AstralFields

Yeah , it is the same telescope. Using the 4x Powermate shifts your focus outwards. I managed to resolve the backfocus issues but it could've been done smarter like Sky-Watcher does it. They provide 50mm extensions and once removed, you get an extra 50mm backfocus.

I've had an 8" Sky-Watcher 200p for about 2 years. While it was the best beginner telescope I could've possibly pick, I started thinking about an upgrade sometime in the future but I decided to wait until I would complete the full Messier catalog. My thinking was along the lines that until I get the whole catalog completed, I wouldn't know if I still would want to stick with this hobby and invest more money into it. Also I just felt I should somehow 'deserve' a bigger telescope. Another reason was that the 8" is an f/6 telescope and while my budget eyepieces worked reasonably well there, I was afraid that in the brighter f/5 they would look pretty lousy, so there was not only the investment into a new telescope but into new eyepieces as well. And yes , I was right about that part, the budget stuff is simply not suitable for telescopes under f/6. They should ideally be used at f/8. At the same time I discovered a nice APM 30mm UFF clone from Aliexpress for about 180$, which is a great deal as well as a Panoptic 14mm clone for like 83$. I bought both of them and they are just amazing in the 12" f/5, which is great as these two focal lenghts are used in my observations for like 90% of the time, the rest is done with my SVBony Zoom 7-21mm which is a great performer in the new scope as well. So basically I will be selling most of my budget stuff along with the 8" during this year. On to the scope itself. It arrived in two big boxes, the guy in the van warned me that this is some heavy stuff. I've been worried about the weight of this thing from the moment I clicked the order button. The whole thing cost me 1020$ , although I could've saved 30$ had I used my local currency. Had no idea they accepted it in the neighboring country. Very good price anyway as locally they go for 1100 to 1200$. Why did I choose the GSO and not my beloved Sky-Watcher? Well.. Sky-Watcher apparently is not that great after all. Sure the mirror is superb with Pyrex and all of that but the rest is nothing to write home about. GSO Mirror is BK7 - so a bit worse on the thermal side of things but it comes with a fan at the bottom pre-installed.. so no biggie. Also I store it in my cellar which is not that bad temperature difference wise to the outside. And I also am more of a DSO than planetary guy.. so there is that. Focuser - Hands down the GSO is two leagues better than Sky-Watcher. It also has dual speed. The adapters of the GSO are a LOT better with compression rings allowing for a tighter more accurate fit. So good that my Cheshire and Laser collimator finally agree. I could not get them to agree in the Sky-Watcher due to lousy adapters. GSO - A big downside is the absolute lack of backfocus... Most stuff focuses quite closer to the secondary mirror even when there is a lot of focus travel to use, never-mind there is also a 35mm extension for 2" which doesn't get any use. Raising the primary to the maximum the screws allow alleviated a bit the problem but it was so bad that I barely got focus after raising the primary in my 3x Barlow. Sky-Watcher is so much better here and it provides the possibility to focus and attach a DSLR. That is not