Author Topic: Understanding Reflectors...  (Read 1124 times)

Bryce Roberts

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Understanding Reflectors...
« on: December 31, 2017, 08:17:55 AM »
Hi all,I think my first telescope was actually a reflector, a small 4.5" one. Long gone now of course. I had a refractor back then too, a really poor one.Anyhow, 20 years or more later, I'm back on telescopes.I'm trying to wrap my head around reflectors and their use today for <em class="bbc">astrophotography purposes. For visual, I actually like reflectors since they're big and bright. But for AP, I'm curious where their place is for everyone. I currently have an 80mm refractor (600mm F7.5) and a 152mm SCT (1500mm F10) and I image with both of them. I read a lot about people saying to use shorter focal length refractors for imaging for DSO and the like. And to use SCT's for planets and the lunar surface, etc. Granted you can use anything for anything really, but some things just are better for imaging an object than another. For example, refractors just don't seem to have the reach for a planet that a SCT does. And SCT's seem to be pretty slow for DSO, unlike refractors. And that sort of walks me into reflectors again. At first my allure to the reflector is the speed of the system, being able to get F4 systems right out of the gate, but they will not stay F4, and require some extra adapters to calm down and flatten the field, etc. I've seen people use huge reflectors for imaging DSO. And I've seen of course people using refractors for the same thing.So I'm trying to figure out, if a reflector eventually makes sense for my uses.I'm currently imaging DSO with my 80mm refractor. To get a faster, larger refractor, it would cost more thousands basically (to get something on the order of 5" or 6" for a refractor). Mean while, my refractor isn't great for what I like regarding the moon and the moon surface, so I use my SCT for that stuff. I use my 6" SCT for planets (Jupiter, Saturn, moon, etc, with an ASI178MC) and mostly use it for lunar surface. I use my refractor for DSO (with an APS-C).I know I'm not going to get tremendous reach from a reflector, unless using barlows or powermates. I also can't even mount and track with a big reflector on my tracking head (Orion Sirius EQG / Skywatcher HEQ5, maxxing out at 30lbs payload, so 15lbs would be my safe zone of imaging instruments, though maybe I could push closer to 20lbs if it was worth it, I don't do super long subs). So I was trying to think of what an 8" reflector, like the Orion 8" F4.0 astrograph would be able to do for me. As is, with a field flattener, it seems it would be good for bright, fast subs, wide field and large object DSO. It would cut my subs down to less than half the time compared to my F7.5 refractor, and it has similar focal length. It's more than twice the aperture size than my current refractor, so it gives me more light to gobble for the fainter stuff. It seems to me that it could potentially take a 4x power mate for planets or lunar stuff with that aperture. I'm just curious if that would be worth it or not.So again I'm just trying to see if I can wrap my head around the use of a reflector in my current practice and future imaging.1. Suitable to replace my current 80mm refractor for DSO (assuming 8" reflector, I can't handle larger than that with my mount)?2. Does it compete or compare to my current 6" SCT (again regarding an 8" reflector) for moon, moon surface, planets?3. I have APS-C as well as ASI178MC for cameras right now, if that helps solve any questions.I'm currently thinking about potentially getting an 8" SCT in the future for the moon and planets.But, I'm also wondering if I get an 8" reflector, how that might compare or add to my kit overall.I don't feel like my refractor is holding me back, as I'm still learning, but I'm curious about a reflector doing the same job as the refractor (DSO) and giving me more light for the exposure time, and wondering if a big reflector will compare or do anything that my SCT (6") is doing before I consider another one just yet.Just seeing if I'm thinking right regarding the reflector.Would love to hear everyone's thoughts on reflectors in general and why you use them over refractors or SCTs and for what.Very best,



adviconno

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Re: Understanding Reflectors...
« Reply #1 on: January 01, 2018, 10:18:02 AM »
Quote
At first my allure to the reflector is the speed of the system, being able to get F4 systems right out of the gate, but they will not stay F4, and require some extra adapters to calm down and flatten the field, etc.
You can get f/2.8 newtonian astrographs with built-in coma corrector and field flattener. You can also get a coma corrector that will change any f/4 newtonian into an f/2.8. An f/3.6 version is also available.

If you go with Celestron's FASTAR system, you can even get down to f/2.Clear skies!
Thomas, Denmark

ransgesislu

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Re: Understanding Reflectors...
« Reply #2 on: January 03, 2018, 08:57:12 AM »
Good point, though I'm sure those F2.8 systems cost considerably more than an 8" F3.9 astrograph.

