Is the larger aperture worth the money?

[wuthton]

I'm fully clad, head to toe in my flame proof suit for this discussion as I've probably missed something obvious.

I'm fancying a little more focal length for some galaxy hunting. I've been looking at the 6" and 8" Ritchey Chretiens of various badges (reduced X 0.67) or the Skywatcher MN190 (native FL), to be paired with my Atik 314L+.

Now... with that massive reduction applied the two RCs get very close in what they yield. The field of view is only differs by a few minutes and the 8 is a smidge faster. If I buy the 6 I can afford to pop a Moonlite on it straight away, the 8 would need to be a homebrew motor focuser for now.

Is there any benefit to an extra 2" of aperture as neither scope will ever be adorned with an eyepiece or does it not matter because the MN190 would be a better choice?

[kroy]

If you are looking for astrophotography of DSO/planet my advise would always be to invest in a good motor driven stand.

If its just for visual purpose then the bigger the aperture the better it is.

I used 8" reflector with eq5 guided stand with sysnscan and believe me a motorised stand is worth it

Sent from my iPad using Tapatalk

[wuthton]

If you are looking for astrophotography of DSO/planet my advise would always be to invest in a good motor driven stand.

If its just for visual purpose then the bigger the aperture the better it is.

I used 8" reflector with eq5 guided stand with sysnscan and believe me a motorised stand is worth it

Sent from my iPad using Tapatalk

Many thanks for the reply. I should have mentioned that they will be mounted on my NEQ6. I currently use a separate guidescope but I'll probably go off axis for this setup.

[michael.h.f.wilkinson]

I think I would go for the MN190, unless you also want to image at F/8, the native focal ratio of the 8"RC. The MN190 has roughly the same speed and focal length as the reduced RC, but will give a larger flat FOV. If you move to a bigger chip camera, you are future proof.

[ollypenrice]

First let's think about focal reducers. If you are going after small galaxies which will fit on the chip without the reducer then the reducer brings no advantage. The faster F ratio does not speed up capture. This is the F ratio myth. (Olly dons fireproof overalls and Harry Page crash hat as well...)  Why? Because it brings in no new photons from the galaxy. The number of 'object photons' depends exclusively on the aperture. So that, really, is the primary answer to your question. The idea that a fast F ratio speeds up capture is not wrong but it is based on the idea that, for a given focal length, you speed up the F ratio by increasing the aperture. That is obviously true. But that is not what focal reducers do. They don't increase the aperture, they reduce the focal length, so your object covers fewer pixels and those pixels fill faster. However, you have lost resolution and image scale. If you worked without the reducer and simply shrank the final presentation you'd be in the same position. Focal reducers are great if you want everything that is in the wider FOV and you will get a good signal to noise on that wider FOV in less time, according to the usual rules of F ratio and exposure. But if you don't want all that's in the wider FOV ditch the reducer. It is useless.

Next up, focal length. You need it for galaxies. I have a TEC140 with a similar FL to the MN190 and I don't consider it a galaxy hunter. Take out M31, M33 and M101 and what does that leave for a metre? M106, just about. The Leo Triplet as a triplet. Nope, a metre doesn't cut it. The closer you can get to two metres the better, provided you can guide it. In my view the EQ6 is out of its comfort zone beyond a metre but it can certainly be persuaded to cope as many people have demonstrated.

The 8 has more FL and more aperture so it will open up more small galaxies. Both John and Sara have shown what these scopes can do though.

Olly

[Ikonnikov]

First let's think about focal reducers. If you are going after small galaxies which will fit on the chip without the reducer then the reducer brings no advantage. The faster F ratio does not speed up capture. This is the F ratio myth. (Olly dons fireproof overalls and Harry Page crash hat as well...)  Why? Because it brings in no new photons from the galaxy. The number of 'object photons' depends exclusively on the aperture. So that, really, is the primary answer to your question. The idea that a fast F ratio speeds up capture is not wrong but it is based on the idea that, for a given focal length, you speed up the F ratio by increasing the aperture. That is obviously true. But that is not what focal reducers do. They don't increase the aperture, they reduce the focal length, so your object covers fewer pixels and those pixels fill faster. However, you have lost resolution and image scale. If you worked without the reducer and simply shrank the final presentation you'd be in the same position. Focal reducers are great if you want everything that is in the wider FOV and you will get a good signal to noise on that wider FOV in less time, according to the usual rules of F ratio and exposure. But if you don't want all that's in the wider FOV ditch the reducer. It is useless.

Next up, focal length. You need it for galaxies. I have a TEC140 with a similar FL to the MN190 and I don't consider it a galaxy hunter. Take out M31, M33 and M101 and what does that leave for a metre? M106, just about. The Leo Triplet as a triplet. Nope, a metre doesn't cut it. The closer you can get to two metres the better, provided you can guide it. In my view the EQ6 is out of its comfort zone beyond a metre but it can certainly be persuaded to cope as many people have demonstrated.

