Astrophotography: Newtonian v SCT

I've been looking into buying a scope (currently using an 80-200 2.8L) for astrophotography. A friend recommended a Schmidtt-Cass, and I can see the benefits as they are fairly compact compared to a Newtonian. However, prices on the SCT are much higher than a comparable Newt.

Does anyone else use a Newtonian telescope? Aside from the size, are there any major disadvantages?

Tyguy wrote in post #18326980
I've been looking into buying a scope (currently using an 80-200 2.8L) for astrophotography. A friend recommended a Schmidtt-Cass, and I can see the benefits as they are fairly compact compared to a Newtonian. However, prices on the SCT are much higher than a comparable Newt.

Does anyone else use a Newtonian telescope? Aside from the size, are there any major disadvantages?

The advantages of a SCT is a shorter tube , easier for transporting and setup . The disadvantages of a NEWT is longer tube , slightly longer setup and not as easily transported .

SCT uses a primary and secondary mirror and a corrector plate at the front of the scope . A NEWT uses a primary and secondary mirror but no corrector plate . The corrector plate is used to correct the spherical aberration in the primary mirror , otherwords when light enters the scope it's RGB colors are corrected to align properly to eliminate chromatic aberration which is usually is a blue or purple shade at the edges of bright objects like stars or planets or the moon . This is because of the SCT being so short and light is bent 3 times before it exits the eye-piece you look in . NEWTs don't have this problem because light is only bent twice and the tube is much longer allowing the RGB light to align more properly at the exit hole of the scope .

Attaching a camera to a SCT is much easier cause all you need are proper attachments for your camera to fit the SCT . Attaching the camera to a NEWT is a little more awkward and you may have problems with focusing because of the length of focus . Some NEWTS won't come to proper crisp focus with the factory focuser and some imagers change the focuser to a more costly but precise focuser on the NEWT .

Since the NEWTS do not have a Corrector plate at the front of the scope tube the mirror is more likely to collect debris over a period of time which depending if the wind or sand is blowing could cause problems with imaging cause of a dirty mirror and if it gets dirty enough it has to be cleaned and if your not a pro at cleaning scope mirrors you'll need to get a professional to clean it for you . Cleaning the mirror on a scope is not at all close to the same as cleaning your camera lens . Since higher power is used taking images with dirty mirrors can be a disaster in a ruined image .

The Corrector plate on a SCT not only helps align RGB light but it also is mounted at the front of the scope preventing debris from falling inside the scope and allowing a clean mirror much longer . For debris to enter the SCT the eye-piece or diagonal on the rear of the scope must be removed which even then if wind is blowing or sand is blowing it can enter the rear opening that is unprotected . The Corrector plate can be cleaned fairly easy taking great care not to scratch the plate or drop the scope and cause it to be cracked or broke . But the mirrors are best cleaned by a professional also but if care is taking of the scope it can be long periods of time before needed .

Now all the basics needed to attach a camera to a SCT are basically the same for a NEWT with proper attachments . Main thing you have to worry about as I mentioned earlier is the Focuser having a long enough focus draw . But when deciding what type of scope is best will be what your personal desires are , and just how serious you are at wanting to invest in a scope and attachments ? Thing is if you have a scope it does not benefit you fully if all you do is try to image and never just try to observe also . Seeing the objects you want to image in a telescope is highly addictable to the astronomy field hobby and can if you allow it become very expensive over a period of time .

So that's something to really think about investment wise and your attention span . Some find out this is not really the hobby they want after they have been shooting landscape and wildlife and professional images such as portrait images especially after they have invested in high cost cameras and lenses unless they were specific for astroimaging . Anyway hope this helps explain some on the difference between the two scopes .

Celestron alluded to the focusing issue with newts... there is such a thing as a "newtonian astrograph" and if you want to use a newt then you'd really want to make sure you got that variation. The major difference is that the focal length of a scope (in this case a newt) is designed to produce a focused image at some specific distance from the primary mirror. Suppose you have a scope with a 700mm focal length (I'm just making a number up)... that means that 700mm after the light hits the primary mirror it will come to focus. SO... Newts have the focuser designed to hold the eyepiece at that distance (and allow some adjustment for variations in eyepieces.)

The "problem" with astrophotography is that a DSLR camera's sensor is about 2" (about 50mm) behind the lens mounting flange. This is too far away (it would be like trying to get a 700mm scope to focus at a 750mm distance... the mirror isn't designed to focus at that distance. You'd adjust focus inward... but usually you'll hit the travel limit on the focus adjustment before the image is focused.

