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This reminds me of a thing my friend said once: "I don't have many superpowers, but one I have is - being able to sift thru ‘rubbish’ on the net quickly"

Everyone knows that short tube scopes are harder to mount

This can't be true - here is another list https://www.toptenreviews.com/best-telescopes-for-beginners again, same author, similar scopes (not the same - they seem to shuffle them between reviews), same publish date

Don't be too harsh, maybe this article by same author published at same date is better? https://www.space.com/15693-telescopes-beginners-telescope-reviews-buying-guide.html (notice same scopes and again amazon links)

I don't think it's supposed to be any good - it's main purpose is to make cash - direct people to purchase some sort of scope to earn commission.

If by AA+ you mean ASIAir Plus - then look at manual section 4.6 - Plate Solving:

Plate solving. It gives you current camera orientation so you can rotate it until you hit wanted angle.

Hi and welcome to SGL. That really depends where you observe from, what are conditions and your observing skills. Have a look at this video to get the idea (note telescope size and light pollution zone indicated - you can find your approximate zone at lightpollutionmap.info website):

Well, I currently have Heq5 that I'm happy with (within it's limits of course). It is heavily modded - I tuned and did belt mod, changed saddle plate and put it on Berlebach Planet tripod. When all adds up - it is probably twice the price of stock model and already in iOptron 40/45 territory. In such condition it just carries 80mm without any issues. In fact - it can achieve rather good results with 8" 1600mm FL scope. My next mount is going to be a bit more serious than that. At the moment I'm eyeballing E.fric mount. If I were starting now, and I had the budget, I would consider iOptron 40/45 line. I would first do extensive research as I've seen some people complaining about QA, although most praise finish of the mounts. I've seen couple complaints about iOptron customer service as well (to be fair - I never ever thought about Synta/Skywatcher customer service and I replaced cracked bearing on my Heq5 myself). Step up would be iOptron Cem70. I was actually really keen on Cem60 but they are now discontinued and almost the same mount - Cem70 is sold with considerable price increase (I wonder why ).

Heq5 is more than capable of holding 80-100mm frac and image with it. This is up to 2 hours from very light polluted area taken with 80mm triplet on Heq5: Did not bother to process out satellite trails. Same image contains like dozen or so galaxies. Here are few more shots with different cameras (again 80mm / heq5):

80mm of aperture is not a limit - only invested time is the limit - given enough time, 80mm scope is capable of reproducing Hubble extreme deep field - not in resolution though - but in detection of galaxies - sure.

Depends on what you want to achieve and how much money do you want to spend on getting there. Used DSLR is really most cost effective way as far as camera goes. Are dedicated astronomy cameras better - yes, some models are, but they are also much more expensive. With about 700mm of focal length you can get to the limit of what you can achieve in DSO as far as resolution goes. With ~3.8µm pixel size (most modern CMOS sensors have pixels that large) and 700mm of focal length you are already approaching upper limit of 1"/px. 130PDS and 150PDS are both good newtonian scopes for imaging. APO triplet in range of 100-120mm of aperture is also excellent imaging scope. Difference is in ease of use - newtonians need collimation and sometimes have some quirks that need to be addressed - like rigidity of OTA / mirror moving / light leaks and so on, while refractors are mostly trouble free and of course the price - good triplet APO will be more expensive than newtonian scope. Mount will probably be most important thing in your imaging setup - so put the bulk of your budget there.

I have their 80mm F/6 Apo and it is great little scope - I use it for imaging. Do be careful - they have two different scopes - both called Photoline 80mm F/7. One is with FPL51 and other is FPL53 glass. Both are doublet scopes. https://www.teleskop-express.de/shop/product_info.php/info/p7169_TS-Optics-ED-APO-80-mm-f-7-Refractor-with-2-5--R-P-focuser.html (500e without VAT) https://www.teleskop-express.de/shop/product_info.php/info/p8637_TS-Optics-Doublet-SD-Apo-80-mm-f-7---FPL-53---Lanthanum-Objective.html (587e without VAT) FPL53 is supposed to have better color correction - but I'm not sure how much of it you'll actually see if you use scope for visual. Difference can be seen when imaging, and there neither scopes is truly color free, but FPL53 version does have less blue halo around stars (almost none). If you want very cheap 80mm ED doublet scope - then have a look at this: https://www.svbony.com/sv503-80ed-f7-doublet-telescope/ I think it is the same glass as that TS80 FPL51 one (might be the same scope branded differently).

To answer your query about darks - yes, you'll need different set of flats for each setup (if you mean darks - then no, as long as you keep all parameters the same as lights - gain, bias, temperature, exposure length ...).

