RASA 8 vs Refractor

[AXE]

All this talk of the pickiness of the faster F ratio scope is starting to make me look at the refractor again. Yeah, it's slower but its the same ratio as my Newtonian and i'll be able to guide better hopefully given the weight and wider FOV so 5+ minute subs shouldn't be an issue. Plus it's a bit cheaper and just easier as a complete package by the sounds of it.

[The_bluester (Paul)]

Both have a 400mm focal length, though I would imagine the Esprit needs a reducer/corrector for photography? That will reduce it's focal length and increase the field of view.

At 400mm the both of them (Assuming the refractor does not need a reducer/corrector on top of that) should guide similarly, the advantage to the frac being the ability to use an off axis guider than you can't do with the RASA, that has to have a separate guidescope. The RASA might be a little more wind impacted when guiding due to the bigger surface area, the RASA WILL need a heater strap in cooler weather, the frac MIGHT.

I am still tempted by the RASA myself now that some quality images are appearing, I was not prepared to be an early adopter though.

[AXE]

Originally Posted by The_bluester

Both have a 400mm focal length, though I would imagine the Esprit needs a reducer/corrector for photography? That will reduce it's focal length and increase the field of view.

At 400mm the both of them (Assuming the refractor does not need a reducer/corrector on top of that) should guide similarly, the advantage to the frac being the ability to use an off axis guider than you can't do with the RASA, that has to have a separate guidescope. The RASA might be a little more wind impacted when guiding due to the bigger surface area, the RASA WILL need a heater strap in cooler weather, the frac MIGHT.

I am still tempted by the RASA myself now that some quality images are appearing, I was not prepared to be an early adopter though.

The Esprit comes with a flattener included, as far as I've read there's no reducer out there that can be used with it.

[gregbradley]

Riccardi reducers should work with it.

Teleskop Services sell them so does APM.

Greg.

[The_bluester (Paul)]

Originally Posted by AXE

The Esprit comes with a flattener included, as far as I've read there's no reducer out there that can be used with it.

Well, that makes it a simpler comparison then. The FOV will be similar, the speed will be markedly different, guiding will be probably not easier but potentially more accurate with the refractor due to being able to use an off axis guider.


I was actually looking for a good explanation of factoring the F number to imaging speed and found one on the CN forum.

Without using actual figures. Take a given scope and call it F5, say that particular scope at F5 with your camera is at a scale of 1 arc second per pixel and a particular arc second of sky has 100 photons arrive in a given exposure time, those 100 photons will all fall on 1 pixel. Now double the focal length (F10) of the same diameter objective, the image produced at the focal plane will double in size, each pixel is now seeing half an arc second. 100 photons arriving in that same arc second of sky are now falling on a 2 X 2 pixel grid so it is 25 photons each, you need four times as long to gather 100 photons per pixel compared to the "Faster" setup. The resolution of the image is higher, but if seeing limits the resolution you don't get any benefit from that.

[lollywater (Paul)]

F2 though is a bit of a fooler. Its the aperture that counts. F ratio really only refers to the width of field.

I am with Paul here. I have a 8" f/10 sct and 8" f/4 newt. My latest addition to the 8" sct is a f/4 starizona night owl reducer. Already have a Meade 3.3 and 6.3 reducers. Only have to do a 15 sec capture on any camera to see the difference between the Rasa and the other 8" scopes.
cheers

The Rasa 8 is an ideal scope for VA. I want an image with detail in less than a minute. Beautiful images with pinpoint stars at the corners are not a priority for me.
It appears astrophotographers have different requirements and I can understand why the Esprit 80 is a favourite.
cheers
Paul

I often use a lightwave 0.6 focal reducer flattener on the Esprit. The reducer is meant for refractors f6 or more so I dont think you guys would approve of what I do to that poor little scope.
cheers
Paul

[AXE]

Originally Posted by lollywater

I often use a lightwave 0.6 focal reducer flattener on the Esprit. The reducer is meant for refractors f6 or more so I dont think you guys would approve of what I do to that poor little scope.
cheers
Paul

Is this for photography? Do you still use the field flattener or does the reducer also correct curvature?

[lollywater (Paul)]

Yes Axe ,it is for videoastronomy. My typical image is 10 to 15 secs exposure with a live stack of about 5 on sharpcap.I do spend a bit of time on the histogram. I generally dont save the images unless I want to post something on a forum. So things like darks,flats, curvature are not a big concern. I dont like vignetting which happens a lot with focal reduction on the sct's using the bigger sensor cameras like the zwo 294. That doesnt happen on the Rasa. A few small donut stars out in one corner doesnt bother me. It is all very sloppy from the point of view of an astrophotographer trying to get great images but videoastronomy is all about live viewing
cheers
Paul

[gregbradley]

Originally Posted by The_bluester

Well, that makes it a simpler comparison then. The FOV will be similar, the speed will be markedly different, guiding will be probably not easier but potentially more accurate with the refractor due to being able to use an off axis guider.


I was actually looking for a good explanation of factoring the F number to imaging speed and found one on the CN forum.

Without using actual figures. Take a given scope and call it F5, say that particular scope at F5 with your camera is at a scale of 1 arc second per pixel and a particular arc second of sky has 100 photons arrive in a given exposure time, those 100 photons will all fall on 1 pixel. Now double the focal length (F10) of the same diameter objective, the image produced at the focal plane will double in size, each pixel is now seeing half an arc second. 100 photons arriving in that same arc second of sky are now falling on a 2 X 2 pixel grid so it is 25 photons each, you need four times as long to gather 100 photons per pixel compared to the "Faster" setup. The resolution of the image is higher, but if seeing limits the resolution you don't get any benefit from that.

