RCC - same target, different scopes. - Forums

I recently acquired a Vixen Extender for my R200SS, which takes the scope to 1120mm focal length at f/5.6. This was so I could get those lovely little small targets that have always intrigued me.

We've had a precious couple of moonless, clear nights here, so I wanted to shoot the (not so small) M81 region as it's this month's Cloudy Nights beginners' competition target.

One night I used the R200SS...

...and the next I used my 81mm refractor at its native focal ratio; f/7.7

Both images consist of 60x180", but I forgot to take flats for the first image before removing the camera and putting it onto the refractor.

I prefer everything about the second image, taken through my refractor. I think it's because the stars are smaller and crisper and the colours seem richer, which is odd seeing as the first image effectively was taken through a much faster scope.

Advice gratefully received on either image (why is the bottom one so mottled?! Perhaps it's JPEG compression) but particularly on how to get the stars crisper on through the Newtonian. I'm not massively interested in doing it through image manipulation...if they're bigger simply because that's what Newts do, fine with me but if it's technique, I'd love to hear some tips.

Thanks!

This seems like comparing apples with screws. Could you register the larger fov image with the smaller one and show those in comparison?

Frankly to me is just the opposite. The RS200 image's stars are flaring on one side which suggests poor secondary alignment. Star size-wise is all down to different image scale and focal ratio. Were the RS200 image be brought down to the same scale of the smaller refractor then the stars size would look good/smaller too (BTW they look good in the original version if you ask me).

The mottling is due to poor flat technique and likely no LP suppression filter being used.

I'd think I was maybe comparing small apples with large apples, but you make an excellent point and might actually have allowed me to answer my own question!

Here's the image through the Newtonian (R200SS)

...and here's the same field of view through the refractor (SD81S)

My thoughts - the Newtonian image is more detailed and comparatively, the stars are much the same. But also, the much smaller, slower refractor holds up remarkably well, which makes me wonder if I'm still not using the Newt to its full, or whether refractors simply are "better" at this sort of thing?

andrea tasselli:
Frankly to me is just the opposite. The RS200 image's stars are flaring on one side which suggests poor secondary alignment. Star size-wise is all down to different image scale and focal ratio. Were the RS200 image be brought down to the same scale of the smaller refractor then the stars size would look good/smaller too (BTW they look good in the original version if you ask me).

The mottling is due to poor flat technique and likely no LP suppression filter being used.

Interesting observation - I'd noticed that too. The problem I find is the back plate of the secondary holder has small divots in it which means it sort of goes where it wants to go! I have thought about putting a small plate on the back to give the collimation screws something flat to work against but am worried that might move the secondary too far down the tube. Could a couple of mm make that much difference?

The flats were generated using a Lacerta flat panel and calibrated in PIS so I'm not sure I'm using any technique as such - I shall research! As for the light pollution filter, I'm in a Bortle 3 area (assessed myself. not just taken from a map) so I was assuming any LP filter would be counterproductive?

Piers Palmer:
andrea tasselli:
Frankly to me is just the opposite. The RS200 image's stars are flaring on one side which suggests poor secondary alignment. Star size-wise is all down to different image scale and focal ratio. Were the RS200 image be brought down to the same scale of the smaller refractor then the stars size would look good/smaller too (BTW they look good in the original version if you ask me).

The mottling is due to poor flat technique and likely no LP suppression filter being used.

Interesting observation - I'd noticed that too. The problem I find is the back plate of the secondary holder has small divots in it which means it sort of goes where it wants to go! I have thought about putting a small plate on the back to give the collimation screws something flat to work against but am worried that might move the secondary too far down the tube. Could a couple of mm make that much difference?

That's a bummer. I thought the RS200s were built to much better standards. Maybe a replacement secondary holder? At any rate 2mm is far too much for focuser misalignment so you'd need to re-align the focuser to the secondary. But normally you would readjust the secondary to sit square against the focuser with a sight tube, the stem bolt normally being long enough for this sort of adjustments?

