M33 the Triangulum Galaxy

[Menno555]

Hi Tim

Apologies accepted
And here my apologies to you: I don't have many answers because actually I don't know what I am doing. Sounds weird but it's true. So, take a seat and read

I know basically that FWHM has something to do with sky quality, I know what arc sec per pixel means, and so on. And I tried to put that knowledge into practice .... but that made it boring for me. Working the numbers took the fun right out of it for me.
So, I act on gut feeling. I know the basics of AP, learning more each time and I do what feels right and what produces a test capture that feels and looks good to me. I experiment a lot. Just like my fractal art: I have absolutely 0% knowledge on the fractal formulas, don't know zip about their working. Yet I make stunning stuff (not my words ) and that is very hard to swallow for some fellow artists since the crunch the numbers.

But here the things I can answer.
I tried to use the Brain multiple times but always it's giving advise that is too dark for me. I think that's purely due to all the factors combine: the Brain measures a specific thing but doesn't take things like equipment and so on in consideration.
I also did try (and have) a reducer. Also here: I did notice the bigger FOV and somewhat shorter exposure times. But how to "translate" that into values: no idea. I did notice though that the un-reduced FOV appeals way more to me.

My unguided exposures I just measure with a star and the cross hair overlay: while capturing, I zoom in like 400 to 500% and have 1 star in the cross hair center. The CEM25EC mount has the possibility to manually alter the sidereal tracking rate, so I adjust that according on how much shift I see with the movement of that star in the cross hair. And if the polar alignment is off by a fraction, I adjust that on the fly, again according to the shift of the star in the cross hair. This whole process takes around 10 to 20 minutes and after that, the star doesn't move anymore. Okay, a tiny bit under 500% zoom but in normal view that's not visible.
I did experiment with this on how long I could expose and that was 5 minutes before it started to get unstable. But I have exposures of max 180 seconds due to my Bortle class. But all this really is due to the more then excellent EC of the mount.
Then I manually look through the FITS files if any are showing bad trails and so on and remove those. And then in DSS I use 100% of the captures.

So because of the excellent mount, no guiding for me. But also because of the small(er) FOV, which makes it harder to use an OAG because there are less stars in the FOV. So I can not tell you anything about guiding since I never used that.
And yes, the combination if off-putting .... but not impossible. It all depend on what you do with it and accept that certain things are out of reach. I know for example that objects like M33 will never be bright for me. The exposure time would simply be too long for me. So for me it just a matter of exposures between 30 and 180 seconds and then a lot of them to bring out detail. And process it good.

On the carry capacity limit of the mount. In general they are right: using a light traveling mount witch a scope and equipment that is at 55% of the limit (that's my set up), maybe is great for visual but for DSO APO not so much.
But there are always exceptions. In my case I have a SCT, so a short tube with a lot of front weight. And the mount is a Center Balanced Equatorial Mount. My scope is in balance when it is very far forward in the saddle of the mount, meaning that a lot of weight is shifted forward towards the center of the mount. But since it is a Center Balanced mount, this is no problem what soever. It would be different if you have an APO scope that is end heavy, meaning that the weight is shifted away from the center.

So, there you have it
As you can see I am very, very much a hands on guy and experiment a lot with things. Especially when "they" say that something is simply not possible, then I feel challenged to try. Not so that I can say to them that it is possible, but purely for myself. Nothing is impossible until proven by myself that it is

Menno

[turfpit]

Tim

have you tried comparing results obtained using 2x2 binning or a reducer to change the arc sec per pixel scale

Try the CCD Suitability Calculator at https://astronomy.tools/calculators/ccd_suitability
If you select say a Celestron C8 SCT and the ASI294 from the drop down lists, it is easy to demonstrate that bin2x2 improves the arc sec per pixel.

I always use this tool now if contemplating purchase of equipment.

Dave

[timh]

Hi Menno,

(something funny seems to have happened because this my second reply? the first seems to have dissappeared into the ether so if it reappears there will be two replies .. I hope that they are consistent )

Thank you so much for your detailed reply. It does help. It is certainly true that the real core of this hobby is the astronomy and indeed the beauty and art inherent in the objects that we seek to image. The technical bit is just the means to an end. It was also interesting to learn how you manually steer things to get some of those long exposure images and take a pragmatic approach.

My interest is in trying to get a bit more close in detail on DSOs.

For myself I probably tend to be a bit of a theory/ numbers person -not in great depth - but FWHM (arcsec per pixel) is certainly a useful parameter for working out - under different seeing conditions - just how many pixels the central bright core (of non-overblown) stars is spread over and if it is too few or too many. i.e. you don't want a star to be represented in too few pixels or it will be blocky and unresolved from neighbouring detail and neither is there any point spreading it out over too many pixels and wasting the resolution of the sensor.

I currently have a refractor plus reducer set up that is great for quite wide vistas (e.g. I can fit in most of the Andromeda galaxy) but doesn't provide the right image scale to get close in and yield more detail when the seeing is good enough. For example under quite good seeing (say FWHM angular resolution 3 arc secs - which is rare I know) my resolution of ~ 1.6 arcsec/ pixel would mean only ~ 2 pixels FWHM to represent the core of a star which is not enough in my view.

