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Ok, let me first say - no, I'm not going to complain about prices of exotic ED glass - on the contrary! I'm also going to do full disclosure here - I'm suffering cloudy skies syndrome at the moment - you know, have not been observing or doing anything astronomy related for months and I'm starting to fantasize about new scopes, different combinations, examining prices - making up needs and all those weird stuff we amateur astronomers do when we get this syndrome It all started with focusers. I have M90 Monorail focuser that is leftover form an upgrade. I wanted to fit that focuser on My Evostar 102mm F/10 refractor for some time now but I can't seem to find suitable solution. While looking at other focusers and thinking about focusers - it hit me, most are quite expensive in comparison to cheap units that come on most entry level scopes. You can't really find entry level refractor telescope with a decent focuser, and often, focusers themselves cost as much as entry level refractor scopes (achromat types). For some reason I then started thinking about very cheap portable refractor type telescope that I would upgrade to make a decent scope (might have been prompted to this behavior by recent thread - "Show us your small refractor scope" ). Say you take Skywatcher Mercury 705. I believe it to be decent glass, and with decent focuser, diagonal, maybe zoom eyepiece like Hyperflex 7.2-21.5mm - could make very nice portable / grab'n'go / quick peak package. I would also like retractable dew shield if possible. Ok, let's see, how much would it cost? I'm going to quote here TS prices that are in euros and without VAT, so those are not final prices - but they do serve the purpose - comparison purpose. OTA ~ 116 euro Focuser ~ 125 - 145 euro (125 is 1.25" model, and 145 euro and upwards are 2" models - all having 1:10 reduction. There are 1.25" and 2" single speed GSO crayford models that are for newtonian scopes but could be adopted - for about 76 euro each) Adapter to mount focuser onto tube - at least 40 - 50 euro if not more. Ok, lets add that up - OTA + 2" with 1:10 + adapter (cheapest estimate) = 116 + 145 + 40 = 301 euro. On the other hand, you have something like this: https://www.teleskop-express.de/shop/product_info.php/info/p1151_TS-Optics-70-mm-F6-ED-Travel-Refractor-with-modern-2--RAP-Focuser.html And yes, it costs 301 euro and it has ED glass. Maybe not the best ED glass out there, but it will sure beat Mercury 705 in color correction and planetary views. It has retractable dew shield as a bonus. Moral of the story? Entry level refractors are actually expensive for what they provide!

Ultimately, maximum FOV will be dictated by telescope's illuminated circle. Focal reducer just "compresses" that circle into smaller space. If, say, illuminated circle of the telescope is 35mm, using x0.7 reducer will give you 24.5mm illuminated circle. If you use eyepiece that has larger field stop than those 24.5mm - you'll get vignetted field: Important numbers to consider: 1.25" eyepiece has field stop of max ~27mm (max field stop diameter is related to barrel size and is usually 3-4mm less than that. 1.25" = 31.75mm) 2" eyepiece has maximum field stop diameter of ~47mm (again, 50.8mm - few mm). Visual observation is less sensitive to vignetting than imaging. Humans can tolerate up to 50% vignetting without noticing it too much (this is because we see "logarithmic", or in power law - like magnitudes, so 50% is really not "half as bright" for us). Depending on the size of SCT - they might not illuminate very large field. For example C8 has smaller illuminated field than 38mm. With x0.7 reducer it is effectively 26.6mm and that just about fits into field stop of 1.25". This means that you can see equal amount of the sky by using reducer and 1.25" eyepiece of just using 2" eyepiece that has field stop of ~38mm or more - like ES62° 40mm or AERO ED 35mm. It also means that using eyepiece with maximum field stop diameter (46-47mm) on C8 will be waste as it will vignette quite a bit. Smaller cats have even smaller illuminated field.

Ok, get it now. Plate solving works regardless of the scale, however, there are a few things to consider when doing very long focal length plate solving. First is that you need a number of stars to plate solve on. Second is to have decent SNR on those stars so they can be identified as stars. FOV gets into this equation because it restricts number of visible stars to plate solve on. Too constrained and you won't have any stars to plate solve on. Other is SNR, and here its not so much focal length as sampling resolution. If you oversample by a lot - stars are no longer these points of light - they are "blobs" of light and plate solve software might not identify them as stars. Another issue is that you spread light over more pixels and each pixel gets less light as a result - SNR goes down and stars become fainter - another potential issue for star detection. Luckily, there is way to solve over sampling issue - just use binning for exposures that you'll plate solve on (but in reality, you should also use binning for everything else if you are over sampling) and that will handle it. I plate solve at 1600mm FL with 0.5"/px sampling resolution with no problem, so at least you know that down to 0.5"/px it should work ok.

