Choosing an imaging scope - your views?

[ollypenrice]

Hi All,

I'm tryng to put together a short piece for my website on how to choose a scope for deep sky astrophotogaphy and would welcome your opinions on this as a draft, in the unlikely event that you have nothing better to do! It might also be worthwhile to kick the ball around together and create a joint SGL piece? I am a refractor nut but have tried to be impartial.

How to choose a DS imaging scope;

The three optical fundamentals here are aperture, focal length and their relationship, focal ratio. For imaging purposes forget aperture for now and think only about focal length and focal ratio. Choice of aperture will follow naturally from your deliberations. In imaging, aperture does not rule.

Focal length. This will decide your image scale, or what will fit on your chip. To make a small object fill the chip you need a long focal length and to fit a large object on the chip you need a short. There is no 'better or worse' here. They simply suit different targets. The question is, what do you want to photograph? For clusters and nebulae you need a wide field (short FL) and for most galaxies and planetaries a narrow field (long FL).

Then, unfortunately, there is a 'but'... A long focal length effectively magnifies the tracking errors of your mount. To shoot at long focal lengths you need very accurate autoguiding and this is likely to be beyond what is possible (or at least easy) on the less expensive mounts. For a rough rule of thumb I'd say that for the HEQ5/6, a focal length of 500mm is very easy, 1000mm should be okay but will be noticeably less reliable, 1.5 metres is getting difficult and 2 metres will require fine tuning and expertise. (I base this on using a couple of EQ6 mounts very regularly. They vary. One of mine is a little better than the other. Since this guide is for beginners I think that what comes out of the box is more relevant than what fine tuning can achieve.) Exactly at what focal length you start to need a premium mount is debatable but for me I would start considering it for focal lengths of a metre and for 1.5 metres of focal length I would be wanting something classy. Be aware that the price jumps up by a factor of at least four after the EQ5/6 series mounts so long focal length imaging is going to be expensive. Many imagers also feel that the integral fork mounts on which some SCTs are sold are not as satisfactory for imaging as the sales pitch implies.

Long F-lengths are also more susceptible to both wind and seeing. Active Optics devices are a huge benefit at long focal lengths but introduce considerable complication and are probably best left out of a beginner's system.

Focal ratio. If anything 'rules' in an imaging scope it is probably this. A 'fast' system will cut down your exposure time (or collect more signal in the same time) than a slow system. Exposure time increases by the square of the focal ratio; f7.5 requires 2.25x longer than f5, all other things equal, so you can see how important it is. In terms of what is now available, fast means f2 to around f5 and slow means more than that. Most people would prefer not to fall below f7.5 and f10 is getting to be seriously problematic. There are desirable f9-ish imaging scopes but they are chosen in spite of their f- ratios and not because of them!

However, beware of the 'f ratio myth.' The galaxy (say) in a fast scope of aperture 'x' is only brighter than the same galaxy in a slow scope of the same aperture because it is smaller. Crop and enlarge it to the scale it would appear in the slow scope of aperture x and you have gained nothing.

Longer exposures require more cloudless time plus better tracking/autoguiding and polar alignment. Note, therefore, that a combination of slow optics and long focal length is really going to make demands in this area. The reverse, fast optics and short focal length, is the most user friendly option and is widely recommended for beginners.

If you don't have great weather, think 'fast.'

Fast focal ratios are intolerant at the focus stage and require the camera to be orthogonal to a high order. Poor focusers will easily sabotage good fast optics.

Next, what type of optical system, refractor, reflector or catadioptric?

Refractors;

For; ease of use. They need no collimation, are least susceptible to dew, have no image shift affecting focus, have high contrast and produce no diffraction spikes (though some like these. Not too nice on small CCD chips, I think?)

Against; cost. They need to be apochromatic and can be exotically expensive. Only the good ones have near perfect colour control, especially around hot blue stars. As aperture increases it becomes impossible to create colour correction at fast focal ratios so, as they get bigger, refractors slow down.

Doublet and even triplet apos will often need a field flattener on larger chips. The spacing to camera has to be correct if one of these is used and the flattener needs to be compatible and known to work with the scope. Ask on the forum.

Reflectors.

For; you can have both a long focal length and a fast f ratio at moderate cost. There is no false colour issue.

