Help needed choosing a new telescope for imaging

[Lee_P]

1 hour ago, The Admiral said:

Just to muddy the waters further, have you looked at the Altair triplet scopes? I had a Wave 102ED triplet and that gave some nice results. I see that there is a new 130mm triplet, which with the reducer gives about 700mm effective FL at f/5.6. Might be a bit heavy though. These scopes might exist as other brands of course (i.e. TS APO range for example). They all come with an optical bench test results sheet.

Ian

Good shout, the 115 looks pretty tasty.

[vlaiv]

23 minutes ago, raadoo said:

So, if you under or oversample your images, is that the end of the world?

Under sampling is never a problem.

Over sampling - it's not a problem in itself if you accept two things:

1. You will spend more time imaging to get the same quality image

2. When viewed at 100% zoom - your image will not look as sharp

Luckily - you can choose to correct both of these things after you are done imaging. With CMOS sensors there is no difference between binning in firmware and binning in software and you can choose to bin in software later on.

 

[Lee_P]

45 minutes ago, vlaiv said:

AstroImageJ gives quite different results. Something else seems to be off

Average FWHM seems to be around 3.5-3.6px.

FITS header of that file is also very strange - it gives:

Pixel size seems to be reported correctly, but focal length is set to 749mm and hence pixel scale is 1.0356"/px?

Do you plate solve or did you manually enter 750mm FL for some reason? Is this image from that 400mm scope?

If image is from 400mm and all else is correct apart from wrong FL reported - then your FWHM values are about ~6.75", which means that you should be sampling at about ~4.2"/px or image should be about 2.2 times smaller (it is over sampled by factor of x2).

Indeed - if we reduce it to 50% (a bit less than it should be - but easy to do in order to remove bayer matrix), and crop it to 100% "zoom level":

It still looks good and sharp (above is just linear stretch - white/black point to show what is there).

Aaaah, the plot thickens. That's odd. I use an ASIAIR Plus for my imaging, and the FL there is correctly stated as 400mm (or 402mm or something, from memory). It uses this info to plate solve correctly. I've no idea where 749mm comes from -- unless it's a sign from above that I should get the 107PHQ, which has a FL of 749mm 😝 

I've got a fair few telescope options to choose from. I appreciate the benefits of going for something like a 6" reflector, but fear it would be a bit fiddly and, for me, not as fun to use. The MN152 looks appealing, but hard to get hold of now. As for refractors, I'm still drawn to the 107PHQ (FL 749mm, f/7). I haven't quite got my head around speed and binning and the like. So, with that telescope in mind, would you be so kind as to summarise the differences I'd get between it and my current FRA400 f5.6? I *think* I'd get "closer" views, would see more detail (e.g. separating close stars), but would need longer integration times to reach my desired SNR. Or maybe longer subframe lengths and the same total integration... I'm getting in a muddle and am not sure of anything 🤣🥴 

Thanks vlaiv!

[ONIKKINEN]

3 hours ago, Lee_P said:

Ok, thanks. So what if I didn't bin (i.e. BINx1) with the PHQ107? Better resolution but slower speed (0.6x my current?)

The resolution will depend on your seeing conditions mostly, if the mount plays along nicely, Vlaiv has gone into that in detail.

But the speed will still increase, even if you dont bin/keep debayering using interpolation. Your focal length will increase by 1.87x but aperture area increases by 2.2x so it will still be a little bit faster in terms of integration times. Your subexposures will need to be a bit longer, but swamping read noise in bortle 8 is not difficult even with narrowband filters so this is something you dont have to worry about either.

[Lee_P]

29 minutes ago, ONIKKINEN said:

The resolution will depend on your seeing conditions mostly, if the mount plays along nicely, Vlaiv has gone into that in detail.

But the speed will still increase, even if you dont bin/keep debayering using interpolation. Your focal length will increase by 1.87x but aperture area increases by 2.2x so it will still be a little bit faster in terms of integration times. Your subexposures will need to be a bit longer, but swamping read noise in bortle 8 is not difficult even with narrowband filters so this is something you dont have to worry about either.

Thanks! Re: speed, earlier in this thread it was said that "You can work out a measure of the “speed” of your current system- it’s aperture squared multiplied by image scale squared."

FRA400 = 72^2 * 1.94^2 = 19,510
107PHQ = 107^2 * 1.035^2 = 12,264

Given that method, the 107PHQ is 0.63 (let's say 2/3) the speed of the FRA400. But you're saying that the 107PHQ would be a bit faster. So there's something I'm missing, or not understanding. For clarity, and I think this is what you're saying, the subframes would be slower, but the total integration time would be a bit faster?

