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I would say L-eNhance unless you for any particular reason want to separate OIII and Hb lines, or rather eliminate Hb lines. Main difference between the two filters is that L-eNhance captures both OIII and Hb in green and blue channels of your DSLR, while L-eXtreme captures only OIII. You won't be able to separate OIII and Hb with L-eNhance, but it gathers more signal because of additional Hb (that is present in some nebulae). L-eXtreme passes less light pollution, but you are in 20 mag skies so difference will be minimal (even in heavier LP difference would be very small given band passes of both filters).

I would actually put 294 first of the three because it has the largest sensor area. Largest sensor means that you can pair it with larger scope of same F/ratio to get same FOV and consequently larger aperture - more light gathering for the same integration time. Red FOV is 533 paired with 130PDS. Green FOV is 294 paired with 150PDS. 33% more light gathering - same (or larger FOV - due to difference in sensor aspect ratio). Blue FOV is 294 paired with 200PDS. 136% more light gathering for about same (or a bit smaller in this case? - again aspect ratio) FOV.

You want something small, portable, cheap to put on a tripod and observe moon and planets with? How about this then: https://www.firstlightoptics.com/maksutov/skywatcher-skymax-90-ota.html

Just to add - while LP filter is not very usable for visual there are filters that are really good for visual applications. UHC probably being most versatile one - very useful for emission type nebulae.

In that case, how about this one: https://optcorp.com/products/celestron-nexstar-6se It is still 6" instrument so you won't be loosing any light grasp over 6" newtonian. It is much lighter and compact - cheaper GOTO Alt/Az mount should be able to carry it without too much trouble. It is F/10 instrument and hence provides same focal length as F/12 Orion Apex 5" Mak - same FOV. Only drawback is that it is limited to 1.25" eyepieces and won't provide you with wide field of view. Here is what 32mm Plossl gives you (about as wide as 1.25" go): Only drawback of these types of scopes that you should be aware of - is dew and cool down time. Both Mak and SCT are susceptible to dewing up and all scopes needs some time to reach thermal equilibrium with surroundings. Scopes with front corrector plate and mirrors take more time than other (open) designs.

It also depends on type of telescope and coatings applied. Mass produced newtonians in particular need to take this into account. For example, 6" F/5 newtonian (28% CO) with regular "enhanced" coatings will have light gathering close to: (75mm ^2 * pi - (0.28 * 75mm)^2 *pi) * 0.94 * 0.94 = ~14390.3mm2 which is equivalent to 135.36mm of clear aperture. (mind you, refractors also need to take into account losses, but these are fairly small, something like 0.5% per Air/Glass surface. You have 4 of these for ED doublet, so total losses are about 2%).

This is fairly recent thing. My first impression was that they changed mirror cell for weight savings / cost cutting. Most of the scopes that don't have collimation screws are small scopes mounted on very light weight mounts. Not many reports coming in on these, but what I did hear - people say that such scopes are ok and well collimated out of the box. Time will tell if missing collimation screws is as big issue as it seems now (at least to me). F/6-F/8 is good range of focal ratios for general usage. Field of view depends both on design of the telescope (internal baffles and focuser size) and focal length. Focal lengths of about 600-900mm are considered best for general viewing (both wide field and high power with suitable barlow lens). For planets you want higher magnification, but that is easily solved with barlow lens / telecentric amplifiers. Shortening focal length is problematic. Slower scopes have longer focal lengths - for example even 4" F/13 scope will have larger focal length than 8" F/6 scope and hence narrower field of view. For this reason Maks are usually considered planetary scopes, although they can be used as general purpose scopes with a bit smaller FOV. Slower light beam is a good thing - cheaper eyepieces will give you nice / sharp image with slow scopes while faster scope require expensive multi element (8 or more elements) eyepieces to be sharp to the edge of field of view. Here is nice tool that you might want to check out: https://astronomy.tools/calculators/field_of_view/ It will give you idea of field of view that you can expect with different telescope / eyepiece combinations. For example: Pleiades with F/5 6" scope and 25mm X-Cel eyepiece - very nice framing.

