Esprit 100ED or RASA 8"

[Obi Wan Ken00bi]

Hi SGL,

I've been imaging with a Newton 8" on a EQ6-R Pro for quite a while now, using a OSC, the 6 megapixel QHY8L. Here are some results, just for you to see where my level is at (and we always like to share our work anyway eh?!)

I'm taking the next steps in this wonderful (and sometimes frustrating) hobby. The budget is around 4000 dollar/pound.

The need:

• I have developed a special interest in nebula and wider field targets (like Horsehead, Heart/Soul, Rosetta, M42, M45, larger clusters, etc)
• I want to up my game, by getting more and finer detail in my images and don't like the collimation process too much to be honest
• I would like more control over the colors and being able to, for example, pull out more detail on Ha, OIII and SII spectra. Also I'd like to play with the Hubble palette
• Easier guiding by reducing focal length

Therefore, I've decided to get an APO refractor for the first time, in the range of 400-600 mm (to complement my 1000mm Newt). To keep a good pixel size and do narrowband imaging, I also need a new camera, and I'm kinda set on a mono CMOS, the QHY163M with 36mm filterwheel with LRGB, Ha, OIII and SII filters. That's 2000 dollar/pound spend.

As for the scope, I thought, why not get the best of the bunch within the 2000 dollar/pound price range: the F5.5 Esprit 100ED? I've been pretty much set on this beauty and have already placed a pre-order.

BUT, then I saw this review by Astrobackyard on an interesting new scope, the RASA 8", which is 400mm at F2!!!! His review was very positive (like most of his reviews to be honest) and I was blown away by the speed. I live in Denmark and getting long subs in is sometimes a problem with clouds and fog/dew, but then again, I'm not in a rush with this hobby to be honest. The price is similar, but the light gathering of the RASA is so much better on paper. I know there is some tricky focusing because of the speed, and you can't attach a filter wheel, but using 60sec subs instead of 10 minutes ones more than compensates for this and you can do mono with a filter drawer right? The thing is however, I can't find really amazing images from this scope (or it's bigger brother the 11") on astrobin, while the Esprit 100ED comes up with a lot of pretty, fine detail images; the stuff I'm after. Ease of use also is a big deal for me, I love fiddling around during the day and home, not in the field.

It's a first world problem, but what would you do in my situation?

(btw, I'm not interested in using the new scope for visual btw)

[Whirlwind]

If you don't like collimation I would probably stay away from the RASA.  At F2 getting collimation spot on is going to take some time (and may need to be tweaked every time you set up).  In comparison refractors are reasonably plug and play once you have the flattener distance dialled in.  This and that the Esprits have been out for longer is likely the reason you see more images using them.

   

[Datalord]

As an owner and user of the RASA for a year (which in UK means about 15 nights), I have a few things you need to know.

1. The filters have to be specifically fast filters for f/2. Make sure you get them in a bundle for your order. When I ordered mine, I accepted them to be sent later, but it turned out Baader didn't make a new set until last fall.
2. Focus is harsh. I didn't have a motor and I think that caused me some grief. I would buy the celestron focuser they made specifically for their scopes.
3. Because of 1 I only used a one shot colour. Oh the ease of use, I miss that.
4. The short exposures are real. Really real. Not only do you get faster to saturation of your camera's pixels, it also means you get more photos in a stack, faster, which means you have better noise control AND guiding becomes less of a pain.
5. Because of 1 and that I wanted to use NB, I got upgraditis and didn't get to really tune the RASA well enough. My pictures shows it, I'm afraid. It could very well also be because I got better at processing I now look back and don't really think they are awesome.

Here's a few from that time.

The M106 below has some history. I had 4 hours of data, processed it quickly and posted it here. Got some feedback and asked if more data would make it better. Obviously yes. So I ended up having 17 hours of data on this one and I kinda like the outcome. There are two quasars visible and NGC4231 and NGC 4232 are easily distinguishable, both being about 330M ly away. It's a fun picture to go digging in. NGC4226 at 380M ly is also quite clearly visible.

It also shows that I never mastered coma correction properly. I will set it up again this summer and get some proper tuning on it.

So, the RASA is not an easy scope. It is also bloody heavy. But it has some very unique benefits and the potential is huge if you can tame it.

