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You have uneven background due to vignetting even after calibration and that is a sign that calibration was not successful. Point of calibration of the data is to remove those imperfections. There could be several sources of this: 1. Improper calibration procedure performed 2. Proper calibration performed but with calibration files that are themselves flawed somehow. As far as first point - you'll hear very different opinions on how to perform calibration of DSLR data. Some will say - use darks, some will say don't use darks, some will say use bias and so on... I'll write about what is proper calibration and why it is hard to do with DSLR - and the way to do it with DSLR. Proper calibration is one that uses: - darks matching lights (in exposure, temperature and capture settings like ISO and so on). These are taken in total absence of light - flats. With flats you have to be careful that they are properly exposed - enough signal but no so much that you end up with saturation / clipping - flat darks. same as darks but matching flat exposure (same exposure length, iso and so on as flats) Problem with DSLR is that you can't guarantee matching darks because you don't have control over temperature of the sensor. There is no set point cooling, and even small change in temperature can have significant impact on dark current (dark current doubling temperature is about 6°C - so even degree or two will change dark current by 10-20%). Luckily there is way to compensate for this (and it is mostly successful) - and that is "dark optimization". It works only if you have good bias subs for your DSLR and if your bias behaves properly. With some cameras that is not the case. This technique relies on math and computer to determine multiplier for dark current - in reality to do a best guess on it so that dark current in dark subs matches dark current in light subs even if they are taken at different temperature. In order to do it in DSS - you need darks (which you have), bias (which you have), flats (which you have) and flat darks (which you don't have). If your flat exposures were reasonably fast (less than 1-2s) and were taken at same ISO setting as lights and darks - then you can use your bias subs as flat darks - although that is not proper calibration procedure (but it will have to do in this case, unless you can shoot another set of flat darks at same temperature as flats - again problem of set point). We can only hope that bias for your camera is proper bias. You need to check dark optimization check box in DSS stacking settings. Use simple Average stacking method for darks, bias and flats. In the end - I'll address briefly point 2 from the beginning - proper calibration but with improper calibration files. This means that you follow proper calibration procedure but something in your calibration files is wrong and that results in wrong calibration. You can have light leak while taking darks for example (even IR leak). Always cover viewfinder of your DSLR when taking bias and darks. There is small plastic cover for it that came with your camera. Make sure you match ISO settings and exposure length between lights / darks and flats / flat darks. Don't over or under expose your flats. Make sure you have good flat panel. If you have mechanical shutter - take longer flats exposure so that motion of mechanical shutter does not produce artifacts in flats (gradient from closing shutter).

Well, your calibration is not very good. I can deal with noise in various ways - but I can't fix poor calibration. When I stretch image hard - I get this: Here - look, almost no noise: but calibration issues remain.

Well, that one has not been explained well. Focal point sits somewhere behind the scope. When focuser is fully racked in - you have some distance from that point to actual focus point of the telescope - that is called back focus of telescope. In order to be able to reach focus with your camera and accessories - total optical length of all of the gizmos in the optical train need to be less than this distance - if it is larger - camera will sit beyond focus point of the telescope when focuser is fully racked in - and no racking out of focuser will bring it to focus point. Number that you mention - 55mm is related to flange focal distance of T mount (related to T-thread used with telescopes). Camera lens have something called flange focal distance - and is very similar thing to back focus of telescope. It is distance from lens thread/connection to where sensor should ideally sit so that lens has full range of focus from close to infinity. It is "operating distance" of lens. Telescopes adopted T/T2 connection (which is really M42x0.75 in metric thread specification) and with it - majority of simple correctors are designed with T/T2 flange focal distance in mind (for what ever reason as it plays absolutely no part in telescopes). This means that most simple coma correctors for Newtonian telescopes and field flatteners / reducers for refractors require to be positioned at 55mm away from sensor. This is their operating distance and yes, in most cases - you need to get them to exactly 55mm and change of +/-1mm will be noticeable. On the other hand - some of them are actually designed for slightly different distance and you often need to "dial in" correct distance with spacers - again, they will be sensitive to variation in distance. Third ones state that they are not overly sensitive to distance - and that means they will work ok in range of distances around their prescribed working distance. All of this is listed in specification of said accessory. For example at FLO page for Skywatcher CC, you'll see this: This means two things (which is the same thing when you think about it): a) coma corrector needs to be placed at 55mm distance from sensor b) telescope needs at least 55mm of back focus as coma corrector adds 55mm to optical path because of distance from its thread to sensor which is 55mm This brings us to last point - two are sometimes and somewhat related - telescope back focus and corrector working distance. They are related in two things: 1) Like shown above - you need to have enough back focus of the telescope to be able to properly focus with all your accessories (do include any filter wheels, off axis guiders, adapters - etc) 2) Sometimes, due to their optical properties - reducers / extenders change focus position of the telescope. Almost as a rule - reducers bring focus point inward (shorten back focus of the telescope) and extenders (like barlows) move focus point outward - make back focus of the telescope longer. Point of all of that is to be able to figure out if you'll be able to reach focus with your planned accessories, and if you'll need perhaps additional extenders if back focus is too long. Makes sense?