You take a set of flat darks (dark flats - or any other word order ) and you create master from those (stack them). You take your flats and you stack them and subtract master flat dark - this creates master flat. You create master dark like you normally would (no dark scaling so you need to match exposure of your lights exactly). Or in formulas: master_flat = stack_of_flats - stack_of_flat_darks (flat darks match flats in everything) calibrated_light = (light - master_dark) / master_flat (darks match lights in everything). You need flats, flat darks, darks to do proper calibration (with any camera, and CMOS in particular).

Under mounted scope means that one is using telescope on a smaller mount than would be suitable for that particular telescope. In the end it is subjective thing - there are no rules except weight limit of the mount (and even that is a guide line in most cases). It creates general bad experience of using scope - things get shaky and it takes long time for view to settle. Whenever you touch eyepiece or focuser - resulting vibrations take multiple seconds to settle. Sometimes it is very hard to properly focus in such conditions - you are never sure you have right focus due to motion blur of the shake. This mount can handle telescopes up to 13lbs. It would be a good idea if someone who actually used this mount would step in and give their comments on mount performance. As for telescope itself - both models probably have same optics. Mass produced mirrors from China (not meaning they are bad - they are quite good optically). It is optical tube (OTA) that differs. With this particular model, you get rather rudimentary 1.25" focuser. I would not be surprised that it is in part plastic: You want your focuser to be smooth, precise and rigid enough. Having 2" focuser extends range of eyepieces that you can use (they nowadays come in mostly two flavors 1.25" and 2"). 2" are very nice for wide field low power viewing. Another "drawback" of this basic model is that it does not have collimation screws for primary mirror. It comes precollimated from factory and that is it. Sometimes optics gets out of alignment and you need to adjust it - collimation. Regular newtonian scopes come with collimation screws to do that (rather simple procedure). People say it is not big deal and scope is well aligned, but I would not feel at ease owning the scope that I can't adjust if need be. I would not suggest this scope for visual - it is too fast. It has focal ratio of only F/4. With Newtonian telescopes, primary aberration is coma and it depends on speed of mirror. Fast scopes will have more coma and there are coma correctors for this purpose (mostly used for photographic applications but there are a few models used for visual). Fast scopes also require more expensive eyepieces in order to have well corrected image. If you can - look for F/6 scope for visual. F/5 is still ok but it is considered fast scope. If you can avoid it - don't use Equatorial type mount with Newtonian telescope. Due to focuser placement on newtonian telescopes and the way EQ mount tracks - eyepiece ends up in very awkward positions and this usually requires you to rotate telescope in rings. This gets rather tiring after a while (unlock tube rings, loosen them, rotate tube, lock tube rings ....) EQ mounts are better with telescope designs that have focuser at the end of the tube (refractors, folded telescope designs). Number of elements is important because it shows what type of design eyepiece could be and it is believed that fewer optical surfaces means more light throughput (and it was certainly so some time ago, but with modern optical coatings differences are minimal). For first upgrade / first set of eyepieces, look at these: BST Starguider range or Celestron X-Cel range.