Against; They are bulky, need careful collimation and will need some kind of coma correction. This can be a simple after market coma corrector lens or a sophisticated built in astrographic correction system as seen on Orion Optics AO scopes, Takahashi Epsilons, etc.

The Newtonian design has limited focus travel so it is important to be sure that your camera can come to focus on the telescope you choose. Ensuring that this is so is not always as simple as it should be since mis-information abounds. While they are, in principle, good budget instruments, Newtonians can be let down by mis-aligned focusers at the wrong focal distance for a particular camera. Coma correctors can introduce vignetting. (I have no personal experience of this. Anyone like to confirm or deny it?)

Catadioptrics.

For; compact and fairly affordable at long focal length.

Against; they are prone to dewing and will need both electric dew strap and dewsheild. Those with moving mirror focus really need an aftermarket Crayford to allow critical focus to be found. Focus is of primary importance for a good result. Mirror shift can be an issue. They usually need focal reducers which then require careful chip-to-reducer spacing which is only easily maintained on high end Crayfords which also hold the reducer. Reducers can introduce vignetting. See also the inherent difficulties of long FL imaging discussed above.

Conclusion; in a nutshell the easiest system to master is a short focal length, fairly fast refractor with a field flattener/reducer known to be compatible with your telescope and at the correct distance from your camera chip. To ensure that that is what what you end up with you need a reliable retailer and/or confirmation from SGL members that the system you have in mind is going to work. A good budget alternative would be a Newt with coma corrector provided the focus distance issues have been addressed before you buy.

Okay, there's the draft. All additions/deletions/corrections/refinements welcome.

All the best,

Olly

[Euan]

Conclusion; in a nutshell the easiest system to master is a short focal length, fairly fast refractor with a field flattener/reducer known to be compatible with your telescope and at the correct distance from your camera chip. To ensure that that is what what you end up with you need a reliable retailer and/or confirmation from SGL members that the system you have in mind is going to work.

I think this is right, I'm reasonably confident at this stuff now and I'm still trying to work on how to get sharper shorter focal lengths shots before going further in.

It's much easier to learn the ropes with something like an ED80 where you don't have to worry about collimation & guiding as much.

The downside is your average beginner might be using a DSLR with this kind of system and when it comes to galaxy season it's pretty useless.

I take it this is purely DSO and not for planetary / lunar?

[Mr Spock]

From what I read the premise here appears to be wide field imaging and not deep sky.

[ollypenrice]

'I take it this is purely DSO and not for planetary / lunar?'

Yes.

I agree about the frustrations of galaxy season but there are nice targets, none the less. Markarian's Chain, maybe mosaic-ed into more of the Virgo Cluster. M101. M81-82, M106 and friends... But you're right, times is 'ard for the small refractor right now!

Olly

[Psychobilly]

Seems sensible to me

If only I had read something like this before buying a fork mounted 8" f10 SCT as my first "proper" telescope...

Peter...

[ollypenrice]

From what I read the premise here appears to be wide field imaging and not deep sky.

Oh, I don't think so, really. I mean 450mm gets you M42/43/NGC1977 on a large (15mm) CCD chip. F = 328mm gets ic1396 on the same. No, I'd call that deep sky.

Olly

[Euan]

f only I had read something like this before buying a fork mounted 8" f10 SCT as my first "proper" telescope...

That must have been some challenge

It might also be worthwhile to kick the ball around together and create a joint SGL piece?

There are so many people who post asking about their first imaging telescope, and the general consensus from what I've read seems to be a small refractor. You would think the ideal beginners setup would be:

Modded DSLR on an ED80 plus flattener

Webcam or small CCD on ST80 for auto-guiding

HEQ5 (EQ6 if possible for future-proofing)

EQMOD

[Deneb]

I think that summerises it quite well, possibly a stickie is in order, if there is not one already this is similiar to what type of primer I would want on my website, since I have experimented with those 3 types of Telescopes.

It's just getting time to write one.

Cheers

Nadeem

[RobH]

Generally fine Olly, but although I would agree that aperture doesn't rule for imaging, it is something to take into account nonetheless.

More aperture and a fairly fast scope can mean shorter subs on a particular target, so less chance of tracking errors causing trouble.

