Ok - ive got analysis paralysis.. SOS! (Which OTA..)

[Emuhead]

Hi all,

Ive reached a point where i think ive read every article there is, some of them twice or three times (slow learner), watched every youtube video to the point where i think im just watching peoples videos about gear.. essentially its a home shopping channel, and im really now just at a point where im ready to just blindly hope for the best.. which is something id rather not do when spending a large sum of hard earned.. so.. this is where im at and if you are willing to listen and offer advice, id be grateful.

I want to take mind melting astro photos.. much like these wizards here (https://www.astrobin.com/users/GWLopez/) and here (https://www.astrobin.com/users/Tudor_Chibacu/)

Ill get it said that I dont want to do any visual, nor take planetary images, or moon images, or even galaxy images... im all about nebulas, dust & clouds & colour. Ill farewell some of you there im sure.. sorry.

If youre still reading, brilliant, ill continue. So I dont have endless funds but I am willing to pay the price to get the job done once & right if possible.

Im looking at these scopes with a keen eye:
Skywatcher Esprit 100ED 550mm f/5.5 Triplet (optional x0.65 reducer)
Skywatcher Esprit 120ED 840mm f/7 Triplet (optional x0.65 reducer)
William Optics GT102 703mm f/6.9 Triplet (optional x0.8 or x0.72 reducer)
William Optics Zenithstar 103 710mm f/6.9 Doublet (optional x0.8 reducer)
William Optics Zenithstar 126 970mm f/7.7 Doublet (optional x0.8 reducer)

Anything more expensive im certain are amazing, but i cant justify the price of admission there.. Looking at you AP, SV, Takahashi..

I was also recently looking at SCT's (a third time - im seriously on loop ruling things out and then back in again) only because of the huge light gathering power they have, but they dont sound like are without their numerous downsides either. I was looking at f/4 Newtonians briefly but then saw some images and decided that id much rather round stars only, so they are out.

I purchased an EQ6-R with a payload capacity of 20kgs.. probably only usable to about 15kgs max (im guessing, happy to be corrected). Im also thinking i should probably stay somewhere between a 500mm to 1000mm focal length for now based on the things id like to shoot, but ideally 700mm to 1000mm from the telescope simulator i used.. (and that is where im at right now. Simulating a telescope - i have the passion just without the scope).

Ill be autoguiding with a 60mm guidescope and an ASIAir Pro for now. Potential move to Stellarmate, Astroberry, or a miniPC/NUC, but will see how the ASIAir goes first. Shooting with a Fuji XT2 APS-C mirrorless camera, for now, however im sure the obligatory 1600MM with all the filters wont be too far down the road.

So.. that long story cant be shortened, but long story short.. are those 5 scopes decent? Will they help get the images im chasing? Do they have downsides people dont speak of? Some better than others from your experience? Can i even shoot at f/7 (for the longer scopes) or will i have to get a focal reducer bringing it down to f/5.5 anyway. Is a Triplet really that much better than a Doublet? Is there a sweet spot for focal length, and for focal ratio for that matter? Do SCTs really lack contrast due to the secondary mirror getting in the way. Are smaller diameters better to reduce 'muddiness'. Do i need help? (yes :D)

Strehl, spot size, mirror flop, image shift, 3"+ focusers... this is a great hobby but pretty full-on to get ones head around at times. Especially this one.

S O S

[gregbradley]

I have seen great results from the Esprit triplet APOs. Not to say the WO scopes aren't great too. I would steer away from doublets as claims they are APO are usually marketing BS. Agema Telescopes excepted.

The bigger the better, If you have the budget the standout there to me is the Esprit 120.

I would consider:

Triplet and has an FPL53 element.

A decent focuser.

Plenty of examples of user images using these scopes. Astrobin is the best resource for that.

WO are great at machining. Their machined parts are works of art. I am not so sure about their scopes. Perhaps they are. But they also BS about APO and call FPL 53 triplets fluorite sometimes. Fluorite is a crystal and a high end lens element like Takahashi and TEC use. It is superior to FPL53 which is why they call it that. FPL53 has a lot of fluorite in it but its not fluorite.

You don't need a 3 inch focuser for an APSc camera like Fuji XT2. 2.5 inch would work.But if you plan to go full frame one day 3 inch is safer.




Greg.