[ONIKKINEN]

31 minutes ago, Lee_P said:

Thanks! Re: speed, earlier in this thread it was said that "You can work out a measure of the “speed” of your current system- it’s aperture squared multiplied by image scale squared."

FRA400 = 72^2 * 1.94^2 = 19,510
107PHQ = 107^2 * 1.035^2 = 12,264

Given that method, the 107PHQ is 0.63 (let's say 2/3) the speed of the FRA400. But you're saying that the 107PHQ would be a bit faster. So there's something I'm missing, or not understanding. For clarity, and I think this is what you're saying, the subframes would be slower, but the total integration time would be a bit faster?

I wouldnt bet on my math, so lets say i got it wrong. Come to think of it, when focal length doubles the speed of the system is not halved but goes down to 1/4. So you are right, it will be a bit slower. 

I still think you should bin x2 by either superpixel debayer, bayer split or just bin the linear stack x2 after stacking. You will lose nothing but gain SNR.

[vlaiv]

3 hours ago, Lee_P said:

So, with that telescope in mind, would you be so kind as to summarise the differences I'd get between it and my current FRA400 f5.6? I *think* I'd get "closer" views, would see more detail (e.g. separating close stars), but would need longer integration times to reach my desired SNR. Or maybe longer subframe lengths and the same total integration... I'm getting in a muddle and am not sure of anything 🤣🥴 

It is not easy to summarize the differences.

I can give it a go, but I'm afraid it will be all but "summarized"

- in same conditions 107mm of aperture has potential to out resolve 72mm of aperture. How much? That really depends on other conditions as these don't add up linearly. I'll give you few examples (good seeing, excellent mount, then medium conditions and poor conditions). Mind you - this is for diffraction limited optics (we need to discuss in some detail what I mean by this).

In 1.5" FWHM seeing and 0.5" RMS guide error

72mm aperture will produce 2.46" FWHM which corresponds to 1.54"/px

107mm aperture will produce 2.17" FWHM which corresponds to 1.36"/px

In 2" FWHM seeing and 1" RMS  guide error

72mm aperture will produce 3.45" FWHM which corresponds to 2.16"/px

107mm aperture will produce 3.26" FWHM which corresponds to 2.04"/px

In 3" FWHM seeing and 1.5" RMS guide error

72mm aperture will produce 4.89" FWHM (3.05"/px)

107mm aperture will produce 4.75" FWHM

-------------------------------------------

72mm aperture performs 13.36% worse, 5.83% worse and 2.95% worse with respect to FWHM size depending on conditions (from best to worst).

If you want to really take advantage of larger scope - you need to have good mount and good skies and difference will be significant.

Another thing to notice is that improvement in sharpness is not proportional to aperture size - double the aperture and you won't get double the resolution, so even 150mm aperture wont help much in poor conditions.

Mind you - above are theoretical values for diffraction limited aperture.

Most scopes that we use for imaging are not diffraction limited! Most of them will in fact produce more blurred image. Primary optics is almost always diffraction limited in center, but we almost always use correctors of some sort - like coma corrector, field flattener, focal reducer - you name it.

These often make optics perform under diffraction limit. Take look at Askar FRA400 spot diagram:

RMS radius is 4.873um in center of the field and growing as you move away from center.

As a comparison - RMS of Airy disk of 0.662" for 72mm of aperture, which at 400mm focal length gives 1.28um RMS (0.516"/um for 400mm).

That is x3.8 larger Airy disk because you are using quintuplet astrograph instead of regular refractor.

Note that in average conditions we calculated expected FWHM of ~3.5" for diffraction limited optics - and we measured about 6.75" FWHM in your image. How come that we have such huge difference?

That is down to having optics that is not diffraction limited. If I do another calculation for same parameters (2" FWHM seeing and 1" RMS guiding) but this time instead of 72mm I use 19mm of aperture - which gives x3.8 larger airy disk - I get expected FWHM to be:

6.66" FWHM star profile - which matches what we measured in your image.

What does this mean for 107mm vs 72mm comparison?

Well, we can't really tell until we see spot diagram of 107PHQ and we run some calculations. Will there be some improvement - I think so.

Probably most improvement will come from larger aperture coupled with very good field flattener with excellent spot diagram. Alternative is to use telescope with longer focal length that has flat field without additional corrector and already good spot diagram and to bin those to get to your target resolution.

 

 

[Lee_P]

4 hours ago, vlaiv said:

It is not easy to summarize the differences.

