Newtonian - how fast can I go? Or Hyperstar?

[Tommohawk]

So I'm thinking that sometime soon I'll want to get more photons and am considering an upgrade. Probably something like a SW Quattro 10" (what was wrong with metric 250 I wonder?) or Orion f3.9 10".

Anyhow I cant find anything faster that these, and I'm sure there are good reasons for this.

However, in my investigations, I gather that an SCT can have the secondary replaced with a corrrector lens and be connected straight to a camera - Hyperstar type thing. In this configuration the mirror is f2 I think.

So, can I ask a few questions please?

1. How is it possible to get an f2 mirror on an SCT, but not in a Newtonian?

2. Why bother with an SCT design and then strip out the secondary - and then need a corrector lens?

3. Following from Q2 above - does anyone make a scope with a fast paraboidal mirror, and with camera attachment in place of the secondary?

I hope this all makes sense!!

Thanks in advance

[ronin]

The answer is really that a parabola is not the correct shape for a perfect image, therefore as you go faster you make the mirror sort of "more worse". In effect there is a point at which you get a faster mirror but a worse image so it is unusable.

Parabolic mirrors are better then spherical mirrors, that is not the same as they are the best/ideal profile.

SCT's and others simply put a corrector in to take care of the faster mirror, or to make a slow spherical mirror into an acceptable fast mirror.

In 3 you would have to have a corrector made for the fast parabolic mirror to get a usable image. Lots of the "professional" scopes have the camera at the secondary, no one looks through a professional scope so no need fopr eyepiece etc.

Not sure what you are asking in (2).

[Tommohawk]

The answer is really that a parabola is not the correct shape for a perfect image, therefore as you go faster you make the mirror sort of "more worse". In effect there is a point at which you get a faster mirror but a worse image so it is unusable.

Parabolic mirrors are better then spherical mirrors, that is not the same as they are the best/ideal profile.

SCT's and others simply put a corrector in to take care of the faster mirror, or to make a slow spherical mirror into an acceptable fast mirror.

In 3 you would have to have a corrector made for the fast parabolic mirror to get a usable image. Lots of the "professional" scopes have the camera at the secondary, no one looks through a professional scope so no need fopr eyepiece etc.

Not sure what you are asking in (2).

Hi and thanks for input. I think I get the point about the natural limit of a parabolic - so f4 ish as per the Quattro is presubably about it then?

<<Lots of the "professional" scopes have the camera at the secondary, no one looks through a professional scope so no need fopr eyepiece etc.>>

So does anyone make an f4 ish scope like the Quattro but with a camera attachment to replace the secondary? I cant say I've seen any.

Or maybe could I get/make/buy an f4 and replace the secondary with a camera attachment - but again I havent seen such an adapter.

In 2, all I meant was that if the SCT has an expensive secondary, which you then remove to replace with an expnsive corrector, this doesnt seem very good use of your £s!!

[Dave_D]

In 2, all I meant was that if the SCT has an expensive secondary, which you then remove to replace with an expnsive corrector, this doesnt seem very good use of your £s!!

best not look at a Takahashi Epsilon180ED f/2.8 then

i've often thought about how to place cameras at prime focus and do away with the secondary but there are problems... you'd either be limited to a few makes of OSC or have to manually change filters if mono as a filter wheel would definitely be a no-go. also, great big thick wires trailing across the optical path. i'm surprised some enterprising ccd manufacturers haven't made their cameras wireless yet and then use 2 of the spider arms to carry power

unless of course you can afford something the size of the Hale 200" then you could stick whatever kit you want at prime focus 

[Peter Drew]

Most SCT's have F2 mirrors, they can use these because they are spherical, the main problem with spheres is that they are subject to spherical aberration. This can be corrected by the usual corrector plate, the secondary mirror amplifies the F2 focal length to F10 wich is a good all round visual configuration. To use one at F2 by removing the amplifying secondary calls for extra correction, hence the addition of a sophisicated, and expensive system such as Hyperstar.

A parabolic mirror in a Newtonian is the correct shape to form a good image on axis, dropping off in quality off axis in relation to the focal ratio. At F4 and faster a parabolic mirror will require further correction by way of a coma corrector to increase the field of good performance.

If you want a really fast reflecting system you would have to consider a Schmidt camera, these can be made faster than F1.     

