Mount vs Exposure time vs Guiding vs Star trails?

[jimjam11]

As a beginner I have done a fair amount of reading and it would appear the heq5 is almost ubiquitously recommended as the minimum for imagining. However, a number of people seem to get outstanding results using lighter mounts both guided/unguided.

I think it would be hugely useful if there was a catalog of various mounts vs exposure time before trailing both guided/unguided and more importantly what kind of objects require longer exposure times and thus necessitate the better mounts. It might then be possible for a beginner to easily(!) see what each successive mount unlocks in terms of potential...

I guess my interest originates from questions along the lines of:

'Would a 150p on a guided eq3 outperform a 200p on an unguided eq5?'

'Would a 200p on a guided eq5 outperform a 200p on an unguided heq5?'

'What objects would I struggle to image on an eq5 that I could readily image on a heq5?

Tia.

[nmoushon]

The limit to exposure time for unguided imaging on any mount depends on how accurate you polar align it. Then the next thing to effect it is how much weight is on the mount and whether or not that mount can handle the weight. So you can get decent unguided images on an EQ3 but you wont be able to put much telescope weight on it. Which is why HEQ5 is so popular. It can handle the weight for most imaging set ups. When you set into guided mounts then things change a bit. Polar alignment is still important but not as much. So then you have to go back to how much weight your mount can handle. But you have to consider all the extra weight that the guiding equipments adds now on top of all the normal imaging gear. You will only have trouble imaging the faintest of objects with any mount and any scope. Exposure time is really what dictates what you can and cannot image then your focal length. So I would suggest putting your money towards a HEQ5 first then decided on what scope you can afford. That will be your safest bet to get good quality images and future proof yourself from having to buy it in the future to upgrade. Saves you money in the long run.

[Capricorn]

To me the "problem" is one that all the scopes you ask about are visual scopes on a visual mount, fundimentally you are asking how much you could get away with using a setup that was intended for another purpose.

As you say in imaging an HEQ5 is regarded as the minimum, try the typical an EQ6 instead.

In imaging the typical scope is not a reflector, an apo of 80mm maybe even as big as a 100mm refractor is used.

The focal length is usually short, not as long as the 200P as tracking errors are amplified as the focal length goes up, and the focal length of the 200P is really too long.

Really it is time to seperate visual and astrophotography, as much by actual equipment as well as outright cost.

In visual you can accept some shake, 99% of the time the observer causes the shake, you move the scope, you adjust the focuser, hell you even go as far as looking down the eyepiece and so actually touching it when in use.

In astrophotography shake in any degree is a complete no no.

In visual if the object drifts a little then so what as long as it stays in view.

A little drift in astrophotography is a disaster.

So to "compare" what appears to be becoming typical systems:

An EQ5 and a 200P, add goto for ease.

An EQ6 and an 80mm apo, with goto and guide system.

Hardly a straight comparison.

[jimjam11]

Thanks for the responses. At this stage I am trying to asses what the options are in a measured way. I have a habit of getting carried away with things and buying completely over engineered solutions!

Would it be fair to say an 80mm app would be poor for visual use because it wouldn't gather enough light, which isn't a problem for imaging because the cameras are so sensitive?

[pete_l]

I think there are too many variables: inside a mount, with the way it was set up and the position it's pointing while imaging to be able to make any accurate comparisons - especially with low-end mounts. The phrase YMMV could have been invented for telescopes, mounts and their tracking ability.

[Ags]

If you keep the EQ5 and 200p for visual use and planetary imaging, you have a good and appropriate setup.

Take the 200p off the mount and put your camera with a camera lens on the mount and you have a great setup for imaging!

[jimjam11]

If you keep the EQ5 and 200p for visual use and planetary imaging, you have a good and appropriate setup.

Take the 200p off the mount and put your camera with a camera lens on the mount and you have a great setup for imaging!

That appeals to me. I like the idea of spending money on lenses because I can use them everyday!

[nmoushon]

You are kind of correct on the 80mm APO. Thats what I have for imaging. Its great. Love it. Visually its not bad. You cant get much mag out of an 80mm so you are limited to widefield visuallly. So large clusters and such are the most you will see. You wont get any detail out of galaxies. Even Andromeda is just a large smudge. The most on planets Im able to see are the silhouette of Saturn and its rings and was just able to make out the two large stripes in Jupiter. But got to remember that a great imaging setup is not usually a great visual setup and vise versa. So you really need to decide which side you want to lean towards--imaging or visual. If you get a HEQ5 (not a EQ5) with a 150p that is a vary common imaging setup that I think you wll be happy with. You will also get some decent views from it also. You can use just a camera and lens on a mount but you wont get the quality image you would get through a telescope unless you have a very good lens. Which are usually about the same price as a telescope.