The 8 has more FL and more aperture so it will open up more small galaxies. Both John and Sara have shown what these scopes can do though.

Olly

I presume in theory at least that a CCD with smaller pixels coupled with high QE (e.g. some of the newer Sony sensors) could go some way to help acquire galactic images at a reasonable level of detail using a more modest focal length? You would still need high guiding accuracy but could get away with a smaller OTA which may help the mount achieve this.

Paul

[Scott]

I too was wondering about the leo triplet with an ed80+414ex asi was intending giving it a crack when camera arrives. what is it exactly that doesn't cut it?

I'm not arguing the point, I really don't know. with a resolution of 2.22" and this fov, I thought it would be screaming to be imaged with this set-up

First let's think about focal reducers. If you are going after small galaxies which will fit on the chip without the reducer then the reducer brings no advantage. The faster F ratio does not speed up capture. This is the F ratio myth. (Olly dons fireproof overalls and Harry Page crash hat as well...)  Why? Because it brings in no new photons from the galaxy. The number of 'object photons' depends exclusively on the aperture. So that, really, is the primary answer to your question. The idea that a fast F ratio speeds up capture is not wrong but it is based on the idea that, for a given focal length, you speed up the F ratio by increasing the aperture. That is obviously true. But that is not what focal reducers do. They don't increase the aperture, they reduce the focal length, so your object covers fewer pixels and those pixels fill faster. However, you have lost resolution and image scale. If you worked without the reducer and simply shrank the final presentation you'd be in the same position. Focal reducers are great if you want everything that is in the wider FOV and you will get a good signal to noise on that wider FOV in less time, according to the usual rules of F ratio and exposure. But if you don't want all that's in the wider FOV ditch the reducer. It is useless.

Next up, focal length. You need it for galaxies. I have a TEC140 with a similar FL to the MN190 and I don't consider it a galaxy hunter. Take out M31, M33 and M101 and what does that leave for a metre? M106, just about. The Leo Triplet as a triplet. Nope, a metre doesn't cut it. The closer you can get to two metres the better, provided you can guide it. In my view the EQ6 is out of its comfort zone beyond a metre but it can certainly be persuaded to cope as many people have demonstrated.

The 8 has more FL and more aperture so it will open up more small galaxies. Both John and Sara have shown what these scopes can do though.

Olly

[ollypenrice]

I too was wondering about the leo triplet with an ed80+414ex asi was intending giving it a crack when camera arrives. what is it exactly that doesn't cut it?

I'm not arguing the point, I really don't know. with a resolution of 2.22" and this fov, I thought it would be screaming to be imaged with this set-up

LEOTRIPLET80ED.PNG

No, it does cut the Leo Triplet very well indeed (though it will crop the tidal tail in the Hamburger if you are head-banging enough to want to go after it!)  But my point was that this is still a framing of three galaxies. In order to go after the thousands of small individual galaxies out there I think you need more FL than a metre. At 980mm in the TEC M106 still needs a friend to make a decent image  on a 15mm squqre CCD chip.

I'm sure Paul's right that small pixels at a metre would help resolve fine details but still, for galaxy hunting, I'd want more than a metre. I just don't think it's enough. I've found it to be a focal length which has really only come into its own once allied to a full frame camera. In this mode you can attack widefield targets with more resolution than is usual and do exciting things. But for galaxies... no, it's too short for me.

Olly

[Scott]

cheers Olly, as I said, I am/will be very new to ccd so was truelly unsure , thanks for the clarification 

[wuthton]

The 8 has more FL and more aperture so it will open up more small galaxies. Both John and Sara have shown what these scopes can do though.

Olly

Thank you for the input, interesting read. I've only ever imaged fast and short so I'd never considered using it at F8 but... hey ho, why not? My mount is well tuned and guides very well so it's worth a go.

[ollypenrice]

Thank you for the input, interesting read. I've only ever imaged fast and short so I'd never considered using it at F8 but... hey ho, why not? My mount is well tuned and guides very well so it's worth a go.

I think I remember Tim saying, of imaging galaxies at long FL/slow F ratio, 'You'll wonder what all the fuss was about.'

My own galaxy imaging was with Yves' 14 inch at 2.4 metres and F6.8, so long but not that fast. If chasing faint outer parts we did go for 20 to 22 hours (M51, M101) but some of the small bright ones were done very quickly. These two were in the five to six hour bracket - to my surprise.

Olly

[DragosN]

I have seen this two scopes at work. The 8''RC from GSO, and the MN190. What I can say is that MN190 give me a better impression. Better build quality, better correction,  smaller obstruction, and probably, a better mirror. And in my opinion, it's easier to make a longer focal length with a barlow, than to reduce it. Not to mention the collimation, which is a pain with the RC (I've helped the owner of the RC twice to collimate his scope, and it took us hours, using a collimation program).

