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I think you are right - I calculate exit pupil like this: EP FL / Scope FR, so in case of F/5 scope - 5mm eyepiece will give 1mm exit pupil. Or better remembered like this: Eyepiece with focal length equal to F/ratio of scope will give 1mm exit pupil, and around half of that will be max theoretical useful magnification (based on resolving alone - perfect optics).

On the other hand, if interested in orthos only, TS has nice selection of Kokusai Kohki Fujiyamas, no 8mm, but they have 6, 7, 9 and 12.5 (among others) - again prices seem to be right (around 100e, this time VAT included )

How about Vixen SLV line? I don't have personal experience with those, but from what I've read, performance is Ortho like, eye relief more than comfortable, and line happens to cover focal lengths that you might find useful (6, 9, 10, 12). Price is, I believe also within requirements (a bit more, but smack on 100euro without VAT )

If you are looking for a cheap upgrade to dual speed, have a look at this: https://www.teleskop-express.de/shop/product_info.php/info/p2625_1-10-micro-transmission-for-retrofitment-of-Crayford-focusers.html I've fitted it to my Skywatcher 8" dob, and it works great, quite a difference on fine focusing for planets.

I was just thinking about that, if Takahashi could do it with CN212, how hard can it be? But I would still prefer Cassegrain configuration to be with small secondary and long focal length - for planets.

All I can say from experience is that TS/GSO RC8" is worth the price. Someone mentioned focuser on this new Cas above, and it is the same unit as on RC 8" model - I ended up replacing mine. Although standard focuser (Monorail 2") delivered with scope is quite usable unit (for visual), there is serious drawback for its primary use (as astrograph) - it does not have threaded connection so tilt due to any mismatch in 2" interface is likely to happen. But if you just count a set of 8" F/8 RC mirrors - of quite good figure quality (I tested mine to be >=0.94 strehl) - at that price, I believe it is very worth it.

I know that one, and yes, I would like one of those CCF F/20 units at a price of GSO F/12 scope

I'm not sure about that one ... Looks like too much compromises. I would expect classical Cassegrain to be at least F/18, but it seems that GSO opted for F/12 for a number of reasons: 1. Reuse of tube and other components from existing RC model (both 6 and 8 inch) - F/18 or longer would require longer tube 2. Trying to make all rounder rather than specialist scope - bigger market share.

I believe worm period on HEQ5 is 638s (135 teeth so 10 minutes and 38s) while on EQ6 is smaller (180 teeth, larger worm ) - 479 seconds, or a second under 8 minutes. Recommended "dose" of PEC is at least 5-6 worm periods, so indeed 40 minutes is minimum for EQ6, but I would not go less than that - it is like regular stacking, more data, greater SNR and more precision in PEC curve.

I do also believe using PHD2 log and PecPrep to be more accurate. There are couple of video tutorials on PecPrep - worth looking at (they are short and easy to understand): If you follow next videos - you will get each part in a row.

I don't have permanent setup either, but for PEC you don't need to have it. PEC records imperfections in manufacturing of different components of drive train. It can't correct all imperfections - only those that are periodic in nature (like different gears not being perfect circles) - and only if they are harmonics of certain base period. EQMod PEC can only work with harmonics of base worm drive period. I've seen some question raised about viability of PEC when using different scopes on same mount. I think if one models PEC of mount - it should not matter what sort of scope you have on it - PEC needs to correct for drive error irrespective of the load. I use it with both small and large scope and it works. You can do PEC in early evening (not astro dark yet), or during full moon when you would not otherwise image. Do it only once and use it for subsequent sessions. You don't need to do it each time. I've done it just 3-4 times so far (after fiddling with mount - belt mod or regreasing / changing bearings). I need to do it now once more since I changed my laptop and lost PEC file (I actually saved file but sync was lost because of fresh EQMod install). You can even do it when there is high level clouds - as long as you select bright enough star and PHD2 is able to track it long enough (without star lost message). One more thing - best to select star near/on Equator - PE will have the largest amplitude there and most precise recording of it will be possible. PecPrep takes into account DEC of guide star (either read from log file, or you need to enter it manually, depending on guide app you are using - PHD2 its automatic). I'm a bit confused, what mount do you have? In your signature NEQ3 is listed. As far as I know, Rowan belt mod is available for HEQ5 and EQ6 models only. Other thing that you might consider is changing bearings on your mount (irrespective of model). They are pretty much standard - get high quality SKF and replace each bearing - it will also help smooth out mount tracking.