Not sure what exactly are you asking, but yes, you can plate solve without knowing any parameters of the image - it is called blind solving. Takes a bit longer, but in principle - any scale, angle, and parity should be matched against catalog and result produced.

Get 178 and bin pixels x2 when capturing. ASI174 will have too large pixels at 5.86µm and you'll need to use barlow to try to dial in optimum sampling rate. ASI178 will have too small pixels at 2.4µm natively, but if you bin them, then you'll be very close to optimal sampling rate for this telescope at 4.8µm Scope is F/15.5 (60mm at 930mm of focal length) and 4.8µm pixel size is ideal for F/14.64 in Ha wavelength. For full disk solar imaging you'll need to use x0.5 reducer of course, but then you don't need to bin pixels and you'll be again at optimum sampling rate to capture all the details.

Hm, if you are after triplet refractor - than have a look at this: just be careful - that scope is going to push Heq5 to its limits, it really needs a bit heavier mount. It is the scope I was initially going to recommend, but new is rather more expensive than those listed above.

I guess most convenient way is to do it after / before filter change. For example - you start with Luminance, then after you finish - setup flat panel, do flats, switch filter and if you have motor focuser offset dialed in - just refocus without checking on stars and do another flat for that filter. On next filter change do nothing and then do one more flat session between last filter change (this assumes 4 filters / LRGB and having motor focuser and dialed in offsets). If you need to refocus on stars - then you need to take flats after you finish with that filter for the night.

I would say that these should be sensible advice for you to get started: 1. forget filters, barlow and 10mm and 6mm eyepieces for now until you get some experience (you won't really be using barlow and filters for DSO). Use 25mm as main eyepiece and 15mm if you want to get a bit closer (more zoomed in) to target. 2. Judge how transparent skies are by looking at the number of visible stars / forget observing if the Moon is out - you need clear moonless night to go after DSOs. 3. Wait until later in the night when people turn off their lights and traffic dies down a bit (car have headlights and those cause LP as well). If you can - get out of town to darker location - this is probably the most important. 4. Shield yourself from immediate light sources - get behind walls or even use some sort of blanket over your head 5. Download Stellarium software to start learning your way around the night sky - learn to star hop. At first, try to point telescope at bright star and use those to hop to less bright stars near by. 6. Start with something easy - now it is perfect time to observe some very easy objects - Orion's nebula - M42 and for example M45 - Pleiades. These you should be able to spot with naked eye (for Orion's nebula - you'll see like a few stars in sword of Orion so you'll recognize its location). 7. Have patience. Both at eyepiece - wait to get dark adapted and don't look at light while waiting, but also have patience as you will learn to observe DSOs. Most are very faint, but as time goes by and you get more observing experience - you'll start seeing more and deeper.

Hi and welcome to SGL. Fact that you decided to go down triplet refractor route does pose a problem. Refractors, especially triplets are expensive scopes and if you want to shoot a bit of everything, well, then you want larger refractor. Something with about 120-130mm of aperture. These instruments don't come cheap. You want very good mount as well. I'm not sure I would be able to put together rig that will suit your needs and wants within a given budget. Let's give it a go, "bare minimum" for what you need: https://www.firstlightoptics.com/skywatcher-mounts/sky-watcher-heq5-pro-with-rowan-belt-mod-upgrade.html With FTDI cable that is roughly ~£1000 Refractor in 4"/100mm class will set you back additional ~£1500, here are some offers: https://www.altairastro.com/altair-wave-series-102mm-f7-super-ed-triplet-apo-102715-452-p.asp https://www.altairastro.com/altair-wave-series-115-f7-ed-triplet-apo-453-p.asp https://www.firstlightoptics.com/esprit-professional-refractors.html Another ~£1000-1200 for mono CMOS camera: https://www.firstlightoptics.com/zwo-cameras/zwo-asi-183mm-pro-usb-3-cooled-mono-camera.html https://www.firstlightoptics.com/zwo-cameras/zwo-asi1600mm-pro-usb-3-mono-camera.html And there goes your budget ... We still did not look at field flattener/reducer £200-300, filters (Baader 1.25" LRGB set for £200 and £250 for Narrow band set), Filter wheel (£175 electronic), Motor focuser (well, if you have electronic filter wheel, then why not motor focuser as well), guide scope and camera another £300 or so ... Various adapters ... Easily £5000 or more. In order to hit your budget, I would say we need to cut corners here and there. If you want to do it all, I would say maybe instead get this scope: https://www.firstlightoptics.com/reflectors/skywatcher-explorer-150p-ds-ota.html It is £230 and more aperture while having good focal length for general work - 750mm. You'll need coma corrector with it, but that is another £130. You can also save up if you don't get narrowband set of filters right away, get manual filter wheel and skip motor focuser. Get above mount (heq5) and camera (ASI1600), guide scope and camera and you are roughly in your budget range.