[Tommohawk]

best not look at a Takahashi Epsilon180ED f/2.8 then

i've often thought about how to place cameras at prime focus and do away with the secondary but there are problems... you'd either be limited to a few makes of OSC or have to manually change filters if mono as a filter wheel would definitely be a no-go. also, great big thick wires trailing across the optical path. i'm surprised some enterprising ccd manufacturers haven't made their cameras wireless yet and then use 2 of the spider arms to carry power

unless of course you can afford something the size of the Hale 200" then you could stick whatever kit you want at prime focus 

Agree with all this - and I think in the end whilst replacing the secondary with a camera seems neat, you also have the issue of focussing adjustment + cables as you say. A newtonian secondary solves all this. With the hyperstar the focussing is achieved by moving the primary, so I can see how this works quite well, and of course you have the option of mounting the camera in either F2 or F10 set up so quite versatile. Sadly Santa didnt bring me a Takahashi  !

Most SCT's have F2 mirrors, they can use these because they are spherical, the main problem with spheres is that they are subject to spherical aberration. This can be corrected by the usual corrector plate, the secondary mirror amplifies the F2 focal length to F10 wich is a good all round visual configuration. To use one at F2 by removing the amplifying secondary calls for extra correction, hence the addition of a sophisicated, and expensive system such as Hyperstar.

A parabolic mirror in a Newtonian is the correct shape to form a good image on axis, dropping off in quality off axis in relation to the focal ratio. At F4 and faster a parabolic mirror will require further correction by way of a coma corrector to increase the field of good performance.

If you want a really fast reflecting system you would have to consider a Schmidt camera, these can be made faster than F1.     

Hi Peter.

I'm still trying to get my head round the corrective elements in the SCT - and it seems they do vary a bit. Could you please tell me if I understand this correctly:

When using a Hyperstar, it seems the Schmidt plate remains in place to correct apherical aberration, and the Hyperstar corrects - or minimises -  other aberrations ie field curvature, coma, astigmatism?

If so, when using an SCT in its normal configuration, the plate will still correct spherical error, but with only a spherical secondary, how are the other aberrations dealt with?

Maybe it does this mostly by virtue of its increased f value?

[Peter Drew]

Hello Tommo. I think you have now got it sorted in your head . I'm not sure , however, if it's possible to focus the Hyperstar unit by moving the primary mirror ?. Happy New Year.

[nytecam]

Agree with all this - and I think in the end whilst replacing the secondary with a camera seems neat, you also have the issue of focussing adjustment + cables as you say. A newtonian secondary solves all this. With the hyperstar the focussing is achieved by moving the primary, so I can see how this works quite well, and of course you have the option of mounting the camera in either F2 or F10 set up so quite versatile. Sadly Santa didnt bring me a Takahashi  !

Hi Peter.

I'm still trying to get my head round the corrective elements in the SCT - and it seems they do vary a bit. Could you please tell me if I understand this correctly:

When using a Hyperstar, it seems the Schmidt plate remains in place to correct apherical aberration, and the Hyperstar corrects - or minimises -  other aberrations ie field curvature, coma, astigmatism?

If so, when using an SCT in its normal configuration, the plate will still correct spherical error, but with only a spherical secondary, how are the other aberrations dealt with?

Maybe it does this mostly by virtue of its increased f value?

As Peter says - that's about it.   The regular SCT uses all three optical elements eg corrector plate [CP] , spherical primary and secondary to correct all aberrations.  When the secondary is removed, as in the f/2 Hyperstar, the complex Hyperstar optics correct all aberrations except spherical ab corrected via CP.  The Hyperstar uniquely corrects for a flat-field over a full-frame camera [43mm diag] for a huge fov.  

As an aside I'm currently running my 30cm f/10 Meade SCT @ f/2 to get a 'faster' and wider fov using the Meade f/3.3 FR .  The images are vignetted in the corners [corrected with a 'flat'] to produce fare results with my 1/2" Lodestar camera in very brief exposures that I use - see M1 in 20s exp below.    SCT are compact but complex optically for reasonable cost for large apertures 

[Tommohawk]

As an aside I'm currently running my 30cm f/10 Meade SCT @ f/2 to get a 'faster' and wider fov using the Meade f/3.3 FR .

I still have a lot to learn! I didnt even know you could use a reducer to achieve that sort of F value with an SCT. 

[laser_jock99]

I use a range of Newtonian scopes (6", 8", 10" & 12") which are native F4 scopes but fitted with the right coma corrector/reducer optic all become F2.9 scopes.