In an ordinary newt, you could shim the primary mirror forward 2" (and just use a 2" extension tube on all eyepieces to compensate) but that means the primary mirror is now closer to the secondary mirror and some of the light is spilling off the sides of the secondary instead of reflecting to the eyepiece. It's "as if" you bought a smaller diameter scope because now you're losing light.

A Newtonian astrograph takes all this into account... the primary mirror is in the right position, the secondary mirror is sized appropriately, etc. and the whole thing works much better for astrophotography (and you can still use it for visual astronomy by putting a 2" extension tube on the focuser -- many of these scopes just include the extension tube.)

But the really big difference in the Newt and the Schmidt Cassegrain is the focal length.

Most Schmidt Cassegrain scopes (or "SCT's" or just "CATs") are f/10 focal ratio scopes and they tend to have long focal lengths. An 8" CAT typically has a 2000mm focal length. A 10" scope is about 2500mm. I have a 14" CAT -- it's about 3500mm.

In contrast, most Newts are lower focal ratios (maybe f/4) and the focal lengths are much shorter. An 8" f/4 scope would have a focal length of roughly 800mm.

So there's huge difference in the area of sky you'll be able to image at 800mm vs. 2000mm (comparing two 8" scopes). Large-ish structure objects do better in shorter focal length scopes. The long focal length scopes do better at smaller objects. If you wanted to image... say the Andromeda galaxy using a 2000mm scope, you'd need to take several "mosaic" panels and piece them together.

There is one more issue/complication... which is that the longer the focal length, the more sensitive it is to any tracking errors. Short focal lengths are a bit forgiving but long focal length scopes are not. This means astrophotography in a long focal length scope is a bit more challenging.

TCampbell wrote in post #18328103
Celestron alluded to the focusing issue with newts... .

No , I didn't allude anything and how rude of you to pick that out and make that comment ! I said all that was needed about the focuser which you know good and well it to be true that focusers have been a problem on NEWTS when it came to imaging !! He wanted to know the advantages and disadvantages of both , I gave proper info to all basics of both to each scope . I just didn't feel like trying to be a professor of science and projecting that I knew everything better than anyone else here when there is NO PROFESSOR on this forum !! . I'm glad I don't go to your club cause you and me would not get along very well ! You make it really unpleasant to be here at times . You give so much info here that's not needed and never produce images yourself .

Tyguy wrote in post #18326980
I've been looking into buying a scope (currently using an 80-200 2.8L) for astrophotography. A friend recommended a Schmidtt-Cass, and I can see the benefits as they are fairly compact compared to a Newtonian. However, prices on the SCT are much higher than a comparable Newt.

Does anyone else use a Newtonian telescope? Aside from the size, are there any major disadvantages?

Heya,

You can do astro with any scope.

But each one has a strength for different forms of astro. A newtonian for example is a short fast instrument and an SCT is generally a longer slower instrument. Both have different attributes that you'd have to correct for in some way. But there's nothing wrong with an inexpensive newtonian reflector, nor a small SCT. As long as you have something that can hold them, they can do the job. Depending on what you really want to image, a refractor may be a better choice. But again, it all comes down to what you're wanting to image, and how.

Really though, before getting into it, you would have to have a tracking mount capable of carrying a lot of weight to even begin this.

Do you already have a tracking mount?

Very best,

Celestron, TC, Mal - thanks for all the info and advice (especially Celestron, a comprehensive and well-thought out post).

It seems I'm still unsure what type of scope would be best yet. I've been having fun imaging with my 200mm, but don't have experience with longer focal lengths. Maybe it would be wise to try a more affordable scope before committing long-term with more money. SCT's seem to be several times more expensive than comparable Newts.

The corrector plate sounds important for keeping the mirrors clean, especially when buying used (which is likely the road I'll go).

I have a mount for my camera, but it's not suitable for a scope. I intend to build another mount for the scope when the time comes, but the design will depend on the type of scope.

Heya,

Mount first, scope second. You can't image with a scope without an appropriate mount that can handle the weight and the focal length.

Mounts like: Orion Sirius / HEQ5 / EQ6/ Orion Atlas / CGEM / CGEM DX, are where I'd start looking. Opening new cost of $1200~2,000. Used can go a lot less if you find local.