Hi and welcome to SGL. If you are set on 6" Newtonian then that StarSeeker IV is configuration that I would recommended. That is Alt-Az goto mount + Newtonian OTA. I'm not sure that I would recommend that particular model. What is your budget like? With that particular model, my concerns are that it will be under mounted scope and scope that is of not too high mechanical quality - rather basic model (due to weight savings). If you like purchasing as a single package and you don't mind it being rather basic model, then yes, by all means - go for that one. Alternatively consider these (I'm going to link you to website I know stock these, but you'll have to do a bit of searching for USA retailers that sell them): https://www.firstlightoptics.com/reflectors/skywatcher-explorer-150p-ds-ota.html Mounted on something like this: https://www.firstlightoptics.com/skywatcher-mounts/skywatcher-az-eq5-gt-geq-alt-az-mount.html or mount like this: https://www.firstlightoptics.com/ioptron-mounts/ioptron-az-mount-pro.html Telescope model is basically the same as Orion one (same parent company for Skywatcher and Orion - same mirrors used) but has dual speed 2" focuser that is higher quality and in general optical tube will be of higher quality as Skywatcher version that I linked is meant for astrophotography as well. Two mounts that I linked are sturdier, higher capacity and overall better mounts - much more stable. Alternative that you might consider is something like this: https://www.firstlightoptics.com/dobsonians/skywatcher-skyliner-200p-flextube-goto.html Version that I linked to is 8" version, but I believe there is also 6" goto dob version of that scope (not 100% sure though). I prefer dob mount and only time I really want for driven mount is when observing planets / moon (I'm not double star observer but there driven mount is good thing to have too). I guess only difference between the two will be in observing position. Tripod mounted one is more suited for standing type of observing while dob is more suited for seated down type of observing. Yes, that SQM reading is not ideal, but it's not bad either. If you are simply near/far sighted - that really does not matter for telescope observing as focusing will deal with that. If you have astigmatism or other issues then you'll need to wear glasses when observing and you'll need to pay attention to choice of eyepieces. Choose those with longer eye relief so you can comfortably observe with glasses on. Not really. I'm in SQM 18.5 and I happily star hop. You'll see plenty of stars with your telescope. To me, goto or rather tracking mount makes most sense for planets (as mentioned). Some people enjoy finding stuff and navigating night sky and star hopping is part of experience for them. Others prefer for scope to find object for them and concentrate on observation. Depends what your personal preferences are. Everything will be a fuzzy blob . Even in large scopes in darker skies - things are still fuzzy blobs - at first, but as time goes on and you gain observing experience - things get sort of brighter and description fuzzy blob shifts to obscure NGC galaxies and such Planets - check (again don't expect too much), globulars - very nice rendition, galaxies too (after a while when you gather some experience), open clusters - check. There will be plenty to see and it will all be either very exciting or very underwhelming - depends on your expectations. Don't expect Hubble type images or anything close to that and you'll be fine. Btw - if you can mange it in terms of weight and budget - ideal scope for you to start with would be something like 8" dobsonian telescope (with goto if you choose so - but there are other means of tracking for dobs - like EQ platform - those don't find you stuff, but let you track objects - again good for planets / moon, or anything that you observe in high power). Not a brand, nothing meaningful except that they are probably rather poor quality (3 element - 60 degrees AFOV - yeah, not going to be good). There are different eyepiece designs and each of them has certain properties that you might like or dislike. That is whole separate topic. What you should know is that stock EPs are usually lowest quality and only good to get you going. It is probably the first thing being upgraded - plan to leave some budget for that. Yes, bunch of things, but this is why we are here for If you have any more questions just fire away. Ok, this puts things into perspective, and much of my recommendations above don't make sense, note to self - read the whole post before starting to answer . Hopefully at least some of what I've written will be helpful to you.

No need to use coma corrector when doing planetary type imaging. Each newtonian scope has "coma free" zone at the center of FOV. There is formula to calculate this zone and in most cases when you use small sensor and a barlow, you'll be operating within this zone where coma is smaller than airy disk and therefore not really an issue. Taken from this page: https://www.telescope-optics.net/newtonian_off_axis_aberrations.htm formula for diffraction limited field (strehl>=0.8) r = F^3 / 90 This means that for F/4.7 without barlow this gives radius of 1.1536mm, or diameter of 2.3mm. With x2 barlow / tele extender, this is diameter of 4.6mm ASI183 has diagonal of about 16mm - so only 1/4 of it will be usable, but that is still plenty - just use ROI and go for something like 1200x800 central ROI (that should be 1/4). Only issue is if you are trying to do lunar with larger FOV, then coma correcting barlow might be better option: https://www.apm-telescopes.de/en/optical-accessories/barlow-lenses/1.25-barlow-lenses/apm-comacorrecting-1-1-4-ed-barlow-2.7-x.html or https://www.apm-telescopes.de/en/optical-accessories/barlow-lenses/2-barlow-lenses/apm-2-inch-coma-correcting-telecentric-1.5x-barlow

No harm, but not much point either. As bias is contained in both in flats and flat darks - you can either remove it from both or let it cancel out (since flat darks are subtracted from flats) - result will be the same.