Hi and welcome to SGL. Your telescope should have T2 thread on rear opening. T ring will screw into that thread and will attach to camera as regular lens. This is enough to get your telescope connected to your camera. Some other scope designs do require barlow element because they can't reach focus regularly. This is usually the case with newtonian type telescopes. Magnification as a concept is not applicable to astrophotography. Magnification works by magnifying angles - when you look at eyepiece you get the image where object looks as if covering larger angular part of the sky. In daytime observing like with spotter scope - you can think of magnification as "bringing object" closer - but it will look "larger" when closer because it subtends larger angle to your eyesight. Astronomical objects are effectively at infinity (as far as optics is concerned) - and no moving back or forth will change their apparent size. With imaging it is about projection - how much pixels on sensor will be covered by certain angle on the sky. This is governed by focal length of telescope and pixel size of camera. You can't change that type of "magnification" easily. You either need to extend focal length (like using barlow lens) or reduce it (focal reducer) or change pixel size (change camera / bin pixels). In the end when you make image, "magnification" of object really depends on how far away you stand when looking at the image, so image itself does not have magnification. There are things called filter wheels, filter drawers or just simply filter attachments. These have T2 threads (or other common threads) on both sides and can hold filter, so attaching a filter would be like Telescope T2 - filter drawer with filter inside - T ring - camera https://www.firstlightoptics.com/zwo-accessories/zwo-2-filter-drawer-m42-m48.html or maybe this item: https://www.firstlightoptics.com/zwo-filters/zwo-t2-to-125-filter-holder.html Be sure to fully understand effect of filter on astrophotography before you use it (or get additional gear to use it) - it is not the same as for visual Best way to take photos with telescope is to do it remotely - either by remote shutter or intervalometer or by computer control of camera. Best way to take planetary photos is something called lucky imaging and DSLR type of camera is not well suited for that. Yes, that would be best way to proceed, but do first look at some tutorials on how to capture and process planetary imaging using lucky imaging technique. It is very different to what we consider "taking a picture". Involves quite a lot of software processing and it would be good that you get the idea of what it all means.

Hi and welcome to SGL. I'm quite happy with my ES62 5.5mm although fastest scope I used with is F/6. It is quite sharp in center and has some eye relief (not enough for eyeglasses though). From what I've read 9mm should be good as well, so that is something I would recommend. For 15mm you can go with quality plossl. If you don't mind narrow AFOV - then have a look at these - I've heard only good things about them - they are in your price range and should be very sharp: https://www.firstlightoptics.com/vixen-eyepieces/vixen-slv-eyepieces.html

All fair points. How about comparing it to something like iOptron CEM40 with RA encoders then? No need to guide, so it saves some weight and although mount head is not quite 3.3Kg, more like 7.2Kg is that weight difference really significant in portable setup (given all other components and their weight)? I'm approaching this from strictly performance / price ratio thing rather than anything else.

That makes perfect sense, guide exposures need to be <1s and yes, infrared is the way to go in order to beet the seeing, but even in that case, such mounts are really in 0.5-0.8" RMS territory and that is fine - but not £4000 fine (at least in my book)

Let's do a little math, so we have 60" P2P (a bit more than ZWO with 40" P2P) in 430s. That is 0-60 and then 60-0 in one cycle, so 60" is covered in roughly 215s. If we assume uniform motion - that is 0.25"/s give or take, but in reality - it will be more than that (on at least some parts of curve). No wonder these mounts can't be guided below 0.5"-0.8". If we take too short exposure - seeing gets in the way and if we use 2-3s guide exposures - mount will move 0.75" from where it should be! For mount to guide well it needs to have smooth varying slow PE curve. Above is simply too fast / too large P2P.

As far as RPI4 goes - I did see 3A requirement, but I also saw people powering it with 2A and having no issues.

Out of interest, what are the stats on RST-135 - P2P periodic error and guide performance? (given that it approaches in price to a serious mount).

I think that it is one of those things that happened to someone and is therefore perpetuated in "folklore", and if it is true that sky has brightness of about 1.5-3 magnitudes, then I can see it happening for mag0 star for example?

There is interesting account that I heard. Apparently, at the bottom of a very deep well - you can see stars during the daylight. Maybe very long and carefully crafted dew shield that will act as baffle tube would help with daylight observations to minimize stay light? I just found small text that puts estimate on daytime sky brightness at about 1.5 to 3 magnitudes per arc second squared. I guess that with careful dialing in of resolution - go with as high resolution as possible while keeping star disk as small as possible - it can be done.