I can give it a go, but I'm afraid it will be all but "summarized"

- in same conditions 107mm of aperture has potential to out resolve 72mm of aperture. How much? That really depends on other conditions as these don't add up linearly. I'll give you few examples (good seeing, excellent mount, then medium conditions and poor conditions). Mind you - this is for diffraction limited optics (we need to discuss in some detail what I mean by this).

In 1.5" FWHM seeing and 0.5" RMS guide error

72mm aperture will produce 2.46" FWHM which corresponds to 1.54"/px

107mm aperture will produce 2.17" FWHM which corresponds to 1.36"/px

In 2" FWHM seeing and 1" RMS  guide error

72mm aperture will produce 3.45" FWHM which corresponds to 2.16"/px

107mm aperture will produce 3.26" FWHM which corresponds to 2.04"/px

In 3" FWHM seeing and 1.5" RMS guide error

72mm aperture will produce 4.89" FWHM (3.05"/px)

107mm aperture will produce 4.75" FWHM

-------------------------------------------

72mm aperture performs 13.36% worse, 5.83% worse and 2.95% worse with respect to FWHM size depending on conditions (from best to worst).

If you want to really take advantage of larger scope - you need to have good mount and good skies and difference will be significant.

Another thing to notice is that improvement in sharpness is not proportional to aperture size - double the aperture and you won't get double the resolution, so even 150mm aperture wont help much in poor conditions.

Mind you - above are theoretical values for diffraction limited aperture.

Most scopes that we use for imaging are not diffraction limited! Most of them will in fact produce more blurred image. Primary optics is almost always diffraction limited in center, but we almost always use correctors of some sort - like coma corrector, field flattener, focal reducer - you name it.

These often make optics perform under diffraction limit. Take look at Askar FRA400 spot diagram:

RMS radius is 4.873um in center of the field and growing as you move away from center.

As a comparison - RMS of Airy disk of 0.662" for 72mm of aperture, which at 400mm focal length gives 1.28um RMS (0.516"/um for 400mm).

That is x3.8 larger Airy disk because you are using quintuplet astrograph instead of regular refractor.

Note that in average conditions we calculated expected FWHM of ~3.5" for diffraction limited optics - and we measured about 6.75" FWHM in your image. How come that we have such huge difference?

That is down to having optics that is not diffraction limited. If I do another calculation for same parameters (2" FWHM seeing and 1" RMS guiding) but this time instead of 72mm I use 19mm of aperture - which gives x3.8 larger airy disk - I get expected FWHM to be:

6.66" FWHM star profile - which matches what we measured in your image.

What does this mean for 107mm vs 72mm comparison?

Well, we can't really tell until we see spot diagram of 107PHQ and we run some calculations. Will there be some improvement - I think so.

Probably most improvement will come from larger aperture coupled with very good field flattener with excellent spot diagram. Alternative is to use telescope with longer focal length that has flat field without additional corrector and already good spot diagram and to bin those to get to your target resolution.

 

 

Thanks vlaiv for the thorough answer, as always. I appreciate your efforts to "summarise!"

Here's the spot diagram for the PHQ107. Does it help with the comparison with the FRA400?

 

You say that to take advantage of a large aperture telescope, I need a good mount and good skies. Well, I'm stuck with the skies I've got. But I could consider a mount with a higher tracking accuracy and larger weight capacity. (Not sure which one, exactly..!) The FWHM percentage improvements you calculated don't seem that significant, so should I conclude that if I want something that will be a decent step up, I should be getting a new mount *and* a large aperture telescope?

Thanks again...

[vlaiv]

20 minutes ago, Lee_P said:

You say that to take advantage of a large aperture telescope, I need a good mount and good skies. Well, I'm stuck with the skies I've got. But I could consider a mount with a higher tracking accuracy and larger weight capacity. (Not sure which one, exactly..!) The FWHM percentage improvements you calculated don't seem that significant, so should I conclude that if I want something that will be a decent step up, I should be getting a new mount *and* a large aperture telescope?

Well, yes.

I often advocate at least 6" of aperture in order to push down to 1.5"/px and 8" for going below that value.

However, given above spot diagram - I think that you will see a difference with 107mm scope.

Not because of aperture size - but because of better looking spot diagram.

First thing to note is that spot diagrams are given in micro meters and not arc seconds. Here longer focal length gives a bonus because it makes error in micrometers smaller in arc seconds (same thing as being closer in with longer FL - but in opposite direction).

~4.9um RMS spot diagram on axis for FRA400 is really 2.53" RMS

~3.16um RMS spot diagram on axis for 107PHQ translates into 0.87" RMS

So spot diagram alone is x2.9 times smaller. That will improve things as far as detail goes.

Mind you - adding focal reducer will most likely skew spot diagram and make RMS bigger again. I would use it at native FL.