[Obi Wan Ken00bi]

Thanks for the advice. As I feel not very pro yet, and the issues you mention are there, I’ve decided on the easier esprit 100 ED with mono cam. It’s less of a beast, for good and bad. In the future I’ll get myself an 8 inch rasa (the 11 inch is too heavy for my eq6R pro)

[newbie alert]

I'm reading speed alot in this thread.. f ratio is fl to aperture ratio.. 

As a example of what we have here is m16 with a 400fl x 200 f2 scope verses a 1000 x200 f5 scope..

So a (faster? ) f ratio captures more background sky which is great for big nebulae such as m16, moving onto galaxies it's not so great as it's shorter fl hasn't got the reach of the 1000mm fl ..

[Datalord]

9 hours ago, newbie alert said:

So a (faster? ) f ratio captures more background sky which is great for big nebulae such as m16, moving onto galaxies it's not so great as it's shorter fl hasn't got the reach of the 1000mm fl ..

I would think of it as more photons per pixel on the camera. That will ultimately get higher SNR.

[ollypenrice]

11 hours ago, newbie alert said:

I'm reading speed alot in this thread.. f ratio is fl to aperture ratio.. 

As a example of what we have here is m16 with a 400fl x 200 f2 scope verses a 1000 x200 f5 scope..

So a (faster? ) f ratio captures more background sky which is great for big nebulae such as m16, moving onto galaxies it's not so great as it's shorter fl hasn't got the reach of the 1000mm fl ..

No, this is incorrect and confuses the issue. For a given chip the field of view is governed by the focal length and nothing else. For a given focal length, chip and binning the photon flux per pixel is governed by aperture and nothing else of any significance in principle. (Secondary obstruction, anti reflective coatings, etc.) So a faster F ratio has no effect whatever on the area of sky captured. The increased area of sky comes from the reduced focal length.

The moment you start to alter both these variables at the same time the discussion heads straight up a gum tree!

If you want to image at Hyperstar or RASA focal lengths (as well you might and I do regularly - 520mm in my case) then the instruments are very fast. They have more aperture per unit FL than either fast apos or regular Newts. They catch more light so they are faster. But how good are they? That's a lot more complicated. And fast optics are a lot more complicated. They have a very shallow depth of field so they are dependent on almost perfect focus. If there is the slightest tilt part of the image will be out of focus. They demand more of their filters. They demand more of the person collimating them.

DS imagers are not stupid. They know perfectly well that, at a given FL, more aperture will reduce exposure times. They also know perfectly well that theoretical resolution is locked into aperture in diffraction limited optics by the physics of the Dawes limit. So why don't they all use Hyperstars and RASAs? Read the first sentence of this paragraph... 

Olly

[newbie alert]

Thanks for reply Olly,.. I didn't change any settings on chip (they was the same) binning I didn't mention,   and to keep the aperture the same as in the  200mm and to compare a f2 scope to a f5 scope the fl had to change..that was what I was trying to say..

Ok as another instance, I have a w/o scope, it's native f ratio is 6.9, focal length 555mm, 80mm aperture..if I want to speed up the optics as is commonly said i add a reducer..then making it f5.5, fl 444mm with still the 80mm aperture.. have I made it a faster scope? Optics are the same

To my mind ive shortened the fl with the reducer,  I've now got more background sky..

[andrew s]

Olly is absolutely right technically and as he says it is a lot simpler to use an apo. The question for me is do you want to be like all the rest? 

Same targets, same kit...

There is nothing wrong with this it is a personal choice based on how you want to enjoy the hobby.

If I wanted to go into imaging I would go for a scope that maximises the data I could collect in my  very infrequently clear skies and poor seeing. Very different to Olly's situation.  However, this is a complex issue trading off raw capability with failures due to technical problems and also taking account of personal capability.

In the end you pays your money and takes your choice ?

Regards Andrew 

[ollypenrice]

1 hour ago, newbie alert said:

 

Ok as another instance, I have a w/o scope, it's native f ratio is 6.9, focal length 555mm, 80mm aperture..if I want to speed up the optics as is commonly said i add a reducer..then making it f5.5, fl 444mm with still the 80mm aperture.. have I made it a faster scope? Optics are the same

To my mind ive shortened the fl with the reducer,  I've now got more background sky..

