vlaiv

I have that GSO (Revelation) x0.5 1.25" focal reducer, and here are some things that you should know about it: 1. Focal reduction depends on distance between focal reducer and sensor - greater distance, greater reduction - use this in combination with above diagrams - start with close distance and small reduction to see if you are getting usable field at all - then extend distance up to point where it still works. 2. I had to reverse lens in mine - it was set one way at factory but I found that for imaging it works better by reversing the lens in holder (don't ask me how I got idea to try it out - probably read something somewhere). You might want to try which way it gives you better image, by undoing retaining ring and just flipping it around. Use some sort of soft cloth to handle the lens - don't touch it with your fingers to avoid staining it. You can do this at prime focus of scope with your guide camera to avoid any OAG related complications.

I'll try to explain with diagrams, first just reducer physical length: Regular diagram: Diagram with reducer in place: As you see, in order to focus both guide camera and imaging camera at the same time, because you added focal reducer before guide camera and thus pushed it back, you need to add optical path between OAG and imaging camera. Next diagram will try to explain additional distance needed because of the way simple reducer works: This one is trickier to understand because reducer bends rays - this is why we need inward travel of focuser for simple reducers like mentioned two element 1.25" x0.5 reducer. Actual sensor will lie where bent rays meet to be in focus, but at the same time imaging sensor needs to be at a distance equal to where guide sensor would be without bending of the rays - further out. Hope this makes sense

Forgot to add, if you can, guide with ASCOM driver and higher bit depth, rather than native drivers and 8bit.

It will not work (at least I think so, below is why). All it can do is shrink available field onto smaller region of sensor, but ultimately field of view is limited by pick-off prism and more importantly opening in prism holder. I currently guide with a bit larger sensor - ASI185, which has diagonal of ~8.6mm at 1600mm FL - F/8 beam. I get vignetting on this setup, here is screen shot: Other things that might interfere with using focal reducer are: - do you have enough back focus to fit body of reducer between sensor and T2 connection of OAG? Additional distance must be mirrored between OAG and main camera as well. Focal reducer moves focus point inward - this means another change in imaging sensor position vs OAG (not sure if this will bring it back forward or does it need still being pushed further out - probably this second point). What about fixing problem of guide stars in another way? Things that you can do to guide on fainter stars: 1. Make sure your OAG is focused properly. Although you can guide on slightly defocused star, this is not a good thing if you want to use faint star for guiding. You want star light to be concentrated in smallest possible area to maximize star profile and SNR. 2. Position prism as close to light beam aimed at the sensor - just barely avoiding prism shadow on your imaging sensor. This means rotating prism so it sits next to longer sensor edge. Further out in the field stars become more distorted due to different aberrations like coma or astigmatism - look at difference between stars in above image - in right part they are more concentrated than in left part of the field. You want to pick up stars in the least distorted part of the field - again has to do with star light concentration into smallest region and SNR 3. At longer focal lengths you can use a simple trick because with small pixel guide camera you have enough precision - bin your guide camera output. It does not matter if it is true CCD hardware binning or CMOS software binning - it will increase SNR of stars and make it possible to guide on faint stars. 4. You can always increase guide exposure (up to mount limit), if you are used to guide at 1s or 2s exposures and can't find guide star - why don't you try 3s or 4s guide exposures? Sometimes in poor seeing I go as long as 6s or more with my HEQ5 (it is tuned and belt modded, and that is above max exposure that I would otherwise use on it but if seeing is poor, guide performance is not going to be the best possible anyway).

F/ratio plays significant part in performance of CC - SW CC is good for F/5 scopes, although I'm not sure how much SA there is with that F/ratio. Scope used in comparison of coma correctors is F/4. 8" F/4 has focal length of 800mm - and that puts it in right spot for 1"/pixel with ASI1600. F/5 8" with x0.9 reduction will provide 900mm focal length - that is 0.87"/pixel with ASI1600 and I would personally rather be on north side of 1"/pixel than below it - that would mean ASA x0.73 CC - and that one will not provide fully corrected field for ASI1600. It will for ASI183, and I think that would be really good combination, but then again I would bin ASI183 as well for effective resolution of 1.36"/pixel rather than leaving it at 0.68"/pixel. There is characteristic signature of SA on star shapes - most SCTs have such stars (SA depends on wavelength with SCTs, and also with primary/secondary distance that changes when you focus) - almost "button" like rather than point like - smooth vs sharp so to speak.