I've decided to forget the 10 inch RC I was going to get, and am now adding a 14 inch LX200ACF to my arsenal, and will probably also get a 12 inch F4 too, as my 6 inch TMB, although crisper than a packet of Walkers, does mean that on some targets (for example with say SII narrowband), I'm having to go up to 30 minute subs, whereas a big lightbucket could do the job much quicker.

Cheers

Rob

[jgs001]

Olly, I think that is well written. Being of the pair of small fracs on a guided heq5 (when I've dealt with the Gremlins) I have to whole heatedly agree with what you've written. They work well, and comparatively easily (technical glitches aside .

[ollypenrice]

Generally fine Olly, but although I would agree that aperture doesn't rule for imaging, it is something to take into account nonetheless.

More aperture and a fairly fast scope can mean shorter subs on a particular target, so less chance of tracking errors causing trouble.

I've decided to forget the 10 inch RC I was going to get, and am now adding a 14 inch LX200ACF to my arsenal, and will probably also get a 12 inch F4 too, as my 6 inch TMB, although crisper than a packet of Walkers, does mean that on some targets (for example with say SII narrowband), I'm having to go up to 30 minute subs, whereas a big lightbucket could do the job much quicker.

Cheers

Rob

Thanks to all for the input.

Rob, I think what I was trying to do was to let aperture 'emerge' from the issue of f ratio. When you see a need for more speed at a given focal length then aperture is, of course, the only way to get it. If beginners put aperture first then they may mistakenly overlook f ratio and end up with a large slow scope which does the opposite of what they are expecting. It will take its light in sips rather than slurps!

I'm slightly disappointed to hear that the RC has been cancelled since I was wondering how it would shape up... Was your previous big Meade not an ACF? I'm sure a big SCT will do well on your mount. There is a guy here who uses a C14 to great DSO effect but, like Martin, he has an AO unit in operation. Any thoughts on that? My impression is that they work wonders at long focal lengths and really let the scope strut its stuff. However, personally I cringe at the thought of more snakes in the snake pit.

As for pensioning off the big refractor - well, Humph!! But I take your point. That's why, for me, if anything is king its f ratio in this game.

Cheers,

Olly

[RobH]

Hello Olly.....yes, I agree that many beginners star off thinking that aperture is king, and you are right to emphasise the point that it isn't.

An 80mm scope and simple mount will work wonders

Re. the big 'frac.....it's certainly not going to be pensioned off, and will most likely stay as my main imaging scope, but I have missed having the long FL big scope, especially in the springtime with all those tiny but fascinating galaxies out there.

Also, I think I'm heading down the route of using 2 or 3 scopes on an image these days, and I want to go in really close on some targets.

My old 14 wasn't an ACF, and I had to spend hours during processing dealing with dodgy stars, hence the move to an ACF this time, which will mostly be used at F5 or F6, and hence be really good at gobbling up the photons.

Also, the old one was on the Meade mount and wedge...this time I have a much better platform in the AP1200, which will mean I can push things a lot further.

As regards the RC....I got more and more convinced that if I have diffraction spikes on my images, I want control over them, rather than being stuck with them due to scope design, and RC images always have diff spikes on them!

I will most likely go down the active optics route, as the images that people like Martin are producing using big SCT's and AO are the way forward for high resolution stuff I think.

Cheers

Rob

[sgazer]

Excellent Olly, I agree with everything there. Perhaps just one thing to add would be to emphasise the importance of focal ratio on exposure times by saying something about the exposure time increasing by the square of the focal ratio eg. f7.5 requires 2.25x longer than f5 (alll other things equal).

Personally, having used a f7.5 frac, I would recommend pushing more towards f5 to reduce exposure times, which really rules out any apo fracs for beginners, apart from very short focal lengths. The aperture required for an f5 apo frac >500mm would push the price up significantly. That would really only leave a reflector, but then there's the "maintainance issues" of course, which aren't so bad really.

[Euan]

The aperture required for an f5 apo frac >500mm would push the price up significantly.

My scope is f5 (Equniox 80 @ f6.25 with 0.8x reducer), only slightly more than the ED80 so it can be done

That's 400mm mind you if you want >500mm then you are talking big bucks

[ollypenrice]

SG and Euan, thanks for the input. I have added the f ratio formula.

Euan, your setup sounds the business. I get similar numbers from a Tak FSQ but if I weren't using it for the business I very much doubt that I would have got it past my conscience.

