vlaiv

What is the rant quota per user? It's been a while since I brushed up on my CoC familiarity - two times a year? three times a year? I feel I'm entitled to one, so here it comes ... What's with SCTs? What could possibly be making them that much more expensive? Celestron C5 - £569 vs SW Mak127 £265 vs Bresser MC-127 £312 Catadioptric - check Spherical primary - check Glass for corrector plate - check Similar tube size - check Funny mirror focusing mechanism that causes image shift - check I don't really see much more craftsmanship involved, if anything - less glass is used for corrector plate (much thinner, isn't it?). All of that must mean twice the price - there is no other explanation ???? /rant over But seriously, this has stopped me from recommending this scope as "do it all" budget option for novice astronomers. Not much available as a mounting option either - well nothing sensible.

I called multiple times for people to do an experiment. I did it once and simple resizing won. In part because DSS threw some artifacts in drizzled image and in general because SNR loss was obvious and there was no gain in resolution in favor of drizzle. Want larger megapixel count in your image? Why don't you simply do one of the two: - take all your calibrated subs prior to stacking and just scale them up x2 or x3 or x4 or even x3.5 - what ever number you choose - and stack them like that. Use a fancy resampling algorithm that produces good results like Lanczos3 or Splines or similar. - Do the same but after you are done stacking. In fact, I think that first approach might even do as much as drizzle in terms of improving resolution (real or not) and it certainly won't lose SNR since it will provide sample for each pixel from each sub instead of spreading pixels around and not covering all the pixels of output with every sub. Back to original idea, I'll propose experiment and someone else can do it with their own data (partly because I have not imaged for ages and in part because I want to exclude any sort of bias on my part - too many parts in there? ). It does not matter if your data is properly sampled or even over sampled - it is rather easy to create under sampled data - just bin it in software (do it after calibration). For our purposes it is just like using larger pixels. Do 3"/px or 4"/px - that way you'll be certain you are under sampling. Experimental protocol: 1. Produce under sampled data by taking your calibrated subs and then binning them. PI has it as integer resample - use average method. 2. Take one sub as reference frame for other subs to be aligned on. 3. Do drizzle stacking x2 without any fancy stuff - just simple average (no weighing of the subs, no sub normalization - just plain average) 4. Take all subs and scale them up x2 using Lanczos3 resampling and then do regular stacking - again just simple average no fancy stuff 5. Do comparison by selecting 2 regions on both stacks. Try to select same regions on both stack (this is why I said take same reference frame as stack should be roughly aligned this way). One region should be pure background - without anything but pure dark background - try not to select stars or anything. Measure standard deviation in this region on both stacks. Second region should be uniform part of target (just take some nebulosity without too much features - end of a spiral arm or galaxy halo or whatever) - measure average pixel value here. Divide respective measures and compare. This will show how SNR fares between two approaches Also measure FWHM of resulting stacks - this will show you what happens with resolution.

Hi and welcome to SGL. I'm afraid that you won't be able to do everything you want on that sort of budget. Taking pictures is very demanding aspect of hobby and it usually involves dedicated equipment. Recommended SkyWatcher MC127 is very nice scope and you can get it within your budget in Goto variant. It is also well suited for imaging of planets, but you won't be able to image DSOs with it (it can be done, but it requires great skill and knowledge to do so). It is also not the best scope for observing DSOs in that price range. It has larger focal length and you will be using larger magnification most of the time. That is fine for smaller objects and planets, but you'll be missing out on wide field views that are very nice. ASI 120MC is ok camera for planetary work - but not good for DSO imaging. Maybe you should start by realizing that you won't be able to do it all and just making a priority list - what is most important for you, and we can then give more reasonable recommendations that will satisfy major points on your list.

So do I Pixels are not little squares. Well, they are, but they are not. I know that this is confusing. Pixels are square on sensor, almost, but once image is read - pixels are no longer squares. They are values with coordinates - just dimensionless points. Once you have image, even if it is very under sampled - stars will be dots. It then depends on how you render those pixels if they look square or not. Here is very under sampled image: I would not say any of the stars is square? Here is 600% enlarged part of above image: Now, this is enlarged using nearest neighbor algorithm. Same image segment enlarged using different algorithm - no more "square" stars.

Only problem with that is - there are no square stars.

Thank you for sharing these images. Nice data, but I feel it should be saved as 32bit float after stacking. Histogram clearly shows signs of posterization. This is M42 data. I'll give it a go and see what I can produce.

Here is a simple recipe that will help you tease out even more detail: - bin your data 2x2 in software. Or even 3x3 - it will still be same size when fit to screen is done (FireFox says it shows it as 29% scaled when opened in new tab).