This won't really get you closer in to your targets, but it will get you sharpness. Given that you use OSC camera, I would do super pixel debayer and effective resolution would then be 2.07"/px - but I think it would be "true 2"/px".

[raadoo]

14 hours ago, 900SL said:

Having just gone through the same process and candidates, I've decided to get an ES 152 MN. Now waiting for them to arrive in stock. 

I reached out to TS to ask about the ES 152 MN. Copied and pasted below:

I see that we have ordered them, but honestly don't know if we can get them delivered.I find only discordant informations.  in any case  I asked to check this fact to our purchase office, in cae the webshop will be corrected

[Lee_P]

38 minutes ago, vlaiv said:

Well, yes.

I often advocate at least 6" of aperture in order to push down to 1.5"/px and 8" for going below that value.

However, given above spot diagram - I think that you will see a difference with 107mm scope.

Not because of aperture size - but because of better looking spot diagram.

First thing to note is that spot diagrams are given in micro meters and not arc seconds. Here longer focal length gives a bonus because it makes error in micrometers smaller in arc seconds (same thing as being closer in with longer FL - but in opposite direction).

~4.9um RMS spot diagram on axis for FRA400 is really 2.53" RMS

~3.16um RMS spot diagram on axis for 107PHQ translates into 0.87" RMS

So spot diagram alone is x2.9 times smaller. That will improve things as far as detail goes.

Mind you - adding focal reducer will most likely skew spot diagram and make RMS bigger again. I would use it at native FL.

This won't really get you closer in to your targets, but it will get you sharpness. Given that you use OSC camera, I would do super pixel debayer and effective resolution would then be 2.07"/px - but I think it would be "true 2"/px".

Blimey, this is intense! So, the PHQ107 would get me sharper views, but not "closer"; and would be about 0.6x the speed of the Askar FRA400 (calculated earlier in this thread) thereby necessitating longer total integration times? That doesn't sound like a great deal for a £2500 'scope, compared to the Askar FRA400's ~£1000 price tag... It does seem weird that increasing the aperture by such a large amount doesn't really yield much in the way of any benefits, and it's actually down to the quality of the optics that there would be an improvement 🤯 

And is it the case, flagged earlier, that ~700mm is an effective limit on focal length for any system if I stick with my current mount?

[vlaiv]

18 minutes ago, Lee_P said:

Blimey, this is intense! So, the PHQ107 would get me sharper views, but not "closer"; and would be about 0.6x the speed of the Askar FRA400 (calculated earlier in this thread) thereby necessitating longer total integration times? That doesn't sound like a great deal for a £2500 'scope, compared to the Askar FRA400's ~£1000 price tag... It does seem weird that increasing the aperture by such a large amount doesn't really yield much in the way of any benefits, and it's actually down to the quality of the optics that there would be an improvement 🤯 

And is it the case, flagged earlier, that ~700mm is an effective limit on focal length for any system if I stick with my current mount?

Sharper is closer but the problem is that people don't really think this way.

We think of being closer in terms of FOV, right?

Above image of M13 is not very close in, right?

How about this image:

That is close in, right?

Same image. It was taken with 80mm scope. Sampling rate is 2"/px - same as your image.

When it is zoomed in 100% do you feel it is soft and too much zoomed in by looking at the size of stars? I think it has good sharpness - and this allows for it to be cropped and looking much "closer in".

[scotty38]

It might fall foul of your weight criteria but what about the Esprit 120? @vlaiv I'd be interested to see how that would that stack up numbers wise please......

[vlaiv]

5 hours ago, scotty38 said:

It might fall foul of your weight criteria but what about the Esprit 120? @vlaiv I'd be interested to see how that would that stack up numbers wise please......

That really depends on flattener used with it and spot diagram.

120mm of aperture can resolve something like 1.5"/px under right conditions - for example, in 1.5" seeing and 0.7" RMS guiding (good seeing and something HEQ5 should be able to achieve on regular bases, at least tuned version) - it hits 1.5"/px.

It has 840mm of FL - which makes it operate on 0.92"/px when using regular debayering (and I would advise against this) - or 1.84"/px when using super pixel debayering or split debayering which is ok.

Here is an example of what Esprit 120 can achieve as close up:

 

 

[scotty38]

Thanks Vlaiv, appreciated as always!

[Lee_P]

On 21/07/2022 at 21:28, vlaiv said:

Sharper is closer but the problem is that people don't really think this way.

We think of being closer in terms of FOV, right?

Above image of M13 is not very close in, right?

How about this image:

That is close in, right?

Same image. It was taken with 80mm scope. Sampling rate is 2"/px - same as your image.

