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I had ST102 and while good, it is a bit crude - no retractable dew shield, focuser is rudimentary and overall scope feel is quite agricultural. Recently I started looking for maybe another small and light weight wide field instrument and this one caught my eye (I probably not going to get it as I might go for ED glass but I do believe it is a bargain): https://www.astroshop.eu/telescopes/omegon-telescope-90-500-ota/p,43767 Lighter than 100mm scope and more light grasp than 80mm one. Same focal length as 4" F/5 but being F/5.5 and only 90mm - probably less CA.

I think that 150PL is going to be a bit much in focal length - it is 1200mm fL if I'm not mistaken. It is also rather large scope and will have big arm momentum. I used 8" F/6 scope on heq5 and it was no picnick. If you want to go that route - 6" and above meter of focal length - take look at this scope: https://www.firstlightoptics.com/stellalyra-telescopes/stellalyra-6-f9-m-crf-ritchey-chrtien-telescope-ota.html I have no idea what the WO GT-81 is like, but if it's anything like TS Photoline 80mm f/6 apo (often these scopes share same source only different branding and accessories) - it will be very good. I have later and I'm very pleased with it.

Well, I'm not sure what it is that I'm seeing here. I suspect that is maybe a bit off all of there cases that I listed above. First optics. It definitively has some residual color issues, here are examples: These are stars in the center of the image (color balanced to show issue / blue boosted): This is red, green and blue channel separated and montage made out of them. Red has larger stars, green looks good, and blue has some color fringing around stars - unfocused light. When color combined, here what stars look like: If you look carefully you'll be able to spot that red / bluish edge on some of the stars. Second - you need some sort of field flattener. This scope has rather curved field and it shows in corners. Again, next to astigmatism, there are color issues as well: Again, red, green and blue channels - all three showing astigmatism "cross". When color balanced and color combined, here is what it looks like: Again, rather colorful as R, G and B don't really match in bloat level. Stars are rather big even at this sampling rate - 3px FWHM at center and going even up to 7px-8px in the corners (thanks to field curvature). This data will benefit from being debayered with super pixel mode, or there is another way to do it - binned in software x2.

FWHM is full width at half maximum. Basically a measure of how "fat" the star is by specifying "waist" diameter It was noticed that regardless of the star intensity - most stars in the image share this value (or are very close in value). Stacking software often calculates this for you, or you can use software like AstroImageJ (java based open source image processing software based on ImageJ - which was made for scientific analysis - primarily for microscope images) to find this values. It will be expressed in pixel units, but if you know your sampling rate - you can convert to arc seconds. Very high resolution images have this value at about 1.5". Professional observatories on mountain peaks have this value often below 1". For the equipment that you are using, you should be happy with 3-4" FWHM stars (star FWHM depends on aperture size as well among other things). Super pixel debayering is just a type of debayering often used in stacking software. Since Bayer matrix on OSC sensors has every other pixel "colored" differently - there are pixels that capture red, green and blue light and are interleaved, regular debayering algorithms interpolate missing values. This makes image soft and does not correspond well to actual sampling rate of OSC cameras. Super pixel mode uses different approach - it takes 2x2 group of pixels that have different colors (one red, one blue and two green) and creates one single pixel with three colors out of them. It effectively reduces image size by 2 in both height and width and acts very similar to binning of mono sensors in that regard. Result is smaller (in terms of resolution / pixel count) image that is sharper when viewed at 100% zoom. RMS is short for "root mean square" and is just that - when you have bunch of values it calculates square root of mean of their squares. This is sort of average for errors and such - when you have both positive and negative values that represent some error and would cancel out in regular average - way to calculate their average without them canceling. It shows how good your guiding is as it represents average error of where your mount is pointing (as opposed to where it should point). If you use PHD2 for guiding it will be displayed to you in stats window: Values below 0.3-0.4" represent exceptional guiding results. values that are in 0.4-0.6" represent very good guiding. 0.6-0.8" are good guiding, 0.8"-1.0" average and above 1" is below average guiding. Mind you, even if you guide and have round stars - it does not mean your guiding is good. How tight your stars are - that is true measure of guide performance. Another important point - above scale is just rough guide and actual guide performance depends on working resolution. Good rule of thumb is that you want your guide RMS to be at or below half of imaging resolution. This means that if you image with lens at ~100mm and resolution of let's say 6-8"/px, then 1" RMS guiding will be excellent guiding with respect to impact on the image. Same RMS guiding value when imaging at 1"/px is going to be very poor result. Do you by any chance have image that is still linear, prior to any stretching? Any measurement or inspection of the data is best done when data is still linear. You can't measure FWHM on stretched image as stretching changes the "shape" of the stars.