After reading posts like this one: http://www.astrofotoblog.eu/?p=856 Not really convinced in perfromance of coma correctors and fast newtonian systems. Granted 200PDS is not that fast at F/5 - but slower the newtonian - longer the tube and harder on the mount and guiding it is.

I agree with you on that one - aim for final resolution, but this is where I found RC to be the best match. Let's say that we want ASI1600 and 1"/pixel resolution with 8" aperture on HEQ5. That means focal length of about 800mm. There are couple of options to go here: 1. F/4 Newtonian with native pixels, or F/5 with reducer CC 2. F/8 RC with binned pixels. For option 1, we need good CC. I'm not sure that I've found CC that will correct coma to good degree but not introduce SA or something else, over large enough field. Best CC that I've seen in terms of correction would be ASA x0.73, but it has 17mm corrected circle (or there about). It also means F/5 scope - and that will add its own complexity in terms of bulk being put on HEQ5. Somehow I can't seem to find good 8" Newtonian candidate for 1"/pixel - maybe I just have not searched enough. With option two we have nice compact design that sits well on Heq5. It is flexible in terms of resolution / focal length as there are reducers for it - x0.67 or x0.75 (I would say that any reduction past x0.72 will not work well on this RC and asi1600 due to corrected field - x0.67 reducer can be "spaced" to x0.72 - x0.75, but there is also very good reducer - Riccardi FF/FR - x0.75). Binning ASI1600 in software is exactly the same as using camera with larger pixels and having larger read noise - one of 3.4e read noise - still better than most CMOS cameras out there in terms of read noise. Only drawback for RC is FOV, and for galaxies it is of course less important. So if we go by aperture at resolution (around 1"/pixel), I think RC is very viable if not the best option in 8" class to be mounted on Heq5 (with given camera). Being true mirror system it has other advantages - broader use as astronomical instrument - for spectroscopy, photometry and astrometry - I mean it is most widely used by professionals in scientific role because of its characteristics.

Hi, yes, I ended up upgrading it precisely because of lack of threaded connection. Stock focuser was otherwise quite usable - never had slippage issues with it, but I try to keep my imaging train light. I plan to use that monorail focuser on my SW Evostar 100 F/10, just need to get suitable adapter to it (OTA tube to M90). This is the one I upgraded to: https://www.teleskop-express.de/shop/product_info.php/info/p6970_TS-Optics-2-5--Rack-and-Pinion-Focuser---holds-Acc--up-to-6kg---travel-53mm.html I also have this extension before focuser, screwed into OTA: https://www.teleskop-express.de/shop/product_info.php/info/p2773_TS-Optics-50-mm-Extension-Adapter-for-M90x1-thread.html At the time of my purchase it was not included into delivery (I have TS version - regular one, not carbon fiber), but from what I see, TS includes it now with their RC units. I also added rotator and suitable thread adapter (because need to mount 2" filters and reducers): https://www.teleskop-express.de/shop/product_info.php/info/p9781_TS-Optics-360--Rotation---Thread-Adapter---M63-to-M68--M54-and-2-.html + https://www.teleskop-express.de/shop/product_info.php/info/p6400_TS-Optics-Adapter-from-M54x0-75-to-M48---T2-Focal-Adapter-for-M54x0-75.html

+1 for RC 8" - I have such setup and very pleased with it. I also bin x2 in software for around 1"/pixel. It is on edge of HEQ5 capability - in terms of guide performance, you need nicely tuned mount to be able to guide at around 0.5" RMS total. Scope hold collimation very well - I setup each session and I've collimated scope only twice (well, one could say that it was single collimation, because I did not get it spot on in first round, so I had to repeat the next day).