Originally Posted by lollywater

F2 though is a bit of a fooler. Its the aperture that counts. F ratio really only refers to the width of field.

I am with Paul here. I have a 8" f/10 sct and 8" f/4 newt. My latest addition to the 8" sct is a f/4 starizona night owl reducer. Already have a Meade 3.3 and 6.3 reducers. Only have to do a 15 sec capture on any camera to see the difference between the Rasa and the other 8" scopes.
cheers

The Rasa 8 is an ideal scope for VA. I want an image with detail in less than a minute. Beautiful images with pinpoint stars at the corners are not a priority for me.
It appears astrophotographers have different requirements and I can understand why the Esprit 80 is a favourite.
cheers
Paul

I often use a lightwave 0.6 focal reducer flattener on the Esprit. The reducer is meant for refractors f6 or more so I dont think you guys would approve of what I do to that poor little scope.
cheers
Paul

This is the original article that started this F ratio myth debate:

http://www.stanmooreastro.com/f_ratio_myth.htm

Greg.

[lollywater (Paul)]

Oh dear!
Not that article again.
One criticism of that article I have read is that "true S/N" is a cropped version of "image S/N".
There is a lot of discussion on other forums as well as this forum done by very informed people and it is obviously not clear cut. Paul's explanation is the way I see it .
So ,what do I see? The Rasa is much faster than my other 8" scopes with/without focal reducers.
cheers
Paul

[gregbradley]

The way I see it is a wider field of view captures more total light.

A faster scope broadens the FOV letting more total light in. A slower F ratio lets in less light and it is more tightly focused on less pixels as you mentioned.

But if you cropped the wider FOV image and compared a similar area of crop to the slower F ratio at the same aperture what would you get?

Greg.

[Slawomir (Suavi)]

I have noticed an increase in SNR when I used a reducer on my refractor as opposed to imaging at the native focal length (used the same camera in both scenarios).

A 15-minute exposure at f/4.5 gave a slightly better SNR for extended objects than a 20-minute exposure at f/6 (same scope, same camera). I tested that for multiple filters and for multiple exposures.

Faster f-ratio does speed up imaging of the extended objects (nebulae, galaxies) if the same camera is being used and same scope, but for the stars it is less of a difference, since these are approximately point sources therefore aperture dictates the signal detected from the stars, not the f-ratio.

[The_bluester (Paul)]

A quick read of the link, I think this undoes any other argument he puts on the page.

Varying CCD exposure-time or f-ratio varies the number of photons per pixel.

That, very simply put is exactly what is being talked about. Vary the F number, vary the number of photons on a given pixel in a given time, lower F number = more photons on a given pixel in a given unit of time = faster.

Focal length (and thus f-ratio) has absolutely no effect on the number of photons collected and delivered.

This statement is correct but omits half of the equation, it omits how the collected and delivered photons are distributed. Lets say that the entire target fits on the sensor at both focal lengths of a scope, with and without a reducer. The "Faster" ratio with the reducer fitted will take the same number of collected photons from the target and deliver them over a smaller number of pixels (Shorter focal length, smaller image formed) The image will be smaller and brighter as the same number of photons have been distributed over a smaller number of pixels. That is it in a nutshell.

[gregbradley]

Seems right Paul. I think also a reducer focuses more of the light that is missing the sensor back onto the sensor surface so it is focusing more of the light the optics are receiving back onto the sensor. So the total amount of light collected is greater for the same sized sensor.

The cone of light the scope receives is usually quite a bit larger than the sensor so the sensor is missing a lot of the light the scope has collected. Reducers put some more of that onto the sensor.

Greg.

[The_bluester (Paul)]

You can look at it that way to a degree, a faster ratio with the same objective diameter means a steeper light cone, but that does not add any light to what is already on the sensor, it moves light from objects that was missing the sensor on to the sensor by compressing the view - expanding the field of view. Stuff that was just missing the sensor now falls on to it, everything that was already visible occupies less sensor area and that compression in size delivers the same number of photons per hour from any object (Use a single star as an example) to a smaller area of the sensor, so less pixels divvy up the same amount of photons per unit of time between them.

[gregbradley]

A very few scopes have large corrected fields like the FSQ106 (88mm from memory) and Astrophysics scopes (often 100mm).

So how much of that is being missed by the sensor I wonder? Probably a lot.

Greg.

[The_bluester (Paul)]

Well, depends on the sensor of course, but putting a bigger sensor behind them wont give you a brighter image, just more sky in one frame. I hate to think what a sensor to cover a 100mm image circle would cost.

[Slawomir (Suavi)]

Originally Posted by The_bluester

Well, depends on the sensor of course, but putting a bigger sensor behind them wont give you a brighter image, just more sky in one frame. I hate to think what a sensor to cover a 100mm image circle would cost.

And a set of filters to cater for a 100mm sensor! Sensor + filters alone would be more than a scope+mount+camera's body+FW+OAG+kidney

[The_bluester (Paul)]

I reckon at least one liver is probably on the table too.

[AXE]

Well I pulled the trigger on the 80mm. For anyone who might be interested the things that pushed me towards the refractor, in no particular order, were:

- Easier to set up and use, a plug and play solution
- Can do imaging and visual
- Small and portable
- Collimation isn't really an issue
- Option to move to mono imaging with a filter wheel if I want to
- Skywatcher scope comes as a kit with flattener and diagonal for visual
- Slightly cheaper on it's own, way cheaper when you take into account special filters and other accessories.

In the end the faster F-ratio and aperture didn't outweigh the downsides for me. I'm sure i'd have been happy with the RASA but it seems as though it's one of those things that needs to be 100% dialled in to work. The refractor is just more forgiving and for wide field imaging should do the job just fine.