I'm not sure that works well. I have one, I tried it once and I did not like what I saw. I still need to try it some more to be 100% sure of how it performs as the issues I saw might have been related to tilt. Main thing to realize is that 8" is corrected for up to 26-27mm diagonal. This means that you should be able to use APS-C sensor size without reduction, however, once you reduce the focal length - you effectively "increase" sensor size. Said 27mm diameter gets squeezed to x0.67 of its original size and that is around 18mm. Have sensor that has more than 18mm diagonal? It is likely that you'll start seeing stars that are not as nice on the edges. For this reason, you'll often hear that people use this reducer more at x0.72-x0.75 "setting" (you can change magnification of reducer by changing distance to sensor). This of course makes more sense as with x0.75 reduction - it gives about 20mm of corrected reduced field and that fits better with most sensors up to 4/3 size. This reducer will be useful if you have smaller sensor, otherwise maybe look into dedicated reducer/flattener or perhaps Ricarrdi x0.75 reducer/flattener - I've read some reports that it works very good with RC scopes.

That is very nice looking image. Yes, stars seem a bit bloated, and there are a few possible reasons for that. First would be use of 80ED scope. This is ED doublet scope so color correction will not be the best. Fact that you are using OSC camera is going to make things a bit worse (people with mono camera and RGB filters can refocus between filters which helps a bit). In order to see how bad things really are due to this point - just look at R, G and B channels separately. If possible - measure star FWHM for each channel on same stars and see if you can spot the pattern. If blue channel is the worst, followed by red channel and then green channel (this is the order I would expect from an ED doublet) then telescope might be the culprit for bloated stars. Solution to this one is to either switch to triplet scope with good color correction or possibly use additional UV/IR cut filter that is a bit more restrictive - like Astronomik L3 filter for example. It cuts away most offending parts of the spectrum. Second reason can be related to guiding / tracking and working resolution. What is your guide performance like in arc seconds? You really need to be below 1" RMS to get tight stars. Fitting belt mod will certainly help things there. Third possible cause is light scatter due to high humidity. This creates "wings" around stars more than anything else. It looks more like some sort of glow around tight star cores than bloated star, but if you stretch your data aggressively (and we always tend to do that when processing image for faint detail) - your stars will end up looking large. You'll see if this is in effect by checking for actual FWHM in the image and remembering what sort of weather was on particular night. This high humidity happens in fall (at least in my climate) when weather is rather nice during the day - high pressure, sunny and no wind. Seeing on those nights is usually good but due to high pressure and no wind - there is going to be a bit of mist/moisture in the air. Things dew up rather easily in these conditions. I just looked up your scope and it is FPL-51 F/6.8 doublet scope. My money is on the first cause. There is a way to process image in special way as to minimize this effect. Works good with galaxies but not as good - or rather poorly with emission type nebulae. Use green channel as luminance layer. Green channel is often the sharpest of the three if there are issues with residual chromatic aberration. Green channel will have tightest stars. Another good thing is - with OSC cameras you have twice as many pixels capturing green then blue and red and green is the most sensitive region of camera response - it will have best SNR for broad band targets. You should also consider using super pixel mode debayering. I was not able to view your image at 1:1 because flickr offers bunch of resolutions but no indication of which one is native. In any case, super pixel debayer mode will make smaller image in pixel width/height but it will look sharper on 1:1 inspection. Another little thing - background looks too dark / clipped. Maybe try not to clip the histogram as much?

It seems that these are not quite the same mounts. Here is what FLO has on AzGte versus AzGti: Freedom find means that you can release clutches, turn the scope to point the way you want it to and Goto and alignment will be maintained. With AzGte you don't get that option. Not sure how big a deal that is going to be for you. I've had AzGti mount for almost a year now and I've never used this capability (although usage time was limited this year to hand full of sessions with this mount). Honestly, I don't know. I have not put that much weight on my mount - either in Az or EQ mode. Yes, this little mount can be converted to work in EQ as well if you need it to. I'm currently using it as wide field imaging setup, although I've done some planetary imaging with it and my Mak102. It handled it and ASI178mcc (cooled version, a bit heavier than regular one) without any issues what so ever. SkyWatcher does sell it as a bundle with the scope you have: https://www.firstlightoptics.com/maksutov/sky-watcher-skymax-127-az-gti.html At least we know it can handle your scope for visual without much problems (or at least SkyWatcher seems to think so). Only thing that sort of bothers me with this mount is - lack of hand controller It works with mobile app, and it works fine, but mobile phones don't have regular keypads and there is no tactile feedback of any sorts. You need to look at you phone screen when you want to for example make corrections to mount position and that means moving away from the eyepiece. I'm seriously thinking of getting myself one of those SkyWatcher ver 5 hand controllers to be used with both small mount and my Heq5. I purchased my Heq5 as SynTrek version (without goto) as it was meant as imaging platform connected to PC. I now use it like that exclusively but sometimes with to just use it for visual with heavier scopes and don't feel like taking out laptop and setting everything up. In any case, that bundle with AzGte seems like a good value - you get another scope that you can either sell or keep for quick grab'n'go sessions when you don't feel like waiting for mak to cool.