[h0ughy]

Simple answer is a RASA 8 and a asi294mcpro, you will get heaps of nebulae in spades and tonnes of dust

[Outcast]

Have you looked at the size of objects you wish to image & what image circle you will require in relation to your camera sensor size?

Longer focal length will result in smaller image circle; for example, when I started my imaging journey with an 8" f10 SCT, I was unable to capture objects such as m42 & eta carina in there entirety. Even with a 0.63 reducer, I could only just squeeze them in. I now image with an f6 80mm triplet & 130mm f5 newtonian & they happily cover my interests.

Longer focal lengths also usually require longer capture times.

If you want to get a feel for fov, put your sensor specs & each scopes specs into stellarium, you can even add in the reducer. Then pic targets & get a feel for how they will frame.

Also, there are ccd calculators that will show if you are under or oversampling with particular combinations, I think there is one on the bintel site.

Double vs triplet, well I think that depends on the quality of the glass used. If choosing between two scopes of equal quality, I'd choose a triplet over the double assuming I could afford it...

I'm sure other, vastly more experienced imagers will chime in with more advice but, that's a snapshot of what I've learned over my journey that relates to your questions.

Hope it's helpful

Edit: I see they already have... :)

[Emuhead]

Thanks for the fast response Greg. I have spent a fair amount of time on the old astrobin checking out what these & other scopes can do, and its a great resource for sure.. even have my own little page on there (https://www.astrobin.com/users/EmuHead/).

So all 5 of those have FPL-53 glass as it goes, and im sure all have pretty decent focusers (some rack & pinion, some not). I am ever so slightly leaning toward the 120mm but can i shoot at f/7, or will i have to spend the extra to get the x0.65 reducer which will end up netting me what could essentially be a very expensive Esprit 100ED (more or less).

[Emuhead]

Hey H0ughy, I was pretty set on the Rasa 8 and still may head down that path.. one shot colour though given the price of 2" filters and a mono camera.. I was hoping to get away with using my mirrorless Fuji for a while first, and it can be done I've read, but I was hoping to go a little deeper.. and the Rasa 11 (& 14/36)is way out of my price range.

[Emuhead]

Hi Outcast,

I didn't know that longer focal lengths need more time on target.. that is good to know.

I have played too much with telescope simulators, like Telescopius, getting an idea of the fov I'd get with my sensor & potential scope. It's actually pretty fun. I also have an impressive spreadsheet with combinations of oversampling & undersampling, which for my sensor is the reason why I was looking around about the 100mm diameter and up region.

Thanks for the points given, all good stuff to consider.

[Camelopardalis]

Not sure which reducer you are referring to, but Skywatcher don't make a reducer for the Esprit range. They come with their field flattener included, but that is 1.0x so doesn't change the focal length.

I've owned my Esprit 100 for 5 years now and it's an excellent scope for the money. But you really need to consider the targets you will chase as the extra aperture and focal length would come in handy for some targets.

Of course, the FOV you get depends on the camera you use too...so that's another factor to think carefully about.

[Emuhead]

It's a Starizona APEX x0.65 reducer. Which would make your f/5.5 a f/3.575 for the 100ED, or the 120ED down from f/7 to f/4.55.

So what I'm trying to understand now is how to compare the 100ED to the 120ED, as they both have different apertures and different f numbers. While the 100ED is faster at f/5.5, the 120ED has a larger aperture but its f/7. If both scopes had the same aperture this would be easy of course, but hunting for a formula now to properly compare these in assumably photons per second, or something similar (for the same light source and same camera sensor).

[The_bluester]

It is very easy to get in to paralysis mode. I bought my Evostar 72 and reducer as a toe in the water and quickly realised I enjoyed the image scale and was going to want something better, but it took the best part of a year to settle on the Stellarvue I eventually bought (Thankfully before the exchange rate really tanked during the earlier days of the COVID 19 event)

It is a bit hard to advise anyone else as everyones wants, needs, budget and pet peeves are different. But I would say from my own experience that if you consider any moderately priced doublets, really cruise Astrobin for images produced with them and a similar or the same camera that you plan to use and look for issues like blue bloat etc.