The 'magic' optic is the ASA Keller 2Korrr

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p4685_ASA-2--Newton-Korrektor-und-Reducer-0-73x.html

Granted this is not a cheap optic (in fact more expensive than any of the scopes I use it with) but I can use it in all four scopes to give a wide range of fields of view - all at F2.9

The caveats of working at F2.9 is collimation has to be millimetre perfect- so a high quality laser collimator is also a must. With the cheap GSO F4 Newtonians I work with a few modifications to the mirror cells were necessary to help the mirrors remain in collimation during the imaging session.

Focus must also be spot on at F2.9, which may mean checking focus between targets on steel tube Newts.

The pay off is ridiculously fast imaging times- and another word of caution here too. Fast optics and light polluted skies don't mix well either (unless you're strictly narrow band?). Dark sky sites and DSLR or cooled OSC CCD's seem to work well. I've yet to try mono CCD in these scopes owing to having no camera!

It's possible to buy a ready made 'pre-tuned' fast Newtonians with better mirror cells, carbon tubes and nicer focusers- but the basic optics in these scopes are GSO as far as I can tell.

http://www.teleskop-express.de/shop/product_info.php/info/p5034_TS-10--F4-UNC-Newtonian-Telescope---optimized-focus-position---Carbon-tube.html

The ASA Keller 0.7x reducer/corrector is offered as option at the bottom. 

I 'made do' with the cheap steel tube variants!

http://www.telescopehouse.com/acatalog/Revelation-10--f-4-M-LRN-Optical-Tube-Assembly-OTA-2-1.html

Finally a 'taster' image at F2.9- NGC7000 single 159s sub

[Tommohawk]

I use a range of Newtonian scopes (6", 8", 10" & 12") which are native F4 scopes but fitted with the right coma corrector/reducer optic all become F2.9 scopes.

The 'magic' optic is the ASA Keller 2Korrr

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p4685_ASA-2--Newton-Korrektor-und-Reducer-0-73x.html

Granted this is not a cheap optic (in fact more expensive than any of the scopes I use it with) but I can use it in all four scopes to give a wide range of fields of view - all at F2.9

The caveats of working at F2.9 is collimation has to be millimetre perfect- so a high quality laser collimator is also a must. With the cheap GSO F4 Newtonians I work with a few modifications to the mirror cells were necessary to help the mirrors remain in collimation during the imaging session.

Focus must also be spot on at F2.9, which may mean checking focus between targets on steel tube Newts.

The pay off is ridiculously fast imaging times- and another word of caution here too. Fast optics and light polluted skies don't mix well either (unless you're strictly narrow band?). Dark sky sites and DSLR or cooled OSC CCD's seem to work well. I've yet to try mono CCD in these scopes owing to having no camera!

It's possible to buy a ready made 'pre-tuned' fast Newtonians with better mirror cells, carbon tubes and nicer focusers- but the basic optics in these scopes are GSO as far as I can tell.

http://www.teleskop-express.de/shop/product_info.php/info/p5034_TS-10--F4-UNC-Newtonian-Telescope---optimized-focus-position---Carbon-tube.html

The ASA Keller 0.7x reducer/corrector is offered as option at the bottom. 

I 'made do' with the cheap steel tube variants!

http://www.telescopehouse.com/acatalog/Revelation-10--f-4-M-LRN-Optical-Tube-Assembly-OTA-2-1.html

Finally a 'taster' image at F2.9- NGC7000 single 159s sub

Well - I still have a lot to learn about this!. I was planning to get a reducer at some point just because some targets - like NGC700 - are too large for F5 scopes like mine. I hadnt really appreciated that this also means the f value is effectively reduced. The ASA you mention is a bit outside my bracket I think, especially as I plan to update both mount and scope soon. Any thoughts on something for the budget conscious skinflint ?

Also, the main objective for me in getting a reducer will be for larger subjects - that being so, whats the limit on reduction? Is there anything like 0.5? It looks like for Newtonians you need flattener/coma corrector, and thats where the limit occurs - yes?

That is a very impressive image! I enjoy pretty dark sky round here, so hopefully no problems on that front. I happened on your other thread with image of Sadr - equally impressive! What's all that emission doing there??

[laser_jock99]

There are two things to consider when looking at imaging scopes- both the focal length of the scope (which along with the size of the CCD chip determines the image scale or amount of sky covered) and the F ratio or optical speed of the system (and thus the exposure times) .

