vlaiv

Maybe try something like this: https://richardhaw.com/2017/10/21/repair-infinity-focus-calibration-12/

Maybe add this as well for testing purposes: https://www.firstlightoptics.com/other-collimation-tools/astrozap-artificial-star.html That way reducer + scope combo can be tested even when it is cloudy?

The way you did it would be the way I'd do it. Only thing I would additionally do would be to not stretch the stars so much. That removes their outer halo / glow and keeps them smaller.

I understand what you are saying and this one is not so much about F/ratio myth or anything like that. It is just common sense - we use focal reducers to try to capture as much sky as we can given the sensor size. If sensor is already big enough to cover usable field of scope - then there is not much point in reducing that field - from perspective of capturing more sky - sensor is already capturing as much as can be captured. For smaller sensor it makes sense - focal reducer lets you capture more of the sky in "single go" - or shell we be precise - more of the sky having fully corrected image. Only when you start thinking about the speed and you put pixel size into the mix then we are entering the "realm of F/ratio myth" - and from that perspective, what you are saying makes perfect sense - capturing at F/10 vs F/7 will match your experience - provided that you don't do anything to change pixel size. However, if you change pixel size things start to change and F/10 can be faster than F/7 - but that was not what I wanted to talk about - I just wanted to point out that as far as capturing more sky - you don't have to do it with focal reducer if you already have the sensor large enough to capture the whole corrected field. Having smaller sensor and using focal reducer still makes perfect sense from this perspective.

Here is the thing - why use reducer in the first place if you end up cropping away stuff because of poor edge correction. This means that sensors already capture all possible information from the scope (whole corrected field). Maybe sampling is not tuned in, but you have software binning to take care of that.

I think that both scopes can be effectively reduced with such reducers as long as you understand "process of reduction". For example, RC 8" has corrected and usable field slightly less than APS-C sized chip. Already at APS-C size, you'll notice slight field curvature in the corners (depending on what exact sensor size we are talking about - is it 26.7mm or 28.4mm). Let's say that we have close to perfect correction up to 26mm. If you use x0.67 reducer - you are effectively squeezing down same field and you can't expect field much larger than 26mm to be corrected when squeezed down - this reducer has very mild field flattening effect, but it is not dedicated field flattener. 26 * 0.67 = 17.42mm This means that you'll have roughly corrected field up to 18-19mm diagonal. Even ASI1600 will start showing aberrations in corners with this reducer simply because you are using field that is curved and has aberrations in far corners (ASI1600 has about 22mm diagonal so you are effectively imaging 32.8mm unreduced field with this reducer and ASI1600). Many people on internet suggest to use this reducer with ~ x0.72 - x0.75 reduction factor. Reason is given above, 26mm x 0.75 = 19.5 and with slight field flattening that could almost stretch to 22mm of ASI1600 diagonal. If you have ASI183 or ASI533 then yes, you can use this reducer at even x0.67 and it will work ok on RC8" It should even work better on CC - because it is slower beam and there is less field curvature, but again - one needs to know how large fully corrected field is (not fully illuminated - we don't really care about illumination as much as pin point stars here) and shrink that accordingly.

If this is indeed what is happening, then I'm not sure it would be resolving power of 8" but rather that of 7.3". For resolving power, you need rays from each point of aperture to reach focus - if rays from edge of mirror don't reach focus point - it does not matter if they are "removed" by aperture mask or by secondary - result is the same - they simply do not reach secondary. I'm actually not sure if undersized secondary is making that effect of scope feeling less bright. This effect might be due to better baffling and less light scatter. Let's examine some quotes from that article: I'm going to assume couple of things here: - C8 has 97% reflective coatings and two air glass surfaces - 99.5% each, 64mm secondary obstruction, 8" / 203mm diameter - CC has 96% reflection on mirrors, CC has ~7.3" mirror (186.5mm) and 58mm secondary obstruction - 150mm refractor has decent AR coatings (which really means x4 air glass surfaces with 99.5% coatings). So image seems to be about equal between 150mm and CC and CC is FAR dimmer than C8. Let's compare the three: 150ED: 75 x 75 x 0.995^4 = ~ 5513.341 C8: (101.5 x 101.5 - 32 x 32) x 0.995^2 x 0.97^2 = ~8642.825 CC: (93.25 x 93.25 - 29 x 29) x 0.96^2 = ~7238.765 Ratio of brightness between 150ED and CC is x1.313 and ratio between CC and C8 is x1.194 If explanation was that mirror is stopped to about 7.3" and all other specs are fine, one would not be able to conclude that 150ED is about the same brightness as CC and that CC is FAR dimmer than C8. By the way, x1.194 in light grasp equates to a bit less than 0.2 magnitudes of difference. I'm not sure that I would be able to spot that sort of difference in light grasp and to judge that it is darker.

