The "No EQ" DSO Challenge!

[Manners2020]

Yeah I'm in Canada.  I've not found anything by lunt online. That's one of the reasons I asked actually. I've not found that many online.  I'll take my time on this one I think. Just until I'm sure which set up will work. Thx.

[The Admiral]

I don't think the make of reducer is critical, so long as it's suitable for the focal length. I see that the Lunt white light 'scopes are made by APM, so anything suitable for those should be fine. Just make sure that the back focus is adequate if you intend ever using a filter wheel. For the Photoline one it's ~56mm at a FL of 715mm. It's also useful if the reducer can attach to the focuser by screw thread, thus ensuring stability of the optical train. Mine doesn't, and I use the Baader Click-Lock screwed onto the focuser to hold the reducer central and square.

Ian

Edited April 11, 2018 by The Admiral

[Filroden]

21 hours ago, Manners2020 said:

Is it as straight forward as it's going to make things a little faster  and easier or is there anything it's going to make harder at the same time?

Just something to add onto Ian's good points above: adding a focal reducer will not make imaging faster. It only increases the field of view (of the same scope) by trading it off with a lower resulting resolution (for the same camera). If you don't need the additional field of view you are better not using the reducer. Or to put it another way - have the reducer available, but only use it when you need the additional field of view.

Edited April 11, 2018 by Filroden

[happy-kat]

That's interesting.

That's read like the same as the crop sensor comments, again it is not changing the magnification it is only changing the FoV seen when using that lens on a crop sensor.

So a focal reducer is changing the FoV seen but is not actually making an f10 scope f6 for argument example. That makes sense as the light still has to travel the same distance. Or have I misunderstood you @Filroden?

[The Admiral]

1 hour ago, Filroden said:

adding a focal reducer will not make imaging faster.

Well now Ken, this subject raises a lot of contention, and often has 'you know who' wading in with the 'f-ratio myth' . He said it again in a recent post I noticed.

However, I'm afraid I have to say I don't buy it, or at the very least, I don't understand where I am at fault! If the FoV is increased using the same equipment as before, it follows (to me!) that the photon flux/pixel from an object must necessarily increase, because all the photons being emitted by that object will be squished over a smaller area of sensor, i.e. fewer pixels. Now if exposure time is governed by SNR, won't the use of a reducer allow the target SNR to be reached quicker, simply because the pixels will be 'filled' at a faster rate? I accept that the total number of photons entering the telescope will be governed by the objective area, but it's not the total number of photons that we are interested in here, surely? What am I missing?

Ian

[Filroden]

I agree that the SNR will increase because you have reduced resolution but your overall image brightness (in the original fov) will be lower if you lower exposure. So SNR will be higher but brightness lower in the area covered by the original fov. So you absolutely should use a reducer if your imaging target is larger than the original fov. You have a choice to make about the reducer if your target fits within the unreduced fov. Do you only want SNR or do you also want brightness. You can push the processing through more aggressive stretching to increase brightness, but then you're also stretching the noise. Does one equal and cancel out the other? I suspect the overall net effect is that whatever gain you make in SNR is lost through lower brightness, so you're back to the same exposure length to achieve the same final quality image and the only parameters that really changed were the fov and resolution.

But I'm tired and this sort of logic makes my head hurt!

[The Admiral]

Sorry to feed the headache , but wouldn't using a reducer increase the brightness, as well as SNR? Surely, that's how the higher SNR is achieved. Is it not the equivalent of using a shorter longer FL eyepiece in visual observing, where a lower overall magnification gives you a wider field of view and a brighter image?

Ian

Edited April 11, 2018 by The Admiral

[Manners2020]

I only wanted one to decrease exposure times so if that's the case I wont bother however I should have known there are no concrete answers Haha   well for now I'll hold off until I k ow at least something about these things. Right now I know nothing. Thanks for all the info everyone  

[Manners2020]

But just to see if I'm understanding  this correctly. A fr  would initially make an f/7  a lower f number however the reduced light would be dimmer which would counteract the initial speed boost ?