Simple fact is that mono + LRGB is faster - in terms of achieved SNR in the same amount of time. Mind you - it is not much faster. Difference between the two comes down to: 1. Interference filters have higher QE than absorption filters used in Bayer matrix 2. Time spent on G channel with OSC will be spent on L channel with mono +LRGB L captures more signal per unit time than G simply because it is wider band filter. Imagine you image with OSC for one 4 hours. Each of R, G, G and B pixels of Bayer matrix get 25% of light, so we can argue that each R, G, G and B get one hour of imaging time (25% of 4 hours). With mono + LRGB - in 4 hours, if you spend one hour per each filter R, G and B will get the same time as OSC R, G and B, while L will get the same time as remaining G from Bayer matrix. Here you have edge in L filter. Mono has other advantages - like ability to vary filter contribution. One might decide to do half an hour in RGB and remaining 2.5h in L. Human eye/brain system is more sensitive to noise in luminance than in color information so spending more time on L makes sense. Narrowband filters use is another advantage. Drawback is time spent on filter change / refocusing (very small percent of time since, especially with stepper focuser as you can memorize focus shift needed when changing filters, so refocus is just moving remembered offset). Each filter requires its own flat - additional time spent if you don't have permanent setup and shoot flats on regular basis. OSC has one more advantage. If you get interrupted mid session - you still have complete data to work with. In order to achieve that with mono+filters, you should really "cycle" filters after each exposure, but refocusing and filter switch time would increase greatly.

I think that focuser is made for some strange S88 or similar, not ID 86. You can't attach it directly to tube with inner diameter of 86, at least I think so. In any case, you should have focuser with 96mm ID and you'll need Steeltrack to ID 96mm adapter - but again, do check. Yes, it is better to have FF nosepiece removed as it just adds distance which can make it difficult to focus properly (nosepiece just goes inside focuser normally and does not count towards optical path). Check if you have thread under nosepiece. I read somewhere that some FFs have nosepiece removable and that there is also M48 thread there, or it might be M54 in that model - not sure. If it's different thread - then yes, you'll need different adapter from S58 to that thread (S58 is strange designation for steeltrack focuser thread).

If it does - that should be 2" filter thread - or M48. I linked suitable adapter for baader steel track focuser above. Hopefully it will have that thread and you'll be able to use it.

I'm just looking at focusers, and I think that best choice for you would be Baader Diamond Steeltrack refractor focuser. You only have to make sure you have proper connection to your telescope to replace your stock focuser. It will probably be either for flange ID 86 or ID 96 (that are two standard tube inner diameters for Skywatcher). If it's ID 96 - then it is easy - you have ready made adapter: https://www.firstlightoptics.com/baader-diamond-steeltrack-focusers/baader-steeltrack-diamond-rt-adapter-for-synta-sky-watcher-celestron-otas-with-id-97mm.html And in the end, you only need M48 adapter: https://www.firstlightoptics.com/baader-diamond-steeltrack-focusers/baader-diamond-steeltrack-m48-adapter.html

Comments on that one you linked say that it can use both 2" and M48 connection. Does 2" nose piece unscrew from flattener? Does your focuser have any sort of threaded connection?

Just realized that FF/FR for ED80 is probably not suitable for Equinox 80. What Field flattener are you using and does it have threaded connection?