Whether or not a focal reducer 'speeds up the optics' depends on the image you are trying to take. A focal reducer is not a camera lens diaphragm. With a diaphragm we can be sure that opening it up by one stop will double the area of clear aperture and so double the incident light and halve the exposure time. The optics become faster because the aperture has increased. No debate. But a focal reducer leaves the aperture unaltered so, from an object which will fit on the chip without the reducer, no new photons are brought in by the reducer. In the case of such an object the reducer will put the object's light onto fewer pixels and so 'fill' them faster but give a smaller, less resolved result. You don't need a reducer to do this. You can use the camera binned or you can software bin the captures. Even just resampling the image downwards will go a long way.

However, if you want the new field of view because it contains objects you wish to include then you will get a less resolved image with equivalent S/N ratio in less time by using the reducer. So it's slightly complicated and target dependent. Two bits of graphics which aim to show when the reducer is useful and when it isn't:

I can quite understand the attraction of ultra fast optics, especially for those who don't have our clear sky time. However, those making these products downplay the difficulty of setting them up and using them perfectly and my aim is to sound a cautionary note amidst the hyperbole of advertizing. For fast optics from the best source and in the best hands check this thread: 

https://stargazerslounge.com/topic/333395-regulus-leo-1-dwarf-galaxy/

Olly

 

 

[Obi Wan Ken00bi]

Yeah the fiddling around with focus and collimating put me off the fast optics for now. I’d rather spend an extra night or 2 on a target and at least enjoy getting good subs most of the time, without caring for the equipment too much. I enjoy being out there and having equipment that works for you, not against you. 

Your experience is very much valued here Olly, thanks again. 

 

[Jkulin]

6 hours ago, ollypenrice said:

Whether or not a focal reducer 'speeds up the optics' depends on the image you are trying to take. A focal reducer is not a camera lens diaphragm. With a diaphragm we can be sure that opening it up by one stop will double the area of clear aperture and so double the incident light and halve the exposure time. The optics become faster because the aperture has increased. No debate. But a focal reducer leaves the aperture unaltered so, from an object which will fit on the chip without the reducer, no new photons are brought in by the reducer. In the case of such an object the reducer will put the object's light onto fewer pixels and so 'fill' them faster but give a smaller, less resolved result. You don't need a reducer to do this. You can use the camera binned or you can software bin the captures. Even just resampling the image downwards will go a long way.

Very nicely explained Olly ??

[andrew s]

@ollypenrice just one minor quibble.  While a reducer will not add photons to that part of the image that fitted on the ccd without it, it will add photons to the surrounding part of the image that would have otherwise missed the ccd. This is the advantage of a reducer over just binning.

Regards Andrew 

[Datalord]

12 hours ago, ollypenrice said:

For fast optics from the best source and in the best hands check this thread: 

Uhm, for putting someone off a very fast optics system, that didn't work well. He basically got to focus with an LCD screen, a bathinov mask and processed it in 10 minutes. With that kind of result. If anything, it shows what kind of impressive images you can take with short exposures with such systems.

[ollypenrice]

7 hours ago, Datalord said:

Uhm, for putting someone off a very fast optics system, that didn't work well. He basically got to focus with an LCD screen, a bathinov mask and processed it in 10 minutes. With that kind of result. If anything, it shows what kind of impressive images you can take with short exposures with such systems.

My whole point in posting the link was not to put people off fast optics but to put them onto good fast optics.  It's worth noting that it took Maurice some time and effort to bring the Epsilon to collimation, too, and he had a fight with tilt, but once he'd cracked it his system works incredibly well as all his images confirm. He has a genuinely fast, productive, portable rig giving outstanding results. This is why I posted the link. Celestron are not the only suppliers of fast optics and they are certainly not the best.

What do you think of this from the Starizona website?

Exposures that take an hour at f/10 take mere seconds with the Hyperstar lens. Replacing the secondary mirror on your Schmidt Cassegrain telescope, Hyperstar converts it into an f/2 imaging system. Yes, we said f/2. That means your imaging canl be up to 28 times faster. Which means you can go to bed sooner...or, stay up all night and take 28 times more photos.