No idea what those numbers mean - but here is rather simple way to see what image has higher SNR. Take two regions in both images (try to do same regions at least roughly), first being empty background without stars - just a patch of sky without anything in it and second being part of target that is more or less uniform. Try to select same regions for both images (this might not be straight forward because drizzled image will be larger, so you can't simply align them and use same selection). Measure stats in both regions - in empty region measure standard deviation. In target region measure average value. Make sure both images have been stacked the same - same reference frame, same normalization of subs - everything apart from drizzle / no drizzle. Alternatively, you can wipe the background in both images. If you treated both images the same - target patch should produce roughly the same average value. Compare standard deviation between empty patches - it represents noise floor - higher the number, noisier the image.

Well spotted - I did not realize there was EQMod involved. According to EQmod prerequisites page, it will work, but one needs to do a bit of tweaking: http://eq-mod.sourceforge.net/prerequisites.html

Can't you use wifi with ASCOM driver? I controlled my AzGti via wifi from laptop with ease. You can't control camera and other bits that way, but for mount - you don't need any special hardware.

Issue with speed and narrowband filters is that speed of telescope is effectively negated by filters. Focal length remains the same, but aperture decreases. It is the filter that decrease aperture of telescope. Maybe a graph/image will explain this best: Here we have one dielectric layer on the filter - made specifically for a certain wavelength of light. If light comes at an angle, then wavelength with respect to filter layers becomes smaller and interference won't work as expected. We get the same thing as if light had different wavelength. Narrower the filter band pass - less angle it will tolerate. Solar Ha filters being extremely narrow band - like 0.5-0.7A and even smaller (that is 1/20 of single nm - or about x100 narrower then regular Ha for night time imaging) need almost collimated beam of light or something like F/30. With fast lens some of the light is coming at shallow angle and some at steep. Filter will effectively block out all the light where angle is larger then some angle determined by filter band pass. Another image might help here: If filter is very narrow and optics are fast - resulting stopping down of aperture can have more drastic impact on result then using wider band filter.

Crossed my mind to mention that as well. Mesu200 uses encoders, right?. As far as I know - any mount using servo motors need to have shaft encoders in order for servos to maintain precision spinning rate. These mounts don't advertise use of encoders - because these are not absolute encoders. Neither are those of CEM120, but from that mount advertising - one might get the idea that they are absolute encoders.

I'm finding this discussion valuable myself as I'm also contemplating obsy class mount for my future obsy (oh will this Corona related state of emergency ever end so I can continue with construction?). Two contenders are, believe it or not - CEM120 and Mesu200. CEM120 for precisely reasons that @RayD mentioned - a lot of gizmos that are useful - USB3.0 thru the mount cabling with all other cables, decent capacity for my current needs and of course - half the price of Mesu200. It also features some mechanical improvements over regular worm gear mounts - like spring loaded worm gear and such. I'm not considering EC version at all. I'm a bit worried now by some of comments. Is CEM120EC that much better guided than regular version? Because if it is - then regular version must be rather poor.

Although I see your point, I have to say that using encoders is not removing error before it develops. It is the same thing as guiding does. Maybe on a finer scale, but then, question arises - how rough is your mount when you need to correct error at time scales less than 1-2s? You have both mounts - Mesu200 and CEM120EC. I know that these two mounts don't cost the same, but CEM120EC2 is comparable in price to Mesu200. Do I need to ask which one do you prefer?

If you are already at 1.5"/px why do you want to drizzle? It is very likely that you are already at proper sampling rate. Measure FWHM of stars in your subs (or after integration while still linear). If they are 2.4" FWHM or larger - you are good where you are and you should not go with higher resolution. Drizzle works under very controlled circumstances. You need to dither and dither very precisely. If you intend to drizzle x2 - you should dither so that your offsets are always aligned at half a pixel for best results. Similarly if you plan to drizzle x3 - you should align your dithers every 1/3 of a pixel (this means dither of 2.666 is fine and so is 3 and 4.333 but you should not dither for 3.5px for example). Another important thing - you need to be under sampled for drizzle to make sense. With amateur setups - you simply can't control your guiding and dithers precisely enough to make sense. Drizzle will lower your SNR. Because it will use only 1/4 of samples for each pixel of output image (in case of x2 drizzle, 1/9 in case of x3 drizzle, etc ...) it is like stacking only quarter of your subs, so instead of imaging for let's say 8h - you'll get only 2h worth of imaging. I think that people are just better off resampling their subs to larger size prior to stacking then using drizzle in amateur conditions.