If that's a scary thought, then don't buy a telescope for imaging. Just being as frank as possible to avoid you throwing a few hundred down on a scope only to find out it takes $2500 to image, entry level, realistically with telescopes.

Very best,

Not scared about the mount. As mentioned, I plan to build one. My current mount works very well using a 280mm lens, has an ASCOM driver for autoguiding, and a plan for significant improvements is in place for the next one.

I can definitely appreciate your advice though, and wouldn't look into a telescope without also making plans to mount it. But at this point I am much more concerned with the type and size of scope.

Mal above uses a C6 Celestron SCT and is an excellent scope for imaging . Just check out his sun images and you can see what I mean . But a 6" SCT is very light and Celestron makes an excellent scope at affordable prices .

Heya,

Choosing a scope type will have a lot to do with your mount's limits, the camera sensor size (relating to FOV) and the subject matter.

A newtonian's strength is the potential speed of the focal ratio and the larger aperture for the least cost. The offset is that the focal length is not long and the size/weight is pretty enormous (they're proverbial wind sails when under-mounted), require correction options (adds to cost), etc. They can be great for imaging DSO and general visual use. They can be ok for planets, but only with larger apertures and expensive powermate barlows to get enough magnification to get some pixels on the subject. Pretty good for lunar, if the aperture is large enough, again, needing powermates to really get the magnification. The nice thing of the newt reflector is the inexpensive quick access to F4 optics with a decent focal length for DSO (such as a 6 inch F4 astrograph for about $299 new). Add correction ($150) optics. Much cheaper than a comparable refractor and SCT's can't get here for this cost.

A SCT is a long focal length slow focal-ratio instrument that is great for planets and lunar. They can be ok for DSO, but only with a very good setup and focal reduction and other adapters (like faststar, etc). Traditional SCT setups are not great for DSO without serious mounts, guiding and lots experience. SCT's are very easy to image with for planets and lunar though and that's where they really shine right away without anything special added. SCT's are compact and fairly easy to mount at 8 inches and under, but over that the weight goes up fast and the mount requirements go up with it. If you're into planets & lunar surface, an SCT is an ideal way to start. Also, SCT's can be cheap used, such as the C6 ($250), C8 ($450), C9.25 ($650) on the 2nd hand market, for what you get with them.

Refractors are a great do-all tool, ideal for short focal lengths and color correction and flat fields and wide FOV for large DSO. If you're into DSO, a refractor is the way to go. I recommend a refractor for everything short of planetary work honestly, as a starting point. Combine with a large sensor camera and the FOV is wide and you can tackle the big DSO's. Big refractors become extremely heavy and extremely costly quick. I'd stick to short refractors aimed at imaging, like an apochromatic with a focal length of 400~600mm and F6~F7.5 or so. You will want a flat field corrector and potentially a focal reducer. Used market is huge for these as they're commonly traded (the small ones).

I image with SCT & Refractor.

I also do a lot of ultra narrowband imaging (solar in hydrogen alpha) but I do this with refractors.

Very best,

Celestron wrote in post #18328289
No , I didn't allude anything and how rude of you to pick that out and make that comment ! I said all that was needed about the focuser which you know good and well it to be true that focusers have been a problem on NEWTS when it came to imaging !! He wanted to know the advantages and disadvantages of both , I gave proper info to all basics of both to each scope . I just didn't feel like trying to be a professor of science and projecting that I knew everything better than anyone else here when there is NO PROFESSOR on this forum !! . I'm glad I don't go to your club cause you and me would not get along very well ! You make it really unpleasant to be here at times . You give so much info here that's not needed and never produce images yourself .

I'm not certain what I've ever done to deserve this hostility from you. I generally get positive comments from people who appreciate that I provide helpful responses.

Those responses are meant to be helpful for the person who posed a question. If you don't personally like them, then just skip past them. It's that easy and it doesn't have to bring you stress in your life.

That could have easily been avoided had you not pointed me out and saying I Alluded information . This is not the first time you have done this to me . You insulted me , therefore I responded best I could .

Celestron wrote in post #18329666
That could have easily been avoided had you not pointed me out and saying I Alluded information . This is not the first time you have done this to me . You insulted me , therefore I responded best I could .

"Alluding" is an insult?? You might want to look up that word. It's not a word typically used to denigrate.