My experience is with EQMod (there has been some debate if PEC helps or whether guiding is fighting PEC, but for EQMod it seems that PEC helps quite a bit) and here is how I do it: I turn off guiding and use PHD2 to create log file (there is option in PHD2 to disable guide commands). I usually create about hour / hour and a half of log file. While doing that - at some point I press time stamp button in EQMod. EQMod auto PEC is also turned off. Next I load PHD2 log data into PecPrep and create PEC curve. After that I just tell EQMod to use that PEC curve and work "normally" from then on. I usually recreate PEC curve on significant change in system, or if sync is lost (HEQ5 does not have absolute encoders, and one must park each time after session to preserve mount position to PEC synchronization - if there is power failure or something and sync is lost - PEC curve needs to be generated again). In general PEC is meant to be recorded on "raw" mount data / motion - so don't have anything running that will disturb this - like guiding or whatever.

No, I have not heard about that one, but I'll have a look. I don't have a particular place for images (yet, I'm sort of working on it), but to tell you the truth I don't have much to show off. I started using RC last summer, and managed to get just a couple of images with it - most L channel (no NB or color taken with RC yet - I did combine color data from other scope on one of images but it did not turn out that great). Weather is just not cooperating for at least past 6 months (only one session, and that was short one). I post most of my images here in DSO imaging section, but to save you trouble, I'll link in ones taken with RC: http://serve.trimacka.net/astro/2017-07-18/ http://serve.trimacka.net/astro/2017-07-21/ http://serve.trimacka.net/astro/2017-08-27/ http://serve.trimacka.net/astro/2018-05-07/ Just open links and click on png. One of ngc7331 has a lum only (that is pure RC) and color one - mixed with data from another scope and another camera (TS80 F/6 apo and ASI178mcc - so not much color data and it is not that good). You can use search here on SGL for my posts in DSO imaging to see capture details for above images.

No, I did not use collimator at all. I found this very informative blog post (let me see if I can find a link) that gave interesting instructions for collimating RC. https://deepspaceplace.com/gso8rccollimate.php I adopted it a bit for my gear. I did it once with Bahtinov mask, but found that it is not as precise as I would like it to be, so I switched to SharpCap and measuring of FWHM for defocus measurement. I did all as recommended, but when I focus on star in center field then I inspect all corners for FWHM and pick the one that has the biggest value to adjust. It actually took me two rather short sessions (of about 15-20 minutes) to get it quite good over ASI1600 chip. First one was with Bahtinov mask, I did I think 2 iterations and proceeded with imaging, results were not that good, so on next night I repeated with FWHM and additional 2-3 rounds of tweaking. Here are results: Top left corner prior to any collimation : After first round of collimation: (improvement but still some elongation) After second collimation: or this one:

Honestly, I don't know I did not learn it all in one place, it is a product of reading multiple sources on the web and then simply applying basic logic to it to draw conclusions. Some of it was from personal experience since I've got the same gear. First thing that I've learned related to above is importance of collimation on RC scopes - you don't collimate it well you will end up with stars that don't look nice. You can find formula for reducer magnification (or minification? ) here - http://www.wilmslowastro.com/software/formulae.htm#FR but I've seen it elsewhere on web. It works for barlow calculations as well (very similar formula - barlows have negative focus). On AP website there is a document for CCDT67 technical data: http://www.astro-physics.com/tech_support/accessories/photo/Telecompresssor-techdata.pdf There you can see that FL of this reducer is 305mm, and that housing is such that you need 85mm from thread to be at 101mm for x0.67. TS RC specs can be found on TS website. I've also read on multiple places online that reducers (similar to barlows, but unlike field flatteners) work over the range of distances. Problems with tilt I experienced first hand on different scopes, so that I something I pay attention to.