I was just pointing out that most people who don't guide suffer from periodic error and if they want to continue imaging without guiding - they are better of having mount with encoder/encoders. It is a trade-off, you don't spend money on laptop, guide camera, guide scope/oag and all those bits and you spend your money on encoder. You then don't worry about parameters for guiding and just image. This approach has its limitations - like you say, you can't do 20 minute subs in high resolution or whatever - but for some people - it is the way they like doing things. Otherwise - just guide. I'm fan of guiding and I would not image unguided. I'm still not sure what about my recommendation was misplaced. I simply recommended EC version based on whether one expect to guide or not. Maybe this is interesting point: Could you be more specific about how encoders help guiding immensely? For example, would 120 mount be guided 0.7" RMS without encoders and 0.3" RMS with encoders or something like that? From specs (which might be wrong, but let's not complicate discussion with that as well) - PE is less or equal to +/-3.5". That is 7" P2P and 240s worm period. Let's assume that all of 7" was traveled in only 1/3 of worm period, then RA drift rate is 7/80 = 0.0875. Let's assume that you guide with 4s cycle, that is 0.35 error. It follows that you can't really have more than 0.35" RMS in RA. How much can encoders improve on that?

These are screen shots from iOptron website. This would imply that one should have less than 0.15" RMS periodic error over 240s. According to this: http://celestialwonders.com/tools/driftRateCalc.html 2 arc minutes of Polar alignment error in worse case scenario (largest drift) - gives about 0.5"/minute of drift. Most people that don't guide, image at shorter focal lengths and resolutions around 2-3"/px. That gives you 4 minutes of drift before Polar alignment error moves you single pixel. According to specs, you should be able to do easily a few minutes unguided, and even 5minutes and more if you do your polar alignment right (not sure what sort of precision can you get with SharpCap polar alignment routine for example)?

Why would anyone pay a thousand extra for encoders that don't work?

I would not dismiss going to the dark site. I did some calculations between my current site and future one - first is 18.5 and second is 20.8. That is about 2 mags of difference. For my imaging gear an intended targets that means x6 in imaging time! I usually image for 4h a night. That would mean that one night in dark site would be equal to almost a week of good weather and imaging every night. Bortle 6 -> 4 is similar if not the same difference in sky quality, and don't be fooled by the fact that you'll spend some time getting there and that you might spend less time on target. Results will be worth it.

Until you decide on shorter focal length scope - why don't you try another approach with your F/4 newtonian? See what sort of FOV you'll be getting on particular nebula and decide on how many panels you need to do. Start simple to get used to doing mosaics. Let's say that you want to do 2x2 mosaic. First make sure you have software support for making panels (SGP has mosaic assistant as an option, EQMod also, not sure about others). Do 1/4 of allocated time on each panel. Processing will be a bit different - since you used only 1/4 of time (or rather 1/4 of number of exposures), you'll need to make up for lost SNR by binning your data 2x2. That will in the end create same size image that you are used to (in pixel count terms) and it will be as if it was shot by twice shorter focal length. In fact - it will be like using twice shorter focal length F/4 scope. Not something that can easily be found. Only issue is processing / stitching of panels. You need to calibrate them properly and remove background gradients for them to blend in seamlessly. There is software out there that can combine panels into mosaic for you (not sure, but I think APP can do it, probably PI as well, and if you don't have licenses for those - ImageJ has plugins that can stitch panels - but it is a bit more manual work to get panels to blend together).

You really always want a bigger scope - you just don't want to spend much on it, haul it around, store it, set it up and all that

In theory flat calibration will never work on camera that does not have set point cooling. Simple as that. You can get it good enough so that you don't notice it's not working well. But that is about all you can do. Closest to getting it to work would be this: - shoot bias - shoot darks (aim to actual ambient temperature lights were shot at) - make sure you don't have any light leaks when doing darks (or any subs with covered scope - flat darks and bias included) - shoot flats - make sure you are not clipping them - shoot flat darks Calibrate with dark optimization turned on. For this to work, your camera has to have stable bias. Let's hope it does. You need bias for dark optimization and you need dark optimization because you really want your darks to match your lights. Any temperature change will either add or subtract some signal from calibrated sub (either more dark current or less dark current). When you divide with flats - that creates either over or under correction - because you have some signal that did not come thru lens and is not influenced by flat field but you are trying to correct it with flats. For flats to work properly, only signal that is corrected needs to be light that came down telescope tube - so no light leaks, no residual dark current - nothing, just pure "vignetted" light signal.

I would not generalize it like that. This is closer to the truth: - you always want better seeing - you always want better tracking/guiding - you always want darker skies - you don't necessarily always want a bigger scope (but many people do )

Here is google translate of french review - it does not sound best either: