vlaiv

I've heard good things about this one: https://www.teleskop-express.de/shop/product_info.php/info/p3041_TS-Optics-PHOTOLINE-115-mm-f-7-Triplet-Apo---2-5--RAP-focuser.html Also, maybe consider use of Quark Combo instead of regular quark if you intend to do imaging alongside visual. With Quark Combo, you'll be able to do close up shots / viewing but also full disk viewing. Trick is to use both aperture masks and telecentric barlows. You can get x2 and x3 telecentrics from ES - they are supposed to be good, and simple aperture mask can help make scope be F/20 or F/30. You can get full disk viewing with up to 1800mm focal length. This means that you need something like 400-450mm FL scope with regular quark to get full disk viewing. Above scope is 800mm FL so you would not be able to get full disk with regular quark, but take x2 telecentric lens and make 80mm aperture mask. 1600mm FL and 80mm aperture - gives you F/20 system and full disk viewing with something like x120 magnification with ease. Want to get very close in? Put x3 telecentric and you are at F/21 with 115mm of aperture. x200 mag should be doable without too much trouble. BTW, put Riccardi FF/FR on above scope and you'll have 115mm aperture F/5.2 600mm FL wide field instrument for imaging.

Semi apo filter is nice addition. I have F/10 4" achromat and use Baader Contrast Booster as minus violet filter. It does work. Out of focus blue and violet are much easier to spot in images than with eye - which is good for visual but bad for astro photography. This is because camera sensor is more sensitive to blue part of spectrum than human eye is. Human eye is most sensitive in green part of the spectrum. As for accessories, I don't think that you should worry about it now. Part of the fun (for me at least) is discovering what you need and anticipation and the thrill when you get new piece of kit and wait for a chance to test it out. Here is something that you should consider, but like I said, it will depend on your interests / taste. I would consider 2" diagonal mirror if you don't already have one and wide field eyepiece. 6" refractor is very good scope for wide field viewing of Milky way and large open clusters. If you are serious about astrophotography - consider getting autoguiding kit at some point. That means guide scope (you can turn your finder scope into a guide scope or add separate guide scope instead of finder scope) and planetary camera. Planetary cameras can be a lot of fun. You can take better images of the Moon and the Planets with them and even some Deep Sky imaging - that is how I started into AP. Scope you have is not well suited for AP because it is achromatic design (it is a bit better corrected than regular achromat, but as you have seen - bright objects will have that blue / purple halo around them). Luckily there is something that you can do about it. - You can use Semi APO filter or even regular #8 Wratten yellow filter. Very good filter for that is also 495 long pass filter (from baader - it is deep yellow filter). Problem with yellow filters is that it will skew your color balance, but that can be corrected in processing phase - Another thing that you can add is aperture mask. That is something that you can't purchase, but you can make one, even out of cardboard. With nowadays 3d printing - it is very easy to print one to suit your needs. Aperture mask is just a mask with smaller aperture than original aperture of telescope. Chromatic blur of achromat telescope depends on clear aperture size. Reducing this size removes some of chromatic blur. Don't think that your scope won't be able to produce good images - it is just a bit more complicated and appropriate technique needs to be used. Here is example of what F/5 achromat (2 lens simple design with a lot of CA) can do with planetary type camera: Of course, to get better images - you will need DSLR camera and adapter (T2 ring) for that camera. Canon cameras are probably easiest to work with for astrophotograpy because of software support and being able to shoot raw images (without any in camera processing).

Hi and welcome to SGL. You should wait with field flattener / focal reducer for your scope until you start doing AP. You might not need it at all. Scope that you have is actually 4 lens design and that means it could have flat field as is. According to TS website - in fact, this is modified Petzval design and it has flat field: source: https://www.teleskop-express.de/shop/product_info.php/language/en/info/p7786_Bresser-4852760---152-mm-Refraktor--f-760-mm--OTA.html Bigger problem will be chromatic aberration, but you won't know until you start imaging with said scope - again, here 4 lens design is meant to reduce chromatic aberration usually associated with refractors.