For visual there might already be suitable focal reducer. It's not very affordable, but it is not overly expensive either. If I had the scope, I would certainly try to see how it works. It is x0.67 reducer CCD47 which is copy of CCD67 - AP reducer. Only problem is that it requires around 85mm distance and that means it can't be used in front of 2" diagonal as 2" diagonals are around 100mm or more of optical path. Possibly T2 diagonal from Baader - it should have shorter light path, or 1.25" diagonal with some sort of M48 adaptation. I tried to use this focal reducer on my Evostar 102 F/10 achromat and 2" diagonal - but it was too much reduction and I could not reach focus as it requires too much inward focuser travel.

You know what would be nice addition to StellaLyra line of scopes? x0.5 and x0.33 focal reducers. I can see 6" scope becoming very interesting all rounder scope with above two focal reducers (if done affordably). Natively, scope will be excellent for planetary and lunar work, double stars and such With x0.5 reducer, one should get very about 25mm of usable field - that would make it comparable to 6" F/6 scope and provide very nice wide-ish field with 32mm plossl or other eyepiece that has 26-27mm field stop. With x0.33 reducer - it would be very interesting EEVA platform. It should provide around 12-13mm of usable field in this configuration and F/4 system with 6" of aperture - what is not to like with ASI533 / ASI183 type cameras?

@FLO You might want to revise your tech specs for these scopes - a bit of Copy/Paste I believe, both 6" and 8" CCs have same secondary size of 58mm? I think that 8" version should have more like 67mm obstruction size. There is also conflicting data on weight of 6" version - some report it to be 5.4kg, while you and Agena Astro put it at 6.3kg - almost 1kg of difference? Difference is even more significant for 8" version 9.1kg vs 7.5kg.

As far as I know it should be the last thing on the exit at eyepiece side - you should be able to see it.

Yes indeed. You need to both account for expected distance of of blocking filter from imaging plane and F/ratio of system. Slower the system - larger sun disk you can have as vignetting will be less. At for example F/15 and 50mm distance from blocking filter to sensor, you will have 21 - 50/15 = 17.666mm of field without vignetting. We have seen that 1800mm of focal length is going to give you 15.7mm sun disk, so you should be fine - with some room to spare for tracking and centering issues.

Avoid focal reducers. Full disk viewing is only possible up to 1800mm because of the size of blocking filter. It has only 21mm of clear aperture. Radius of solar disc is about 1/4 of a degree (1/2 for diameter). Tan(0.25) * 1800mm = 7.854mm This makes disk of 15.7mm in diameter at focal plane. Blocking filter is some distance from focal plane, so to avoid vignetting - Solar disk must be smaller than 21mm at focal plane. Want to use ASI290 to capture full disk? First calculate what is critical F/ratio for 2.9um pixel size. It is F/8.85 - this is value above which you should not sample - there simply is no point. Now let's calculate focal length at which sun fits on ASI290 sensor. It has height of about 3.2mm - so let's take 3.1mm to be solar diameter at focal plane. Radius will then be 1.55mm. You need something like 330mm to get full disk onto ASI290 - that is smaller than focal length of 80ED. Maybe consider using larger sensor instead? How about ASI178? It has 5mm of height so needed FL will be about 530mm. Use x0.85 FF/FR or get ASI174 or ultimately use focal reducer - but most available in 1.25" format are low quality. Btw, optimum sampling rate for ASI178 is F/7.32. In order to get that sampling rate with small pixels and telecentric lens - you should consider using aperture mask and pixel binning.