[Ags]

Agreed, lenses get expensive. But if you are into both astronomy and photography, it is a good compromise. From my research, I would say ther are a fair nomber of targets between 100mm and 200mm focal length. I imaged M31 last week, and got quite good size at 135mm.

And although camera lenses lack focal length, they have phenomenal speed - my 50mm f1.8 is usable at f2.8 - show me the apo that can come close :-)

[nmoushon]

Agreed, lenses get expensive. But if you are into both astronomy and photography, it is a good compromise. From my research, I would say ther are a fair nomber of targets between 100mm and 200mm focal length. I imaged M31 last week, and got quite good size at 135mm.

And although camera lenses lack focal length, they have phenomenal speed - my 50mm f1.8 is usable at f2.8 - show me the apo that can come close :-)

True that very few scopes can get down to f2.8 but at f2.8 even lenses start to get color adoration and stars bloat if you take long exposures unless you have a VERy expensive lens. You can get some AMAZING shots no doubt. I want to get a nice widefield lens at f/2.8 some day. If you want to go widefield then a 12-20mm lens and a 100-300mm lens that are in those ranges is what you might want to consider. You can even capture a couple DSO with lenses. But anything that is much smaller than M31, M42 and some of the open clusters you wont get detail shots of. You will capture them but you wont get the detail. If you want shots of galaxies and small nebula then you need a telescope. Even a small one at 80mm is a huge improvement on FL over the lenses.

[RikM]

I used to be able to get 90-120sec unguided subs with a 150P on an EQ3-2. Stack enough of them and you can get a presentable deep sky image but...90-120sec doesn't give you a very deep image and with the faintest of puffs of wind, the sub was ruined. I used to loose 40-50% of all the subs I took. That is not very efficient and is very frustrating.

Putting the same scope on an NEQ6 with an autoguider I can get 600-900 sec subs and I can keep almost all of them.

Why do I use a 150P for imaging? Ask what makes for a good imaging scope:

Apochromatic - check

Fast - check

Light - check

Short focal length - ish

Flat field - well...I bought a coma corrector so it's okay now

Maintenance free - erm...no! This is where Newt's loose out to refractors and it can be a major pain.

Affordable? - check (most Apo's are out of my price range, the 150P is less than £180!)

It ticks enough boxes for me!

Ask the same questions of the 200P and you can say no to light, short focal length and affordable (£320 is second hand ED80 money and I would far rather have one of those) It can be used for imaging, and in the hands of someone with the skill and determination to make it work, very nice images can be produced, but I feel the 200P is much more suited to visual astronomy.

[jimjam11]

Agreed, lenses get expensive. But if you are into both astronomy and photography, it is a good compromise. From my research, I would say ther are a fair nomber of targets between 100mm and 200mm focal length. I imaged M31 last week, and got quite good size at 135mm.

And although camera lenses lack focal length, they have phenomenal speed - my 50mm f1.8 is usable at f2.8 - show me the apo that can come close :-)

I have the 100-400 f4.5-5.6L on my wishlist for F1 anyway so it looks like that might work quite well; perhaps another reason to justify that lens!

[jimjam11]

I used to be able to get 90-120sec unguided subs with a 150P on an EQ3-2. Stack enough of them and you can get a presentable deep sky image but...90-120sec doesn't give you a very deep image and with the faintest of puffs of wind, the sub was ruined. I used to loose 40-50% of all the subs I took. That is not very efficient and is very frustrating.

Putting the same scope on an NEQ6 with an autoguider I can get 600-900 sec subs and I can keep almost all of them.

Why do I use a 150P for imaging? Ask what makes for a good imaging scope:

Apochromatic - check

Fast - check

Light - check

Short focal length - ish

Flat field - well...I bought a coma corrector so it's okay now

Maintenance free - erm...no! This is where Newt's loose out to refractors and it can be a major pain.

Affordable? - check (most Apo's are out of my price range, the 150P is less than £180!)

It ticks enough boxes for me!

Ask the same questions of the 200P and you can say no to light, short focal length and affordable (£320 is second hand ED80 money and I would far rather have one of those) It can be used for imaging, and in the hands of someone with the skill and determination to make it work, very nice images can be produced, but I feel the 200P is much more suited to visual astronomy.

Thats precisely the kind of data I was hoping for because its easy to see what upgrading the mount provides in terms of hit rate and exposure length...

[jimjam11]

I used to be able to get 90-120sec unguided subs with a 150P on an EQ3-2. Stack enough of them and you can get a presentable deep sky image but...90-120sec doesn't give you a very deep image and with the faintest of puffs of wind, the sub was ruined. I used to loose 40-50% of all the subs I took. That is not very efficient and is very frustrating.

Putting the same scope on an NEQ6 with an autoguider I can get 600-900 sec subs and I can keep almost all of them.