Silly prices is what surprises me the most

How about some math to fix that? Say you have A and B that are some quantities (Ha and OIII for example) and you get X = 0.9*A + 0.1*B Y = 0.8*B + 0.2*A Y - 8*X = 0.8*B + 0.2*A - 7.2*A -0.8*B = -7*A A = (8*X - Y)/7 And similarly you can get B as well In another words - even if you have mixed Ha and OIII in your red and green (and blue) channels - you can extract pure Ha and OIII components with some math performed on your channels. If you don't have exact QE efficiency of your duo band filter and camera (or you doubt published graphs) - then you can establish those by shooting stars of known temperature (with known spectrum) and calculating what is QE based on your per channel results.

Yes. Sometimes people report much worse guiding graph when switching to OAG. Some of it is due to reporting of guiding results in pixels instead of arc seconds and some of it is because of guide resolution/precision. With increased precision they are now getting actual results and that surprises them.

Just did some research, and maybe someone will find this useful. Using power bank to power camera cooling in the field. Problem is that power banks are 5V and we need 12V, right? Well, appears that above power bank and many others that support QC 2.0 (quick charge protocol 2.0) can deliver 12V. We just need to figure out how much running time we can get from one of these fully charged. In above video we have two important bits of information - 20000mah (that is at internal cell voltage of 3.6V) which equates to 20ah at 3.6V which again equals to 72wh My camera - ASI1600 uses up to 2A at 12V - which is 24W of power at maximum load. In above video we saw 80% efficiency at 30W, so that would make it about 57wh at 24w or about 2.4h of operation. If we don't assume 100% cooling utilization all the time (in colder climate, or not trying to reach maximum deltaT) - we could get 4h out of one charge - but at max load - I'd say top limit is 2h By the way - in order to power it - one would need special cable https://m.ubitap.com/12V-USB-to-DC-5-5mm-x-2-1-mm-Quick-Charge-QC-2-0-3-0-Trigger-Cable-p249434148 That one triggers Quick charge protocol and outputs 12V. You can also get step up - but these probably cause some power loss, so get that qc trigger cable - or use trigger module and put your own cable onto it. https://www.amazon.com/5-Pack-JacobsParts-Adjustable-Voltage-Trigger/dp/B08NFK8BPP?th=1

Maybe something like this:

PHD2 will report what it can calculate, and actual figure can be lower or higher, but it should not concern you too much as it is on lower side of things - which is good. It will also depend on DEC of target - higher the DEC - less resolution there is in measurement (error is smaller in pixels). Ultimately, performance will reflect in stars in the image - if stars are tight and round, then you should not really worry too much about it all.

That is very close to resolution limit of guide system. I'm not overly confident in accuracy of reported RMS. Some estimates of guide system resolution are between 1/16th to 1/20th of pixel size. In your case, if we take 1/20th - we get 0.176" as accuracy limit (for 1/16th it is even higher). In my estimate, you need at least three times higher resolution of guide system than RMS figure you are trying to measure. With 0.176" resolution - I'd say you are relatively accurately measuring RMS down to 3 x 0.176 = 0.528. This is very close to figures you reported and I'm not sure that measured figures are very accurate.

What is your guide resolution?

I'd like to see stock EQ6-R or CEM40 that run better than 0.5" RMS

Actually, you'll have hard time getting stock either CEM40 or EQ6-R to perform below 0.5" RMS guided. You need to spend at least double that to get mount that will go below 0.5" RMS guided reliably. 1"/px mentioned is pretty much lower bound for any mount and any amateur imaging system. You simply can't get sharper images than that in long exposure astrophotograpy (and in fact - it really takes effort and money to even come close to 1"/px). Stating focal length is not very helpful - if you already have lower bound on sampling rate - then you don't really care if people will get 1"/px with 800mm FL or with 3000mm FL - and in fact, mount does not care about focal length of instrument. Only possible benefit of this mount is lack of backlash - and that should enable good / responsive guiding. Stating that lower bound is 0.5-0.8" RMS does not provide much of a confidence. I've seen 0.36" total RMS on my Heq5 (which is heavily modded / tuned). iOptron system with spring loaded worm gear should be similarly backlash free if implemented properly.

You should position pick off prism as close to main imaging sensor as you can. If you put it too far out - you'll get very aberrated stars

Only difference is in sensor size. That is it. It really depends if extra sensor size will be worth to you or not. That sensor is about as larger as my ASI185 and here is what guiding with OAG and 8" RC and that sensor looks like: Vignetting is already starting to show in corners of that sensor. If it was any larger - it would be waste of space really as far as guiding goes. It's up to you to weigh if extra sensor surface will be beneficial to you. For planetary, it might be only useful for lunar and solar - all other planets fit nicely in FOV of 224.

I would consider that for ONAG or maybe IR pass guiding. With 800nm pass filter - you'll get essentially mono sensor very sensitive in IR. IR part of spectrum is the least sensitive to seeing - which is good thing for guiding. Otherwise - it is not as sensitive as other cameras. It has very high QE at 800nm - but only 0.9 and 0.85 of that value. Even if it has close to 90% QE (very unlikely with color filters) - that would put regular QE below 80% in regular part of spectrum.