I think that it is not just misleading but mendacious. At no point does it mention that the 'Exposures' or the 'photos' are not the same with or without the Hyperstar.  If your F10 target were a frame-filling galaxy you would be able to photograph it more quickly but with a fraction of the image scale and resolution. This should be there in the text.

Olly

[Datalord]

12 hours ago, ollypenrice said:

My whole point in posting the link was not to put people off fast optics but to put them onto good fast optics.  It's worth noting that it took Maurice some time and effort to bring the Epsilon to collimation, too, and he had a fight with tilt, but once he'd cracked it his system works incredibly well as all his images confirm. He has a genuinely fast, productive, portable rig giving outstanding results. This is why I posted the link. Celestron are not the only suppliers of fast optics and they are certainly not the best.

Fair point. And correct. In fairness though, the RASA is less than half price of the Epsilon. 

12 hours ago, ollypenrice said:

What do you think of this from the Starizona website?

I think you are right and so are they. ?

There's a reason I went to a slower galaxy hunter in the RC, simply because it actually is a galaxy hunter. As such, comparing an f/10 to an f/2 at same aperture is complete nonsense.

However, if you are more flexible in your target acquisition, the faster optics make a gigantic difference in utilisation of the precious clear skies especially here in UK.

[ollypenrice]

2 minutes ago, Datalord said:

Fair point. And correct. In fairness though, the RASA is less than half price of the Epsilon. 

I think you are right and so are they. ?

There's a reason I went to a slower galaxy hunter in the RC, simply because it actually is a galaxy hunter. As such, comparing an f/10 to an f/2 at same aperture is complete nonsense.

However, if you are more flexible in your target acquisition, the faster optics make a gigantic difference in utilisation of the precious clear skies especially here in UK.

When it comes to hunting galaxies (a noble passtime!) I have found all the rules and theories to be profoundly underwhelming. According to my own thinking on the matter the 14 inch I used to use (not mine) should have been considerably faster than the 5.5 inch I now use on the same targets. But I'm not using the same camera - nor would it be sensible to do so. And it wasn't faster. Ho hum, ours is a strange game.

Olly

[vlaiv]

On 06/04/2019 at 13:16, ollypenrice said:

Whether or not a focal reducer 'speeds up the optics' depends on the image you are trying to take. A focal reducer is not a camera lens diaphragm. With a diaphragm we can be sure that opening it up by one stop will double the area of clear aperture and so double the incident light and halve the exposure time. The optics become faster because the aperture has increased. No debate. But a focal reducer leaves the aperture unaltered so, from an object which will fit on the chip without the reducer, no new photons are brought in by the reducer. In the case of such an object the reducer will put the object's light onto fewer pixels and so 'fill' them faster but give a smaller, less resolved result. You don't need a reducer to do this. You can use the camera binned or you can software bin the captures. Even just resampling the image downwards will go a long way.

However, if you want the new field of view because it contains objects you wish to include then you will get a less resolved image with equivalent S/N ratio in less time by using the reducer. So it's slightly complicated and target dependent. Two bits of graphics which aim to show when the reducer is useful and when it isn't:

I can quite understand the attraction of ultra fast optics, especially for those who don't have our clear sky time. However, those making these products downplay the difficulty of setting them up and using them perfectly and my aim is to sound a cautionary note amidst the hyperbole of advertizing. For fast optics from the best source and in the best hands check this thread: 

https://stargazerslounge.com/topic/333395-regulus-leo-1-dwarf-galaxy/

Olly

 

 

Can I make a point here? I've seen this argument before, and I want to address it.

Using focal reducer is going to speed up your acquisition regardless of the fact that you won't be catching new photons from the target. To be precise, "NO MYTH HERE" refers to both diagrams.

You might wonder, how is it possible to have faster acquisition (faster setup), when using the same aperture, and same photon count fall on the surface of sensor. There is also issue of how those photons are being spread over pixels.

Imagine for a second that you have 100 photons falling on your aperture (in a given time interval, with QE being 100%, no other noise sources, etc, to simplify things), If you capture all those photons with a single pixel, you will have SNR of 10 (signal will be 100, associated shot noise will be square root of 100 = 10, and ratio of those two will be 100/10 = 10).