Processed completely in Gimp 2.10 Ha used as luminace - stretched separately and some light noise management and sharpening applied Created color as SHO color combination - pasted stretched Ha layer on top as luminance and used Channel combination to achieve wanted color feel of the image (no color is pure - all have been mixed in each channel to get this palette).

You can try with this: https://www.firstlightoptics.com/sky-watcher-mount-accessories/sky-watcher-synscan-v5-handset.html and appropriate cable (maybe cable from your handset will fit, but I'm not 100% certain of that).

I think that you will struggle to find any mount that is capable of tracking a target that costs less than your scope. Your scope new is something like £265. Eq3 mount with motor drive would be £176: https://www.firstlightoptics.com/skywatcher-mounts/skywatcher-eq3-2-deluxe.html + another £69 for tracking motor: https://www.firstlightoptics.com/sky-watcher-mount-accessories/single-axis-dc-motor-drive-for-eq3-2.html Well, that is about it - totaling at £245 - just £20 shy of price of the scope. Next option that will hold your scope (and has proper goto and all) is this: https://www.firstlightoptics.com/skywatcher-mounts/sky-watcher-az-gti-wifi-alt-az-mount-tripod.html Which is £215 without tripod or £285 with tripod. Maybe best option would be to reuse tripod from your AltAz goto mount and go for AzGTI mount head.

But that would be up to the "operator" right? Let's say we are after 1.6"/px or there about resolution. We have 1500mm of focal length and pixel size of about 5.71µm. Natively this gives ~0.79"/px, but since we have OSC sensor, simple super pixel debayering will give double that so ~1.57"/px. There you go, no need to do anything else. If on the other hand, one was aiming to match sampling rate of the ED80 - they would simply bin data further. One scope has 1500mm of FL and other has about 500mm FL (with x0.85 FF/FR that ends up 510mm or so?) - that is very nice 3:1 ratio so x3 bin will get almost perfect match between the two. In any case, I would just leave things as they are and go for Mak with Canon 1000D and super pixel debayering to get 1.6"/px, provided of course that mount and guiding support it.

Compared to what? To ED80 at same sampling rate? What happened to aperture at sampling rate? Even if we account for central obstruction and mirror losses, 127mm (which is 5" not 4" as I wrote above) is going to be much more clear aperture than 80mm.

This was very interesting read and I'll need to try something mentioned in that thread. I never tried to take few seconds of pause between exposures, but I did notice case where that might be useful thing to try. Sometimes there is rather strange thing happening with darks - some of them have higher mean value. Others have different standard deviation - as if more noise is there (these are not calibrated so it could as well be some signal/pattern producing higher standard deviation). Maybe this would not happen with few seconds pause between subs. Camera in question is ASI178,

Not only round, but curved as well!

I'd say go with Mak and Canon. First spend some time to understand what is max resolution that you'll be able to achieve with your setup - either Mak or ED80 and mount you have (guiding performance) and skies and then see how to best match that resolution with your camera. Next thing - figure out just how "close in" you'll be getting and learn to accept that . For example, I think that best resolution with 4" scope and good regular mount, that one can hope for, will be something like 1.6"/px. Let's say you want to image M77 that is about 11 arc minutes in diameter. That is 660 arc seconds in diameter or about 410px across. That is it - your whole galaxy will be something like 410px across if you properly sample it.

In a "sport" that aims to make every photon count, you want to crop your sensor and throw away captured photons? Sacrilege!