The down side, MN is heavier. But the friend who has the MN 190 use it with an off-axis guider, and it's working well, on a EQ6.

[swag72]

While I understand fully that if you are taking a picture of a small galaxy, whether you are imaging at f8 or f6 you will only get the same amount of photons hitting your sensor as there is only THAT much available light.

Now if we were to consider a nebula for the moment - How much difference would you expect to see with regards to light captured and better SNR between f8 and f6 say? ...... in simple terms if possible!! Even better if there's an illustration!!!

[ollypenrice]

I have seen this two scopes at work. The 8''RC from GSO, and the MN190. What I can say is that MN190 give me a better impression. Better build quality, better correction,  smaller obstruction, and probably, a better mirror. And in my opinion, it's easier to make a longer focal length with a barlow, than to reduce it. Not to mention the collimation, which is a pain with the RC (I've helped the owner of the RC twice to collimate his scope, and it took us hours, using a collimation program).

The down side, MN is heavier. But the friend who has the MN 190 use it with an off-axis guider, and it's working well, on a EQ6.

John and Sara's images show that there is nothing wrong with a well collimated budget RC... Sure, they need sorting but so does the MN190, as endless threads on here have shown. If you Barlow an MN190 it goes to F10.6. It's a lot bigger than the RC with vastly more polar moment so, at this kind of FL, guiding might get exciting. Not many people combine DS and Barlows. I don't know what happens to the image circle. Anybody know?

While I understand fully that if you are taking a picture of a small galaxy, whether you are imaging at f8 or f6 you will only get the same amount of photons hitting your sensor as there is only THAT much available light.

Now if we were to consider a nebula for the moment - How much difference would you expect to see with regards to light captured and better SNR between f8 and f6 say? ...... in simple terms if possible!! Even better if there's an illustration!!!

If you want to keep all of the image (and not crop it, viz my first post) then the exposure time goes as the square of the F ratio so F6 gives 6x6=36 (call it minutes) and F8 gives 8x8=64 (call it minutes again.) But if you are going to crop the F6 to give the same FOV as the F8 then all that goes out of the window and aperture rules.

Olly

[petevasey]

I have a 10" Truss GSO RC.  Very nice, description and some images on my web site. http://www.madpc.co.uk/~peterv

Of course there isn't an 8" truss version at the moment.  BUT Ian King did one using carefully picked GSO optics - the Ikharos 8".  And there's one for sale at the moment on UK Astronomy Buy and Sell.  It isn't shown as sold so hopefully is still available.  Looks nice, and has a Feathertouch focuser!

http://www.astrobuysell.com/uk/propview.php?view=91632

Might be worth considering ;-)

Peter

[petevasey]

Just to add to the above, here is a compendium of Skymap screen shots showing how some well known galaxies fit on the 285 chip in the Atik314L+ at 1600 mm focal length (8" GSO f8).  (Is it possible to modify earlier posts on this forum??)

Cheers,

Peter.

[wuthton]

I have a 10" Truss GSO RC.  Very nice, description and some images on my web site. http://www.madpc.co.uk/~peterv

Of course there isn't an 8" truss version at the moment.  BUT Ian King did one using carefully picked GSO optics - the Ikharos 8".  And there's one for sale at the moment on UK Astronomy Buy and Sell.  It isn't shown as sold so hopefully is still available.  Looks nice, and has a Feathertouch focuser!

http://www.astrobuysell.com/uk/propview.php?view=91632

Might be worth considering ;-)

Peter

Thanks for that! I had given it some lusty glances but it's a bit more than I want to spend, good price what it is though.

[swag72]

So Olly, if I can just use a couple of examples....... To clarify my thinking , assuming in each example no cropping takes place and the frame remains as is. I am taking a picture of NGC0000, a faint nebula in the constellation of Spamarium....

At f3.9..... 3.9*3.9 = 15 (give or take.... Let's assume minutes)

At f6.8..... 6.8*6.8 = 45 (as above...)

At f8....... 8*8 = 64 (as above...)

So I capture say an ADU of 10k on point x of a nebula in 15 minutes at f3.9, to capture the same 10k at the same point will take 45 mins at f6.8..... And at f8 that would take me 64 minutes.

Have I got that right in my head?

[wuthton]

So Olly, if I can just use a couple of examples....... To clarify my thinking , assuming in each example no cropping takes place and the frame remains as is. I am taking a picture of NGC0000, a faint nebula in the constellation of Spamarium....

At f3.9..... 3.9*3.9 = 15 (give or take.... Let's assume minutes)

At f6.8..... 6.8*6.8 = 45 (as above...)

At f8....... 8*8 = 64 (as above...)