Now, spread those 100 photons on 100 pixels, and each pixel will have SNR of 1, or 10 times less. Each pixel will get 1, square root of 1 is 1, and finally 1/1 = 1.

Same aperture, same time interval, same number of photons, but two different SNRs. Making all photons fall into single pixel speeds up things (you need less time to get same SNR as in spread case, or you get more SNR for same time).

This is what focal reducer does - it alters sampling rate, or photon / pixel spread. It concentrates photons on pixels and this boosts SNR.

We can also address true F/ratio myth here, and compare RASA 8" to some other scope that is much "slower". For example, let's go for 8" RC.

Olly also mentioned that one would use RASA to get short focal lengths. What is usually implied with short focal lengths is wide FOV and particular sampling rate. You can achieve that sampling rate with longer focal length as well - by using larger pixels. Only thing that you can't match is FOV.

RASA 8" with 3.8um pixel camera will sample at 1.96"/px. 8" RC F/8 with 15.4um pixel camera will also sample at 1.96"/px - two systems will have same aperture and same "relative pixel size". They will have the same speed. RASA will have much wider FOV, but RC will have other advantages - like ease of using OAG / Filters, diffraction limited stars, etc ....

 

[ollypenrice]

10 hours ago, vlaiv said:

Can I make a point here? I've seen this argument before, and I want to address it.

Using focal reducer is going to speed up your acquisition regardless of the fact that you won't be catching new photons from the target. To be precise, "NO MYTH HERE" refers to both diagrams.

You might wonder, how is it possible to have faster acquisition (faster setup), when using the same aperture, and same photon count fall on the surface of sensor. There is also issue of how those photons are being spread over pixels.

Imagine for a second that you have 100 photons falling on your aperture (in a given time interval, with QE being 100%, no other noise sources, etc, to simplify things), If you capture all those photons with a single pixel, you will have SNR of 10 (signal will be 100, associated shot noise will be square root of 100 = 10, and ratio of those two will be 100/10 = 10).

Now, spread those 100 photons on 100 pixels, and each pixel will have SNR of 1, or 10 times less. Each pixel will get 1, square root of 1 is 1, and finally 1/1 = 1.

Same aperture, same time interval, same number of photons, but two different SNRs. Making all photons fall into single pixel speeds up things (you need less time to get same SNR as in spread case, or you get more SNR for same time).

This is what focal reducer does - it alters sampling rate, or photon / pixel spread. It concentrates photons on pixels and this boosts SNR.

We can also address true F/ratio myth here, and compare RASA 8" to some other scope that is much "slower". For example, let's go for 8" RC.

Olly also mentioned that one would use RASA to get short focal lengths. What is usually implied with short focal lengths is wide FOV and particular sampling rate. You can achieve that sampling rate with longer focal length as well - by using larger pixels. Only thing that you can't match is FOV.

RASA 8" with 3.8um pixel camera will sample at 1.96"/px. 8" RC F/8 with 15.4um pixel camera will also sample at 1.96"/px - two systems will have same aperture and same "relative pixel size". They will have the same speed. RASA will have much wider FOV, but RC will have other advantages - like ease of using OAG / Filters, diffraction limited stars, etc ....

 

It seems to me that I agree with this and have already said so, no?

Whether or not a focal reducer 'speeds up the optics' depends on the image you are trying to take. A focal reducer is not a camera lens diaphragm. With a diaphragm we can be sure that opening it up by one stop will double the area of clear aperture and so double the incident light and halve the exposure time. The optics become faster because the aperture has increased. No debate. But a focal reducer leaves the aperture unaltered so, from an object which will fit on the chip without the reducer, no new photons are brought in by the reducer. In the case of such an object the reducer will put the object's light onto fewer pixels and so 'fill' them faster but give a smaller, less resolved result. You don't need a reducer to do this. You can use the camera binned or you can software bin the captures. Even just resampling the image downwards will go a long way.

Olly

 

[andrew s]

I agree with @ollypenrice if you just focus on the native image then neglecting read noise  and reducer aberrations,  using a focal reducer or binning have the same effect on resolution and s/n.

Regards Andrew 

[vlaiv]

2 hours ago, ollypenrice said:

It seems to me that I agree with this and have already said so, no?