To make an example of my Evostar 72, they were originally marketed as an Apo scope, but they really are not, for the price I could forgive it (And was not expecting miracles) but being a bit of a technical wonk and perfectionist I outgrew it and while I got some pretty decent results out of it, the less than tightly focused blue did not appeal to me. Currently it is acting as a guidescope while I wait for a new and better OAG to arrive than the old Thin OAG I had before now. I will probably look to sell it and it's reducer as an intro package once my new OAG arrives and I can retire it from that duty.

I guess the TL;DR version of that is be careful about setting a tight budget on the glass. You already have a mount that should carry an imaging scope pretty well, but I think the decision on scope has to be between toe in the water (Evostar 72 in my case), a fairly solid but moderately priced scope and a maybe lifetime scope (My Stellarvue SVX80, which cost four times as much as my Evostar while only having 8mm more aperture) I did have the advantage when I realised I wanted more of being able to borrow the little brother to the SVX80 off a friend, which in some ways was a bit of a mistake, as soon as I started shooting with it I knew I wanted one like it.

I looked long and hard at a RASA 8 as well and very nearly pulled the trigger but I could not find many images from one that would have convinced me at the time. H0ughy's images in this thread would have made the decision harder again! They are even taken with my current camera. Many images you see from a RASA 8 seem to exhibit loads of reflection issues. The huge advantage of the RASA if you can tame the reflections is F2! On a good night you could shoot enough subs to produce a pretty handy image of any less than insanely dim target!

[cometcatcher]

Nebula don't really care about aperture. Aperture is for faint stella objects. Nebula pics come faster with low F ratio, though you can get them with any F ratio, if you have the time. Scope quality is probably the biggest issue, as we all like to push process our images until imperfections show. The better the scope, the less imperfections.

You WILL want a good mono camera. The camera is half the optical train. No OSC ever comes close to a mono. You will also eventually want narrow band for nebula, and filters... There goes the bank account.

I would probably go with a refractor with a little longer focal length, and use a reducer for wider stuff. That way you get the best of both worlds in one scope.

I was looking at the Starizona 065x reducer also for my F9 refractor to get it to f5.85. Would love to know what the quality is like, and the effective corrected image circle.

[Emuhead]

Ok Im probably realistically going Esprit 100ED or 120ED.

I really am trying to now understand how to compare the 100ED to the 120ED, as they both have different apertures and different f numbers.

While the 100ED is faster at f/5.5, the 120ED has a larger aperture but its f/7.

Of course if both scopes had the same aperture this would be easy but hunting for a formula is not as easy as you would think.. Id like to boil these down to a comparable unit of measure, like photons per second or similar (for the same sensor & target).

Given Mono is down the track.. im sticking with a mirrorless for now, the time it takes to capture the image is a concern given the longer exposure time, the more noise.. want to keep the SNR down. If only i could boil these 2 scopes down to comparable units.

[Camelopardalis]

Comparing without running them side by side is hard :shrug:

The 100 will appear roughly 1 f-stop faster, so at the same camera settings, so exposures about half the time of the 120.

The flip side is, that for the extra time, the 120 is going to give you more zoom from the focal length.

I'd be sceptical about those reducers...the included flattener sets the bar pretty high, and again it depends on the camera you will use. Starizona specifies an imaging circle of 30mm, which is far from the native, and I'd be concerned how my stars are going to look in even an APS-C sized sensor like that. It's a case of buyer beware...unless you track down some info that demonstrates how well it works...in which case post here :D

The mono (and cooled) camera really brings the noise under control, and you a calibrate with darks. It really would get the best of either scope.

[gregbradley]

FPL53 doublet probably is quite good but really in my experience you really want the triplet. Doublets are really non APO but are called APO for marketing.
Even Tak doublets are going to fall short for imaging. I had a high end Tak FS152 fluorite doublet and it was superb visually but for imaging it tended to get false colour on brighter stars - a blue ring, which is the usual thing you see with non APO scopes.

The Esprits though seem pretty good for imaging.

Greg.

[The_bluester]

Personally, in addition to being faster and giving a wider field of view, the 100 is going to be smaller, lighter, easier to mount, easier to guide, cheaper. So long as the focusers were comparable if I were buying I would probably go with the 100.