Therefore I was interested if a setup like yours would be an answer. Your Pelican nebula image for example was very nice although I appreciate now that it must have been a fair bit of work and long accumulation. Anyway with that in mind I ran the numbers ---just as Dave (turfpit) mentioned and worked out that at under ~ 0.5 arcsec per pixel the f 2000 mm/ ASI 296 set up must be oversampling under more or less any likely sky conditions. So for example with FWHM spread over more than 8 pixels under OK seeing with FWHM arcsec of 4.

I don't think oversampling matters terribly much but it just means that you have to work harder than necessary for the same result. So as Dave says the natural option would be to 2x2 bin. As Robin explains elsewhere on this forum it will improve the signal to noise by a factor of 2 (so you could image for half the time and get the same result or the same time and a better result). It is easy to try-but you will need to set up a 2x2 binned flat and dark library. That was why I asked the 2x2 bin question ---if you can get the mount to support 1 arcsec/ pixel resolution (which is where 2x2 binning will take you) for exposure lengths of up to 50s or so then I think -even at F10 (2.5 F stops more than my dob) it would be a worthwhile addition for me - and even more so with a reducer.

All above is only pertinent to deepsky. As you have demonstrated --it is in any case a pretty good set up for planetary work. That will become a more alluring prospect once Jupiter and Saturn slowly wheel into higher parts of the sky from our Northern latitudes.

best wishes
Tim

[oopfan]

Tim,

---if you can get the mount to support 1 arcsec/ pixel resolution (which is where 2x2 binning will take you)

I could be misinterpreting what you meant but I think it is the other way around. My resolution at bin 1 is 2.3 arc-seconds per pixel. When I switch to bin 2 it becomes 4.6 arc-seconds per pixel. Thus there is less demand on my mount.

Brian

[timh]

Hi Brian,

I think that we are on the same page? i.e 1x1 binning is (in theory) 0.5 arc sec/ pixel and bin 2 doubles it..therefore 1 arc sec/ pixel ..in this particular anyway oversampled case? ...which is highly demanding on the mount..but at least a lttle less so.

Tim

[oopfan]

Tim,

That's where I was having difficulty following the conversation. Whether or not it is more or less demanding depends on where you are coming from. That was a very long conversation that required multiple readings and yet I still made a mistake.

May I ask a question? Why are we talking about 0.5 arcsec per pixel? I mean, no one lives (at least no one I know) where seeing conditions are that good. Mine are typically in the range of 3" to 5" FWHM. So even if my OTA is capable of 0.5" it is being wasted. Such a high resolution can only be put to good use for lunar and planetary work, not for deep-sky.

Brian

[timh]

Hi Brian,

I agree. Intrigued by the nice photo of the Pelican it started with me asking Menno about his setup - noting the high level of oversampling and therefore wondering whether he has or might try 2x2 binning.

For my own part --- I don't want to buy a new mount (I have an HEQ5 PRO) and was wondering whether it was worth going down the road of getting something like Menno's scope to put on it. For planetary use certainly but with short enough unguided exposures (maybe up to 45s or so - and enough of them) maybe also steady enough for DSO use with binning and a 0.7X reducer?

Tim

[Menno555]

I haven't tried binning yet. Well I did but that was a failure since I didn't realize that the calibration files also have to be at the same size
But going to try it for sure ... after Brian's challenge
As for my stats, this is the data from the 2 nights I did M33. Exposure 60 seconds, Gain 120, Brightness 4. Info from DSS: night 1 FWHM 5.46 Sky Background 0.51% / night 2 FWHM 6.66 Sky Background 0.47% And like I said, I have no clue what those values mean I do know that the seeing was mostly poor.

As for your mount Tim, planets with an 8" f/10 with a 2x Barlow is indeed no problem.
The DSO indeed is somewhat more difficult. My mount has a resolution of 0.14 arc seconds, realtime PEC and because of the optical encoders, it has <0.3 arcsec RMS. I don't know the specs of your mount but if you can manually alter the the tracking rate, then 45 seconds should be possible. Before this mount, I had it's little brother (CEM25P) and with that and manually altering the tracking rate, I could go as far as 90 seconds. No reducer and no binning.

And yes, the oversampling is there and I knew that when I got my setup. I used the site Brain mentioned and found the info there. But for me it's fine, I am finding new ways with processing to make that oversampling look less. I did find a camera there that would be perfect for my scope but $7800 for a cam I did find a bit much

[timh]

Thanks once again Menno,

I suspect that your mount is better than mine. If my understanding is right those are really very good FWHM scores and -even if the Bortle score is high - also implies that actually the seeing was quite good as well as a testament to your mount being steady over 60s - i.e. averaging the the DSS measured FWHM/ pixel scores you can calculate the effective seeing in arc sec.

so e.g. FWHM (arcsec) = FWHM (pix) * (pixel size/f) * 206.3 = about 2.9 averaged over the two nights

So - empirically - based on the quality of your images as well as the FWHMs it just seems to work! (unless there is something that I am misunderstanding about DSS FWHM scores which is always possible)

Anyway first chance that I get I will do an experiment. Get the HEQ5 set up as best I can - all nicely polar aligned - and then probably use the Sharpcap drift graph facility to measure the unguided drift in pixels at different exposure times and focal lengths.

best wishes
Tim