Very best,

holdfontrosci

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Re: Understanding Reflectors...
« Reply #3 on: January 04, 2018, 09:24:59 PM »
Imagining Newtonians. Fast, need a coma corrector (choosing one can be hard) and a good mount.

SCTs. Jack of all trades, master of planetary. Annoying aberrations, marginal focusers, slow.

RCs. The choice of many professionals. Inexpensive ones tend to be touchy to get working well (some just give up), expensive ones with corrective lenses are _very_ expensive.

With unlimited budget, I'd have an FSQ106 with reducer and extender for wild and medium fields, a corrected RC for the small stuff. Oh, and an SV70T with reducer for really wide fields (but I actually have that &lt;grin&gt;).

All mounted on an Avalon M-Uno. No backlash, no meridian flip. very smooth periodic error for easier guiding. That RC will need to be smallish and carbon fiber/titanium.

longtichaten

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Re: Understanding Reflectors...
« Reply #4 on: January 11, 2018, 02:13:35 AM »
Would love to hear everyone's thoughts on reflectors in general and why you use them over refractors or SCTs and for what.

I don't use reflectors over refractors &amp; CATs, I use them in addition to the other designs. Big aperture, simple optical path, and low cost per inch / aperture makes a lot of sense to me. When I got the comet hunter bug back in the early 80s, I didn't build myself a 6" f/4 achromatic RFT - I ground an 8" f/4.5 mirror. Learned a lot, and had a decent DSO hunter at a price I could afford.

Today I can buy what I want (within reason!). I have no desire for a 6" or larger hand-crafted APO. My Dream Scope is a vintage 10" / 12" Classical Cassegrain (and the Observatory to house it), though the brand spanking new CFF Casses sure look nice. Meanwhile, my XT12g, 6" Tinsley, and the rest of the menagerie keep amazing me with what they can show, and the collection (in my signature block) didn't cost a small fortune.

Brandon Belknap

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Re: Understanding Reflectors...
« Reply #5 on: January 13, 2018, 07:21:16 PM »
As you correctly pointed out, not anything is for anything.
Same goes here. Your refractor is 600mm, SCT is 1500mm and Orion 8" F/4 will be 800mm - so not a direct replacement for any of your current scopes in terms of FOV.
You will need a coma corrector with the newt. The weight increase is pretty significant, but also I found the distribution of weight in 8 f/4 is challenging. Since camera is mounted on the side, it adds a multidimensional disbalance which cannot be corrected with standard counterweights. This makes tracking less accurate. And don't forget the need to keep the newt perfectly collimated, or your images will be washed out.
One more point, aperture is not as important in AP as in visual. It's the focal ratio that counts. So maybe something like AT6IN could be a better fit as a refractor replacement?

From personal experience - I actually swapped 8 f/4 for a 80mm refractor and never looked back. Even though I now have to do longer exposures, I find it being easier than imaging with the newt (more portable, easier to balance, sharper images, no need for collimation - which I suck at, wider fields - which I like).

Douglas Preece

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Re: Understanding Reflectors...
« Reply #6 on: January 17, 2018, 12:29:49 AM »
Hrm,

Good food for thought.

I keep thinking of adding a reflector in the 8" range. Prices are variable for different scopes, but I'm a budget imager, so I'm looking at the $500~$1,000 range for a basic tube with more aperture than my refractor for DSO stuff. But not to replace the refractor as it has it's place too (I like it for solar, where the filters are not ginormous).

But I'm just trying to wrap my head around it as if it's worth while, compared to imaging with my current 80mm refractor (600mm F7.5). Going to a 800mm F3.9 reflector, that is. Gain quite a bit of speed, just under 2 stops, which drastically shortens exposure time. Seems like a plausible way to go, instead of trying to get a large, really expensive refactor that is closer to that speed and that focal length.

Also seems like if visual was going to be the way to go, I'd want to have a big reflector to look through for bright views.

++++++++++++

But realistically, maybe my refractor with a guide scope is overall the better generalist for DSO imagine right now. I'm not moving to buy anything soon, mostly just gathering information about reflectors since I already have a frac &amp; sct, so adding a reflector eventually might be interesting to have another option.

++++++++++++++

On another thought, maybe a 6 inch reflector is a happy medium? I'm seeing 600mm F4 reflectors out there for imaging for good prices, not too huge, not too heavy, and still get access to that faster F4. Just looking over the Astro-tech 6" F4 imaging newt. Competes with my refractor for similar FOV, but gives me significantly shorter subs.