[Emuhead]

Starizona Apex 0.65 Reducer (https://www.astrobin.com/gear/104300/starizona-apex-ed-065x-reducer/) for Esprit scopes

[Xeteth]

You can easily get bogged down with all the different numbers and specifications - I remember buying my first scope and it's a bit overwhelming. You've picked some great scopes there, any of them would do the job, but which would do it best? Well, that comes down to what you want to get out of it.

Given your enthusiasm and the examples given I think the Esprit 100 won't have enough focal length for you. My old ED102 had a focal length of 714mm and for me it wasn't quite enough - sure, you could fit the majority of eta Carina in but other nebula appeared a bit small for my liking. At 550mm I think you will quickly outgrow the Esprit 100. The 120 at 840mm is a much better choice I think.

Yes, you will have to image for longer with the Esprit 120 at f/7 - but really so what? If it's going to give you the kind of photos you want then that's what matters. I understand you're concerned about SNR but again, that's just one of the things you'll have to account for to get the photos you want. With proper cooling the noise can be kept under control quite well.

One other thing - this hobby takes an incredible amount of patience. Those amazing images you see - you probably won't be able to get that kind of quality until you have a lot of practice. I too am chasing the ability to take those breathtaking shots, be patient with it otherwise it's easy to get frustrated along the way :P

[cometcatcher]

Sensor size will give a certain field of view also. Small sensors like the 183 won't fit in as much as the larger 1600 or the full frame versions. Pixel size matters too.

Just buy 2 of everything and you will have it all. :D

[gregbradley]

Its aperture not F ratio that determines exposure time.
So the 120 should need less exposure time than a 100 as its got 20% plus larger aperture = more light collected.

F ratio is more the field of view (its not quite intuitive because of how camera lenses get rated often by F ratio).

Everything is easier with a 100m scope and there are lots of widefield objects. the 120 840mm focal length opens up a lot of targets. These smallish sensor ASI183mm or ASI1600 also means you can get a lot of objects as well.

I must say though that 100mm scopes provide images I find most people like the best.

Greg.

[Atmos]

Not quite right there Greg, its the interplay between f/ratio and pixel size that determines exposure time.
A perfect example of this would be with what I get from my backyard. Using the same sensor on two telescopes that have similar aperture but vastly different focal lengths. A 10” F/10 and 8” F/3. One is as 0.4”/pixel while the other is at 1.67”/pixel.

In the first instance I can do 120s exposures under full moon and bortle 7 skies and still not quite be getting close to covering read noise; this is with a OSC mind you so it picks up a lot of light pollution.
Alternatively I can do 100s exposures under Bortle 2 moonless skies and be 10x read noise with the 8” F/3.

Another example would be between your 12” Honders and a 50mm lens both at F/3.8. Your Honders captures some 390x the amount of light (with a 50% obstruction) but they require the same exposure time with the same sensor. You wouldn’t expect a 50mm lens to take nearly 400x the amount of exposure.

This is of course for extended objects like galaxies, IFN and nebula, point light sources like stars are determined purely by aperture.

[Slawomir]

My understanding and experience agree with what Colin has written about F-ratios and photographic speed. Large aperture will get you more stars and they will be brighter largely irrespective of the F-ratio, while faster F-ratio will give your subs a greater SNR for extended objects. (This is actually good news when using pixels with shallow wells on a fast small aperture telescope, because stars do not saturate very quickly while we get good signal on extended objects)

For example: 15-minute subs at f/4.5 still had a higher SNR than 20-minute subs at f/6 (same aperture and same camera, same mount etc) - verified on a number of nights when I was going for over 200 hours on Helix.

With a short refractor - personally I would not want to go faster than f/4.5. Focus shift becomes significant and precise orthogonality to ensure a well-corrected field is also becoming a challenge when using small pixels. I also agree that the corrected flat coma-free field shrinks rapidly when we attach stronger reducers.

[The_bluester]

That is interesting Suavi, I never really thought of it like that which might be one reason why you see such good results around from combinations like a RASA 8" and ASI1600 which used as most people do, has effectively about 8K pixel wells, so very shallow. At the relatively short focal length the "Point source" of stars is much more like a point source as the airy disc will not be as prominent where the nebulosity will benefit a lot from the F2.

Makes me think when they finally release the reducer/corrector for my SVX80T I should buy it.

[Emuhead]

If we had two 130mm scopes, one at f/10 and one at f/4, focal length aside (and visual astronomy also aside), 30 seconds on the same target would yield very different results right?