When it is zoomed in 100% do you feel it is soft and too much zoomed in by looking at the size of stars? I think it has good sharpness - and this allows for it to be cropped and looking much "closer in".

Thanks vlaiv, that's a really useful example.

I'm understanding that perhaps I should bite the bullet and get a new mount to improve guiding and allow more options for an OTA. An EQ6-R PRO should fit my DIY pier and so be a relatively straight-forward upgrade. It's been mentioned before that with my current mount and sky conditions, a FL of 700mm is likely a realistic limit. I wonder, how is this worked out? What would be a realistic FL limit on an EQ6-R PRO? And how much better would this new mount be compared to my Orion Sirius EQ-G? The EQ-G's manual says that its resolution is "0.144 arc sec (or 9,024,000 steps/rev)", while the EQ6-R PRO is "9216000 Counts/Rev., approx. 0.14 arc-second". Sounds basically the same..?

Something else I've learned from this thread is about the "speed" of an imaging system. I just want to check I'm getting this right... If I calculate aperture squared multiplied by image scale squared for the FRA400, I get 19,510. For a much bigger telescope -- the Askar 130 PHQ -- the result is 10,177, i.e. 52% of the FRA400's speed. In real terms, would that mean that I'd need twice the integration time from the Askar 130 to get a comparable SNR from the FRA400? It's hard to get my head around, considering the slower telescope has a significantly larger aperture.

Thanks again to everyone helping with all these questions! 

[vlaiv]

4 hours ago, Lee_P said:

I'm understanding that perhaps I should bite the bullet and get a new mount to improve guiding and allow more options for an OTA. An EQ6-R PRO should fit my DIY pier and so be a relatively straight-forward upgrade. It's been mentioned before that with my current mount and sky conditions, a FL of 700mm is likely a realistic limit. I wonder, how is this worked out? What would be a realistic FL limit on an EQ6-R PRO? And how much better would this new mount be compared to my Orion Sirius EQ-G? The EQ-G's manual says that its resolution is "0.144 arc sec (or 9,024,000 steps/rev)", while the EQ6-R PRO is "9216000 Counts/Rev., approx. 0.14 arc-second". Sounds basically the same..?

First of all, I would not think of terms of focal length but rather working resolution / pixel scale. Mount has no idea what is the focal length of telescope that it is carrying nor it cares - it will behave the same (if weight and size of scope is the same).

Second thing - I would not think of mount as being hard limiting factor for certain resolution, but I would rather think of mount being able to do two things:

1) support the weight of the scope and accessories with certain ease (overloading mount just makes second point worse)

2) being able to be guided to certain precision

With respect to first point EQ6 is step up over HEQ5 class mount, but with respect to second point - not really.

Given certain care for the mount (tuning and modding) - both of them are capable of guiding down to 0.5" RMS.

Next tier up, mount wise would be something like iOptron CEM 120 (maybe we can even consider CEM70 to be step up over EQ6/HEQ5 in both points, but I'm not 100% certain of this - not sure if it will reliably guide below 0.5" RMS most of the time).

Following tier for me would be mounts like Mesu 200. It does not need be that bulky or expensive - there is mount that is same tier but is less weight / cost - take a look at E.fric for example:

http://www.geminitelescope.com/efric-friction-drive-mount-german-equatorial/
 

With that last tier you basically remove mount from the equation, or at least that second point. You get mechanical side of things - tracking / guiding as well as it can get.

In any case - these could be steps up the mount ladder:

1) 1" and above RMS (EQ3  / EQ5 type mounts)

2) ~0.8" RMS - this is common for stock EQ6 and HEQ5 mounts

3) down to 0.5" RMS - this can be done on tuned EQ6 / HEQ5 mount

4) ~0.4" RMS - I think iOptron CEM120 should be capable of this (version without encoders - I'm not considering encoders in this)

5) ~0.2 - 0.3" RMS - this is really top tier for mounts - this is where guiding is good enough - it stops being determining factor for most part

You have idea of what your current RMS is and this will help you decide what would be step up for you (I'm at point 3) with my HEQ5 that is tuned and modded, and hope to jump to number 5) at some future time).

4 hours ago, Lee_P said:

Something else I've learned from this thread is about the "speed" of an imaging system. I just want to check I'm getting this right... If I calculate aperture squared multiplied by image scale squared for the FRA400, I get 19,510. For a much bigger telescope -- the Askar 130 PHQ -- the result is 10,177, i.e. 52% of the FRA400's speed. In real terms, would that mean that I'd need twice the integration time from the Askar 130 to get a comparable SNR from the FRA400? It's hard to get my head around, considering the slower telescope has a significantly larger aperture.