possible with the GSO if I ever wanted to get one. The only way I got my security/astro 4K cam to focus was to put the whole thing into the 2" extender... well at least I got it to work. Finder - the finder is a regular straight view 40mm finder. After unboxing it and checking it out I put it back in the box. I hate optical finders as I find a laser and RDF combo so much better and so much lighter. With a 19kg OTA, every gram I can save counts. Eyepieces - The GSO here wins hands down. Sky-Watcher cheaps you out with a 10mm and 25mm Plossl (a.k.a are you freaking kidding me??). GSO is extremely gracious that it gives you a 30mm SuperView Erfle which normally goes for like 70$ to 100$. Not a bad value at all. I will probably sell it as I have the far superior 30mm UFF now. It also gives you a 9mm Plossl which I will also sell as the eye relief on those things is abysmal. The bearings Alt / Azimuth are also better, the GSO comes with a lazy susan type. Maybe the Sky-Watcher 300p also has it, not sure. But I can say that the motions of the 12" are a lot better than my 8", both Alt and Azimuth. Another big plus for the GSO. Also the GSO is a lot cheaper than Sky-Watcher... so really, GSO all the way in my book. Collimation was totally messed up from the factory, the secondary was like 15mm off vertically. I don't think they care at all. Doesn't matter, I am pretty good with collimation so I got it spot on. Transporting the thing was not as bad as I had feared, especially after I installed some wheels and carrying straps. It basically made no noticeable difference to my observation preparation, drive and setup.. like maybe an extra 5 minutes out of the one hour it takes me to prepare, pack, drive and unpack at the destination. The weight is perfectly fine once you have a good way to strap that thing. I use two carrying straps and one extra to put over my shoulder. My son is 20kg and I carry him all the time.. so it is not the weight itself but more about figuring out a way to properly attach that weight and distribute to the body. The base is basically the same weight as the Sky-Watcher, so no difference there at all. The first night observing was a real blast. M51 - We all saw the dust lines pretty clearly that night as it was at 90 degrees up in the sky on a clear night M104 - Looked amazing and showed clear structure. Leo Triplet - Fit nicely in the 30mm UFF and they looked very clear, bright with easily recognizable structure. The Ring Nebula was also amazing as was the Cat's Eye. Checked also M13 and M92 and again, very very nice in the 14mm panoptic clone. The Beehive Cluster in the sharp edge to edge 30mm UFF was out of this world, simply great to look at. Going through the Virgo cluster with the 30mm UFF yielded a ton of NGC galaxies, easily visible with direct vision and actually confusing us from identifying the Messier galaxies we were looking for. A big step-up from the 8" where only the brightest galaxies are easily visible. I also got to compare the Kellner 32mm eyepiece to the SuperView Erfle 30mm and the APM UFF 30mm clone. It really showed nicely why the first one costs 45$ , the second 100$ and the third 180$. I will be making a video focused on this and why there is such a difference. That is about all for now, I will be making a proper detailed review half a year down the road once I've gotten some field experience with the scope under various conditions and done some imaging.