This one is easy Although not quite coherent. If you follow the graph from foot to nautical mile, you will conclude that: nautical mile = 10 cable = 10 x (100 fathom) = 10 x 100 x (2 x yard) = 10 x 100 x 2 x (3 foot) = 10 x 100 x 2 x 3 x foot = 6000 x foot = 6080 x foot (if you go directly) We have 80 feet (11520 poppy seeds) missing in our calculation - or about one shackle

While we are on the subject, can anyone explain this graph:

According to wiki (I had to look it up since this is the first time I saw that English and Imperial units are different thing), American units are further evolution of English units. Therefore - not two steps behind, but two steps behind and one to the side https://en.wikipedia.org/wiki/Comparison_of_the_imperial_and_US_customary_measurement_systems

I regularly bin my color guide camera with OAG since "base" sampling rate is somewhere around 0.48"/px and I don't need that much resolution for my guide system.

Yes, other way around. It really depends on mount used. Higher quality and better performing mounts don't need as frequent corrections. Many people use 1-2s exposures for guide cycle. That is often fast as seeing influence can be quite big on those scales. Better mounts tolerate 4-8s guide cycle, while top level mounts can go over 10s of seconds for single correction - mount simply stays on target that long. Guide exposure depends on several factors - one is seeing, you need long enough exposure to average seeing effects. Other is quality of polar alignment. There is a tool that will calculate DEC drift rate depending on polar alignment error. In most cases, this rate is something like 1-2 arc seconds per minute or less. From that you can calculate guide exposure length - just choose what is maximum offset in DEC you will tolerate for single guide exposure. Similarly, another factor, acting in RA is periodic error. This can be as much as 30 arc seconds peak to peak in one worm cycle or as low as few arc seconds. Depending on how smooth it is (for example, pure sine wave is probably the smoothest form you can have - but it is rarely so), you can again calculate max drift rate in RA. Based on that and wanted max correction - you can calculate guide exposure. Ideally - you want longer guide exposures as that means that seeing effects will be reduced, your polar alignment is good enough and your periodic error is low enough and smooth to be able to use longer guide exposures. Sometimes in the future it can even happen that imaging exposures become shorter than guide exposures ( less read noise and better mounts requiring corrections less often) - but that is easily handled by "summing" (stacking) multiple short imaging exposures to form single longer guide exposure - thus still being able to both guide and image with single camera.

More affordable scopes mean more people can use larger aperture scopes and hence higher magnification? Knowledge on optimization of high power viewing is now readily available online?

It is actually related to read noise of sensor. With modern low read noise sensors, we are approaching moment when no additional guiding system will be required. As has been pointed out, at the moment, there is discrepancy between imaging exposure and guiding exposure - a few magnitudes of a difference. Imaging exposures tend to be in hundreds of seconds while guiding exposures are in seconds. Since difference between long exposure vs short exposure image quality depends only on read noise (or more precisely - it's relation to other noise sources) - we still need to keep our imaging exposures at least few minutes long. With advent of very low read noise sensors - this time will reduce and at some point - exposure lengths will match and then you'll be able to guide on imaging exposure. In fact - something like that is already partially possible in what is called EEVA - short exposure live stacking where exposure lengths are tens of seconds long. I don't think software is yet capable of doing it, but in practice EEVA software would benefit from such guiding as it would be able to dither and improve results.

Some sort of statistical analysis should be considered here. Besides already mentioned variables - observer plays a role as well. Some people tolerate slightly soft but larger image as it allows them to see detail easier. Others prefer detail to be small at edge of resolving but overall image to be sharp. I see this in imaging also. There is not much difference in x2 sampling resolution in terms of what can be seen in the image and perceived sharpness. That is x2 in "magnification". As for myself, I think I went thru different phases. Before - it was about magnification / image size - it allowed me to see better, but as time went on, I found that I prefer lower magnification and perceptually sharper image. Btw, actual magnification that allows you to see all there is to be seen is quite low. For 8" aperture it is less than x100. Everything above that is just magnifying image to make it easier to see without additional detail being revealed.

Hi and welcome to SGL. This is rather interesting question - many are reluctant or can't afford to set aside such a large budget to start with. I think that probably best approach would be: - buy the best mount you can afford - start with a decent starter scope and OSC camera. - maybe throw in guiding as well Decent starter scope and OSC camera won't cost that much (about a third of the budget?) and can be sold easily when you feel ready for upgrade. Good mount will save you trouble at the start and also having to upgrade later. Scope should be up to 600-700mm FL class scope, and camera something like this: https://www.firstlightoptics.com/zwo-cameras/zwo-asi-294mc-pro-usb-30-cooled-colour-camera.html You'll need a laptop with that to capture images. Mount recommendations up until recently would have been iOptron CEM60 (basic version) - but these are no longer being made and are replaced with CEM70 which will be more expensive (not sure if that will be justified or not).