[Manners2020]

Also I have a 9.25 sct. I've considered buying the fr  for that. Would that make for a better imaging scope with the 6.3  reducer than my f/7 refractor? 

[The Admiral]

3 minutes ago, Manners2020 said:

But just to see if I'm understanding  this correctly. A fr  would initially make an f/7  a lower f number however the reduced light would be dimmer which would counteract the initial speed boost ?

That's not my view, but happy to be proved wrong .

Ian

[Manners2020]

Ian. Sorry if this is an impossible question but in your opinion how much of a reduction would you say you would get. You have a 102mm f/7 right? 

[Manners2020]

In exposure time I mean.

[The Admiral]

Difficult to answer really, it's not something I've attempted to demonstrate, principally because with Alt-Az imaging the exposure is constrained totally by field rotation and by tracking quality. This as you probably realise depends on both altitude and azimuth, and I've taken the line of using just a 30s exposure for everything as a matter of operational convenience, unless I particularly want to reduce it to deal with wide dynamic range. Some folk have managed to get decent results with a minute or more exposures, in preferred directions, but my mount wouldn't be able to achieve that I don't think.

Given the constrained exposures, I have taken the view that a reducer would improve things, based on my, perhaps naive, understanding of theory. You would not use an fr to reduce exposure time; you would maximise the exposure time within the imposed constraints. I believe it will improve image quality, and I can convince myself that that has been the case . Not only that, but the effectively reduced focal length means that the effects of mount tracking deficiencies are less obvious, enabling me to keep a higher proportion of subs. It has also allowed me to image more of the Rosette Nebula and M31 for example, and given me a bit of room for cropping off the field rotation artefacts.

Hope that helps,

Ian

Edited April 11, 2018 by The Admiral

[Manners2020]

It does yes. I did actually mean get more out of current exposure lengths but you explained it better. 

[alacant]

14 hours ago, The Admiral said:

He said it again in a recent post I noticed.

He's unlikely to ever look here so we're quite safe! There was one thread where it seemed to make sense though; the 2 metre reflector at La Palma. That's f10. So it must need a longer exposure than a f7 skywatcher refractor? Then I got it! HTH.

13 hours ago, Manners2020 said:

In exposure time I mean.

Just to support @filroden above, AFAICT you have not changed the aperture of the telescope so the exposure time is the same with or without the reducer.

--- --- ---

But hey, who cares? Let's just get out there and take photos!

Cheers and clear skies.

Edited April 12, 2018 by alacant

[Stub Mandrel]

1 hour ago, alacant said:

He's unlikely to ever look here so we're quite safe! There was one thread where it seemed to make sense though; the 2 metre reflector at La Palma. That's f10. So it must need a longer exposure than a f7 skywatcher refractor? Then I got it! HTH.

If you point both scopes at the same (large) grey card, the skywatcher would need a shorter exposure.

http://telescope.livjm.ac.uk/TelInst/calc/

 

[alacant]

1 hour ago, Stub Mandrel said:

point both scopes at the same (large) grey card

Ahhgghh! So I haven't understood. What if I pointed them at m82?

[calli]

Different explanation: (maybe I am totally wrong)

Point the scope on a big grey card which is emits 100photos per square meter and second. We choose the spacers so that the image will cover our complete sensor (also square)

1 second exposure.

Without FR we get a FOV which covers 1 m² == 100 photons.

FR 0,5x we now get a FOV od 2x2 m² == 400 photons on the same sensor area!

 

Not sure what this will mean in practice or for "point" lights of stars. And of course if the object will fill the FOV without FR we will not gain anything when we crop our FR image later to one m²... So IMHO it is "faster" but we compare apples with pairs because we basically don't take the same picture... But then imagine taking a 2x2 mosaic without FR, sould be the same image as with FR (when it is perfect and does not swallow any photons).

Carsten

[Stub Mandrel]

1 hour ago, alacant said:

Ahhgghh! So I haven't understood. What if I pointed them at m82?