Me too! Right now working on an unboxing / overview video which I will put out in a week or so, but the detailed review will be few months from now, to ensure I get some experience/feelings by using the telescope over a range of targets.

I love them. When I decided to buy the 12" , I bought and got the wheels first . I am never dragging another DOB base without these on it , especially not a 12" Base

Yes I know ... sorry Central Europe.. it is cloudy, raining and even snowing 😄. At least I had ample time to put together and now to collimate.

For those who may be interested to check on how it went

The 8" is in the background yes .. and they say , always upgrade at least a magnitude higher

I always say, get an 8" and get most of the Messier catalog done with that telescope. Do some Solar System observing and imaging and then you will know if you should get a bigger scope. Been there done that.. and I know now! Bigger it is! So what is it??

The thing about the controller.. I already have one laying around here, got it back last year where I built my own planetary motor out of Lego (long story) .. so it is just a matter of rewiring it to the platform. I did settle down on a constant speed towards the end of the observation but it just felt like I could use some minor micro tuning of the speed back and forth.

Mission Accomplished. As they say, pictures speak louder than words. This is M3 with a more or less OK alignment. I don't have a proper DSO camera so this was with my cellphone through a 15mm eyepiece on my 8" DOB .. 80x magnification. the Aluminum segments that you find in most plans are definitely not needed. Wood is a lot easier to deal with and lined with a bit of door tightening rubber it runs extremely smooth.. no jerky motions of the objects whatsoever even at 800x. There is some drift at this power, but it is smooth and mostly to not exact alignment/speed setting. I captured all of this on video of the Moon and will be compiling it into a nice 10-15 minute video of how I made the platform from start to finish with the key lessons learned. The price tag is really small for this one, given the performance I am getting. Next time I am doing it with Plywood. When you break it down , there are just few parts: Plywood - about 30$ EQ 2 Motor - 37$ from Aliexpress with shipping Wheels and pre-cut iron rod 0.8mm - 10cm long - about 10-15$ Some spherically curved bolts - 2$ Regular wood screws - 1-2$ optional: Li-Ion rechargeable 9V batteries - about 8$ (which I bought) A hand-held controller for the speed of the motor - they go for about 2$. I will make one as it was very frustrating having the fine tune the speed by going back and forth from the eyepiece to the motor. All in all , a nice price tag of around 80$. A new one (albeit prettier) sells for about 500$ here with the same motor and same performance.. I may make a couple of these for a 12" DOB and sell them for 250$ each. Already two people expressed interest. It is far below my hourly wage when I count the time I need but it is fun and always nice helping out an astro buddy :). The biggest advantage of building one your own is you get to simply make it fit your telescope exactly and you understand how it works. Also you can cut the segments exactly for your needs. By the looks of it, this one should be OK for the 12" as well , which I already have on the way. It provides tracking for about 90 minutes, which is great and so comfortable that I found myself forgetting to reset the telescope after an hour of observation.

Almost there. There have definitely been some lessons learned along the way. 1) Don't use any drill adapters to cut wood. The damn thing broke 80% along the way and I am having to cut the rest manually. Just get a small hand held wood cutter. I probably will for my 12" DOB version. 2) Use plywood. The type of wood I am using is not the ideal, the top board broke in two so I had to patch it up. It will likely add a bit of bounciness to the system but shouldn't be too drastic. I can always replace it later on with plywood. 3) Don't store your only 9V battery in a box full of metal parts. Few days later - voltage ZERO. Just when I was going to turn on the platform for the very first time. 4) Aluminum is a tough nut. Not all Aluminums are alike. I was going to use a 3mm bar to cover the segments but that thing was tough to cut. I even gave up of putting it under the pivot. I am just going to hollow out a small hole in the wood. So I just made the segments out of wood and put some rubber on top of them to cushion the roll. All in all, few cosmetic cuts remaining but it really came along very well. Maybe Thursday I will get to test it on the Moon.

Yeah .. To put things into context, the errors I am speaking of are like 1 degree per hour of tracking.. pretty minor but worth sharing for any new buiders.

Also to come back to the printable segments shared in this thread. They are incorrect for all the latitudes. Seems like Reiner has counted with a tilt degree for the segments of 15 degrees but in the plans it is 20 degrees. For some of the latitudes he did not apply the segment tilt at all, making the error bigger. In practical terms this will induce some errors necessitating to change the speed of the motor during the tracking and some minor drift. After I fixed it for my latitude, the difference between 15 degree tilt or 20 degree tilt is obvious Sorry for the bad news to everybody who printed the segments for their platform. On the bright side, I guess they can be easily replaced. I may create a fixed version after I finish my platform for all latitudes as it is pretty easy using Libre Office - Draw Another mistake I have noticed in this thread, people have printed the segments at the wrong scale. Somebody asked about this and someone else took a picture of a ruler over the segment. When you open them in Libre - Draw using the in-app rulers for the page, it is clear how big they should be. One can even draw a simple line and see exactly to mm how long the segment should be. This is likely induced by the incorrect handling of PDR printing by the printer and some auto-scaling stuff going on due to printer margins. To avoid, it is best to import the plans in Libre Office - Draw. Fix the segments there and print the segment directly from the app. In order to perform the fix, one may need to temporarily increase the page size to 100x100cm as the ellipse creating the segments is that big. The exact dimensions of the segments are very important because the distance in the plans is fixed at 244mm between them (meaning they should be at a specific size). Again all of this can be verified in Draw. Not having them exact is again inducing minor errors in the system.