Yes, that is what speed of optical system is in nutshell.

How much effective aperture you have (surface) to capture photons and how much you spread those captured photons (surface of pixel).

F/ratio is good measure of speed if you keep pixel size constant. This means F/7 scope will be slower than F/5 scope in that case.

However, F/ratio is not general measure of speed. This is because we don't need to keep pixels at constant size. We can use different camera, or we can utilize binning. When we do that - things can change drastically.

Take for example 4" F/10 refractor vs 4" F/5 refractor. F/10 is way slower, right? "No one in their right mind would image at such slow speed" , right?

Just for a moment consider this. Both scopes have exactly the same aperture size, so they gather same number of photons - same amount of light for given time. Neither is "faster" in that respect.

F/10 will have longer focal length and will spread light over more pixels. Each pixel will thus have less photons and signal goes down. This is why it's slower, but what if we make pixels twice as big (simple as bin x2)?

Focal length is twice as big in F/10 versus F/5 and if pixels are twice as big - then effective pixel size remains the same.

Suddenly F/10 scope is as fast as F/5 scope - it gathers same amount of light and it spreads that light over same number of pixels.

Most people would not image with 4" F/5 achromatic refractor due to chromatic aberration, but never even consider using 4" F/10 achromat for imaging - believing it is "too slow" - although it has decent color correction (not perfect). In fact F/10 achromat with 80mm aperture mask and x0.67 reducer is cheaper than SW ED80, equally good for imaging and more versatile scope (but I'm starting to digress here).

So yes, just because scope has larger aperture - it does not mean setup with such scope will be faster (as scope is only a part of setup). Similarly - just because scope has "fast" F/ratio - it does not mean setup with such scope will be faster than one with slower scope.

We often don't consider binning and long focal length because we are afraid of loosing FOV or having too few pixels in our image - but in reality, majority of target out there are rather small.

This is why I asked you in the beginning what do you expect. Diffuse nebulae (Ha regions, dark nebulae and alike) in Milky way are among largest targets and require wider FOV, but if you want to do general imaging - most targets span only dozen or so arc minutes - x5-x6 less than one degree, or about 1/2-1/3 of a full moon. Even target like M13 is less than a degree in FOV.

APS-C sized sensor will cover one degree with something like 1200-1300mm of focal length.

 

[raadoo]

4 hours ago, Lee_P said:

 I just want to check I'm getting this right... If I calculate aperture squared multiplied by image scale squared for the FRA400, I get 19,510. For a much bigger telescope -- the Askar 130 PHQ -- the result is 10,177, i.e. 52% of the FRA400's speed. In real terms, would that mean that I'd need twice the integration time from the Askar 130 to get a comparable SNR from the FRA400? It's hard to get my head around, considering the slower telescope has a significantly larger aperture.

It's probably hard to get your head around it because it doesn't seem like focal length comes into play at all. When in fact, image scale is quite affected by focal length, as the formula for image scale is PixelSize* 206 / FocalLength. So the expanded formula for overall system speed becomes:

A²*(μ*206/F)²

A = aperture, μ = pixel size and F = focal length.

This helps to draw the, now obvious, conclusion: a system gets faster as aperture and pixel size increase, while focal length decreases.

To add to @vlaiv's (a zillionth) comment about binning, if you bin, you essentially *2 the above formula.

So if we take the scopes that we've been discussing in this thread and apply the formula using your 2600MC, we can see how the ones with the larger aperture and lowest f/ratio (affected by focal length) result in the faster systems:

• Vixen AX103/825: 9374
• APM LZOS 100/800: 9409
• Askar 130PHQ: 10281.96
• ES FCD-100 127/952: 10582
• Skywatcher 150PDS: 10609
• TS 115/800: 12188
• ES FCD-100 CF 102/714: 12361
• TS CF 102/714: 12361
• Askar 107PHQ: 12622
• TS 106/700: 13843
• FRA400: 19510 👈 you are here
• Vixen R200SS: 24087
• Boren Simon 8" (at f/4): 24087

For the three newtonians I factored in their central obstruction before going through the formula.

If getting something speedier than the FRA400 is top of your list, there are options if you must stick with refractors, though you may not like them:

• Televue NP101is: scores 21153 but costs €6000 and has a focal length of 540mm, so not that far off from your Askar. And then there's its bigger brother the NP127is which scores 22458 and costs €11000 🤯
• ES FCD-100 127/952 with the 0.7 reducer: scores 21703 but is large and long. But it's within budget, the CF version is light enough and even with the reducer it comes to 666mm. This speed increase applies to all the refractors above if you stick a reducer in their tail end. I specifically chose this one because even reduced it still falls within that ~700mm focal length range you're after.
• Sharpstar 121SDQ: scores 19027 but costs €4500 and weighs 9kg and is discontinued (but maybe you can find some new old stock?)
• Askar FRA600: scores 19410 is within budget but only slightly more focal length than the smaller FRA400.