Although recommended distance is 85mm, I think you will find that your combination of equipment is not going to work well on "default settings". I have same gear (TS RC8", x0.67 reducer - ccd47 variety from TS, and ASI1600). Main problem with this setup is FOV. 8" RC has fully corrected field less than the size of ASI1600. If you have well collimated RC and use ASI1600 at native - you should get round stars up to the edge with slight field curvature showing - stars close to edge will be a bit larger / slightly out of focus. Best to focus about 3/4 from optical axis - if you use Bahtinov mask or measure FWHM when focusing - place star somewhere around 2/3 to 3/4 between frame center and one of the corners. This way you will "spread curvature" over sensor (stars at the edges will be closer to focus, and stars in center will be slightly out of focus, but almost impossible to tell). This is of course in case you will be keeping whole FOV. If cropping out - focus closer to center of FOV. Now, back to star shapes and reducer - With RC8" it is advertised as being able to do 30mm without corrector. I would say that is probably pushing it and corner stars will suffer. ASI1600 has about 22mm diagonal. When you apply x0.67 reduction, you are squeezing larger field on smaller surface. Equivalent to this is using larger sensor. How much larger? 22mm / 0.67 = ~32.8mm. This way you are imaging out side of corrected circle. Edge stars will surely suffer because of this. I would say that going up to 28/29mm imaging circle would be appropriate (even then, prepare for very specific focusing - finding focus place that gives best focus over FOV). This reducer acts as slight field flattener - but not proper one, so field will be a bit flatter but you will still have to deal with field curvature. Now let's see what sort of magnification one should get to cover 28/29mm "imaging circle". Again some math 22/x = 29 -> x = 22/29 = x0.75. If you do a search on web about this combination - RC8 / x0.67 you will find that people often say - best results are not at x0.67 but x0.72 - x0.75 (above is the explanation why it is so, some will even say it works good with "native" x0.67 - but that is simply because they are using small sensor - 11-17mm, for APS-C that is larger than ASI1600 they probably need to go as low as x0.75 or lower). What would be proper distance for x0.75? Go for distance of 55-58mm (correct distance will not be of big importance, just use spacers that you have) . You can actually experiment and start with 55mm and then move reducer further and further until you reach point where things start to fall apart. So to reiterate: 1. Well collimated scope. 2. Check that there is absolutely no tilt in imaging train and focuser is well collimated as well. I had to switch to threaded connection because of slight tilt. 3. Reduce distance to 55mm and work your way up from there - try to find sweet spot (best reduction with the least star distortion) - experiment with focus position for best results. 4. Understand that you introduced another optical element in optical train and that it is not perfect - so star shapes will suffer (just how much depends on optical quality of item). HTH

