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Refractor or Reflector?


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15 minutes ago, Ravi Hegde said:

In 102/1000mm refractor, how sever the CA will be?

https://www.youtube.com/watch?v=BUtLS-rkLuY

https://www.youtube.com/shorts/qIxFAG8ctsM

https://www.youtube.com/watch?v=BIRivcUcMxU

Do note that the CA is often worse on video recording / image as sensor is more sensitive in short wavelengths then human eye is.

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Here is another good video - this time high power view of moon thru 120/1000

Note that 102/1000 will have less CA - and how CA on moon shows in crater shadows as slight violet cast instead of deep black

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33 minutes ago, Ravi Hegde said:

In 102/1000mm refractor, how sever the CA will be?

Only you can decide that. It's different for everyone. I had a Tal 100RS - a 100/1000 achro refractor. Personally, I thought it was very good, others may disagree. Only way to know for sure is to try for yourself.

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I detest CA, though I realise it doesn't bother many other people. That's why I went for good ED glass when I bought my first refractor: a Starfield 102 ED. It has FPL-53 and Lanthanum components. It has worked out very well - I can't detect any at all. That all comes at a price, of course.

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A good alternative to a long achro is the Maksutov. Near-apochromatic views, compact tube, don't really have to worry about collimation, excellent for planets and moon. Not expensive.

Downsides? Slightly narrow field of view, but not much difference from a long refractor, might take a bit longer to cool.

Swings and roundabouts for all types of telescope. Have you given any thought to what you want to observe?

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2 hours ago, Ravi Hegde said:

Nel rifrattore da 102/1000 mm, quanto sarà severo il CA?

There is this table. For a acromatic 120/1000 wich has f/d = 8.3 CA have a filterable level.

Tabella cromatismo rifrattori.jpg

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I talked a friend into buying a Helios 100mm F10 achromat back in 1999. I was very impressed by its performance on star fields and brighter deep sky and decided to buy one for myself. After placing the order I had a change of mind and ordered the Helios 120mm F8.3 achromat instead. I didn't really notice any distracting level of CA in the 100mm, but I expected significant CA in the 120mm. When the telescope arrived I was thrilled by the good quality equatorial mount and the silver top plossl eyepieces that came as standard. That first night there was a first quarter Moon and a clear sky. As I aimed the scope and brought it to focus I expected the F8.3 would show a chromatic glare, but to my amazement it wasn't there. Don't get me wrong, the scope did show some chromatic aberration but it wasn't destructive or distracting. For the price, the 120mm achromat was awesome and a nice all round performer. The 100mm has slightly better colour correction due to its greater F ratio, but you'd be hard pushed to detect the difference visually. It's when you get to the 150mm F8 that the CA becomes a problem, but really only on the moon and planets. For deep sky the 150 F8 was a wonderful scope. Over the years I've had them all, and the greatest concern for me is that there is variable quality in the figure of the objectives. If you get a good one you'll have a nice telescope with a 100mm achromat.

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10 hours ago, SwiMatt said:

Thanks all for these answers, I'm wondering about the same as @Ravi Hegde, but then I gotta ask: how much work is collimation really? So much has been written about it on the internet, but somehow it's still unclear to me. Especially in the beginning, when one is still learning, I'm afraid it could be so offputting that I would end up not observing at all.

Also: does the kind of usage of the scope matter? E.g. if one plans on having to walk 30 minutes for each session, would that cause the scope to need collimation every single time, and maybe in that case it's best to go for a low maintenance scope?

Collimation is easy.   It just takes a bit of practice.  The first time is always nerve wracking but eventually you will get it down and it only takes a minute

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18 hours ago, Mr Spock said:

I think we'll have to disagree on that one. My 120mm f8.3 is a Helios. It has good correction for SA, and so is detailed, but CA is very noticeable and it is distracting.

I understand why you might feel that way about the CA. It was definitely evident but wasn't as bad as I'd expected, so the initial impression for me was one of relief. Around that time a few of my friends bought different versions including the 150 F8 and 150 F5. Using several of these and using a ronchi eyepiece,  It soon became evident there was significant variability in figure and SA levels. It was a bit of a lottery as to whether you got a good one or not. Among the group of local observers several of the Helios scopes had to be returned and replaced. The replacements did offer an improvement, but at the time none of us were particularly interested in the Moon or planets. My own interest was in comets, and so the 120 F8.3 and 150 F8 were ideal, and CA wasn't a big issue.

 It wasn't until January 2003 that I had opportunity to observe Saturn through a 150mm F8 Helios achromat and a 102mm Vixen Fluorite. That was the night when my world was turned on its head, and I never used my Helios refractors after that night. It was also the night when my interest in planetary observing was ignited. Looking back, it was the best possible year for that to happen. I bought a Tak FS128 and all the planet's were riding high in UK skies, and Mars was at its largest and was spectacular. By the end of the year my enthusiasm for planets and for fluorite refractors was on fire.

