloudy it gets, you'll always see stars:P:P:D:D