Olly

[BlueAstra]

I've had a few thoughts on your article (which I wish I'd had when I started!). I hope you don't mind, I thought it was best to present them as edits to your text. My stuff is in bold.

The three optical fundamentals here are aperture, focal length and their relationship, focal ratio (also known as F/Number, which equals focal length/Aperture). For imaging purposes forget aperture for now and think only about focal length and focal ratio. Choice of aperture will follow naturally from your deliberations. In imaging, aperture does not rule.

Focal length. This will decide your image scale, or what will fit on your chip. To make a small object fill the chip you need a long focal length and to fit a large object on the chip you need a short. The relationship here is Image Radius (mm) = Focal Length (mm) x tan(angular radius of target). There is no 'better or worse' here. They simply suit different targets. The question is, what do you want to photograph? A way of assessing your focal length requirement could be to take the above relationship with a range of focal lengths, and the angular sizes of your targets of interest, compute the typical coverage on your chip. You can then determine what focal lengths would cover say 20-80% of your chip. In general, for clusters and nebulae you need a wide field (short FL) and for most galaxies and planetaries a narrow field (long FL).

Then, unfortunately, there is a 'but'... A long focal length effectively magnifies the tracking errors of your mount. To shoot at long focal lengths you need very accurate autoguiding and this is likely to be beyond what is possible (or at least easy) on the less expensive mounts. For a rough rule of thumb I'd say that for the HEQ5/6, a focal length of 500mm is very easy, 1000mm should be okay but will be noticeably less reliable, 1.5 metres is getting difficult and 2 metres will require fine tuning and expertise. (I base this on using a couple of EQ6 mounts very regularly. They vary. One of mine is a little better than the other. Since this guide is for beginners I think that what comes out of the box is more relevant than what fine tuning can achieve.) [Comment- Does your above assessment assume autoguiding or just polar alignment with no autoguiding?]

Long focal length imaging is also more susceptible to both wind and seeing. Active Optics devices are a huge benefit at long focal lengths but introduce considerable complication and are probably best left out of a beginner's system.

Focal ratio. If anything 'rules' in an imaging scope it is probably this. A 'fast' system will cut down your exposure time (or collect more signal in the same time) than a slow system. Exposure time increases by the square of the focal ratio; f7.5 requires 2.25x longer than f5, all other things equal, so you can see how important it is. In terms of what is now available, fast means f2 to around f5 and slow means more than that. Most people would prefer not to fall below f7.5 and f10 is getting to be seriously problematic. There are desirable f9-ish imaging scopes but they are chosen in spite of their f ratios and not because of them!

Longer exposures require more cloudless time plus better tracking/autoguiding and polar alignment. Note, therefore, that a combination of slow optics and long focal length is really going to make demands in this area. The reverse, fast optics and short focal length, is the most user friendly option and is widely recommended for beginners.

If you don't have great weather, think 'fast.'

Fast focal ratios are intolerant at the focus stage and require the camera to be orthogonal to a high order. Poor focusers will easily sabotage good fast optics.

Next, what type of optical system, refractor, reflector or catadioptric?

Refractors;

For; ease of use. They need no collimation, are least susceptible to dew, have no image shift affecting focus, have high contrast and produce no diffraction spikes (though some like these. Not too nice on small CCD chips, I think?) [You can add them in processing if required]

Against; cost. They need to be apochromatic and can be exotically expensive. Only the good ones have near perfect colour control, especially around hot blue stars. As aperture increases it becomes impossible to create colour correction at fast focal ratios so, as they get bigger, refractors slow down.

Doublet and even triplet apos will often need a field flattener on larger chips. The spacing to camera has to be correct if one of these is used and the flattener needs to be compatible and known to work with the scope. Ask on the forum.

Reflectors.

For; you can have both a long focal length and a fast f ratio at moderate cost. There is no false colour issue.

Against; They are bulky, need careful collimation and will need some kind of coma correction. This can be a simple after market coma corrector lens or a sophisticated built in astrographic correction system as seen on Orion Optics AO scopes, Takahashi Epsilons, etc.