But then again, with an autoguider, maybe this is all just not worth while as a 600mm F7.5 80mm ED refractor with an autoguider scope/cam is going to do the better all around job for DSO, just taking a little more time?

Very best,

Jeremiah Greer

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Re: Understanding Reflectors...
« Reply #7 on: January 20, 2018, 07:52:26 PM »
"instead of trying to get a large, really expensive refactor that is closer to that speed and that focal length."

Lol, good luck with that  Of course you said "closer" and f/6 iscloser, but if you were talking about really close you'd be talking about 8" f/4 refractor which might not even exist

Just to reiterate on this "with more aperture than my refractor for DSO stuff". Bigger aperture doesn't buy you anything for AP.
However, being able to use the scope for visual could be a good reason for consideration. For visual 8" will beat both other scopes you have and youwill see the difference.

handthedemo

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Re: Understanding Reflectors...
« Reply #8 on: January 23, 2018, 04:35:43 AM »
Astrophotography is about 3 things: focal length, focal ratio, the mount.

Focal length determines how big stuff is on the focal plane.
Focal ratio determines how bright stuff is on the focal plane.
The mount determines if all those photons get smeared out as you expose.

4 things actually: post processing work.

engoecircming

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Re: Understanding Reflectors...
« Reply #9 on: January 23, 2018, 07:14:26 AM »
Hrm,

Ultimately maybe it would be better for what I'm doing to look at a faster refractor in the long run for DSO, weight wise, and overall trouble wise. Was looking at the 120mm range refractors and they're roughly affordable. Would definitely like to find something F/5ish in terms of that, to potentially cut my exposure time in half from F/7.5. But again this is a long term work towards goal. Ultimately it may not matter if I stick to guiding and just go with longer subs. I just like the idea of shorter subs, lots more of them, rather than long subs.

Very best,

Scott Bentley

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Re: Understanding Reflectors...
« Reply #10 on: January 23, 2018, 09:01:42 AM »
Until you are ready to replace the mount with something that can carry a larger load and keep the stars round I would stick with a modest weight refractor. Once you have a quality mount that can handle a 45+ pound imaging load you will be ready for other options. I recently bought a Teleskop-Express f/4 10 inch carbon fiber Newtonian that I added to my scopes that I use for imaging and visual. It has replaced my C11 EdgeHD for most of my imaging.

Lesego Dowdy

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Re: Understanding Reflectors...
« Reply #11 on: January 25, 2018, 01:11:14 PM »
I think I understand.

I think the benefit of a reflector really is in the larger aperture ones where you're getting all that aperture for the cost making them truly bang for buck for areas where aperture is helpful. My mount definitely can't handle one of those large reflectors. Plus I don't want something so big I won't want to carry it around to use either.

Looks like I will probably shop &amp; save up for a better refractor in my future, and a slightly larger SCT. I like my 6" SCT but I think I'd like to get an 8" ultimately for what I use it for (planets &amp; lunar surface). My refractor is fine and doesn't need to be replaced, but I actually kind of think a shorter faster refractor would be more to my liking. I have a 600mm F7.5 but I actually think I'd prefer a 400mm F5 or something, if something like that exists around. I generally enjoy wider field on the refractor and the larger DSO's.

Very best,

Teflon Mayorga

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Re: Understanding Reflectors...
« Reply #12 on: January 31, 2018, 02:26:48 AM »
Esprit 100mm ED Triplet APO Refractor - 550mm f5

Takahashi FSQ-85ED Astrograph - 450mm f5.3

Takahashi FSQ-106ED Astrograph - 530mm f5

I'm sure there are more but..

Guy Cleveland

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Re: Understanding Reflectors...
« Reply #13 on: February 09, 2018, 09:00:31 AM »
Quote
Astrophotography is about 3 things: focal length, focal ratio, the mount.

Focal length determines how big stuff is on the focal plane.
Focal ratio determines how bright stuff is on the focal plane.
The mount determines if all those photons get smeared out as you expose.

4 things actually: post processing work.
This is one of the most concise explanations I've read on CN of the main factors affecting choice of imaging rig!
If only this were on the recent mammoth thread on focal ratios / f-ration myth etc...

tioraigenroi

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Re: Understanding Reflectors...
« Reply #14 on: February 09, 2018, 11:35:02 AM »
Oh I forgot the incredible...

Vixen VSD 380mm F/3.8