Edited by mikeDnight
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On 28/05/2023 at 15:24, SwiMatt said:

how much work is collimation really? So much has been written about it on the internet, but somehow it's still unclear to me. Especially in the beginning, when one is still learning, I'm afraid it could be so offputting that I would end up not observing at all.

Dont be put off by it...heres the good part..if you are not collimated perfectly, its still OK. All it means is the scope is not making full use of the optics. The chance that your scope is delivered to you badly out of collimation is low so you should be able to make a start. If you are seeing stars with strange shapes other than round then you know you need to collimate.

Learning to collimate is a bit like learning to drive a car, initially it seems hard and confusing but in a couple of days it seems routine. And once collimated, you may just need to tweak it once in a while to make sure its still aligned.

Edited by AstroMuni
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On 28/05/2023 at 12:46, Ravi Hegde said:

For visual astronomy, which scope performs better? 130mm reflector or 102mm refractor?

One aspect thats also important is the mount. Get the sturdiest mount that you can afford. Generally the stock mounts supplied with these scopes are its weakest link.

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Collemation is not so hard like as been said scopes arrive collemated more times than not. Plenty of videos around that show you how.Many reflectors explain the process in their instruction manual.One point to remember long focal length scopes are easier and more forgiving of collemation  than short focal lengths. I use refractors , no need to fuss about with them,but that's just my preference. Good luck.

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For someone starting out in visual astronomy on your apparent budget, I would recommend the GSO 6 inch DOB Telescope.  It's a slow mirror, so figure and collimation aren't so critical.  There is no CA at all.  You can get by without a coma corrector as well at its f-ratio.  The Dob mount is rock solid stable compared to any budget Alt-Az or EQ mount.  The viewing comfort is nice with the right adjustable observing chair.

The only reason to not recommend it is its storage and transport size.  If you plan to take it on mass transit to get to your observing site, I would instead recommend a 102mm or 127mm Mak on an alt-az mount to avoid counterweights.

Edited by Louis D
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As others have said, collimation shouldn’t put anyone off getting a reflector and it is much easier to do than explain. The physics of walking or even getting out of bed is challenging and yet we do it every day without thinking about it. I think that is the best way to approach it.

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Ah the old debate...

Truth is, everyone has their own preference and you're not going to know your own until you try some scopes out. Nearly everyone here has bought something, then bought another to compliment or upgrade and so on. I think it's fair to say we're all striving for own own version of perfection on our own budgets, and that's hard! Hell just look at people's signatures on their posts, not many have just one scope, if at all possible, get to a star party and try some stuff, chromatic aberration might bother you, it might not, but there's only one way to find your own preference, look through some scopes 👍

I still don't know what my ideal scope is, and not sure I ever will 🤷‍♂️

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18 hours ago, doublevodka said:

Ah the old debate...

Truth is, everyone has their own preference and you're not going to know your own until you try some scopes out. Nearly everyone here has bought something, then bought another to compliment or upgrade and so on. I think it's fair to say we're all striving for own own version of perfection on our own budgets, and that's hard! Hell just look at people's signatures on their posts, not many have just one scope, if at all possible, get to a star party and try some stuff, chromatic aberration might bother you, it might not, but there's only one way to find your own preference, look through some scopes 👍

I still don't know what my ideal scope is, and not sure I ever will 🤷‍♂️

As they say the scope you use most is the best

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OP:

Between those two telescopes you have listed, I would choose the refractor.

However, in my opinion, a better all-around telescope would be the Bresser 8" (or 10" if money is not an issue) Dobson. This will also need the purchase of a collimator, but FLO sell a few inexpensive ones.

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Hi @Ravi Hegde and welcome to SGL. :hello2:

As previously mentioned, chromatic abberation is the curse with achromatic refractors. To give you an example of what it looks like, below is an image I took last week [Wed. 24 May 2023 @21:26BST] of a five day old Moon, using my Meade DS-2102, an 18mm eyepiece, iPhone 12 handheld over it; mounted on my AOK-AYO and Manfrotto 055PROB tripod.

641B6223-16FE-40B2-85FE-112CEB599E88.thumb.jpeg.6be4a5f780dfb53038929ab4d1f933d9.jpeg

It is the purple halo along the eastern edge.

CC5BBB93-4BAA-4EC0-A22F-636FCB99123E.thumb.jpeg.0854083a2435de70781fd802c48ff41b.jpeg

The image above was taken @22:51 on the same day/night, with same equipment. As you can see, the darker the sky, the more pronounced it becomes.

 

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31 minutes ago, RT65CB-SWL said:

using my Meade DS-2102

There is something seriously wrong with that scope :D

This is moon thru ST102 - single shot with DSLR at prime focus:

moon.JPG

Yes, there is some violet - blue halo (not very visible against black sky) and there is some yellowing on the inside of the limb - but nowhere near as in your shot.

Btw ST102 is shot focal length fast achromat - 4" F/5. It should be really colorful scope - but in reality it is not as bad as your image shows.

 

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