I would avoid x2 barlow with F/10 scope and 2.4um camera - it leads to oversampling, especially in NIR with longer wavelengths.

I have noticed this as well. I think it is more about doing something that makes one happy rather than always focusing on what makes others happy. From time to time, I just start craving that - "this is what I want" fix. It is not necessarily "chase the better" gear kind of thing - it can be "papa's got a brand new toy" thing. It's not impulse buy thing either - a lot of thought and yearning go into it.

Do you have a camera and lens combination that has known QE / attenuation and Gain in terms of e/ADU (although that part you can determine)? Simple incandescent bulb with a diffusing panel could be used than - or flat panel for that matter.

I think that easiest way and probably most precise is to use telescope and camera as calibration source. Procedure is fairly simple - just shoot piece of the sky - preferably away from the milky way with a bright(ish) star in FOV. Star should be bright enough to be in catalog - with known magnitude but not too bright to saturate sensor. Then it is simple matter of doing photometry on that star and measuring background value / dividing with sampling rate so that you get photon count per arc second squared (or just ADU per that unit) - ratio of the two, star ADU count and background ADU count will give you basis for magnitude calculations - take star mag and subtract log of ratio to get sky mag (per arc second squared). Compare that with your device reading in the same region to get one calibration point. Do multiple parts of the sky (different sky brightness values) to get good calibration curve.

I also thought that x2 aperture in mm is some sort of magical number, but it is not. There is actual number based on a bit more science and it will surprise you - for 8" scope it is about x94. Yep, you read it correctly - it is only x94 magnification. Let me explain. Most people with 20/20 vision can resolve 1 arc minute feature (high contrast). Rayleigh criterion (again high contrast) for green light and 200mm aperture is 0.64" - arc seconds. How much do you need to magnify to make something that is 0.64" be 1' large? - that is rather simple 60" / 0.64" = x93.75 - there you go. This basically means - if you use magnification below x94 - you won't see what you can see because you did not magnify enough - you are below resolving power of human eye. Going above x94 - just makes things easier to see. At some point, image just becomes too magnified and blurry - we call that "falling apart" of the image, and it depends on observer, scope and object being observed. I used x500 magnification on my 8" dob and it did not fall apart for me - it was dim and floaters were all over the place, but I could still observe. I prefer now to keep things up to x300 and often at x200. I've got 6.7mm and 11mm 82 degrees ES and 5.5mm 62 degrees ES as higher power eyepieces. I also have x2.7 APM coma correcting barlow - but I noticed that I don't really like to use barlows if I can get wanted magnification with eyepiece alone.

You need to plate solve your image and get coordinates of your star. After that you can search online catalogs like Simbad http://simbad.u-strasbg.fr/simbad/ Alternatively, if you can do a bit of coding, you can use these catalogs: https://archive.stsci.edu/prepds/atlas-refcat2/

New drivers usually require new set of calibration files.

Lovely set of captures. For some reason - bottom of each image seems to have significant blur to it. Do you know the reason?

Yes, of course I just downloaded the first one - it has about 86.5% light level due to vignetting in the corners - so 15% light loss, not that much at all.

That might not be bad as it looks - it really depends on how much you stretch it. Can you post single fits for one filter for inspection so we can see actual values in that flat? For example: These two are the same master flat - but stretched differently - in fact, light loss is only about 20% at extreme corners.

I see similar optical aberrations in all stars in the image per channel. Red is most round. Green shows sagittal astigmatism. Blue shows inward coma like tail. I'm not sure if that is aberration or stacking artifact - maybe stars moved due to atmospheric influence between first and last frame and that tail is sigma clip artifact - or it could be genuine thing. M106 Luminance top right corner: Top left corner: Bottom left corner: Bottom right corner: Same as green in above animation that I made - only significant thing that I see is sagittal astigmatism

Not sure if it does - since all stars show in each channel - I would expect that they have enough signal in each band.

In the meantime ... Ken why don't you try Deep Sky Stacker to see if it will do the same? As far as I know - it has rather "bendy" alignment model (sometimes it can really warp an image if it gets the stars wrong - so I guess it will try its best to compensate for any distortion made by lens or atmosphere).