In your opening post you mention a 10"  Quattro F4 - which has focal length of 1000mm and can't be regarded as a wide field instrument. Bigger mirrors (and lenses) invariably mean longer focal lengths more suited to small sized targets like galaxies. Some of the best wide field images come from more modestly apertured scopes like the Skywatcher 130PDS or humble ED80. So depending on the intended target(s) you need a scope (or scopes) to match.

Thus for imaging wide field targets like M31, NGC7000 or the Veil Nebula for instance a shorter focal length 8", 6" or even 5" scope might be worth considering?

My own wide field, fast instrument of choice is the GSO 6" F4 although the SW 130PDS is a popular choice.

Cygnus is a great target area for imagers - just full of nebulosity!

[Tommohawk]

I guess I've been tying to find a simple "one scope does all" solution which fairly obviously can't be done. For now I'm into galaxies /nebulae - would like to planets, but thats some time in the future.

My 200P has proved pretty good, but struggles with smaller targets, and yet isn't wide enough for big subjects. I think I'll probably upgrade to a 10" F4 for general use - apart from anything else my 200P has the earlier smaller secondary - and get a reducer of some sort. In the medium term I'll get a 130 and this together with the reducer should cover pretty much all the wider field stuff. 

Then in the longer term, maybe get an SCT or similar for planets and also revisit the smaller DSOs.

Make sense? Also, any thoughts on an alternative (cheaper) reducer?

[Uranium235]

Might be easier to keep the 200 for galaxy season, and get a 130pds for the big stuff. With the SWCC it runs at f4.5, but that can be tuned (mine runs at f4.4 at the mo).

Personally, I darent go faster becuase it will introduce all sorts of complications as f4 and below will really test the mechanical/optical integrity of your telescope - as well as a trip to collimation hell.

[Tommohawk]

Might be easier to keep the 200 for galaxy season, and get a 130pds for the big stuff. With the SWCC it runs at f4.5, but that can be tuned (mine runs at f4.4 at the mo).

Personally, I darent go faster becuase it will introduce all sorts of complications as f4 and below will really test the mechanical/optical integrity of your telescope - as well as a trip to collimation hell.

Well I need to change the 200 sometime soon not least cos the smaller secondary is tricky to attach a heater to. So far as collimation goes, I had quite a bit of fun with this at the outset, especially the secondary, but I can do it pretty accurately now in no time at all.... I quite enjoy collimating in fact. Should I get help?! 

How does the SW CC compare to the ASA do you think - I realise its 0.9 rather than 0.75, but quality wise?

A further thought - I had the Baader MPCC in mind which is x1, cos I didn't want to reduce image size.  But I suppose, as discussed above, having bigger field and reduced F is very useful and if I'm after very small target - say ring nebula - this will be so central that coma isnt much of an issue. Sound sensible?

[pete_l]

Also, the main objective for me in getting a reducer will be for larger subjects - that being so, whats the limit on reduction? Is there anything like 0.5? It looks like for Newtonians you need flattener/coma corrector, and thats where the limit occurs - yes?

Let's step back for a second.

Can we restate your question as a wish to image a larger FoV, instead of the more specific and equipment orientated (rather than goal orientated) questions about reducers.

First of all, there are two ways to "get more photons": a bigger aperture or a longer exposure. You might find that going down the route of longer exposure times has some advantages. Esp. since bigger aperture means bigger everything else - or at least bigger everything else that's expensive and gets disproportionately more expensive as size increases (whereas longer exposure time is essentially free).

Second. For larger FoVs, what you actually need is short focal lengths. Have you considered using a prime lens with your Canon? A 200mm F/2.8 is an exceptional lens (I have one) and has a stupendous 6 x 4° field. It's also cheaper (c. £500-600) than an ASA reducer. You may well find that the light weight of this combo will allow you to get very long exposure times, right up to your local sky-fog limit on your HEQ5.

[Uranium235]

Well I need to change the 200 sometime soon not least cos the smaller secondary is tricky to attach a heater to. So far as collimation goes, I had quite a bit of fun with this at the outset, especially the secondary, but I can do it pretty accurately now in no time at all.... I quite enjoy collimating in fact. Should I get help?! 

How does the SW CC compare to the ASA do you think - I realise its 0.9 rather than 0.75, but quality wise?

A further thought - I had the Baader MPCC in mind which is x1, cos I didn't want to reduce image size.  But I suppose, as discussed above, having bigger field and reduced F is very useful and if I'm after very small target - say ring nebula - this will be so central that coma isnt much of an issue. Sound sensible?