All this to say I wouldn't worry as much about overall system speed. A quick run of my own setup through the formula and mine get a measly score of 7970. I'm content with just factoring in more integration time.

[Lee_P]

Thanks vlaiv and raadoo, you're both legends and are being very generous with your time in helping me. I'm going to digest all your comments, and read up more on binning. This is also what ONIKKINEN suggested earlier in this thread.

 

37 minutes ago, raadoo said:

If getting something speedier than the FRA400 is top of your list,

Well it's not that I definitely want something speedier, more that I'm a bit reticent to get something much slower 😁 

[vlaiv]

27 minutes ago, raadoo said:

• Televue NP101is: scores 21153 but costs €6000 and has a focal length of 540mm, so not that far off from your Askar. And then there's its bigger brother the NP127is which scores 22458 and costs €11000 🤯
• ES FCD-100 127/952 with the 0.7 reducer: scores 21703 but is large and long. But it's within budget, the CF version is light enough and even with the reducer it comes to 666mm. This speed increase applies to all the refractors above if you stick a reducer in their tail end. I specifically chose this one because even reduced it still falls within that ~700mm focal length range you're after.
• Sharpstar 121SDQ: scores 19027 but costs €4500 and weighs 9kg and is discontinued (but maybe you can find some new old stock?)
• Askar FRA600: scores 19410 is within budget but only slightly more focal length than the smaller FRA400.

It does not really need to be that expensive.

I would personally go for something like 8" RC + x3 binning - for very nice working resolution of ~1.4"/px.

Speed wise, if we apply formula that you posted, we get:

203^2 * (3 * 3.76 * 206.3/ 1624 )^2 = ~84600

(1600mm FL and 203mm aperture, pixel size is x3 3.76 as we bin x3).

Even if we account for massive central obstruction - things don't change much. This system will be x4 faster than most listed and costs fraction of the price of most of them.

32 minutes ago, raadoo said:

All this to say I wouldn't worry as much about overall system speed. A quick run of my own setup through the formula and mine get a measly score of 7970. I'm content with just factoring in more integration time.

Speed of system plays only a part in total imaging speed. Light pollution is major factor to consider.

I did some calculations and loosing just 2 magnitudes of sky brightness - like going from SQM18 down to SQM20 is equivalent to ~x6 speed increase!

If we have dark skies - then speed of system is not that big of a deal, but for someone imaging from city - it is very important as they are already at disadvantage.

By the way - there are a few other factors that are major contributors to speed equation, that most people don't consider.

Take for example target position and transparency. We might not think much of those, but let's run some numbers.

Say we have target that passes thru zenith as some point in the year but we are impatient and want to image it early (or don't want to wait for next year) and decide to image it at 60 degrees above horizon. We also don't want to wait for very transparent skies (say AOD 500nm of 0.1 and instead image at 0.3). How much speed did we loose?

Instead of imaging thru 1 air mass - we image thru ~1.1547 air masses. We loose 0.16 magnitudes per air mass so we have 0.1547 * 0.16 = ~0.02475 magnitudes loss due to angle alone.

Using AOD (aerosol optical depth) of 0.3 instead of 0.1 we have 0.2 magnitudes of AOD - we multiply with 1.086 to convert to stellar magnitudes so that is 0.2172.

We add two together and get 0.24195 of magnitudes fainter target.

If we convert that into intensity we get ~0.8, or only 80% of light from target reached us instead of 100% - reciprocal of that is ~1.25, or we need 25% more time at best to get same SNR.

So imaging in right conditions is like swapping FRA400 with ~24387 telescope (none of ones you listed manages that)

 

 

 

[Lee_P]

23 hours ago, vlaiv said:

It does not really need to be that expensive.

I would personally go for something like 8" RC + x3 binning - for very nice working resolution of ~1.4"/px.

Speed wise, if we apply formula that you posted, we get:

203^2 * (3 * 3.76 * 206.3/ 1624 )^2 = ~84600

(1600mm FL and 203mm aperture, pixel size is x3 3.76 as we bin x3).

Even if we account for massive central obstruction - things don't change much. This system will be x4 faster than most listed and costs fraction of the price of most of them.