Piers Palmer:

The flats were generated using a Lacerta flat panel and calibrated in PIS so I'm not sure I'm using any technique as such - I shall research! As for the light pollution filter, I'm in a Bortle 3 area (assessed myself. not just taken from a map) so I was assuming any LP filter would be counterproductive?

If it were properly done you should have just small near-linear gradients (unless the Moon is out). Mind you, even in Bortle 1 sites you get gradients, only from natural sources rather than from artificial ones. This said it is hard to judge without having a master light to analyze in PI.

andrea tasselli:
That's a bummer. I thought the RS200s were built to much better standards. Maybe a replacement secondary holder? At any rate 2mm is far too much for focuser misalignment so you'd need to re-align the focuser to the secondary. But normally you would readjust the secondary to sit square against the focuser with a sight tube, the stem bolt normally being long enough for this sort of adjustments?

It's quite an old scope, maybe 15yrs old and a previous owner swapped out Vixen's collimation hardware for Bob's Knobs, which I think might be the issue. Because of the age of the scope I've often thought about replacing it which is part of the reason behind this post - to see if I wouldn't actually be better off with a 120mm refractor perhaps. Having resized the images, as suggested, I think the Newt does fine.

In terms of the mottling, if anyone was really bored and had a really fast internet connection, here's the master light... - https://www.dropbox.com/s/ez3tu2kry0ob0mu/masterLight_BIN-1_6248x4176_EXPOSURE-180.00s_FILTER-NoFilter_RGB.xisf?dl=0
...but I would definitely consider it beyond the call of duty.

I gave it a go and come to the conclusion that there are very little natural gradients (or any of any sorts) so the blotchiness is down to processing and to the optics used, given rise to a widely dispersed scattered light, more prominent around the brighter stars/bodies. There are also faint strands of IFN dispersed in the background that render the treatment of the background a delicate balancing act. A re-processing of the image is shown below:

And here is a close-up of Bode's Galaxy:

You're better than me! Thank you so much for going to the time and effort of doing that - next time you're in the north of England I'll buy you a pint.

Would you say the optics are faulty or is the blotchiness (which you've suppressed superbly) simply a by-product of the scope?

Piers Palmer:
You're better than me! Thank you so much for going to the time and effort of doing that - next time you're in the north of England I'll buy you a pint.

Would you say the optics are faulty or is the blotchiness (which you've suppressed superbly) simply a by-product of the scope?

Not a problem, happy to help. Normally I don't venture much north of Leeds but sure love Northern England so I'll be happy to share a pint.

The optics isn't at fault, it is just the way it behaves or more precisely its coatings. They might have decided to throw out the g line quite a bit so you might get these effects but not really sure that is the case.

Hi Piers,

I think that the limiting factor is most likely the steadiness of the sky - and so although the reflector inherently offers the potential for a higher resolution image than the refractor you are not going to see that on this occasion ? I took the liberty of taking a look at the file that you posted of the refractor master light . If the metadata in the file were correct (they may not be?) it said that your pixel size is 3.76 um, f = 642.63 mm and that you were sampling at 1.207 arcsec/ pixel. On that basis the average FWHM of the linear image is about 3.817 pixels or about 4.6 arcsec. That is not that great and on that basis - for the sky conditions - even the refractor is somewhat oversampling and the reflector about doubly so assuming it is using the same camera.

You are very fortunate having Bortle 3 skies to image galaxies – but maybe you were unlucky with sky steadiness on this occasion?

I think that in principle the Newtonian should be a much better scope for detailed galaxy imaging than the refractor - agree with all your and Andrea's comments above about getting the alignment right - possible resilvering the mirrors - but it may also be that you were unlucky with the conditions?

Tim

One thought – the oversampling with the reflector may be such that you should think about 2x2 or more binning the image in processing? That should give you a 2X improvement in SNR with no loss of resolution and after that it may start to look a better image than that from the refractor? Indeed it may even be worth binning both images - just the reflector more than the refractor.