According to this page: https://www.raspberrypi.org/documentation/hardware/raspberrypi/power/README.md Total power output over USB is about 1.2A, which means that RPI itself needs about 1.8A? Or probably a bit less since GPIO uses 50mA, HDMI 50mA, Camera module up to 250mA,... There is also gigabit ethernet, Wifi - these all draw power, and does this depend on amount of RAM used? 8GB model will surely require more power than 2GB model?

I have one of these: https://www.tp-link.com/en/home-networking/computer-accessory/uh700/ Wonder if I could use one of 1.5A ports to power RPI4?

That sounds like power issue. RPI4 requires quite a bit of juice to power those USB3 ports - I think about 5A or so?

From what I gathered online - xserver won't start if it can't detect type of display device being used. There are several work arounds - from dummy display device driver that will report whatever to xserver, manually configuring display device to using hardware dongle that will report dummy device to system. This means that ubuntu starts normally and one can ssh into it, but can't connect to VNC. XRDP might be working though as it is a bit different to VNC. VNC expect xserver session to be present - VNC can connect to existing session, while RDP connection opens new session (each new RDP connection is new session while multiple connections via VNC can all observe same session). Not sure what is better for astronomy usage though.

This is probably first time I've seen 8" scope being used as finder. I do worry that by the time I'd get up the ladder - target would move out of the FOV and I'd need to get back down to again align the scope

If you look at other images of planets - you will often see much larger planet but not additional detail. You can get the same by simply enlarging this image - but why do that? I prefer images that are smaller in scale but sharp - like your image. Also, do bare in mind that Mars is rather small - it has less than 23" of angular diameter at the moment - that is a half of Jupiter in favorable position or a bit lager than Saturn without rings (only the planet itself). This telescope will resolve up to about 0.29"/px, and this means that image needs to be only 23" / 0.29"/px = ~80px across to be able to show all there is to be shown. Your Mars has exactly this diameter - no wonder it is very sharp looking.

Yes, I would say that is pretty much it with 180mm of aperture, and I think you have some excellent detail captured there.

I don't think that is actual color of nebula - I think he is using night vision device attached to eyepiece and filming that and not camera in prime focus. Green color comes from night vision device.

Not sure if Sharpcap can work with DSLR cameras. Maybe best approach would be to search for Nikon ASCOM driver. I know there is Canon ASCOM driver - there could be one for Nikon as well. With that driver, you can use any ASCOM capture application. Oh, well, this seems to be sorted (a bit of fiddling to install it required): https://github.com/vtorkalo/ASCOM.DSLR

If you can, try to use whole sensor. It does not matter if there is vignetting if you can take sky flats (or have 20" flat panel ). Live stacking can be flat corrected as well. Remember, Live stacking is about observing / detecting in short time (in digital domain) and not about getting nice image - so don't be afraid to really bin your data in software. I think that most people will enjoy final image that is about 1000px - 1500px in size. With 7360px in width, you can easily bin by x6 for massive SNR improvement on single sub.

I would actually recommend you start with that. Only drawback is it not being cooled, but for the time being and with short exposures, I think you can live with that. Additional problem is finding CC that will correct and illuminate full frame sensor, however, I don't think that you need sophisticated one. FOV will also be much nicer and you can bin quite a bit to get really good SNR in short exposure.

With such focal length - I would go with real estate rather than "performance" - however, real estate also costs and that is a problem with limited budget. If you can, depending on your EEVA expecations, go with mono version - it will be better but it will of course only provide monochromatic images (very much like visual on most targets). I don't think you want to mess with filters, so if you really want color - then of course OSC camera is better. ASI294 is probably better choice with respect to your focal length and even that camera will give you half a degree FOV. Not sure if there is focal reducer that will work on F/4 newtonian together with coma corrector and illuminate 4/3 sensor size. What you could try, but that will eat most of your budget, is something like this: https://www.teleskop-express.de/shop/product_info.php/info/p9779_TS-Optics-NEWTON-Coma-Corrector-0-73x-Reducer---2--Connection.html