Nope - round stars - not good indicator. All that tells you is that guide error is randomly distributed between DEC and RA (same RMS in both, and both errors being random). There are two indicators of good guiding, and none is perfectly reliable - guide RMS, and star FWHM. Star FWHM will be subject to seeing, but if seeing is relatively constant (or similar between nights) - star FWHM will be smaller when guiding is good vs when it's not good. Guide RMS is good indicator provided that your guide resolution is sufficient to resolve guide errors and that seeing is not totally poor (not worth imaging, or some serious low level disturbance like chimney exhaust right on optical path). For most part seeing is not causing poor guiding - if you visually observe at high resolution, you will notice that majority of time seeing disturbances are very high frequency - it either blurs target (for example planetary) - this means disturbances are too quick for eye to see as shimmer of you can actually spot waving / shimmering / wobbling (it shows well on Moon) - but you will notice that it is also high frequency - changing position many times per second - such effect averages out in duration of normal guide exposure (2-3 seconds). If you suspect seeing is culprit - just use longer guide exposure - if you have fast changing PE it will worsen RA stats, but it should improve DEC stats/graph (this is how you will know - it will start to look more saw tooth). If you have well behaved mount (no fast PE component - so smooth PE, and not very big) - neither PE nor PA will cause big guide RMS (provided that PA is reasonably good - even if you don't drift align). Most guide errors are caused by mount mechanics surfaces being rough (poor bearings, rough surfaces, mount being loose) - to lesser extent, and wind and other factors (cable snag) to greater extent. You can see this by looking at DEC graph. There is no reason for it to jump around if PA is only reason for DEC to go out of normal position - but DEC tends to jump around pretty much - since DEC is usually not moving when tracking it can only be due to rough mechanic surfaces and external influence like wind. Otherwise it would slowly drift out of position just to be brought back (saw tooth pattern). There is another thing that limits HEQ5/EQ6 class mount in guide precision - that is stepper motor micro step precision. Best you can hope for HEQ5/EQ6 mount, if tuned (belt mod) on windless night of good seeing is around 0.4-0.5" RMS - you simply will not be able to do better with these mounts. On such night you will often see DEC error being a bit larger than RA (if you have your PEC in place and mount is not suffering any short period PE component). It is because RA is in motion, and changes micro step very fast, about 100 times per second or so, and if microstep is missed it will not show in 2-3s guide exposure (mount inertia will make sure such miss steps are smoothed out). DEC on the other hand is in dynamic balance - and stepper micro controllers are known to sometimes miss micro step if under load. So DEC will tend to "oscillate" around position where it needs to be (misses micro step - there is stronger pull to get it back to where it should be - there is small jolt due to air motion, or vibration - it overshoots on other side, guide pulse reacts, sometimes it undershoots, sometimes it overshoots - a sort of random oscillation forms). There is another component that is impacting guiding (if you are using OAG) - how well scope is attached to mount - that includes any slack in tube rings, is there even minute flex between scope and dovetail bar - how well dovetail bar sits on mount. Scope length will play a part in this due to arm momentum. Bottom line - for HEQ5 / EQ6 class mount, best you can hope on a good, still night with tuned mount is around 0.5". A bit of breeze will bump this to 0.6-0.7" or above, depending on scope size, weight and connection to mount. A bit stronger wind and you are looking at 1" or above of guide RMS. For guiding below 0.2" RMS you want premium mount (preferably on solid pier, shielded from wind - then you can go as low as 0.1").

In general I do like it, I love the detail and 3d kind of feel to it. I love color balance and palette. Flame bothers me a bit I must say, composition wise - it somehow detracts from the image - both placing / brightness and color seems to be "out of place" on otherwise superb image. It also looks flat compared to the rest of the image.

Yes, well, given that I suspect them sharing design and optical glass elements, and also there has been another thread on 125mm questioning quality of that scope - for that reason I view them as siblings - somewhat like SW 100ED, and SW120ED. Reviews of both seem to be quite scarce. I've also considered getting 125mm, but like you said it is in different category, both weight and focal length. Aiming for all-rounder, 975mm focal length is not quite suited for widefield, though I have no doubt it would be better scope for planets and DSOs.

Just an update ... After a bit more research (I'm itching to pull the trigger, but cash flow has stalled somewhat, so it's not an easy decision, and there is justification for fifth scope to be made ... ), it turns out that both TS 125mm and TS 102mm have better color correction with respect to SW counter parts - 100ED and 120ED, at least according to two sources: For 102 (Stellarvue Access 102 seems to be exactly the same scope rebranded, even focuser is the same, so Kunming Optical / Sky Rover): https://www.cloudynights.com/topic/578894-stellarvue-access-102-first-impressions/ And for 125 (in french): https://www.webastro.net/forums/topic/161648-lunettes-ts-photoline-12578-ed-ou-1027/ Unfortunately, no data on sharpness is available (except quote of Strehl 0.979 on one item).