You can't lock the mirror since it is not movable. Problem with 6" and 8" models is that mirror cell and focuser attachment are in one piece. There is tilt mechanism for focuser - it is "squared in" by design and manufacture. This as a consequence has a problem if too much weight is hanging of the back of the scope (focuser included) - as it will move the cell together with the mirror. I've checked my scope and one of the collimation screws was indeed a bit loose (1/8 of a turn) - but I think I now wrecked collimation - I forgot that smaller screws are collimation screws and larger ones are locking screws so I tightened up smaller screw (instead of just making sure cell is locked by larger screw). Will need to check it under stars. Don't know if I'm over weight limit, really should not be - using ASI1600, OAG, and filter drawer - all of those are fairly light components (in their class). Putting lightweight step motor and bracket shouldn't really be a problem. I've decided on "serial" configuration, and made sure both fine focus knobs are removable - and indeed they are, simple set screw. Fine focus shaft looks like 4mm (or maybe 3mm I need to finally get myself a caliper one of these days) - and this seems to be problem on its own . Checked all local retailers and they have elastic shaft couplers in every possible combination except one that I might need (3-5mm or 4-5mm, neither is available, every other combination under the sun is in stock).

But I did consider buying one at first - until I calculated total costs and saw that it is quite easy DIY solution - very nice for a small project in winter months when weather does not cooperate. At the moment I think I'll look into building L bracket - lego way - out of two or more pieces. Retailers that deal with stepper motors and "robotic" accessories have generic mounting brackets - so maybe I can figure out some way of fashioning suitable L bracket - bolt here nut there ...

More I think about it, more I'm in favor of "in line" approach vs belted side by side configuration. That would mean removing both fine focusing knobs, but I guess that would be ok as it's "non invasive" operation - I can mount them back without much hassle.

Thanks for that info. I did look at usb focus as an option, but I can't justify the cost at the moment. My dealer has kit listed at around 180e without holding bracket, brackets are around 85e and additional motor is around 90e That would total to about 570e with VAT and customs fees - pretty steep. I believe that I can motorize both focusers for quarter of that price if I choose DIY route.

I just realized that I will not be as straight forward as I thought. Although both focusers are of the same name - TS 2.5" - they differ in fine focus knob and travel per turn. On RC I believe that fine focus knob will have to be replaced with either pulley or coupling, depending on mounting position (below or to the side). On TS80 APO fine focus knob already has proper grooves that I can use to put belt on. I was hoping to leave focusers "intact" and just mount motors to them, but it appears that I will need to mod at least RC focuser. I've found good "summary" resource on this one that helped me understand differences between mounting options: https://astrojolo.com/astrolink-4-0-mini/focusing-stepper-motor-solutions/ According to that page, coupling to fine focusing shaft is pretty good option. Did some calculations and it looks like there will be enough resolution with 200 step motor and fine focus reduction to be able to go as low as F/4 - which is enough for my needs. Now I just need to find where to source pulleys, belts, couplings (I can do it online, but would rather like to inspect items for suitability prior to purchase).

This is pretty much how I thought it will look like - I'm also thinking about Nema 14 motors in combination with Arduino nano and a driver. Found where to source most of the items, just need to see about the belt and pulley, and to figure out a bracket for attaching motors (that will also probably be DIY out of piece of metal sheet - little drilling and bending). Found this open source project: https://sourceforge.net/projects/arduinofocuscontrollerpro/

Thanks! Do you mind if I ask couple more questions since you seem to have similar setup? Since there is no position encoder, I presume that it will be ok to use position marks on focuser tube? Focusers have mm markings on them, and for instance, on RC in certain setup (I will probably have different setups depending on use of focal reducers and components in imaging train) focus position is at about 20mm - so I can set range of motor focuser to be between 10mm and 30mm (with appropriate number of steps) - so each time I want to use different setup, I position focuser on 10mm (or equivalent "start" position) before powering everything up and tell it that it is at step 0, right? How does above relate to focuser calibration in SGP? How often do I need to create full V curve? If there is position change (either due to me fiddling around, or mentioned slip / drift) will SGP cope with that (it should since V curve is just approximation it should find right focus position by half flux radius anyway)?