I see that your love for black border has increased linearly with years passed

This gave me interesting idea. Quite a lot of people image with Samyang 135 F/2 and stop it down to F/2.8 - and for that they need "fast" Ha filters. At F/2.8 their aperture is actually - 48mm, so instead of stopping it internally (and adding diffraction spikes from blades) - they should be using 2" front mounted Ha filter instead - before light starts to converge - that will make their filter work properly even if it is not "fast" Ha filter.

Yes, but if you go with ZWO - you'll be ok since you are at F/4.5 at most and it works ok down to F/3.5 According to above graph - it is still 90% effective at F/3.5 (8 degrees) and you won't reach that.

Looking at it - that ZWO one that you first linked looks like very sensible one to get - it has similar response to L-eNhance and is reasonably priced. It is really UHC type of filter - with narrower Ha part - which is good for imaging but probably does not have much impact for visual.

Actually no. Only some of "rays" are at far ends of F/4 - central ray is at F/infinity. So it is really an integral over "cone" with each line in the converging cone having certain angle - being at particular F/ratio - and hence will have certain transmission. At F/4 - very few angles are really at F/4 - only "border" ones. So total transmission is integral of these partial transmissions over rays at angles. And each point on sensor will have different integral because angles will change as we move away from principal ray (center of sensor)

Also note that above graph is for optical axis (meaning 8° is about F/3.5) Edge of the field will depend on your TFOV. Say you image 2° wide field (in corners) - then you no longer have 8° and 8° on either side of normal - it is more like 6° and 10° because primary ray is tilted as well. Here is effect exaggerated - but it shows how different rays end up being at a different angles to normal

It will, that is why there are dedicated NB filters for fast optics - or one can use wider band like 35nm Ha filter for faster lens. Actual effect depends on type and number of interference layers. Interference layers have certain thickness needed for them to work properly (create reflected wave that interferes with forward wave and effectively cancels it). Wavefront coming in at an angle will see "thicker" dielectric layer - simply because of geometry.

Actually - scratch above - F/7 will actually have angle of 4° on each side, so that is ok. It is ~F/3.5 that will produce 8° angles.

Have you seen this: 8° is about F/7 ( arctan(1/7) = 8.13° ) Anything faster than that will put filter in situation where some of incident light is out of band and will be rejected.

I'm not really sure what is going on, but there are some issues with calibration files and possibly with lights? First flats. I'm surprised to see flats looking like this. This is montage of flat you posted separated by colors - I think this is BGGR order (blue top left, then green top right, then again green bottom left and red in the bottom right). Flat intensity has been normalized to 100% for each channel here. Red and Green look very similar but blue looks much "flatter". Red and green fall of to 75% in the corners while blue falls off only down to 85%. This indicates that system transmission and QE depending on angle varies with wavelength as well. This also means that flat correction will never work 100% - as you can't tell what sort of wavelengths you'll be getting. If you for example shoot Ha which is 656nm and falls into red part of spectrum - you won't be sure if your flat produce with your flat panel will have same fall off as flat panel is not producing exactly that frequency. I don't know if this is due to lens and coatings on the lens or is it down to sensor. Second issue is with dark / bias. Camera adjusts data with arbitrary offset so bias is useless and there is no chance of doing dark optimization. Here is measurement of bias and dark you posted. Bias should have lower mean value than dark simply because dark contains both bias signal and dark current signal that builds up during exposure. In this case bias has higher signal than dark, and both are interestingly close to 2048 - which means that camera firmware adjusts data so that mean level of dark regardless of exposure is 2048. We simply can't figure out true bias level nor dark current value because of this (not sure if there is a way to recover this artificial offset number). In the end - I don't think that gradients in the final image are due to dark after all nor due to flat calibration. I think it is some sort of weird signal present in the image itself. It could be consequence of light pollution and the fact that lens or camera has different sensitivity depending on angle / wavelength thing - similar to flats above, but I'm not 100% sure. This is very stretched bias: and it looks like bias - no surprises there. This is dark - and again - no surprises there, in fact - absence of any sort of gradient is a surprise there. It looks rather nice and flat. In the end - here is what each color components looks like after just flat fielding (both light and flat was subtracted with 2048): This is after LP gradient removal. This strange glow looks different in different color components. I guess that your best bet in processing this sort of image is to use advanced background removal tool to make background nice and flat as I don't really think it is due to calibration but something else - and I can't really tell what it is. Is there a lot of light sources near you when you image? Maybe some strange reflection on lens or something like that?

That actually should not be a problem as darks don't receive any light. F/ratio and focal length are not important for darks, but they are important for flats - you need to take flats with same F/ratio, focal length and focus position as you had when taking lights. That can be very useful in diagnosing the problem. I'll take a look at them as soon as I get some time to do it.