The difference between the SWCC and the ASA apart from speed, is the size of the useable field. The Skywatcher has a larger corrected circle, and the Baader even more so (but with no reduction factor). Its pointless going fast if you cant get good corners (especially when using a CCD).

In addition the SW is prone to some internal reflection on very bright stars and is not apochromatic (you need to refocus between filters), neither issue is evident in the Baader corrector (better coatings/glass).

[Tommohawk]

Hi and thanks for the comments - you make some good points. I have muddled things a bit by mixing a general query about the F value of hyperstars with a discussion about what kit is best for the biggest range of FOVs. I think I have the Hyperstar issue understood - it has big aperture low f by using corrector plate.

Thinking about conventional Newtonians, what I want is the biggest range of FOVs with the least amount of kit. I agree that its a bit daft to have a long focal length scope and then use a reducer, when you could use shorter scope or even as you say a prime lens which I currently dont have. 

I also take your point about exposure time vs. aperture - but I think for me time is a problem especially with the cloudy skies we get here - I note your location with envy!

I don't want to spend bundles of money on high aperture / clumsy kit, but so long as I can handle it I'd rather lean in that direction if it saves me exposure time.

Thinking a bit further in this direction - am i right in thinking that for smaller aperture/shorter focal length stuff the balance also shifts toward refractors? (which kind of ties in with you point about prime lenses)

[laser_jock99]

Thinking a bit further in this direction - am i right in thinking that for smaller aperture/shorter focal length stuff the balance also shifts toward refractors? (which kind of ties in with you point about prime lenses)

Shorter than 300mm focal length is camera lens teritory- but even then you have to use higher quality 'pro' lenses with ED glass where possible. Cheap lenses tend to suffer from chromatic abberation and other deffects. Astrophotography is the most demanding subject.

Very short focal length telescopes are few and far between- the only ones I can think of the are the Borg serries

http://www.firstlightoptics.com/borg-astrograph-telescopes/borg-67fl-f38-astrograph-set-b06738.html

But I think even these are based on Canon lens sets?

There's this offering also from TS that looks interesting in that it should cover a 35mm sized CCD (and hence give a bigger FOV):

http://www.teleskop-express.de/shop/product_info.php/info/p7085_TS-Imaging-Star71---71mm-f-4-9-Imaging-APO---covers-full-sized-sensors.html

As far as short focal length reflectors go there is even less to choose from, most start at 600mm or so. Even my GSO 6" F4 when fitted with the ASA 0.7x corrector/reducer only gives me 435mm FL (albeit at a tasty F2.9!!).

It all just points towards the need to own several scopes (and camera lenses) in order to be able to cover most eventualities.

[Tommohawk]

The difference between the SWCC and the ASA apart from speed, is the size of the useable field. The Skywatcher has a larger corrected circle, and the Baader even more so (but with no reduction factor). Its pointless going fast if you cant get good corners (especially when using a CCD).

Being a bit dim here maybe - are you saying that, depsite the huge cost the ASA doesnt have a fully corrected field? TBH I couldnt run to this anyhow - is there anything between?

I need a CC so I think I'm going to end up with the Baader 1X, on the basis that at some point I'll get a shorter scope for widefield. (see below)

Shorter than 300mm focal length is camera lens teritory- but even then you have to use higher quality 'pro' lenses with ED glass where possible. Cheap lenses tend to suffer from chromatic abberation and other deffects. Astrophotography is the most demanding subject.

Very short focal length telescopes are few and far between- the only ones I can think of the are the Borg serries

http://www.firstlightoptics.com/borg-astrograph-telescopes/borg-67fl-f38-astrograph-set-b06738.html

But I think even these are based on Canon lens sets?

There's this offering also from TS that looks interesting in that it should cover a 35mm sized CCD (and hence give a bigger FOV):

http://www.teleskop-express.de/shop/product_info.php/info/p7085_TS-Imaging-Star71---71mm-f-4-9-Imaging-APO---covers-full-sized-sensors.html

As far as short focal length reflectors go there is even less to choose from, most start at 600mm or so. Even my GSO 6" F4 when fitted with the ASA 0.7x corrector/reducer only gives me 435mm FL (albeit at a tasty F2.9!!).

It all just points towards the need to own several scopes (and camera lenses) in order to be able to cover most eventualities.

I reckon I need a 400mm for widerfield stuff. Seems to be a bit of a gap in the market - no such thing as a 400mm fl reflector so far as I can see, and I cant see any canon catadioptrics, neither is there a 400mm refractor - or is there?