Thanks Tim,

Firstly, the metadata was correct; the camera is an ASI2600MC Pro and the focal length is what my ASIair told me (the scope claims 625mm but perhaps the flattener adds a bit?). Secondly, I'm happy with both images really, I love doing this. Having said that, I'd be thrilled to improve!

Now, dark skies, yes; unsteady skies…probably! I don't know if Cumbria is any more prone to unsteady skies than anywhere else, but I wouldn't be surprised if it is. I rarely consider it when I'm setting up either; on this target I was happy something was overhead so disregarded it completely.

I need to get my head round binning again but I think I get the principle, so that's a good shout. I'm also happy to replace my R200SS with a new one, if that's going to improve things. As I say, my one is old, and I bought it more to see how I liked it than as a long-term proposition. I like it because it's short and very lightweight (less than 10kg fully kitted out). However, there must be a sweet spot for imaging scopes/my camera/my conditions which I have yet to work out. Perhaps I should stick with the reducer I have for my refractor and use the 495mm focal length instead.

Piers Palmer:
I'd think I was maybe comparing small apples with large apples, but you make an excellent point and might actually have allowed me to answer my own question!

Here's the image through the Newtonian (R200SS)

...and here's the same field of view through the refractor (SD81S)

My thoughts - the Newtonian image is more detailed and comparatively, the stars are much the same. But also, the much smaller, slower refractor holds up remarkably well, which makes me wonder if I'm still not using the Newt to its full, or whether refractors simply are "better" at this sort of thing?

Problem is seeing too. Its always super hard to compare two diffrent night. Also with a large focal length you are most likly seeing limitated. Anything under 1,5 arc sec per pixel resolution here in cities is blury from seeing. So the resolution increase is not that extrem many think. I noticed that with 750 mm f5 newton to 1000mm f5 and now 2000mm f5,45 RC. The resolution is higher yes, but the image is more blury. That why i use short exposure times. Or waiting for better seeing.

The "waiting for better seeing" is difficult when you live where I do!

I've been fiddling around with the CCD Suitability Calculator on Astronomy Tools and if I assume "poor seeing" and keeping my equipment the same, then binning 2x2 fits nicely with the Newt and using the x0.79 reducer on the refractor also fits quite well. As for shorter exposures, that's definitely something I need to consider more often. I could even consider a camera with larger pixels, but I'd probably be nudging towards full-frame then and that would come with its own challenges.

Of course, I can take any photo with any equipment I like; there are no rules, but I'd like to make sure any future expenditure is value for money.

Piers Palmer:
As for shorter exposures, that's definitely something I need to consider more often.

Agree with that ...and taken to its logical conclusion you enter the realm of lucky imaging once you get down to 5-10s. The challenge then becomes SNR as well as file size (use smaller frames to save on kb--it is all you need for galaxies). In my case at Bortle 7 I can only use such short frames on the cores of relatively bright objects and galaxies like M82. The great thing about being at Bortle 3 is that the SNR challenge is a lot (~ 8x I think) less than for me so you have a real advantage and short frames with the Newtonian (without extender) definitely worth trying I'd say.

As an example, on a reasonable night I can get 5 or 10s frames and indeed final stacks with my 300 mm F4 or 200 mm F5 down to measured FWHMs of less than 2 - sometimes down to 1.7 arc sec . The trick is to use subframe selector and to discard a lot of frames-keeping only the sharpest. Then - if you stack enough short frames to get reasonable SNR- plus if you oversample (as you are) - then using BlurXt (or PI deconvolution also works fine but requires more technical skill) you can find even more fine detail even down to 1.2 arcsec or so.

For galaxies a lot of the fine detail to resolve is often at the centre which is brightest and the sensible part to image using short frames and deconvolution. However to get a decent entire image of the galaxy including the fainter periphery just using short frames will not cut it -- the SNR is too low. One solution is to use a high dynamic range composition (HDRC in PI) so as to combine your more detailed short-frame 'inner' image with an outer part comprised of much longer exposure frames that better captures the fine fainter detail.

