Which scope for galaxy imaging? 8" f/4 Newton, RC8, or 5" Triplet?

[minyita]

Hi everyone,

long post incoming - excuse my ramblings -

after I finally moved to a place with a secluded and safe roof terrace where I have enough space to put up two rigs, I'll plan to add another rig for smaller Targets next to my CEM25P with an 80mm Refractor (which I'll eventually upgrade to a triplet). Right now, I use a 183 MC Pro, which I'm content with - I like the small sensor, it's very forgiving in terms of FF / Reducers and also collimination for reflectors. I'd definitely keep an IMX183 Sensor, probably the Mono, for an 8" f/4 Newtonian, gives a good sample rate for galaxies and the light gathering of the scope is good enough for such a resolution, plus it's fast. I'd get an 533MM tho for an RC8, reduced it samples similar to IMX183 + 800mm focal length, and the sensor is still small, so I could probably do without a field flattener on the RC8. Other option, and likely the most expensive one, but probably also the most stressfree one, with the fewest light gathering, is going 5" Triplet, a decent Triplet that size would cost me between 2.5-3k, plus field flattener/reducer, so I could run it at f/5.6, which is still fine imo. Sampling would be a bit lower than on an f/4 Newton, but no stressful collimination.
The mount I'm planning to put on this whole setup is either going to be an AM5 (dw, no carbon tripod for those scopes obviously, and I saw people running similar scopes just fine on it), or a GEM45/CEM40 (kinda expensive here in germany right now, and no option to order cheaply directly from china). Maaaybee a HAE43, but I barely find any experiences with it. So something in the 20kg class, which should be enough for imaging around ~0.7" with a scope that size. Off axis guiding would be a given under all those circumstances, plus a sensitive guidecamera.

I'm mainly undecided about collimination on those RC8s and f/4 Newtons, which one is more forgiving / easier to learn how to handle?
On the other hand, a 130mm f/7 Refractor has wayy less light gathering abilities and resolution... also I'm unsure on the IMX183 on a 130mm Refractor. Given I run it at f/5.6 and an 80mm Apo rn, it should only improve, but still... People keep talking about it being a niche camera and only good for RASAs which imo, is maybe a bit limited. I'd probably wouldn't use it on a focal ratio slower than f/5.6, and the focal lenght shouldn't be higher than ~800mm probably, but other than that... why not?

So, that leaves the following options for getting decent galaxy shots:
RC8 Carbon + New Focuser + Reducer + expensive collimination tools + eventually a tilt adapter = 1550€ + 260€ + 255€ + I don't even know how much those cost and what works best, but lets say another 300€ + 110€ = 2475€ -> suddenly, the "economic" RC8 doesn't seem as economic anymore. The 533MM and 183MM are roughly the same price, so I'll leave them out of the calculation for now.

Skywatcher Quattro 200P (settled on that one, because here at least the optics seem toptier, and there's lots of DIY stuff to fix issues with mechanics, focusser also seems better than on the GSO f/4 Newtons) + Aparture Ring / New Spidervanes + Velour in the tube + Coma Corrector + expensive collimination tools = 750€ + 250€ + 40€ + 330€ + idk what I'd need here and how expensive it'd get, lets say 200€ for a really good laser? = 1570€ -> certainly the cheapest way to go, and even with a focuser upgrade, I'd stay under 2k.

Altair Wave 130 Triplet imported from UK + Reducer/Flattener + bigger Dewband, and that's about it = 3000€ with customs, tax and everything + 350€ + 70€ = 3420€ -> obviously the most expensive one, but probably also the most comfortable one. No collimination, Dew is fixed with a Heatingband, no weird cables bringing in artifacts into the picture because secondary mirror heating, easy to do the same target on another day because no spikes that could potentially misalign, no weird artifacts, high contrast, Apos that pricerange usually have a good focuser.. yeah.

The only thing putting me off is that I don't know how good a 5" Refractor reduced to f/5.6 around ~725mm focal lenght + IMX183 really is in terms of light gathering abilities and galaxy imaging... I know, some of them are faint, and more aparture is a trump, also the Dawes limit is like 0.89, which is... greater than my imaging scale, which is probably not good? I wouldn't have that issue with a 8" reflector of any sort, as Dawes limit is 0.58. Seeing here is decent, at around 1.5 Arcsecs usually, sometimes lower, sometimes a bit higher, according to meteoblue. Fully focused on my doublet apo I often get ~2.3 FWHM on my images. Only in the summer nights, seeing is usually not that great, but that's widefield time in my opinion anyways, so I'm not worried about that. I'd be imaging from a bortle 5 rooftop in a 40k suburb, not great, but also not the worst. Going mono should help with light pollution as well.
I'd gladly pay the premium if I know I can actually spend my time imaging and getting good galaxy shots with that 5" Refractor - I'm worried about spending more time troubleshooting / colliminating with a f/4 Newton or RC8 than with the Refractor.