I think I pretty much made up my mind on this one, but would like to hear any pros and cons from people using these. I'll probably go for DIY solution since I have two scopes with same focusers (2.5" R&P TS), and that will help me "split" costs - I'll have two steppers - one on each focuser and single control box (probably attached to mount). I'm not entirely sure what my concern is, I can only see benefits of using ASCOM controlled, motorized focuser - coupled with SGPro. I just want it to be minimal hassle to connect and use. Actually here are my concerns about this: 1. I'm feeling that I'm already approaching weight limit for "stuff" on focuser side for my RC8" - last imaging session I noticed change in outer field astigmatism as scope moved around the sky and mirror was in different positions. It was very slight effect but it points to one of these things: mirror is coupled to focuser so total weight on focuser is causing slight mirror tilt at different positions, or weight of all the items on focuser creates "sag" somewhere along the way (threaded connection to focuser, quite a bit of spacers, extension tubes, so something might have some flex in it), or hopefully, collimation locking screws are a bit loose (did not check it, but will do first thing), so tightening up those will help. Stepper+bracket can add up to 300-400g on focuser side of things. 2. DIY design that I looked mentions temperature compensated design - I don't get this one, nor understand if I need it at all. My view is that I can set up SGP to do refocus periodically (like every half hour, or one hour) and I don't need to monitor temperature and refocus on ambient temperature change. Or is that better approach? 3. Since I don't want to mod focusers too much - I plan on using belt system coupled to 1:10 knob. Somewhere I read that this is not the best approach due to wear&tear of micro focusing system - but this approach allows me to use full steps (no need for microsteps to achieve needed resolution), and it does not require modification of focuser - I just attach bracket with motor and put belt on. This is related to my next concern 4. Both focusers are rack and pinion, and have lock screw that I usually tighten after finding good focus position. With motor focuser I presume that I need to leave this lock screw unscrewed. Will motor be able to "hold" position and focuser from changing focus / slipping (although it is R&P and should not slip but rather "unwind" under load) if I couple motor to micro focusing knob (I guess that can slip due to design)? a penny for your thoughts?

Just had a look, if your 70mm Quad is Altair Astro one, with ASI1600 you should have pin point stars across the whole field. They have spot diagram on their product page and it shows that spot diagram is less than airy disk even at 15mm distance from center - ASI1600 has diagonal of about 23mm so center to corner is less than 12mm. If there is dedicated tilt adjuster - it is there for a reason. Elongated stars in one corner or at one edge mean just that - tilted sensor. Above CCD inspector profile also suggests tilted sensor - just tilt your head to the right when looking at it and you will get much more acceptable looking field. According to description, there is tilt mechanism: "The Starwave 70 EDQ-R Quad APO is intended for astro photography and the focuser terminates in an M48 Male threaded rotator plate with locking thumbscrew. All screws are teflon tipped and grip on a flange for accuracy. The rotator plate includes a "push-pull" tilt adjustment system to enable you to square up the image sensor to the focal plane. " You should try adjusting tilt until you get symmetric looking graph in CCD inspector as a start and then look into any further issues (if there is any left).

Two things come to mind that can cause this: 1. Being quad, or having front lens, and integrated dedicated flattener - just means that flattener is matched optically to front lens, at a correct distance - so you are right, when in focus you should also be at optimal distance for flattener. Question is are all elements in scope properly collimated? It could be that rear element is slightly tilted or something. Does manual mention collimation or adjustment of any kind for this? Refractors ought to be factory collimated and very rarely need additional adjustments, but sometimes they do (for refractors, often that adjustment needs to be performed by skilled individual rather than "general public", unlike collimating newtonian or other mirrored systems). 2. Sensor tilt. Is focuser adjustable? Can you compensate for tilt in any way? It might be that you need separate tilt element to achieve sensor orthogonality to principal axis. No field flattener can provide "infinite" flat field, and usually there is technical spec on imaging circle, including sometimes just corrected field diameter and illuminated field diameter. More serious analysis and specification gives spot diagram over field and also illumination curve depending on distance from optical axis. Often if its just diameter of corrected field - it is more "acceptable" field rather than diffraction limited field, so you can start to see distortion near the edges of the field - something that spot diagram would clearly show. What is the size of sensor and does it match corrected field?