Tim

Piers Palmer:
The "waiting for better seeing" is difficult when you live where I do!

I've been fiddling around with the CCD Suitability Calculator on Astronomy Tools and if I assume "poor seeing" and keeping my equipment the same, then binning 2x2 fits nicely with the Newt and using the x0.79 reducer on the refractor also fits quite well. As for shorter exposures, that's definitely something I need to consider more often. I could even consider a camera with larger pixels, but I'd probably be nudging towards full-frame then and that would come with its own challenges.

Of course, I can take any photo with any equipment I like; there are no rules, but I'd like to make sure any future expenditure is value for money.

With wating for better seeing i mean not expecting the best data during early night and low the horizon. For my long focal lenght equipment I am waiting for the highest point in the horizon and best case 3-4 hours after sunset. Ofc thats not always possible, but ive got the best luminance results during that window. But even with all these factors considered, the results can still be shit😂 but the oods are higher. ive tried a lot of lucky imaging. Even with deepsky. Its really really good for bright nebula. But as Tim said, high SNR is important and walking noise pattern can occur if not dithered well

Tim Hawkes:
...short frames with the Newtonian (without extender) definitely worth trying I'd say.

The extender on my scope doubles as a coma corrector so it's almost a necessary evil. I do have a cheap coma corrector which doesn't change the focal length, it was ok with the small sensor of my 533MC Pro camera but whether it stands up to demands of an APS-C is another matter. Certainly worth a go though as it drops the scope to f/4.

Marc Fischer:

Ah, got you. Bode's galaxy was almost directly overhead during both shoots but it was early in the evening, so that's definitely something else to add into the mix.

I do dither....that's one thing I've always done. Whether I do it well though....

Piers Palmer:
Marc Fischer:

Ah, got you. Bode's galaxy was almost directly overhead during both shoots but it was early in the evening, so that's definitely something else to add into the mix.

I do dither....that's one thing I've always done. Whether I do it well though....

I dither every 5 frames with random 3 pixel shift. I usaly do 60 sec exposure and the waking pattern is gone. It depends on you pixel size and resolution, as well the amount of single frames, how often and strong you need to dither, but thats another topic😂

Piers,

Firstly I'm jealous of your Bortle 3 skies I live in Detroit now under Bortle 7 skies but grew up in Northumberland and remember with fondness the Bortle 3 skies out by my parents house!

I suspect many of us have been on this journey before and there are no right or wrong answers. What I've found is that with the small pixel 2600 cameras once the focal length gets above 600 or so mm other factors control the details one can capture: seeing, mount capabilities etc. So for example I have a WO FLT132 which is f7 native or I can reduce it down with my 0.72 reducer. For me at least the reduced version loses nothing on detail and is faster therefore I always use that set up as I get more light per exposure which ultimately helps my SN.

Also as far as binning goes, as far as I am aware, it works with CCD's but not so well with CMOS, others I'm sure have the math so whether I'm at 1400 mm fl, 0r 660 mm I always bin 1x1. Some have issues with file size and binning makes sense for them but that's not an issue I've ever had.

Bottom line use whichever scope you prefer, or that fits the desired field, but don't expect much change in details once your up over a pixel scale of 1.2 to 1.4. Do remember though that f5 will gather twice the light than f7 in a given time - which really matters in light polluted skies but may be less important to you. That's the reasoning I keep in my head every time I'm tempted to buy yet another scope usually a 10" f4 Newt!!!

Hope that helps

John

John Noble:
Also as far as binning goes, as far as I am aware, it works with CCD's but not so well with CMOS,

It is true that with CCDs there is more to gain with on camera binning than for a CMOS camera. That is mainly because CCDs add more read noise. Neverheless with CMOS 2x2 binning still gives a 2 fold increase in SNR . That is because by combining 4 pixels the signal can increase by 4X whereas the noise increases by only the SQRT of 4X.

With CMOS sensors binning increases the RON by a factor of 2, which might or might not be an important factor in any imaging run. This is especially true if you do short exposures.