The thing is, I have decent seeing, light pollution is tameable with going Mono, but the thing that's not great around here is how often I get clear skies - and that's not often. and if I'm going to waste those precious clear nights colliminating instead of imaging... yeah I don't know.

A f/4 8" Newton is economical, I know it works well with the IMX183 sensor, it gives me good sampling rate and resolution, I think I'd also have less central obstruction than with an RC8 (unsure tho, feel free to correct me!) which should result in a higher contrast... but, collimination is... well. I'll have to build up the scope because I won't leave it mounted on my roof terrace of course, every time I'd image. Does that screw over collimination already? Another thing tempting about the Newton is that I could toss in a good barlow, and do easy planetary with my 183MC if I want to do that at some point, Refractors and RCs are less suitable for that, I guess.

RC8 seemed economical at first, but no way I'm gonna use the IMX183 at around 1100mm focal lenght reduced, not the biggest fan of the square 533MM sensor, but I don't want to go a bigger sensor size, as it gets more expensive in everything - bigger filters (I already have 1.25" SII and Ha laying around), bigger filter wheel, bigger OAG needed, reducer is super iffy on the RC8 with APS-C, field probably not flat anymore, bigger camera, bigger focuser due to all the extra weight due to everything being bigger, so 533MM it would be to achieve the desired sampling between 0.6-0.7"/pixel. It seemed promising at first - what I like about the RC8 is, that well colliminated, it gives nicer and less prominent spikes than a Newtonian, also it's shorter, so less of a wind sail. What I don't like is: Focuser attached to primary mirror, gives issues with collimination - but at least once you dial them in, they keep the collimination, apparently that's not the case with an f/4 Newtonian.

I know aperture is king for galaxy imaging, but both reflector systems have a secondary mirror introducing central obstruction, so how much worse is a 5" Apo really in terms of light gathering abilities?
Also what I'm wondering is, if I'll go mono anyways, can Chromatic Abberation on a doublet be fixed that way? each wavelenght gets focused seperately anyways, right? So a 130mm Doublet might even be more affordable - but I'm unsure, I imaged 3 years now with my 80mm Apo - first with a DSLR, then with a cooled colour cam, and CA is a nightmare on an FPL 53 Doublet imo. I tend to pixel peep, but if I zoom into my images, colour fringes are visible around every star - and that's after seperating the color channels and aligning them on green and putting them together again, which does help a bit, but it's still there. Would that issue disappear on a Doublet if imaging mono?

So yeah - I'm very undecided, and I want to decide on a scope first, before I choose my second mount, which will probably end up whatever is most affordable in the 20kg range at that point, and seems trusted for delivering 0.5" guiding consistently if everything is dialed in correctly (I mean I get that on my CEM25P with 6-7kg load after I did surgery on on her meaning belt and meshing fixing according to iOptrons guides, which reduced my Dec backlash to a sweet, sweet 700ms, should be doable on a 20kg mount with 60% payload as well imo, so I'm not worried about that).

I'm thankful for every insight, recommendation, what to do if I want to image galaxies in the M64 range of size decently

I'll keep my CEM25P + 80mm refractor for nebulas, widefield, etc - it's great for that and literally plug and forget at that point... but galaxy imaging... yeah no.

Thanks and Clear Skies to everyone,

Mina

[vlaiv]

Hi and welcome to SGL.

To be honest - I did not read your post as it is long and I for some reason I lack the patience at this moment.

I did see you mention sampling rate, Dawes limit and such - so here is brief guide how to choose good telescope for galaxies.

First determine your sampling rate that you'll manage to achieve. From experience, I'll say that it is 1.4 - 1.5"/px.

Now, this sampling rate can be achieve with selected camera and range of focal lengths.

Say that you keep IMX183 sensor with 2.4um pixel size. Your target focal lengths will be multiple of 330mm. So 330mm for bin x1, 660mm for bin x2, 990mm for bin x3, 1320mm for bin x4 and so on.

In next step - get the most aperture you can manage at any of those focal lengths (but at least 5" because if you go lower than 5" - realistically you'll have tough time reaching 1.5"/px).

If possible - avoid excessive use of correctors or try to use best ones. Ideally you want just diffraction limited scope over large enough field and sensor.

[SamAndrew]

On 23/04/2023 at 14:10, minyita said:

RC8 seemed economical at first, ....................- but at least once you dial them in, they keep the collimination, apparently that's not the case with an f/4 Newtonian.

  Having owned the Quattro scopes I feel they keep collimation well, and I've got decent results using a cheap Cheshire collimator. You definitely want an aperture mask. 

On 23/04/2023 at 14:10, minyita said:

I know aperture is king for galaxy imaging, but both reflector systems have a secondary mirror introducing central obstruction, so how much worse is a 5" Apo really in terms of light gathering abilities?

  I think once you factor in the central obstruction, the 8" scopes are going to gather about twice as much light as the 5" refractor.