I've found this useful, and modified it slightly for my use (not using Bahtinov mask, but rather fwhm measurement in SharpCap): https://deepspaceplace.com/gso8rccollimate.php

I'll present my view on this topic, it might be a bit controversial, and it is not based on very big experience (although I've got some experience with RC scopes - I own larger brother of mentioned scope - RC8" F/8). I've done a sort of comparison of scopes in this class (6") and here is what I think: Let's take 5 representatives of different scope designs and compare, Newtonian, Refractor (we will limit ourselves to achromats due to price difference, and let's be honest, good APO triplet in 6" class will simply outclass all other designs on most comparison criteria), SCT, MCT and RC. RC vs Newtonian: - Newtonian will have an edge on planets, especially F/8 variant with small secondary obstruction. Such scope will be harder to mount (due to momentum arm, except for Dobsonian mount), but will have smaller FOV than RC (due to 1.25" focuser - because of small secondary). In faster ratios coma will be an issue. RC will also have larger light throughput - due to 99% dielectric coatings, regardless of larger CO - newtonians usually have 94% mirror coatings (enhanced versions) - you can go for special coatings like 97% hilux - then they will be better matched. - RC has better corrected field for AP so you can use pure mirror system without need for corrector. Corrector can be used to lower F/speed and further flatten already pretty flat field. Due to large illuminated circle - you can use 2" eyepieces and combined with focal reducer you can have wider views. - For planetary AP, due to processing and sharpening, central obstruction and loss of contrast have tiny impact, so these two will be pretty much matched (long focal length newtonian). - Price wise they are very close, newtonian being slightly ahead (cheaper). - Light baffling and stray light protection - win for RC. - Collimation ease - newtonian wins here. RC vs SCT - Shorter focal length and larger FOV for visual (f/9 vs f/10, 2" focuser). - Less prone to dew problems, and better thermal properties (open design) - Better photographic field (no coma, less curvature, ...) - Similar "format" for mounting, similar weight - SCT will have very slight edge on planets due to somewhat smaller CO, but light through put will be on RC side (again depends on mirror coatings, but SCT has additional corrector plate). - price +RC, -SCT RC vs MCT - just look at difference between SCT and MCT usually mentioned on internet (thermal stability, planetary performance, smaller FOV) - and apply to previous section (RC vs SCT) - all RC strengths will be emphasized, while planetary performance will lag. - price +RC, -MCT RC vs Refractor - it all comes down to fact that refractor - especially faster like F/8 or below will have very big CA issues - this means less contrast on planets, less contrast on DSO in spite of better ligth throughput - no CO at all. Photographic usability of such scope is limited to narrow band (you can do LRGB or OSC, but CA will have huge impact on final result). - Achromat will be heavier and harder to mount, and it will loose in price department. All in all, for that target budget, I think that RC is very overlooked option for good all around scope - both AP and visual. If you are worried about contrast loss on planets, have a look at this: This is simulated MTF of 8" RC vs 5" refractor (both ideal figure). MTF diagram is usually used to represent contrast loss - X axis represents spatial frequencies (or think in terms of large/small features, large features close to origin, small features to the right) and Y axis represents contrast loss (or attenutation in %, going from 0 at origin to 1 or 100% at top). What you don't usually see is such diagram comparing two different scopes - different aperture sizes and different CO characteristics. When you align spatial features axis (X axis) then you can see that smaller scope, although having "higher" contrast, actually looses on detail, and if you use scope with large aperture and CO with small magnifications (equivalent to what you would use with smaller unobstructed scope - you can actually have less contrast loss). Bottom line, RC might not perform as good on planets as other 6" options, however, it will probably be on par or even better than 4" apo if you keep your magnification the same (up to x200). Just to mention, those RCs seem to be optically very good instruments - I tested mine to system Strehl 0.94

It's a spacing issue, and a tilt issue. Spacing issue is going to be trial and error approach. It depends on size of chip, focal length of scope, focal ratio of scope, so it is not always "prescribed" distance. I've looked at some flattner (with reduction) specs, and they can vary greatly in "optimum" distance based on telescope type (F/ratio and focal length). Also some flatteners are designed to certain corrected field - so one can expect correction over given field, but if sensor is larger, outer parts will suffer and not be fully corrected. Procedure is simple - start at some distance and increase/decrease as long as you see improvement. Combine different length extension tubes and use distancing rings for fine adjustments (0.5-1mm range). First step in solving tilt issue is to go with threaded connection. One can probably fix major issues by using high quality / self centering standard connections (baader click lock and alike), but I think that best solution is threaded connection. Adjusting focuser for being square with lens is another thing that can be done (if there is some play in focuser, or if it can be collimated). As a last resort - there is tilt adapter that can be used (but it adds optical path, and probably best for permanent setups).

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.