Quite right , so my statement "... not energy but rather wavelength" does not make much sense. I was trying to say that it is not only energy that dictates likelihood of interaction. Here I go again writing nonsense There is greater chance of interaction at certain energies than on other? \

I don't think it has to do with individual photons energy, but rather their wavelength. Longer the wavelength, less chance to interact with matter. It is actually mismatch between wavelength of particle and atomic structure of the matter. So both long wavelengths go thru (like radio waves / microwaves), but also x-rays and gamma - very short, so cross section is small. Anyway it can be easily checked. Do google search for "Is plastic IR transparent", and "Is plastic UV transparent". Results should give you some hints which is more likely culprit.

Plastic cover does indeed remove all visible light, but IR might get thru it. Since the mono version does not have IR protection window, it is a good thing to protect it otherwise, like using aluminum foil, or something else that is opaque in IR part of the spectrum. I usually take my darks by placing camera "face down" on wooden desk (covered with plastic cap of course), and have not had issues with dark calibration.

Try stopping down aperture. F/6 will introduce quite a bit chromatic coma into spectrum. Again above mentioned spreadsheet can be of help in determining correct parameters. If you want to minimize seeing impact, go with short focal length, or move grating away from sensor. Another important thing is not to focus on star! Focus instead on spectrum, or part of spectrum that you are most interested in. Due to angle of spectrum (that gives spread over sensor surface), that is made after objective, focal point is shifted between wavelengths - think isosceles triangle, but one side being perpendicular to sensor - other side will not touch sensor surface but rather end some distance to it. So if 0 order image is focused, first order image will fall short of sensor, and if you focus on first order, 0 order will fall behind sensor - therefore it will be defocused at sensor.

Just a couple of points: I think that it is better to aim for lines that are more than single point thick - I'm not sure if line that is made from single printed point is going to be strait enough or possibly have holes in it or something. Go for 4-5 dots per line instead. That will give you 10 lines per mm in case of 2400dpi (~90 dots per mm - 9 dots per line + space for 10 lines per mm, 4.5 dots line width). Grating resolution is determined by total count of lines, so 10 lines per mm can sound very low, but put it on 80mm objective and you will have R800 resolution from it. So depending on aperture, few lines per mm is not such a bad thing in terms of resolution. Another thing to consider is dispersion. More lines per mm you have, higher dispersion, or angle of diffracted light. This is important because it helps you determine how spread the spectrum will be on sensor. Here again you don't want very large dispersion in objective grating because you will need wide field instrument in order to record it. For example SA100 has first order spectrum of 550nm light at angle of ~3.15 degrees (if I'm correct with this calculation). So you need very short focal length instrument capable of wide field in order to record it. My TS80 with ASI1600 has 2"/pixel and it would take 5670 pixels just to record 0 order to 550nm (ASI1600 does not have that many pixels ). But let's look at dispersion for 900nm and 10 lines per mm grating. Again if my calculation abilities are up to task, that would give us something like half a degree. That means that I would need about 900 pixels on my ASI1600 + TS80 to record full spectrum - from 0 to 900nm. This again is not really the best option, as sampling would lower achievable resolution - in this case to something like 2nm. So for 10 lines per mm I would probably use RC8" as it gives me 0.5"/pixel with ASI1600. 0-900nm would cover in this case 3600px (camera has 4600px in width). Sampling resolution would be 5A (pretty good), and the whole system would be seeing limited. So one can fiddle with these numbers to get best match for their instrument (focal length, sensor size and resolution, etc). TransSpec v3.1 that Merlin66 attached above, gives you spreadsheet to play with some numbers, it has objective grating tab, but it is initially geared towards SA100 used as objective grating on short FL DSLR lens. By changing numbers you can see different calculations for this type of printed grating as well. Btw I just realized that Bahtinov mask is very low resolution objective grating And it really works well, here we can see at least 5 orders, and mask it self has grating that has many mm spacing.