Update: All the stuff I need arrived from AliExpress and I have initiated the planning phase. Spent about 2 hours fine tuning the Sector 49 degrees using Reiner Vogel's approximation method. The segment itself was easy but it was important to have it at the right dimensions on the A4 so that it fits with the plan I have that has the distance of the segments at 244. Everything needed to fit, and I had to ensure I can mount them to the board. I used Libre Office Draw to create a printable, at scale' segment I can easily copy on the wood without much hassle. At the same time, I couldn't get off my mind the work HenkSB did here: https://www.cloudynights.com/topic/916052-vns-platform-sector-calculations/ He is using a much more accurate projection and was claiming Reiner's projection is off by 1%. That is something, that was bothering me. So I installed SciLab (was really simple on Kubuntu from the app store) and ran his scripts for my latitude. The ellipse that was generated was a lot more compressed than the one I had calculated using Reiner's plans. So I asked him about it, I thought I was not using his stuff properly but in the end it was a simple oversight. He fixed the code and lo and behold in the end Reiner's projections are only 0.1 degrees for a 90 minute tracking off. That is something I can live with. My woodcutting will likely introduce a lot bigger errors. Either way, great work by HenkSB to confirm the accuracy of the segments. I don't think anybody had done this before. He also made a nice animation that shows the movement of the segments in all three dimensions. They do move and this is something that should be taken into account when spacing the wheels. I am also skipping making the segments out of aluminum and doing them from the 18mm wooden board. Omegon has gotten away with a lot thicker segments, so I am hoping this won't matter much as they do tilt on either side. I was originally going to use some aluminum at the bottom of the segments, but I will try to skip that part.. as long as I polish the wood after the cut, it should be fine.. I hope. With the segments finally resolved. I put everything on the table for one final count that nothing is missing. Of course, stuff was missing, took me 30 minutes to figure out where I had put it and also had to locate a hex mini tool to tighten the wheels. After about an hour of some geometry fun with my daughter's coloring pencils, the board is ready for cutting. I don't have a woodshop and I don't want to make a mess out of my apartment, so I am converting my cheap drill into a wood cutting machine. Will be doing the cutting sometime next week in my shared garage space. Hope I don't make too big of a mess. The cut of the segments will be a bit tricky but I did order some some polishing bits for the drill to make the final touches. I am documenting the whole thing for a video as stuff like this should be out there for future generations. Have not found any simple designs documented on YouTube. The final price tag is right now at about 100$ , including the wood cutting adapter for my drill and some rechargeable batteries for the motor. The wood was pretty cheap, got it for 20$ including shipping as I couldn't find any good wood at our summer house. It is much more beautiful than some old worn-out wood. Maybe I will paint it white, or maybe not. We shall see.

I have experienced lifting the 10" in one go to move it a couple of meters. I have yet to lift a 12" but it should be fine when broken apart in two pieces. I am a relatively strong guy and can carry my 8" in one piece about 20 meters.

Oh yea, glasses are a complete deal breaker with some of these. I can't imagine using the Explore Scientific 100 degrees with glasses. Even without I had to push my eyeball to the eyepiece like I have to do it with a 25mm Plossl in order to see the entire FOV. Sure even when I couldn't see the entire FOV I was still seeing over 80 degrees of FOV from the 100 but this made me feel anxious and uneasy that I am not 'looking through the eyepiece as I am supposed to' types of feelings. APM had a much better eye relief.. the Morpheus as well. But so do my 30$ 'red line' SVBonys. The eye relief on them is amazing.