Thanks vlaiv, an 8" RC is a definite possibility, and FLO have them in stock. Your options are sufficiently cheaper than what I was considering that I may be able to get an EQ6-R PRO too, which would make something heavier, like a Sky-Watcher 190MN an option too.

I've been reading up on binning, and think I'm starting to get my head around it, but am coming stuck with a comment you made earlier in this thread: "RC has 1600mm. Bin your pixels x3 and you have effective focal length of about 500mm (which is increase over your current 400m)." How is it that binning reduces effective focal length? I'm imagining that a binx3 image would have the same field of view as a binx1, but a much lower resolution -- but I'm pretty sure that analysis is wrong!

[Clarkey]

As an RC8 owner, I must say I am a fan. In terms of value for money I think they are excellent. As I am sure you have read, collimation can be a bit tricky initially - but once set it does not need much adjustment. I typically bin 2, but in all honesty, with my seeing bin 3 is probably more realistic. Also, although F8 it never 'feels' slow except when compared to my F4 scope.

[Lee_P]

1 minute ago, Clarkey said:

As an RC8 owner, I must say I am a fan. In terms of value for money I think they are excellent. As I am sure you have read, collimation can be a bit tricky initially - but once set it does not need much adjustment. I typically bin 2, but in all honesty, with my seeing bin 3 is probably more realistic. Also, although F8 it never 'feels' slow except when compared to my F4 scope.

Thanks, I'd be interested to hear more about how you go about collimating it. I read some threads here on SGL and it seems rather tricky. Any and all other thoughts on the telescope would be most welcome too!

[Clarkey]

I think they are generally quite well collimated out of the box. I think many have been made worse by 'tinkering'. My method for collimation is to remove everything xcept the focuser and get the secondary perfectly aligned. I use a TSKOLLI or Reego, but you can use a simple Cheshire for that part. Once the secondary is right I use the DSI method for the primary using a star test.

https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.deepskyinstruments.com/truerc/docs/DSI_Collimation_Procedure_Ver_1.0.pdf&ved=2ahUKEwi62bCmhJX5AhVMPcAKHcvBBws4ChAWegQIKBAB&usg=AOvVaw1TQ8YAhohC9_gr_IRXBo3x

This works for mine but my focuser and primary mirror appear well aligned as I get no tilt issues. If this is not the case it is more tricky - but the DSI method should still work. My main advice would be adjust it SLOWLY and try to remember what you have done to put it back again.

There is not a lot to say about the scope itself. I do have a home-made secondary dew heater, but I have never had a problem with the primary and dew. Cool down is pretty rapid too as it is an open tube. I have only used it with a ASI1600 which gave a good flat field. With a larger sensor you might need a flattener. I still need to try it with my IMX571 camera to determine if it is OK with and APS-C size sensor. I did change the focuser to a Baader ST. The original was OK in terms of tilt, but it did slip slightly with the autofocuser. I hope this helps. Here are a couple of images with the scope. Just remember I am far from an imaging expert!

 

Edited July 25, 2022 by Clarkey

[vlaiv]

52 minutes ago, Lee_P said:

I've been reading up on binning, and think I'm starting to get my head around it, but am coming stuck with a comment you made earlier in this thread: "RC has 1600mm. Bin your pixels x3 and you have effective focal length of about 500mm (which is increase over your current 400m)." How is it that binning reduces effective focal length? I'm imagining that a binx3 image would have the same field of view as a binx1, but a much lower resolution -- but I'm pretty sure that analysis is wrong!

No, you are quite right - binning does not change FOV.

One of things that focal length impacts is of course FOV, but I commented that which you quoted coming from purely sampling rate perspective under assumption that targets you want to image will fit the FOV.

Sampling rate is function of pixel size and focal length. If we keep focal length the same and increase pixel size by factor of x2 - we get the same thing as halving focal length and keeping pixel size the same - in terms of "/px or sampling rate. Similarly if we increase pixel size by x3 (binning x3) we get the same effect as using 1/3 of FL and keeping pixel size the same.

1600 / 3 is roughly 500mm (or 533.333 to be precise) hence my assertion.

RC will be faster than your current setup even if you want to quadruple FOV (double it in each height and width).

When we did "speed" calculations we concluded that RC + x3 binning is about x6 times faster than your current setup. If you image target for 6h with your current setup - equivalent will be 1h with RC.

If the target does not fit ~ 50' x 33'  that 8" RC gives you natively, you can still do 2x2 mosaic for example - that will give you ~ 1° 40' x 1° 06' FOV (probably a bit less due to overlap needed to make mosaic but still 1.5 x 1 degrees) and if you image each panel for 1h (giving you same quality as current setup in 6h) - you will end up with total of 4h of imaging.