You mentioned a few aspects of focusing issues on different types of scopes. The most common focusing issue with a reflector is the spacing... the camera sensor plane often ends up being too far from the telescope's focus plane and the focuser doesn't have enough travel to compensate. That's why I added that to list of things to consider... it's the most common issue.

There was no insult intended and I'm perplexed as to how you could possibly think there was.

But then you said "this is not the first time you have done this to me." I don't recall insulting you any other times previously. I generally get along rather well with people. I feel as if somehow you are taking comments made by me as if they are somehow intended as personal insults toward you. I assure you they are not.

If I see information missing or misstated (or sometimes just wrong) then I might post a comment to make sure the person asking a question isn't further confused. But that's not an insult. I sometimes mis-state things myself. Every adult should (hopefully) be able to recognize that and take it in a healthy, constructive way and not regard it as a personal attack.

Celestron wrote in post #18327427
The advantages of a SCT is a shorter tube , easier for transporting and setup . The disadvantages of a NEWT is longer tube , slightly longer setup and not as easily transported .

SCT uses a primary and secondary mirror and a corrector plate at the front of the scope . A NEWT uses a primary and secondary mirror but no corrector plate . The corrector plate is used to correct the spherical aberration in the primary mirror , otherwords when light enters the scope it's RGB colors are corrected to align properly to eliminate chromatic aberration which is usually is a blue or purple shade at the edges of bright objects like stars or planets or the moon . This is because of the SCT being so short and light is bent 3 times before it exits the eye-piece you look in . NEWTs don't have this problem because light is only bent twice and the tube is much longer allowing the RGB light to align more properly at the exit hole of the scope .

Attaching a camera to a SCT is much easier cause all you need are proper attachments for your camera to fit the SCT . Attaching the camera to a NEWT is a little more awkward and you may have problems with focusing because of the length of focus . Some NEWTS won't come to proper crisp focus with the factory focuser and some imagers change the focuser to a more costly but precise focuser on the NEWT .

Since the NEWTS do not have a Corrector plate at the front of the scope tube the mirror is more likely to collect debris over a period of time which depending if the wind or sand is blowing could cause problems with imaging cause of a dirty mirror and if it gets dirty enough it has to be cleaned and if your not a pro at cleaning scope mirrors you'll need to get a professional to clean it for you . Cleaning the mirror on a scope is not at all close to the same as cleaning your camera lens . Since higher power is used taking images with dirty mirrors can be a disaster in a ruined image .

The Corrector plate on a SCT not only helps align RGB light but it also is mounted at the front of the scope preventing debris from falling inside the scope and allowing a clean mirror much longer . For debris to enter the SCT the eye-piece or diagonal on the rear of the scope must be removed which even then if wind is blowing or sand is blowing it can enter the rear opening that is unprotected . The Corrector plate can be cleaned fairly easy taking great care not to scratch the plate or drop the scope and cause it to be cracked or broke . But the mirrors are best cleaned by a professional also but if care is taking of the scope it can be long periods of time before needed .

<deleted stuff>
.

I believe you have confused spherical and chromatic aberration in your explanation of the SCT corrector plate. The function of the corrector plate is indeed to correct the primary mirror's spherical aberration, which is all of the light not focusing at a single point and has nothing to do with the different wavelengths (colors) of light not focusing at the same point (chromatic aberration). Also the Newtonian telescope does not suffer from chromatic aberration since it uses mirrors to focus and reflect the light (instead of lenses), not because "light is only bent twice and the tube is much longer allowing the RGB light to align more properly at the exit hole of the scope". Note that the eyepiece in a Newtonian telescope may introduce chromatic aberration, but the bare optics (primary and secondary mirrors) do not cause chromatic aberration since light does not pass through them and is only reflected.

I think you need to go back and reread what I wrote I never said a Newt produced CA , I actually said the opposite that Newts' don't have this problem . After you reread that you might look up the difference between SCTs' and Newts . You might find what I said is true especially referring to the reasons for a corrector plate on a SCT . Scopes are similar to camera lens as being referred to as just a big lens only because they do the same job and the only scope that can be truly compared to is a glass lens is a refractor . Both have glass at the front and rear of the tube . The SCT can only be compared to a Mirror lens which is not a popular lens . The Newt cannot be compared to any cause it has no glass or corrector at the front of the tube . All a Newt can be compared as just being a big lens which is just a statement of words for resemblance . But like I said do your research on the corrector plate . I think you'll find I'm right .

https://en.m.wikipedia​.org/wiki/Schmidt_corr​ector_plate