Lets say that we even allowed for the focal length and cropped the wide f/4 image down to match the narrow f/10 field-of-view image. Comparing just those 2 resultant images... the f/4 cropped image would have to have collected more photons just in that specific cluster of pixels right...OR...are we saying that number of 'photons per pixel' would remain more or less constant given the aperture is 130mm for both scopes and therefore the cropped area of pixels would essentially have to have collected a fraction of the total photons (relevant to whatever the crop size was).

So for example lets say the crop covers only 1/4 of the sensor, this would then mean that the cropped f/4 image could only have collected 1/4 of the photons as compared to the f/10 image (which used its entire sensor).

After all there's only so many photons you can cram down a tube..and thats probably agnostic of what f/number the scope is...so that constant has to be governed by the aperture, and its whether you spread those collected photons over a wide area of sky, or a narrow area of sky (ie. focal length as determined the by f number).

So does this mean the 120mm will have to collect more photons per second than the 100mm (regardless of f number)...
Edit: When i have that out loud... its seems obvious.

[Slawomir]

Yes, a 120mm aperture collects more photons than a 100mm aperture, but we do not bring all those photons into a single point on a sensor. If we did, our images would be just one bright point at the centre of a sensor, and larger aperture would make that point, or a blob, brighter. Instead, some/most of the light collected by aperture is being distributed over the entire sensor, and it is the f-ratio, not the aperture, that will determine the brightness of extended objects being projected on the sensor. Stars, on the other hand, benefit from large aperture. And so does resolution of images of the extended objects.

[Atmos]

It ALL comes down to flux; the amount of photons per unit area of sky. Both telescopes have the same aperture so they both capture the same amount of light. The difference between them is their f/ratios squared.

10^2/4^2 = 6.25
So the F/4 is 6.25x faster than the F/10

To show this, let’s use an ASI1600 with 3.8 micron pixels.
With the F/4 and a FL of 520mm, you have an image scale of 1.51”/pixel.

With the F/10 and 1300mm FL, an image scale of 0.6”/pixel.

Let’s say with a 130mm telescope you capture 100e- per square arcsec of sky per second. That’s the sky flux.
With the F/4 it’s (1.51^2)*100=228 electrons/s per pixel
With the F/10 it’s (0.6^2)*100=36 e-/s/pixel

So you’d have to capture 6.33 (rounding errors) times longer with the F/10 to capture the same amount of electrons.

Another example would be to have a 325mm aperture with 1300mm focal length. It’s a 12.8” F/4 so it captures 6.25x more light than the 130mm. It will also be imaging at 0.6”/pixel but the flux increases 6.25x.
So, (0.6^2)*625=225e-/s/pixel

Remember, 130mm gives a flux of 100e-/arcsec so a 325mm gives 625e-/arcsec.

[Emuhead]

Thanks Colin, so we might as well just use my inputs:

Fuji XT2:
24.3 Megapixels
Sensor size: 23.6mm x 15.6mm
Pixel size = 3.9 microns
Resolution = 6000 x 4000 pixels

Esprit 100ED f/5.5 550mm
Esprit 120ED f/7 840mm

So.. 7^2/5.5^2 = 1.62
So the f/5.5 is 1.62x faster than the f/7
Although I think the above is only usable if the apertures were the same size right. So might have to disregard that for now.


550mm with my sensor gives 1.463"/pixel
840mm with my sensor gives 0.958"/pixel
Thanks to this calculator (http://celestialwonders.com/tools/imageScaleCalc.html)

Let’s say with a 100mm telescope you capture 100e- per square arcsec of sky per second. That’s the sky flux.
With the 100mm F/5.5 it’s (1.46^2)*100= 213 electrons/s per pixel

Does this mean that with a 120mm, we can capture more light per square arcsecond? If so, given area of 100mm is 7853mm2 and area of 120mm is 11309mm2, then thats 1.44x more area, so 1.44x more ability to collect photons.. im going rogue here, those who know please correct me..

So then with that, let’s say with a 120mm telescope you capture 1.44*100e- per square arcsec of sky per second. That’s the sky flux.
With the 120mm F/7 it’s (0.958^2)*144 = 132 electrons/s per pixel

So you’d have to capture 1.61 (rounding errors) times longer with the 120mm F/7 to capture the same amount of electrons as the 100mm f/5.5.

Is this about right? Or do you actually capture less light per square arcsecond.. my brain hurts. I bet my assumptions are way off.. or ive allowed for something twice.. or not allowed for something.

[FrancoRodriguez]

I've got an esprit 100. All around great scope, but... the centre of the image is crystal clear but I'm a little disappointed at the field curvature in the peripheries, even on my ASI071MC APS-C sensor. It does <<almost>> fine on a SX814 sensor. So, great with small sensors, not so great with medium sized sensors. Forget full frame

[kosborn]

I have an Esprit 100 and love it. The flattener works well but spacing to the sensor is critical. It has a tolerance of +/- 1mm I think but outside of that, the stars around the edges of an ASI1600 aren't round. Obviously more so with an APS-C sensor or larger. The FOV on the Esprit works very well for most nebulae but I added a Starizona Apex x0.65 recently. It does a great job of flattening the field with a bit more tolerance than the Sky-Watcher flattener and turns the Esprit 100 into a very fast wide field triplet. Of course I fell down the rabbit hole of astronomy purchases and added a Sidereal f/5 254mm Newtonian so with two OTAs I have 1270mm, 550mm and 357.5mm focal lengths ranging from f/5.5 to f/3.575. I would guess that the Esprit120 with the Starizona reducer would give you the best range of focal lengths for nebulae and larger galaxies.

[Camelopardalis]

Sounds like something is out in your optical train.

Skywatcher quote a 40mm imaging circle and I've found that to be quite faithful to the truth on my scope, with only the far corners of the frame slightly out of round with a full frame Canon 6D.

[Emuhead]

Ok, so coming back full circle on this.. can anyone with the formulas to work this out assist in determining once and for sure.. based on any given sensor (but mine is a Fuji XT2) which scope would capture light faster?

100mm f/5.5
120mm f/7

Id be eternally grateful if it could be worked out, and even more grateful if I could understand it afterwards.

Thanks :)

[Atmos]

7^2/5.5^2=1.6198

So your calculations a few posts up were dead on :thumbsup:

The reason I went more complicated with my previous post was to show WHY it’s the F/Ratio that determines photographic speed and not aperture. It can also prove that my 50mm F/1.4 is photographically 25x faster than the 120mm and 15.5x faster than the 100mm :)

[Emuhead]

The part thats gets me is, how this calculation can be true for different apertures.. i would totally get this if we were talking the same apertures (100mm only), but we have 2 different apertures 100mm & 120mm. I just cant get my head around how that one formula can be good for any 2 different apertures. Say a 50mm and a 1000mm, both were f/2. This calculation breaks here.. because 2^2/2^2 = 1. They both cant catch light at the same speed.. the 1000mm would be so much faster..

[Atmos]

It’s not necessarily about the amount of light that gets into the system. What it is really about is how that light is concentrated and the focal ratio determines how concentrated the light is.

As aperture increases the amount of flux per arcsec of sky increases BUT at the same time the amount of sky that each pixel covers decreases proportionally. Let’s take your Fuji as an example.

A 100mm (4”) F/2 will have an image scale of 4”/pixel so it’s very low resolution. The 1000mm (40”) F/2 will have an image scale of 0.4”/pixel which is quite high resolution. If I do 4^2/0.4^2 (image scale) it shows that each pixel in the 1000mm is covering 100x less area of sky than the 100mm. But the 1000mm is also capturing 100x more photons so it balances itself out.

If a 100mm telescope captures 100e- then my 50mm F/1.8 (35mm aperture) only captures 12.76e-/arcsec/s which is considerably less. The caveat is that each pixel covers 16 arcsec as opposed to the 1.463”/pixel of the 100mm Esprit which is 121x more area! It’s only capturing 7.8x less light though which is why it’s 7.8/121=15.4x faster.

[Emuhead]

Thanks Colin - i get it now.. thats actually a relief!

I have decided to save the extra cash and stick with the 100ED for now too, on account of being cheaper, and concentrating more light on the sensor than the 120ED. Whenever i look at a mono camera down the track, ill probably revisit the 120ED then.

Much appreciated once again. :thanx:

[The_bluester]

If it is your first imaging setup, you will thank yourself for the shorter focal length when it comes to guiding it that is for certain. And the shorter focal length scopes can produce some really nice results. I shot these with my 80mm Stellarvue and ASI294MC Pro camera (Which is quite tricky to calibrate nicely with flats and bordering on undersampled at this focal length) The third is a WIP of a mosaic I am trying to finish this season.



https://www.astrobin.com/full/1bouo7/0 (https://www.astrobin.com/full/1bouo7/0/?nc=user)
https://www.astrobin.com/full/bcchy3/0 (https://www.astrobin.com/full/bcchy3/0/?nc=user)
https://www.astrobin.com/full/5qg6os/0 (https://www.astrobin.com/full/5qg6os/0/)

[Emuhead]

So I'm looking directly at the RASA 8 now. :D

7x faster than the 100ED, and 12x faster than the 120ED.

Can you get nice round coloured stars with this scope? Any glaring reasons not to go the RASA?


Hi Paul, I like the second image a lot, the stars look 3D.. really nice image.

[The_bluester]

I think that h0ughy's images there have some of the best looking stars I have seen out of a RASA 8, which proves that it can be done. The trick is that at F2 getting sensor tilt dialled in and the spacing correct it really critical as the CFZ is tiny (Critical Focus Zone, I was trying not to write Critical twice in quick succession, now I have done it three times!)

Get the spacing wrong or the sensor not perpendicular to the focal plain and you will have wonky stars, but the RASA is not alone there. I am still not happy with the corners using my SVX80T, but I have new hardware coming that will change things so settled on "good enough for now"

For something like the RASA I reckon automated focus will be a very good thing to have, though they are probably not as tough to focus as F2 compared to a normal SCT at F6.3 (Reduced) might indicate, and from what I have read they are more focus stable with temperature. The double mirror setup multiplies your pain in the SCT configuration compared to the single mirror RASA both getting it focused and keeping it there. But if you want to be able to leave it imaging all night with sharp subs from end to end automated focus is a great thing, if you like to sleep.

[JA]

Hi EH,

No- no glaring reasons, but since you appear to basing your decision somewhat on the numbers, and there is nothing wrong with that, that's all part of any decision, there are a few perhaps hidden issues to consider, when looking at how fast one system is relative to the other, since you are looking at the mathematical square of the aperture ratio, small differences may matter.

The RASA 8 is a 400mm focal length optic with a 203mm front objective OUTER diameter. By the common definition that makes it a 400/203=1.97 or f/1.97 optic,rounded up Celestron call it an f/2 optic. Rightly so, as this typical definition goes, but if one is looking at a comparison between Reflectors and Refractors, and how much light is truly collected then it pays to consider how much light is lost/uncollected over the central obstruction. (Here I'm not considering any contrast loss, just how much light is not collected over the central obstruction and how that might equate something one could call the effective f-ratio.

The INNER diameter of the front objective (the central obstruction)is somewhat large at 93mm, so whilst the system is advertised as f/2, in light gathering terms it is a little slower, circa f/2.22. That is the number I would consider using in any comparison of exposure or light gather.

Another difference to consider is that the RASA 8 has a 32mm wide "Usable field" of which it defines 22mm as the Image circle, compared with 2 of the other telescopes you mentioned: The SW Espirit 100ED and 120ED, which have 40mm and 42mm image circles respectively. Of course this means you can use larger sensors, if you so desire, but also, as you've already noted you can use reducers to increase effective speed on an appropriate sensor.

So perhaps take that in to consideration when looking at how fast one system is relative to the other, for a given sensor.


Taking one of the examples you suggested, i.e: using the 0.65x reducer on the SW Espirit 100ED f/5.5 that would make it ~f/3.57 and reduce its image circle from 40mm to ~26mm. NOW comparing that with the RASA-8 at f/2.22 effective, that would make the RASA-8 about 2.6 times "faster" if using the entire 32mm "Usable Field" of the RASA-8. If using the 22mm Image circle of the RASA-8 things would be much closer, with the RASA-8 still being about 1.3 times faster, based on the reduction in area alone.


Best
JA

[Emuhead]

This is really helpful thankyou!

I also didnt know that by using reducers you effectively reduce the image circle as well. That would catch a lot of people out im sure.

I have pulled the trigger on the RASA 8, and so with my XT2 sensor (width 23.6mm) i think that wont be too bad at all.