Yes, you have tilt of FF and ideally you want to have threaded connection in order to avoid this. As far as I understand, SkyWatcher dedicated FF/FRs should have straight forward threaded adaptation to SW scopes? Here is screen shot from FLO website: https://www.firstlightoptics.com/reducersflatteners/skywatcher-85x-reducer-flattener-for-ed80.html

Just wanted to add following resource - still not proof, but confirmation none the less: https://www.johndcook.com/blog/2015/12/16/sinc-and-jinc-integrals/ Jinc is defined similarly to sinc - J1/x which is core of Airy pattern (Jinc squared).

I still don't think it is convention of any kind nor do we need to assume that in order to make integral finite / valid. True proof of that really needs mathematical backing, and I'll try to do that (maybe not by solving whole integral thing, but if it is cut off - maybe we can show sum to be zero for some frequency higher than cut off), but in the meantime, let's look at some notable examples that really have finite FTs and acknowledge that FFT does produce effective cutoff (still not analytical proof - but it should behave the same regardless of the fact it is numerical method). I would mention just plain box filter as example - because it is very similar to aperture (one can imagine box filter in polar 2d coordinates as being aperture). Box filter has Sinc function as its Fourier transform. Note similarity between Sinc function and Airy pattern, or rather Sinc squared and Airy pattern: Above is Sinc squared. Now we want Fourier transform of Sinc squared. Look at that, it appears that Sinc squared and Triangle functions are Fourier pairs: We can see that Triangle function has frequency cut off as well - not by convention but by actual value - zero for x values higher than certain cutoff point. http://www.thefouriertransform.com/pairs/fourier.php

I think you are mistaken there. If you have PSF - which happens to be defined as a function of distance from optical axis (in mm or other spatial units), then cut off frequency is mathematical concept - not physics one. Given certain PSF, arguing that cut off frequency is at certain point rather than other - makes about as much sense as arguing if 7 x 5 equals 35 or not. I know that there is greater difference between obstructed and unobstructed PSF then there is between 1/4 wave P-V and obstructed PSF, but it really does not matter since all three are transformed into Fourier domain and we can't judge if their transforms will be similar or different and to what extent - unless we do actual calculation (which btw shows that same apertures have same cut off frequency and larger has higher cut off frequency). Fourier transform is integral, a sum, right? Here is an example how things with sums can be misleading. 7 + 3 + 6 + 6 = 22 8 + 1 + 8 + 5 = 22 8 + 1.01 + 8 + 5 = 22.01 First four numbers (unobstructed) is very different than second set of numbers (obstructed) which is in turn extremely similar to third set of numbers (10% larger with 1/4 wave PV), yet first and second set sum to exactly the same number, while third set does not. Because calculations say so . This is purely in domain of mathematics, which says that if something acts as convolution kernel (PSF) then that is the same as doing multiplication in Fourier domain - hence Fourier transform of convolution kernel is filter. Fourier transforms of Airy patterns for obstructed and unobstructed aperture of given size have same cut off frequency, but significantly different shape. Fourier transforms of Airy patterns of obstructed aperture and 10% larger aperture with 1/4 wave PV spherical - are much more similar in shape but have different cut off frequency. In fact - we have seen that for certain aperture, regardless of obstruction or not, cut off frequency is constant - that is the property of corresponding Airy patterns (PSFs) and their Fourier transforms. What can be argued and is much more complicated to derive is physics thing - which no one called into question here. That is - why does light at circular aperture, obstructed or not - gives Airy pattern at focus plane. I've seen once derivation, and it's not hard, but it assumes simple wave mechanics (not QED). It's a bit like Feynman's explanation for reflection of light where each photon has internal clock (phase of wave at that point) and they propagate in all directions and bounce in all directions and are summed (as vectors) to form a wave in destination point and then magnitude is used as probability of finding photon there (intensity of PSF). All that summing actually ends up being integral over aperture, and wave thing is e to the power of i * pi * theta and finally square is used to get probability / intensity and we end up with Airy pattern being power spectrum of Fourier transform of aperture. It is not convention to have MTF be auto correlation of aperture, but rather consequence of the fact that Aperture --> Airy pattern (power spectrum) Airy pattern --> MTF (frequency spectrum) And then we have this: Here we have that power spectrum is the same as function times its own conjugate.