So I capture say an ADU of 10k on point x of a nebula in 15 minutes at f3.9, to capture the same 10k at the same point will take 45 mins at f6.8..... And at f8 that would take me 64 minutes.

Have I got that right in my head?

This is my understanding too (I await to be corrected) but as you previously mentioned I'd also like to know the expected difference in SNR for these three examples.

[ollypenrice]

Yup, that's my understanding. The absolutely key thing is that in comparing F ratios you do so at a fixed focal length or the F ratio myth wheedles its way in and gets you.

F ratio does matter but, on a given target, the only way to improve it is to get more aperture. Down the road from me Karel Teuwen has one of these; http://dreamscopes.com/pages/07/16inDAtube-14.htm

'Only' I.5 metres of focal length but F3.75. It takes a certain courage, I think, to spend so much on a modest focal length but at that F ratio you can really get on with it and catch the faint stuff.

Olly

Edit, regarding S/N ratio I understand the S/N ratio to impove as the square root of the exposure time so four times the exposure halves the noise. I'll assume that it doesn't matter whether you increase the effective exposure time via increasing the aperture (ie lowering the F ratio) or by adding more time. I don't know if it's a simple as that but I suspect that will be more or less right.

[swag72]

So, if you will allow me to keep the same figures as before for the noise analysis.

At 15 minutes of exposure I get 10 'blocks of noise' at f3.9. To half that noise to 5 'blocks' I'd need 60 minutes of exposure at f3.9.

Now assuming I'm working at f6.8, where I need 45 minutes to get the same amount of data as above (and therefore I assume the same 10 'blocks of noise') - In this instance I would need 180 minutes in order to get 5 'blocks of noise' (That's 45x4).

At f8 - 64 minutes gets me the data and 10 'blocks of noise' and so to half that I'd need 254 minutes of data (64x4) 

Is that right?.......................

Sorry for being so simple!!!

[ollypenrice]

So, if you will allow me to keep the same figures as before for the noise analysis.

At 15 minutes of exposure I get 10 'blocks of noise' at f3.9. To half that noise to 5 'blocks' I'd need 60 minutes of exposure at f3.9.

Now assuming I'm working at f6.8, where I need 45 minutes to get the same amount of data as above (and therefore I assume the same 10 'blocks of noise') - In this instance I would need 180 minutes in order to get 5 'blocks of noise' (That's 45x4).

At f8 - 64 minutes gets me the data and 10 'blocks of noise' and so to half that I'd need 254 minutes of data (64x4) 

Is that right?.......................

Sorry for being so simple!!!

Makes grim reading, doesn't it! But, yes, I think that's about it.

On the positive side I suspect that, when shooting at long focal length, you are less likely to be chasing ultra-faint data which then leads you into extreme stretching and difficulty with noise. The chasing of extravagently faint signal tends to be a widefield passtime - and pass the time it does! Clearly this is not a 'scientific' point but is just a practical observation on what you are likely to be shooting at long and short focal lengths.

Olly

And a PS; if you're a bit short on FL you tend to try to compensate with masses of data so as to go for more software sharpening (and exotic bits of surrounding faintness!) If you have the right focal length to start with this becomes less of a priority, maybe? I'm not sure about this point but float it anyway. Carole did a Helix here at rather a short FL for target but the image could fight back with great outlying details and very strong colour.

[swag72]

Having just compiled a spreadsheet based on these figures, you're right Olly it makes very grim reading. Assuming that my average image with the Tak is probably about 20 hours, in order to get the same quality data with the F8 RC would take a smidgen under 85 hours  - Now THAT is depressing!

No wonder I've been thinking these images are a tad on the noisy side!

[michael.h.f.wilkinson]

I am helping in the analysis of some extravagantly faint stuff in things like the Sloan Digital Sky Survey (just had a paper accepted for a conference), and that is very much a galaxy survey. I am also about to join a project on very low surface brightness dwarf galaxies in Fornax.  So there is interest in extravagantly faint stuff in that area. They also use RC scopes, but generally quite a bit bigger than 8"

[ollypenrice]

Having just compiled a spreadsheet based on these figures, you're right Olly it makes very grim reading. Assuming that my average image with the Tak is probably about 20 hours, in order to get the same quality data with the F8 RC would take a smidgen under 85 hours  - Now THAT is depressing!

No wonder I've been thinking these images are a tad on the noisy side!

If you define quality in terms of SN ratio alone, then yes. However, image quality is also about resolution, the more so in longer FL images I think. For example, if you do a widefield Triplet you'll be chasing the tidal tail whereas at longer FL you'll be more interested in tight galactic core details which don't need such exposure times.  Also you work in narrowband mainly, which is inevitably slower than broadband.

Edit. As Michael says, the professionals chase the faint stuff at long FL but they don't have to pay for their own toys!

Olly