Simple way to check if bias and darks are in your case same thing would be to stack bias to one stack (regular average will do) and stack flat darks to other stack. Subtract the two and analyze result. If result is pure Gaussian type noise with mean value of 0 - then yes, these two are essentially the same. In reality however, they are not. Difference is in dark current. How much dark current there will be depends on your sensor and flat exposure. Some people use short flat exposures (bright flat panel, fast optics) and dark current is minimal in that case, but sometimes people use longer flat exposures - slower optic, sky flats or not very bright flat panel, maybe using paper to further diffuse it or mechanical shutter - that creates motion shadow in fast flat exposure. In any case - longer flat exposures or hot sensor susceptible to dark current will have substantial dark current even in flat exposure and it will show. If you get any sort of pattern in above result - that usually means Bias that is not stable. If you get non zero mean value - that means dark current build up even on short flat exposures. In either case - flat darks not equal bias.

I'll need clarification on this. Flat darks (or dark flats) are in principle always needed - regardless of CMOS/CCD sensor type technology. Flats work when they contain only light signal. When we take regular sub, it contains numerous signal sources: 1. Bias signal, 2. Dark signal, 3. Light signal. Same happens with flats - except light signal being uniformly lit flat panel. In order to have only light signal remaining in our master flat file we need to remove both bias signal and dark signal (however small it is for short flat exposure - if we want to be completely correct). We can do that in couple of ways (right and "wrong" ones): 1. Subtract Flat darks that contain both bias and dark signal - this is what we are saying here 2. Remove bias with master bias and then remove remaining dark signal with some means - like dark scaling or similar 3. Remove bias signal only - wrong in principle but in practice it will work 99% of the time since flat exposures are really short and cooled cameras have low dark current, so overall dark signal in such short time is minimal. CMOS sernsor have issues with bias and it is best not to use bias if one can with CMOS sensors - for this reason, I always recommend number 1. option - for both CMOS and CCD - simply because it works properly. No. Yes. Well it depends. In principle - same thing. If you can remove bias than you are all OK - you can scale dark exposure as dark current depends on time (and temperature but we assume temperature is fixed in this discussion). Problem is that one should not expect bias to be well behaved with CMOS sensors. I've found at least 2 or 3 different scenarios where bias is not what one would expect with CMOS sensors. 1. Internal bias calibration This is present on smaller sensors - usually planetary type cameras / earlier models. Each power cycle, sensor would perform internal calibration and some sort of automatic offset. This means that one can't expect bias to be on the same level between power cycles as offset can be (and often is) different. Solution was to shoot everything in same session without shutting down camera. 2. Multimode operation For some reason offset in very short exposures was different than offset in long exposures. I think this was driver issue rather than sensor issue, but could be both - driver just trying to deal with sensor firmware operation. It means that bias for very short exposure (like under one second - explained by ASI as internal camera clock being used to time exposure as opposed to "bulb" mode where application is timing exposure for subs longer than 1s) was different than bias for long exposure and bias subs are inherently shortest exposure possible. I measured mean value of bias sub to be larger than dark sub on several occasions (and that can be true if bias is correct because dark sub = bias signal + dark current signal).

Yes, you should dither. No, you are not probably not undersampled. What you perceive as little squares are likely to be in fact astigmatic stars in the corners. When dithering you don't need to stack differently, but you can - dithering enables some algorithms to work properly - like sigma clip rejection.

Diameter of Airy disk for 180mm scope is 1.43". Optimum sampling rate would therefore be: ~0.3"/px. With ASI178 you will be at 0.18"/px. That is over sampling by almost double. This is perfect if ASI178 is color model, and if it is mono - I would recommend that you bin x2.

Calibrate at around 0 DEC.

I like the one where scale is glued on the bottom part and top part has part of it cutout and hand is made from simple piece of wire. Let me see if I can find a picture of it. Something like this: This one does not appear to have moving hand - but there is plenty of room to accommodate one. That should help with initial setup - base needs to be pointed in rough direction of north and then hand can be adjusted on a star or similar.

Precision of setting circles was always a questionable thing. Suppose that you can accurately read off half a degree of that scale. This is probably the most precision you can get as diameter of the base is probably around 50cm. This means that circumference is about 157cm and single degree is about 4mm. With 32mm plossl you'll get x37.5 magnification or about 1.33 degrees TFOV. In principle - you should be able to put a target somewhere in FOV of your finder eyepiece. Similarly, regular 7x50 finder has something like 5 degrees TFOV and if you put Polaris in the center - you'll be in principle within one degree from NCP. That should still provide enough precision to land a target in FOV of finder eyepiece. BTW - you can make a slight tweak to above design that will enable you to roughly maneuver mount in position and then adjust settings circle arm to "zero it in". Just attach it in such way that it can move/rotate slightly to provide you with couple degrees of adjustment (remember ~4mm is one degree so you really need only a few centimeters of motion).