On 23/04/2023 at 14:10, minyita said:

Also what I'm wondering is, if I'll go mono anyways, can Chromatic Abberation on a doublet be fixed that way? each wavelenght gets focused seperately anyways, right? So a 130mm Doublet might even be more affordable - but I'm unsure, I imaged 3 years now with my 80mm Apo - first with a DSLR, then with a cooled colour cam, and CA is a nightmare on an FPL 53 Doublet imo. I tend to pixel peep, but if I zoom into my images, colour fringes are visible around every star - and that's after seperating the color channels and aligning them on green and putting them together again, which does help a bit, but it's still there. Would that issue disappear on a Doublet if imaging mono?

You can refocus for RGB, but your luminance is still going to be a combination, and the field curvature will also be worse on the doublet. However with Blur XTerminator you can correct this in post processing; my experience so far is, the worse the optical system, the more impressive the results are.

Of those 3 options, the Quattro would be my choice. My actual choice for galaxy imaging at the moment is the Rasa 8, which combines nicely with 2.4um pixel size. It's more versatile, but you can't use the filter wheel, so if you want to go fully automated it is a bit of a pain. 

[ONIKKINEN]

Hello and welcome to the lounge,

As a newtonian imager myself i will go ahead and inject my own biases here and say that you should go with the Quattro 8 out of the options you have. Really it seems the ideal choice for what you want, large aperture to help with the small faint targets you want to use it for, but not too much focal length like with the RC8 where your focal length and small sensor leaves you with a very narrow field of view and small images (if binned to reasonable resolution). With a 1.0x coma corrector such as the TS GPU you are looking at 0.62''/px which will leave you with a good seeing target of 1.24''/px or a not so good seeing night target of 1.84''/px at bin2 and bin3 respectively. Bin1 will be impossible or at least nonsensical. If i were you i would budget the focuser in the package from the start, or at least look at different focusers and see how much they cost to prepare for that.

As for a laser, you dont need to spend 200€ to get a decent one. Try this one for example: https://www.teleskop-express.de/shop/product_info.php/info/p83_TS-Optics-Newtonian-Laser-Collimator---1-25----precise-collimation-of-Newtonians.html

Its a GSO laser sold everywhere under 10 different names, here with the TS branding. You can remove the 45 degree angled silver surface from the middle and add an aperture stop between it and the laser diode. I just drilled a 1mm hole in some thin cardboard and taped it to the inside of the 45 degree block. This will remove any astigmatism and other aberrations native in the lens (mine was rice shaped at a distance of a meter) and spread the diffraction pattern over a larger area. This allows you to collimate using the shadow of the primary mirror marker instead of the center laser spot itself which is a more accurate method as center target here is way too large (2mm i think). You may want to double check that the factory installed center marker really is at the exact center to an accuracy of much better than 1mm and if not, replace it. At f/4 you are looking at nothing other than perfect collimation to have acceptable sharpness, although your small sensor will make things easier. I am imaging with an APS-C sized sensor (at f/5) and perfect stars corner to corner is really not something i am reasonably expecting to happen most of the times (and it doesn't).

On top of the laser it is important to think what else is between the laser and the primary mirror. Namely the 2'' to 1.25'' adapter, it has to be of good quality. If you have one, then no cost added, if you dont then maybe you will be looking for a price of up to 200€ after all. The focuser of course has to be perfect (and you have to collimate with the laser target at roughly the focal point of the system with the focuser in the same height as when imaging). It will be a challenge, but one you can learn to do.

[Clarkey]

As the owner of an RC8, 115mm refractor and an F4 newtonian I will give my opinion. (I did have a 130mm refractor, but it did not fit my very small observatory). I think they all have pro's and con's and ultimately it is down to your preferences.

RC8 - yes, it is a bit tricky to collimate, but once you have developed a method they are not too bad. Fortunately, my focuser and primary mirror are well aligned so collimation is not that difficult. I normally need to collimate it once per season. For the RC8 I don't need a flattener with the 1600mm pro, but the IMX571 is probably pushing it. In terms of imaging, I use the native FL and bin2 or bin3 which gives 1 - 1.5"/px. I was planning to sell it when I brought the new refractor - but I just can't let it go. I like it that much. Once binned it is relatively fast and gives great images. FWIW I use a ST80 for guiding as I always find OAG's a pain. Collimation can be done with a Cheshire and star test. I do have the worlds most expensive LED light, otherwise known as a REEGO - but you don't need it.

The refractor has only had limited use due to the rubbish weather. In terms of resolution, there is very little difference between it and the RC8 due to seeing. At 805mm native FL it gives around 1"/px which is better than my seeing can support unless very good. However, it is simple and bomb proof - no collimation issues and easy to set up. The negative is that it is relatively slow compared to the 8" scopes.

I have a 6" f4 newtonian and in terms of collimation it is by far the most work. It does take great images but does require the most maintenance.

If it was me - and I had the choice of one scope - it would probably be the RC8 with a CCD47 and a Baader Steeltrack focuser. However, I love the simplicity of the refractor (which is also a good visual scope) and the speed of F4