Why do I use a 150P for imaging? Ask what makes for a good imaging scope:

Apochromatic - check

Fast - check

Light - check

Short focal length - ish

Flat field - well...I bought a coma corrector so it's okay now

Maintenance free - erm...no! This is where Newt's loose out to refractors and it can be a major pain.

Affordable? - check (most Apo's are out of my price range, the 150P is less than £180!)

It ticks enough boxes for me!

Ask the same questions of the 200P and you can say no to light, short focal length and affordable (£320 is second hand ED80 money and I would far rather have one of those) It can be used for imaging, and in the hands of someone with the skill and determination to make it work, very nice images can be produced, but I feel the 200P is much more suited to visual astronomy.

Out of interest what kind of exposure times and hit rates did you get on each mount with your 300mm lens?

[RikM]

As a further comparison this is the same scope, camera, location etc only the mount is different.

EQ3-2

NEQ6 + finderguider

I have only used the 300mm lens on the EQ3-2 a couple of times. 3-5 min unguided are possible with a keeper rate about 60% I guess. I haven't used it on the NEQ6 but I am looking for a 300mm T-mount Pentax lens to go on the front of my SXV-H9 camera. I am confident that 30min subs would be easily achievable and I would only expect to loose ones with cloud damage.

[jimjam11]

As a further comparison this is the same scope, camera, location etc only the mount is different.

EQ3-2

NEQ6 + finderguider

I have only used the 300mm lens on the EQ3-2 a couple of times. 3-5 min unguided are possible with a keeper rate about 60% I guess. I haven't used it on the NEQ6 but I am looking for a 300mm T-mount Pentax lens to go on the front of my SXV-H9 camera. I am confident that 30min subs would be easily achievable and I would only expect to loose ones with cloud damage.

Great comparison, thanks.

[dph1nm]

The limit to exposure time for unguided imaging on any mount depends on how accurate you polar align it.

This depends on what image scale you are trying to image at. I suspect periodic error will dominate most mounts well before a reasonable polar alignment starts to be a problem, unless you have such large pixels that PE is not an issue.

NigelM

[nmoushon]

Rik I dont think that is a fair comparison. For two reason:

1.) The mount is not the big factor in that comparison. The big factor is that one is using guiding while the other is just tracking. Which allows for much longer subs. You could compare a guided EQ5 vs a AP2000 mount and do a 300sec exposure on both and they would produce identical images. The only thing the AP2000 would be better at than the EQ5 would be able to support a larger and heavier telescope at much longer focal lengths and still easily do 300sec exposures. It could probably easily do 30min exposures.

2.) M45 is not a very fair target for comparison. Its nebula is very very faint and thus needs long exposures to bring it out not just total exposure time. So this is a comparison of exposure lengths(which goes back to guiding) and not comparing mounts.

Here is a good example of what a 150p can do - http://stargazerslounge.com/topic/163689-m42-nebula-and-rosette/ The M42 is only made up of 60sec subs while Rosette is made of 300sec subs. Not sure what mount he has but I would assume its nothing less than an EQ5. But it shows what you can get with guided and unguided setups.

Here is a comparison of two lenses. The first being your typical stock 75-300mm lens. http://stargazerslounge.com/topic/161485-andromeda-galaxy-wider-view-at-190mm-fl/ Its set at 190mm so just past half way. You get a nice wide field of view and you could get closer still. But notice the purple tent to everything? Thats because of the camera lens. Its set at f/5 which is not bad but if you took it to 300mm it would probably jump to around f/6.5 which is not very good. But look at his mount--HEQ5Pro ( a setup but still very similar to the HEQ5)--he got 420sec subs unguided! Thats amazing.

The second is with a more expensive lens (and camera) but really shows what a lens can really do! http://stargazerslounge.com/topic/162421-my-first-m31-andromeda-with-my-camera-lens/ But that lens is more than like much more expensive than the 150p. Dont quote me on that but that my guess.

But if you really do think the lens is the way to go I would suggest getting an AstroTrac instead of a mount. You wont be able to put any telescopes on it but its much lighter and smaller which means you can take it with you. http://stargazerslounge.com/topic/162374-andromeda-widefield/ Heres a great example of what the AstroTrac can do with a modest lens.

[nmoushon]

This depends on what image scale you are trying to image at. I suspect periodic error will dominate most mounts well before a reasonable polar alignment starts to be a problem, unless you have such large pixels that PE is not an issue.

NigelM

I would agree with you on that but you also got to take in effect the focal length to. PE will be noticeable first at shorter FL while I think mis-polar alignment would be more noticeable first at longer FL.

[ollypenrice]

My ha'p'orth...

By far the most critical factor is the focal length of the imaging lens. A guest recently took a stunning Milky Way image through a very widefield camera lens (was it about 20mmFL?) He used a stack of 5 minutes subs taken using an unguided driven EQ1!!! (Mounts don't get much worse than this.) So how long you'll get without trailing is predominantly about FL.