First azimuth: Scale is on top circle but hand is fixed to bottom one. Scale rotates and hand is fixed: You setup your scope so that it is pointing north and hand is pointing to 0 degrees on scale. Same is for altitude. You will need some sort of application on your phone / tablet or something similar to convert RA/DEC coordinates of the object to Alt/Az coordinates for your location at a given time (calculation is not trivial - so either pre print some tables of times for each object or use electronic aid).

No, not needed at all. It is doing small amount of "damage" to your imaging. Not much. Such filters usually have something like >95% transmission, so you are loosing about 5% of light or similar (some have 98% transmission while others 90(ish)% - depends on actual filter). Any optical surface will introduce some aberrations and it depends on quality of surfaces just how much it will be. Some filters cause star bloat or not perfectly round stars - but most are ok and have minimal impact. Depends - if you are happy to accept small light loss and don't have any significant image degradation (and I'm guessing not since you use it with LRGB anyway), then this can outweigh hassle of removing filter from imaging train every time you switch between LRGB and NB. If it's not too much work to remove it - then why not as it is not needed.

AstroImageJ will do FWHM - not really small but free for sure.

That approach is viable with CCD sensors and it is good idea to shoot only the longest darks if you use multiple exposures and scale them for shorter ones as longest darks will have the least overall thermal noise (although it seems that the longest dark will gather the most thermal current and hence associated noise - it will be scaled and noise will be scaled as well - signal scales with time but noise with square root of that). With cmos sensors it usually does not work as there are issues with bias files. I've tried 5 different CMOS cameras so far and not one had usable bias subs (for dark scaling). Another place bias can be used is if you don't have set point cooling on your camera. In that case, it is good idea to try dark scaling (algorithm that tries to scale master dark to match that of sub it is calibrating) - again bias needs to be removed prior to dark scaling as bias signal does not depend on temperature. Interestingly enough, DSLR CMOS sensors don't seem to be suffering from bias issues like dedicated astro CMOS cameras (or I could be wrong there).

I would like to further discuss this and similar approaches to star removal if you are up for it. Main issue that I see with star removal is the fact that: 1. Stars often have significant signal, hence shot noise is much stronger than background nebulosity signal. Subtracting star profile from image will not leave background signal but rather noisy patch in place where star used to be 2. Some stars are clipping so we don't really have good means to produce star profile in that spot - even clipped one (not all stars in all subs will be equally clipping and stacking algorithm needs to be aware of clipping in order to produce good data). So far I have identified some important points in star removal process. - Understand per pixel SNR. This is actually much easier than most people believe. Only problem is light pollution - as it is unwanted signal that needs to be remove before we can produce per pixel SNR. There are a few more "gotchas" that need to be addressed here, but basic idea is as follows: We stack a number of subs, and we produce mean value as signal output. We can use stddev stack for noise component. Simple approach would be to take signal, subtract light pollution to represent our signal strength and then use stddev stack divided with square root of number of stacked subs to get noise part. Of course, if we don't use simple average, we need to modify noise part accordingly (like weighted standard deviation, removal of pixels due to sigma clip - we don't have same number of samples for each pixels and so on - these would be "gotchas" mentioned previously). - In stacking method, we would produce clip map of our stack as well. This helps create accurate star profiles - in case of clipping we would know that and use only non clipped pixels in final stack, or do HDR composing or whatever and when creating star profile we would automatically set clipped pixels to NaN. In the end, when we remove stars - by subtracting profile for each star - we examine remaining signal value and if it is below certain threshold - like remaining signal / original noise < 3 we place NaN value in that place. We similarly place NaN values in all clipping pixels. What remains is to "fill in the blanks", or reconstruct NaN values somehow. There are several algorithms to do that, but I'm wondering if this algorithm that uses connectivity can be used/modified and what sort of results it would produce?

Pay no attention - just a nerd checking in ....