The Newtonian design has limited focus travel so it is important to be sure that your camera can come to focus on the telescope you choose. Ensuring that this is so is not always as simple as it should be since mis-information abounds. While they are, in principle, good budget instruments, Newtonians can be let down by mis-aligned focusers at the wrong focal distance for a particular camera. Coma correctors can introduce vignetting. (I have no personal experience of this. Anyone like to confirm or deny it?)

Catadioptrics.

For; compact and fairly affordable at long focal length.

Against; they are prone to dewing and those with moving mirror focus really need an aftermarket Crayford to allow critical focus to be found. Mirror shift can be an issue. They usually need focal reducers which then require careful chip-to-reducer spacing which is only easily maintained on high end Crayfords which also hold the reducer. Reducers can introduce vignetting. See also the inherent difficulties of long FL imaging discussed above.

[Comment- (C8 SCT), I have never had a dew problem when using a good drew strap. The use of Bahtinov masks allows precise focus without a problem. Mirror shift is probably more of a problem with small chip CCDs, never had a problem with an DSLR]

[Comment – There seems to be two broad classes of CCD imagers, those with DSLR cameras (large chip), and those with small dedicated CCD chip cameras. I think large dedicated CCD chip cameras are very expensive and perhaps a minority sport (?). Beginners may well split into DSLR camera users and small chip CCD users. Would it be useful to address these two types generically in respect to the guidelines you’ve put forward?]

Conclusion; in a nutshell the easiest system to master is a short focal length, fairly fast refractor with a field flattener/reducer known to be compatible with your telescope and at the correct distance from your camera chip. To ensure that that is what what you end up with you need a reliable retailer and/or confirmation from SGL members that the system you have in mind is going to work.

Okay, there's the draft. All additions/deletions/corrections/refinements welcome.

[ollypenrice]

Thanks B-A. All good points. I think I might keep away from calculation as it is, for some readers, a sort of 'off-switch.' On dewing, I might go for something like 'an SCT pretty well demands an electric dewstrip while the others suffer dewing less frequently.'

You use a moving mirror focus for imaging? I have to say that I have tried it using FWHM and found it a screaming nightmare on my Meade scopes. I hear that Celestron might be better. In my case the image whizzed about all over the shop and the FWHM never settled convincingly. None of our SCT imaging guests uses the original focus. (The sample is not huge, maybe half a dozen.)

Cheers,

Olly

[BlueAstra]

I should have added that I also use a dew shield along with the dew strip. I only lasted 20 min without them one cold winter, after fitting them I never had a problem.

My C8 SCT has a moving mirror focus. I think its the combination of the focus mask and large DSLR CCD area which makes it rather insensitive to mirror movement. Also using the mask I'm making a visual judgement of best focus rather than a more scientific FWHM measurement. A small shift in the image position isn't really noticable on the large CCD. It would probably be worse with a small CCD. I have heard that the Celestron may be better than the Meade in this respect.

The use of high quality focussers will eliminate it, but they are very expensive. I believe you can get a replacement dual speed 'feathertouch' focusser which helps the fine focus and is not quite so expensive. This doesn't add any distance to the back focus.

[brianb]

I believe you can get a replacement dual speed 'feathertouch' focusser which helps the fine focus and is not quite so expensive. This doesn't add any distance to the back focus.

Nor any weight, nor does it risk fork fouling. But if your SCT has mirror flop it won't fix the problem.

Olly is perfectly right that the short focus APO refractor is a great way to learn ... but he's not emphasising the mount enough. For long exposure imaging, the mount is probably 90% of the equation, the optical tube is not that important (providing it works & delivers halfway decent images. You do not need 0.95+ Strehl quality for short focus imaging!) However in apertures over about 6", apo refractors are just too large, heavy and expensive to be practical ... SCTs and Ritchey-Chretien astrographs come into their own. Dewing is a relatively cheap & easy problem to solve if you have an observatory, and instruments over about 6" really do need an observatory for long focus imaging, because of the size & weight of the mount & because you don't want to spend 80% of your clear sky time aligning the thing.

[ollypenrice]

Thanks Brian. The piece is very much about the scope rather than the whole setup. I might add something on mounts later but what I tried to do here was just give an idea of what focal length the ubiquitous EQs can handle and with what degree of competence. I'll add something for more emphasis though. Thanks.