You can make unguided imaging work by having a short focal length and a very fast optic to cut exposure times.

I'm afraid, also, that I don't thnk you find great images taken unguided (with the exception of those taken on millionaire class mounts, which doesn't count!) You see remarkably creditable images taken unguided but not ones that wouldn't have moved a long way up the quality ladder with the benefits of guiding. Unguided imagers reject many of their subs, so they reduce their data stack and so reduce their quality. It is absolutely vital to get as many sub exposures as you can.

Another variable is distance from the Pole.

But, to answer the OPs question, a guided image from an HEQ5 (the H matters because of the smaller steps of the stepper motors) will roundly beat an image from a better optic on an inferior mount. There is not the slightest doubt about this once the focal lengths of these optics are long enough to require guiding. That, for five minute exposures, is likely to be no more than about 80mm in my view.

Mount-camera-optic is my priority order. Mount we've covered. Camera second? Yes, because amateur optics are not that much better than they were thirty years ago but the cameras are transformed, and an amateur M42 is now far better than a David Malin M42 from a three hundred tonne scope of thirty years ago.

Just buy yourself an HEQ5 and build up the rest later!

Olly

[RikM]

Rik I dont think that is a fair comparison. For two reason:

1.) The mount is not the big factor in that comparison. The big factor is that one is using guiding while the other is just tracking. Which allows for much longer subs. You could compare a guided EQ5 vs a AP2000 mount and do a 300sec exposure on both and they would produce identical images. The only thing the AP2000 would be better at than the EQ5 would be able to support a larger and heavier telescope at much longer focal lengths and still easily do 300sec exposures. It could probably easily do 30min exposures.

2.) M45 is not a very fair target for comparison. Its nebula is very very faint and thus needs long exposures to bring it out not just total exposure time. So this is a comparison of exposure lengths(which goes back to guiding) and not comparing mounts.

The way I look at it is that I spent 4 hours collecting data for each picture. Even if I had a guider on the EQ3-2 I would still have had to toss out half the subs because of wind shear, so I might have ended up with a deeper image but with a lot more noise. The EQ3-2 I have is pretty good as regards PE, it just can't cope with the sail are presented by the scope in the same way the bigger mount can.

Clearly you are right, with the same exposure lengths and number of subs, the mount won't affect the final image much, but it will certainly affect the amount of time / stress / hair it costs to get that image.

[jimjam11]

My ha'p'orth...

By far the most critical factor is the focal length of the imaging lens. A guest recently took a stunning Milky Way image through a very widefield camera lens (was it about 20mmFL?) He used a stack of 5 minutes subs taken using an unguided driven EQ1!!! (Mounts don't get much worse than this.) So how long you'll get without trailing is predominantly about FL.

You can make unguided imaging work by having a short focal length and a very fast optic to cut exposure times.

I'm afraid, also, that I don't thnk you find great images taken unguided (with the exception of those taken on millionaire class mounts, which doesn't count!) You see remarkably creditable images taken unguided but not ones that wouldn't have moved a long way up the quality ladder with the benefits of guiding. Unguided imagers reject many of their subs, so they reduce their data stack and so reduce their quality. It is absolutely vital to get as many sub exposures as you can.

Another variable is distance from the Pole.

But, to answer the OPs question, a guided image from an HEQ5 (the H matters because of the smaller steps of the stepper motors) will roundly beat an image from a better optic on an inferior mount. There is not the slightest doubt about this once the focal lengths of these optics are long enough to require guiding. That, for five minute exposures, is likely to be no more than about 80mm in my view.

Mount-camera-optic is my priority order. Mount we've covered. Camera second? Yes, because amateur optics are not that much better than they were thirty years ago but the cameras are transformed, and an amateur M42 is now far better than a David Malin M42 from a three hundred tonne scope of thirty years ago.

Just buy yourself an HEQ5 and build up the rest later!

Olly

Thanks,

What would be a good target to demonstrate the difference between short (say 90s) and longer subs (say 10mins) if the total exposure time was the same? I assume it will be something with very fine detail?

[nmoushon]

Look at the M45 pictures on the first page. The first one is made of short subs and the second one long subs. Rik doesnt say in the post what the exact details are but im sure he wouldnt mind sharing. My guess would be the first one is a stack of 60-90sec subs and the second one i would guess is probably around 300sec subs.

[RikM]

Not a bad estimate The I think first one was at 60sec but the second only 180sec actually. The stars are so bright that they start to burn out in my unmodded 1000D so I didn't push it too far as I wanted to retain the colour. I think they were both ISO800.

[jimjam11]

So to oversimplify you could conclude a heq5 or greater mount gives the hit rate and the guiding gives the quality and therefore a guided lesser mount could approach/equal the result from a better mount if you accepted the low hit rate and were a very patient person with plenty of hair?