It should take longer to expose, but you get a much bigger image. Only if you use the same sensor.

The myth buster is that if you image at the same scale, i.e. with great big pixels or much binning, the exposure needed will be a tiny fraction of that for the smaller scope.

What would be really useful to astronomers would be a single number that covers aperture, focal length AND pixel size. This would allow direct comparison of the relative 'speed' of any scope+sensor combination

[alacant]

9 minutes ago, Stub Mandrel said:

It should take longer to expose

So, just to be certain. I take m82 with my dslr on a skywatcher. I'm gonna need longer with the same camera attached to the La Palma 2m telescope?

[The Admiral]

I think we are getting into a different realm here by introducing another telescope with a much larger aperture. The thing is, the La Palma scope has a FL of 200 x 10 = 2000cm, whereas a 10cm f/7 scope has a FL of 70cm, and therein lies the main difference. We shouldn't be comparing f-numbers, but focal lengths and apertures. Perhaps that is what is meant by the 'myth'?

For what it's worth, here are a few notes I made to try and get my thoughts straight (assuming I've got the maths correct ). The title may, or may not be, a red herring!

Is the F ratio Myth a Myth v4 .pdf

It's the conclusion that is important here.

If you like, the act of 'concentrating' the photons can be thought as being analogous to using a magnifying glass and the sun on a piece of paper. If the glass is not focused, all the photons coming through it are spread over a large area and there is no effect. As the glass is brought into focus, so the intensity of radiation in the illuminated patch gets higher, even though the same number photons are passing through the glass, until at some point the paper combusts! We are clearly not burning the sensor in imaging , but to my mind we are increasing the intensity of light on each pixel. That's my way of thinking about it anyway, though not saying it's the correct way!

Ian

[Filroden]

Ok, here's my logic:

A reducer will "squeeze" the same signal into a smaller number of cells, so the signal per cell will increase (assuming you don't exceed the full well capacity). It also squeezes any signal noise into a smaller number of cells, so the signal noise per cell will also increase. As signal noise is proportional to signal, so you've not changed the Signal to Signal Noise Ratio. You do, however, hold instrument noise fixed per cell. Fewer cells means these will reduce compared to signal. So a slight improvement of SNR (more improvement for noisier cameras, less improvement for low read noise/low dark current cameras).

Assuming you doubled signal through reduction (not a real example but keeps the maths simple):

S / (SN + IN)    <    (2 x S) / ( (2 x SN) + IN)

Now with some made up numbers:

If S = 4, SN = 2 and IN = 1 then 4/3 < 8/5      <==== a noticeable improvement

So, for a target that fits within the original field of view, you gain a small improvement in SNR by increasing the signal (and signal noise) per cell but maintaining instrument noise. But as instrument noise tends to zero (or if instrument noise is completely swamped by signal) then the reduced SNR will tend towards the original SNR and no real benefit is achieved. You might as well have just used software binning to reduce resolution (which can help mask tracking errors too).

If S = 100, SN = 10 and IN = 1, then 100/11 < 200/21     <==== not really a noticeable improvement

So for signal (and signal noise) that far exceeds instrument noise, there wouldn't be any benefit in reducing exposure if aperture and pixel sizes remained constant.

[The Admiral]

OK, that makes sense Ken, though for the limited exposures used in Alt-Az imaging with a DSLR, I imagine signal noise is unlikely to swamp sensor noise, but I may be wrong. Though do you think that raising the signal by using a reducer would be beneficial to seeing deeper, i.e. getting signal off the bottom of the histogram, given that we're normally a long way from being sky limited? Also, I'm not sure how software binning compares to hardware binning when it comes to noise though, but a minor issue so far as the argument is concerned. DSLR users have no choice but to software bin of course.

So far as reducing the effects of mount tracking issues is concerned, it strikes me, despite my earlier comment, that that advantage is only gained if there isn't significant cropping after the event.

Ian

Edited April 12, 2018 by The Admiral

[Filroden]

*** deleted ***

Had not measured noise just signal in original post

Edited April 12, 2018 by Filroden