That rule x2 aperture in mm (or x5 aperture in inches) is very loose rule and it really depends on how good your eyesight is. Not all people see equally sharp and there is a measure of visual acuity that determines how sharp is your vision. Sharper your eyesight - less magnification you'll need. Theoretical limit for 20/20 vision is somewhere around x115 for 102mm aperture. At this magnification you'll see all you can see at threshold of your visual acuity if you have 20/20 vision. Image will look the sharpest without loss of detail. Sometimes it is easier to see if you bump up magnification so we prefer slightly higher powers that theoretical, but there will be no additional detail to be seen after we pass above threshold. Back to original question - I have Mak102 and have 6.7mm eyepiece to go with it, giving about x194. I think I would not mind having something like 8mm as my highest power eyepiece for that scope - I would not loose much over 6.7mm. In fact - image would be sharper with such eyepiece. But that is my vision and my perception of the image. You might find that you prefer higher powers (I don't usually find them as useful - I like smaller sharper looking image).

What are your intended targets? If you are interested in shooting emission type nebulae / H2 regions and you don't mind color cast on stars, or you don't mind taking additional set of images just for star colors, best filter that won't break the bank would be UHC type filter. Otherwise go for D1 for general LPS filter / broadband targets (galaxies, clusters, reflection type nebulae ...).

This is because DSO / long exposure astrophotography is in completely different regime to high power planetary. It is dominated by blur caused by atmosphere / seeing blur and tracking / guiding imperfection blur - both of which integrated over significant period of time - couple of minutes. This produces blur in the image that is order of magnitude stronger than that produced by optics (for 6-8" aperture, with smaller apertures this difference obviously reduces). Sampling rate for planetary imaging is often 0.1-0.2"/px, while that of long exposure astrophotography is 1-2"/px and higher. Differences between obstructed and unobstructed telescopes on these scales simply can't be seen, and mirrored telescopes have some significant advantages in DSO imaging over refractors - hence are much more often used as scientific instruments.

Yes, we did mention that on one occasion, but I have no idea why would narrower central peak shift cut off frequency? After all, MTF is FT of Airy pattern and integration is complex operation - it is not immediately obvious one way or another. Except for doing analytical integration, best way to check if there is shift in frequency due to narrowing of the peak is to do numerical analysis. This is unobstructed vs 40% CO aperture. As far as I can tell - they have exact same cut off frequency. Here is same measurement "zoomed in" on cut off frequency: Now that I have once again read above discussion, I think I know where error in reasoning in it comes from. Let's examine what is being said: I strongly object this argument (in bold - by me for emphasis) that something that is "nearly" identical - which clearly means different (however small difference - it is not asymptotically small / vanishing) should produce identical results. Author even shows - how "nearly identical" these are: Left side of this image shows PSF of 32% CO and 10% larger aperture with 1/4 PV spherical. These are clearly not identical and I see no reason why their FT should fall to zero at exact same point.

Ah, it has corrective optics. That is probably tilt of some sorts. I had something similar with refractor and focal reducer / field flattener - but not as clear. It was confined to one corner (other was just tangential astigmatism and rest were relatively fine). This is one corner - it shows tangential astigmatism - elongation in direction towards the optical axis. But other corner showed this: Each star started showing a little cross. In my case it was problem that FF/FR and sensor were not squared on optical axis of the scope (poor 2" connection - clamping type not threaded). Your telescope has fixed corrector inside focuser if I'm not mistaken, and my guess is that focuser unit is ok. This leaves collimation as likely cause. Possibly secondary, but I think primary could be the issue as well. Do you have any means to remove corrective element from focuser and do collimation of secondary by looking down the focuser tube?

Not sure where you got the idea that obstructed telescopes ought to perform so poorly? In fact I think I know. If someone says that you'll get Contrast Loss of 55% for 50% CO - you instantly think that it will have half the performance of unobstructed scope. That is simply not the case. Yes, there will be contrast loss in visual in comparison to unobstructed telescope of the same size, however it is not remotely as drastic as it may seem. In fact 20-25% central obstruction gives almost the same views as unobstructed aperture. With 20% CO - most of people would not be able to tell the difference and only experienced planetary observers would notice in side by side comparison to unobstructed telescope. It is also important to understand that resolution is governed by aperture and contrast and resolution are almost synonymous terms. In fact resolution of a telescope is related to frequency at which contrast falls to 0, and blur that we get is gradual per wavelength contrast loss. This means that larger aperture will give you better contrast (or rather resolution) than smaller aperture. All that information is contained in MTF, and that is the best way to compare theoretical max performance of two optical systems (just make sure both MTF graphs are plotted for same values - they often come "normalized" to max frequency, but max frequency depends on aperture).