@Adam J I'm also interested in your view on that one.

https://www.teleskop-express.de/shop/product_info.php/info/p4789_10Micron-GM-1000-HPS-GoTo-Mount-with-Encoders---only-Mount-Head.html

With cooled cameras amp glow is really not an issue. Both cameras that I use have amp glow in one form or another. Here is for example very stretched dark frame from ASI178mcc (used for above test of Samyang lens - but not with native 2.4µm pixel size but rather in super pixel debayer mode): And I get very decent results with this camera - have look at what I recently took with above mentioned Samyang lens: You'll see that image is still a bit blurry / not as sharp as it could be - that is mostly because of the fact that I used lens at F/2 (wanted to do decent image in just one hour from strong light pollution - I probably should have gone for two hours and F/2.8 instead) and that even with 4.8um pixel size - image is over sampled.

First things first - don't worry about under sampling. Two reasons for that - first is that under sampling is not a bad thing in itself. It is just a working resolution and lower working resolution just means wider field of view (over sampling is a bad thing as you loose SNR and gain nothing in return). Second thing - you are worrying about under sampling with camera lens. You should not. Camera lens are not diffraction limited and star image that they are producing is much larger than aperture would suggest. To prove my point, here is measurement of Samyang 85mm F/1.4 lens that I did with artificial star: No filter at F/1.4 - Red: 2.66, Green: 2.48, Blue: 2.30 No filter at F/2.0 - Red: 3.82, Green: 2.28, Blue: 2.42 No filter at F/2.8 - Red: 2.53, Green: 2.36, Blue: 2.31 No filter at F/4.0 - Red: 2.24, Green: 2.27, Blue: 2.29 Values that you see are FWHM of different channels expressed in pixels and pixel size is 4.8µm. Best of these values are around 2.3px or if we convert that into microns - 11.04µm. FWHM of 11.04µm requires pixel size of 6.9µm to be properly sampled. In the case of this lens, and probably most of other lenses (don't think that other lens are much better than this one - this is pretty good/sharp lens) - it is neither seeing nor tracking that produces blur, it is lens itself. They are far, far from diffraction limited optics. Btw, this is what star looks like at F/1.4 And this is what it looks like at F/2.8 (and above sampling with pixel size of 4.8µm): While slightly over sampled (yes indeed even at F/2.8, 4.8µm pixel size is over sampling rather than under sampling), this star looks rather nice. Going further to F/4 will not make much of a difference: Bottom line - don't worry about under sampling, if you are happy with FOV for your intended targets at focal lengths that you work with - go for ASI294. It has best size/£ ratio and it is very decent performing camera.

Just a small update, could not get color to work for some reason - too much bloating in both blue and red, but here is mono version - which is just green channel. Stretched a bit more to show all that has been captured:

Opens normally in Gimp 2.10

USB 2.0 is more than enough for guiding. It is only when you want to use the same camera for planetary imaging that USB 3.0 has advantage. You can still do very decent planetary shots with USB 2.0 cameras, mind you. For guiding, mono is a bit better, but you can guide with OSC as well - there won't be any issues except for slightly smaller sensitivity (bayer matrix lowers overall sensitivity of camera and not each pixel is sensitive in whole spectrum). So far I always guided with OSC cameras and never had any issues because of that.

This technique requires light pollution , so if you have dark skies - bad luck, you'll need flat panel. Another requirement is to use darks and that you dither. You can probably get away without darks. I actually developed this technique few minutes ago when I was asked to help out with processing of rather poor data. About 8 x 10min subs were taken in moonlight with DSLR and lens. Due to wide field there is a lot of LP and gradients. Vignetting is about 50% in the corners. I did not have any other calibration files but I managed to remove vignetting. I did not manage to remove dust shadow as that is too fine feature. Here is what you should do to produce artificial flats: - load your subs and do sigma clip stacking with aggressive values without alignment of subs. I used 5 iteration rounds with sigma 2 for this case. - take resulting image and do feature removal - like background extraction or low pass filter or similar Idea is that we need to remove stars and any strong signal from the image. If subs are properly dithered then stars will be in different place in each sub. If we omit alignment of subs that gives us opportunity to use sigma clip stacking to actually remove the stars. Vignetting and other features that are due to flats will not move since they are sensor related and not sky related. Here are example results: Single green channel sub (debayered frame and then binned x2 to improve SNR) - this is wide field shot of Heart & Soul nebulae region. This is "master flat" derived from flats from R, G and B channels (this is OSC camera so vignetting will be the same - each flat was normalized to 0-1 and then average taken) Single frame after calibration ... Here is what sigma reject looks like on 8 subs:

Probably best course of action. Small bearings will have the most impact on tracking performance because they turn the fastest - one turn per 638s.