Whether or not a focal reducer 'speeds up the optics' depends on the image you are trying to take. A focal reducer is not a camera lens diaphragm. With a diaphragm we can be sure that opening it up by one stop will double the area of clear aperture and so double the incident light and halve the exposure time. The optics become faster because the aperture has increased. No debate. But a focal reducer leaves the aperture unaltered so, from an object which will fit on the chip without the reducer, no new photons are brought in by the reducer. In the case of such an object the reducer will put the object's light onto fewer pixels and so 'fill' them faster but give a smaller, less resolved result. You don't need a reducer to do this. You can use the camera binned or you can software bin the captures. Even just resampling the image downwards will go a long way.

Olly

 

Diagrams are probably misleading, or at least I got them wrong.

I'm sorry I did not read your post thoroughly, as you did explain what will happen. I just concentrated on the included diagrams:

and wanted to point out that using focal reducer does not "produce" f/ratio myth, at least not the main part - that F/ratio determines speed of capture. It does relate to other part of F/ratio myth - how ratios of F/speeds relate to ratios of times needed to achieve target SNR (In heavy LP, using focal reducer will have much smaller impact because LP gets "concentrated" as well and associated shot noise increases as well)

[ollypenrice]

1 hour ago, vlaiv said:

Diagrams are probably misleading, or at least I got them wrong.

I'm sorry I did not read your post thoroughly, as you did explain what will happen. I just concentrated on the included diagrams:

and wanted to point out that using focal reducer does not "produce" f/ratio myth, at least not the main part - that F/ratio determines speed of capture. It does relate to other part of F/ratio myth - how ratios of F/speeds relate to ratios of times needed to achieve target SNR (In heavy LP, using focal reducer will have much smaller impact because LP gets "concentrated" as well and associated shot noise increases as well)

Indeed there are circumstances in which the improved S/N ratio produced by a reducer will get the signal above the various sources of noise but, as a more general principle, I stand by the 'no new photons from small objects' position since that is the one which affects most users.

Olly

[vlaiv]

1 hour ago, ollypenrice said:

Indeed there are circumstances in which the improved S/N ratio produced by a reducer will get the signal above the various sources of noise but, as a more general principle, I stand by the 'no new photons from small objects' position since that is the one which affects most users.

Olly

See now, that is confusing me. What exactly do you mean by: "I stand by the 'no new photons from small objects' position"?

It is obvious what it says - indeed, same aperture, in both cases same number of photons reach sensor from target that fits sensor with or without reducer. I'm just not sure what's your position on "speed" of the system in the case where no new photons reach sensor?

[andrew s]

If I may, if I have understood @ollypenrice position (with which I agree) speed as in F ratio is misleading for fixed aperture telescopes. The "speed" is determined by aperture alone i.e. it sets the photon rate for a given target. 

Concentrating them on fewer pixels can be done, read noise and reducer aberstions neglected, equivalently by binning or by reducer.

The reducer does however, increase the field of view but this is mute if the target fitted the unreduced field. 

Regards Andrew 

[vlaiv]

42 minutes ago, andrew s said:

If I may, if I have understood @ollypenrice position (with which I agree) speed as in F ratio is misleading for fixed aperture telescopes. The "speed" is determined by aperture alone i.e. it sets the photon rate for a given target. 

Concentrating them on fewer pixels can be done, read noise and reducer aberstions neglected, equivalently by binning or by reducer.

The reducer does however, increase the field of view but this is mute if the target fitted the unreduced field. 

Regards Andrew 

Ah, that is where I disagree. Speed is not determined by aperture alone. You showed in the rest of the post that achieved SNR depends on photon / pixel "concentration".

That one is governed by relative pixel size, which can be "changed" in a few different ways for given aperture.

1. Change of physical pixel size - use camera with larger pixels

2. Shortening of focal length for same pixel size - use of reducer

3. Use of binning - which is in some respect the same as 1. but a bit different in terms of things we decided to neglect for this discussion.

So, having a patch of target with uniform brightness - system with reducer in place (having same aperture and same pixel size) - will reach target SNR in less time. It will be faster. That is not the only way you can go about that - you can bin and result will hit target SNR in less time, or you can exchange camera for one with larger pixels (otherwise equal) - and it will also reach SNR in less time.

All three trade sampling rate for speed, each one being slightly different in details we've choose to neglect here