Blue Astra, I'm not persuaded by the idea of chip size affecting the need for critical focus. I would have said you are either sharp or you're not on any chip size. Like most imagers I 'obsess' over focus, not only to extract the sharpest image but also to reduce noise. The only thing I can think of re chip size is that a computer screen is not taking a DSLR image anywhere near full size if the whole thing is to be seen. But hey, I want people to zoom in on mine and see why I spend ten minutes at the FWHM window every couple of hours!

Best,

Olly

[brianb]

I'm not persuaded by the idea of chip size affecting the need for critical focus.

Me neither. But a chip with relatively few but individually large pixels is easier to focus (and more tolerant of tracking / guiding errors, and more sensitive) than a chip with lots of individually small pixels ... even when the physical dimensions of the imaging area are the same. The downside is, of course, that the objects in the frame are less well resolved in the finished image. For some applications (photometry, astrometry) this doesn't matter, but if you're trying to produce impressive pictures ....

BTW many of my attempts as DSO imaging have been affected by focus drift apparently caused by differential thermal expansion of the glasses & metals used in the lens / scope tube. Slightly out of focus images are actually beneficial when attempting precision photometry, that's why I started to specialize in that direction.

[BlueAstra]

I think there is a slight confusion in what I was saying. I agree chip size does not affect critical focus (except perhaps for the effects of pixel size). I think what I was trying to say was that the final effect of mirror slop (shift in full FOV) is less noticable with large CCD chips or DSLRs, or at least I don't notice much movement with my C8 and DSLR when I focus. If you are viewing a magnified star image then the effects of mirror flop are more noticable, although I don't see much movement when I am focussing a star with a mask on the C8. But I suppose I am viewing the magnified liveview image of the DSLR rather than maybe a few pixels on a dedicated CCD, where the effects might be greater.

As a separate topic, SCT focussing sensitivity can be improved with add on Crayfords (with no mirror flop but reduced focussing distance for imaging devices), or replacement 'feathertouch' knobs (minimal mirror flop(?), no effect on BFL).

[ollypenrice]

I think there is a slight confusion in what I was saying. I agree chip size does not affect critical focus (except perhaps for the effects of pixel size). I think what I was trying to say was that the final effect of mirror slop (shift in full FOV) is less noticable with large CCD chips or DSLRs, or at least I don't notice much movement with my C8 and DSLR when I focus. If you are viewing a magnified star image then the effects of mirror flop are more noticable, although I don't see much movement when I am focussing a star with a mask on the C8. But I suppose I am viewing the magnified liveview image of the DSLR rather than maybe a few pixels on a dedicated CCD, where the effects might be greater.

As a separate topic, SCT focussing sensitivity can be improved with add on Crayfords (with no mirror flop but reduced focussing distance for imaging devices), or replacement 'feathertouch' knobs (minimal mirror flop(?), no effect on BFL).

Aha, I see what you mean. The image shift is not a problem on the big chip. I do mention aftermarket Crayfords in the piece and use one myslelf on our SCT. It is only a Revelation but seems fine to me, though having the reducer move in the drawtube would be a huge benefit.

Olly

[Merlin66]

A bit OT but I'm about to take delivery of my first Celestron SCT - ever! a 9.25"

Over the (many) years I've ended up using Meade SCT - 8", 10" and stil have the 12" Lx200.

Mirror flop during focusing is just one of those things... I have to focus the target star on the spectroscope slit from one direction...

The added mirror locks I used again were OK but to get "fine" control I added a #1209 zero shift focuser...

The celestron focusing attchment to the primary mirror is completely different to the system used in the Meade. It has a "solid" connecting bar which appears to give better results ( I hope to be able to confirm this soon!)

The change in the EFL due to varying back focus position of the reducers/ CCD can be any issue with plate scale measurements, and I've been discussing this with Chris Lord...

The attached "modified" spreadsheet ( ReducersV2) seems to give results very close to reality.. but I would appreciate any measured feedback of plate scale v's reducer focal length v'd distance to chip.

Back in the 1970's the weapon of choice for DSO's was the ol' Orange C8 with an OAG and notwithstanding all the cooments did some excellent work with hypered film/ cold cameras etc.... things seem to have swung full circle and we're going back to telelens..... but this time calling them short focus refractors.....

My 2c

Ken

reducers_V2.zip

[brianb]

we're going back to telelens..... but this time calling them short focus refractors.....

Lots of truth in that!