And i def take your point on the central obstruction reducing the actual f/number.. in my case it will be reduced slightly more id say given the camera body will be larger than that again. Oh well.. ive dived in, and to be honest relieved for the moment that a decision has been made, lol.

I will def see how this goes and perhaps return to the alluring world of refractors at a later stage in this hobby.

[jahnpahwa]

Sorry to jump in here, but has anyone an opinion on the celestron edgeHD 8" with fastar at F2-ish? I guess this ends up being comparable to a RASA of the same size (though I assume RASA has different/superior flattening?) but has the added flexibility of being able to image at f10 and also be used visually?

I ask because I'm sort of in the same boat, thinking about an OTA to complement my 8" F5 newt... and figure that the 8"edgeHD gives me focal lengths either side of this.

(again, sorry to hijack!)

JP

[JA]

Hi EH,

I'm sure you will do well with that combination given the APS-c sized sensor on the XT2. If the XT2 uses an internal UV/IR filter similar to the original Fuji Pro1 or XE1, then you'll also find very good Hydrogen Alpha response compared with other unmodified mirrorless or DSLR cameras.


Best
JA

[The_bluester]

From what I have seen and read (As far as I know, Celestron don't make the Fastar any more and have not for ages, but the same comments apply to the Hyperstar) you can get some pretty startling results out of them, however they can be absolute murder to collimate, and at F2 are just as touchy about spacing and tilt as a RASA and you will likely need to collimate every time you fit it. Add to that that by the time you get a new Hyperstar in your hands, between the exchange rate and GST and freight you will spend two thirds of the price of a RASA 8.

If a Hyperstar for my CPC925 came up secondhand at a good price I would be tempted, but if I wanted F2 new, I think I would just go buy a RASA.

[Emuhead]

Yes its actually surprisingly good for Ha.. this image (https://i.redd.it/x86nd6mc61w41.jpg) of mine was 12x120s images.

[jahnpahwa]

Ah yes, the collimation would become quite a drag! Thanks for that reminder :)

[Emuhead]

From what i read, SCT's dont need collimation that often (not like newtonians which need it every time you use it).

[The_bluester]

As a basic scope no they dont. Visually I tweaked my C925 every few months, but sometimes I left it go for a year or more.

Photographically I checked it at the start of each session, but for a Hyperstar they are super touchy and if you remove it you really need to at least re check it each time.

[Emuhead]

So heres an interesting question:

https://www.baader-planetarium.com/en/blog/rasa-8-test-report-with-fujifilm-x-t3-sebastian-voltmer/

This shows an image taken with a Fuji XT3 on the front of a RASA 8, but if you look closely there are 90 degree diffraction spikes in the image... which made me question if this could have been taken with the Fuji? I thought those kind of diffraction spikes are created with dedicated astro camera power cables split at 90 degress, and not a fully self contained mirrorless camera body. Odd right?

Also, I can buy a T-2 Fuji wide-adapter that is 8.3mm thick, along with the Fuji back focus of 17.7mm brings this distance to 26mm, however the RASA 8 back focus is only 25mm (so im 1mm too long...) which is why i looked closer at that image in the first place. However there is a clear filter inside the RASA which im hoping adds that 1mm.. does/can this happen? The article does say to leave that in, so im hoping for the best really...

[billdan]

He may have added lines/cables to deliberately create spikes.
Or added them in post-processing, some people prefer spikes.

Come to think about it, wouldn't the rectangular camera body create spikes if it intrudes into the light path.

I suppose you could sand/file off 1mm off that adaptor if you can't find focus.

[Emuhead]

Filing, yes i suppose I could do that. Hopefully the remaining threads can still hold the camera.. :|

[JA]

Light will diffract at the edges/corners of the camera if it protrudes in to the field of view. Since the central obstruction of the RASA-8 is 93mm in diameter, anything larger, like the 133mm wide Fuji XT-2 or XT-3 could create such issues, although I am a little surprised how sharp and symmetrical the spikes appear.

[Emuhead]

Closing the loop on this, the 25mm back focus 'increases' to 26mm if the filter is added. This was was confirmed by Baader. So should be good to go.

Also, this blog is incredibly helpful too.
https://cloudbreakoptics.com/blogs/news/astrophotography-pixel-by-pixel-part-1