I was fortunate enough to join a Star Party for a night under a sky of 21.30 SQL (measured on the spot by somebody) , so a solid Bortle 3.5. There were a bit of high altitude clouds but it got better as the night progressed. Here are some of the key impressions I took with me, obviously they were based on limited observing time with the big scopes as there were literally lines of people waiting to have a peek at M51 and Orion. Aperture is absolutely everything. Even something as solid as M13 looked amazing in the 20" , not as amazing in the 16" and lousy by comparison in a Bino Combo Double Refractor 2x155mm with premium eyepieces attached. There were literally lines of people waiting to see M51 in the 20" , even the guy with the 16" came to check it out and he noticed a difference in the 20". Galaxy dust lanes are incredibly difficult to visually observe. Even with the 20" it was not obvious to the eye as it is with an astrophoto. Any other galaxies, we didn't even try. I assume observing the dust lanes has a lot more to do with having a Bortle 1,2 sky, instead of having a big telescope. I mean they were clearly there with the 20" , but they were not self-evident and easy to see.. You would wonder if you are really seeing something or the mind is just filling it up based on what should be there, as we know what to expect having seen the photos. This is entirely my personal opinion but eyepieces and the whole discussion around eyepieces is absolutely overblown. I did not see any lines of people to test some premium eyepieces or even many discussions around the types of eyepieces being used. Explore Scientific 100 , APM 84 , Morpheus 76.. it was all pretty much very very similar in my view. In fact the ES 100 did not feel comfortable to my eyes as I had to push really hard and really close with my eyes to get the benefit of the full FOV. I just didn't feel the whole 'space walk' experience... just another eyepiece with a field stop a bit further out than my 68 budget ones. Sure .. they were better, but not 500$ better compared to my 30$ SVBony 'red lines'. Same goes with expensive O3 filters. The guys were putting some O3 filters for the Nebulas and yes they were making the image more contrasty , but I did not see any visible difference over what I am used to have with my cheap UHC filter from SVBony for like 25$ and 40$ for the 2". If there is a difference, it is VERY subtle and in my view not a 300$ difference, which is what those O3 likely cost. GoTo - There were two identical Sky Watcher 400p GoTo telescopes as you can see below. One of them spent the night observing, the other spent the night trying to solve it's GoTo system. As the guy probably didn't know how to Star Hop, he spent this amazing opportunity at this amazing place.. troubleshooting technical issues and playing around with cables and what have you not. A similar situation unfolded also with the amazing Bino 2x 155mm refractor. The guy with the manual 20" spent the night happily observing with zero issues and has little trouble finding the targets even with such a huge scope. This only solidified my opinion that GoTo is not something I want to invest in. Even the guy who had no issues told us that it took him a long time to optimize his battery, cabling and so on to prevent having those issues. Having quite a bit of experience with the 8" was really a good thing. This solidified my opinion that a worthwhile upgrade to my setup is going to 12". The 16" is simply way too much of a beast and clearly a two person scope. The guy who had used to have an 8" and a 12" and he confessed that it is not a good scope to take with you, only if you already have the place and space for a solid fixing. The rest, you can check below:

This one .. very accurate Just found this amazing item on AliExpress. Check it out! US $9.39 50%OFF | Digital Display Inclination Box Precision Measurement with Backlight Magnetic Suction Protractor Inclinometer Angle Meter 1PC https://a.aliexpress.com/_EzyJ7NR

Here: messier-objects (lowell.edu) The top row is missing the checkmark boxes though

Not sure if you mean a 91 total seen or M91 but this reminded me of M91. It is a very cool one with a bit of history. It was the first on the list of my last session with the last 12 objects. Right next to it there is an NGC 4571 galaxy, very clearly and easily visible that it confused me in the first moments and I did not know which is which. Due to some bug also in Stellarium on my phone it was not immediately clear what is the galaxy next to M91. Apparently, before the age of Stellarium, with only paper charts and notes the NGC 4571 was frequently confused with M91 and this is one of the reasons it never got its own Messier number. https://www.jthommes.com/Astro/M91.htm (not my photo but a great one)

While the list began as one of objects to avoid when hunting for comets, it evolved quite a bit over the history of Astronomy. Regardless of its original intention it is still an amazing list. Out of the 110 Messier objects, 41 are Messier’s observations. The first edition of the catalog came out in 1774, containing only 45 objects. Successive editions expanded the list, with another edition appearing in 1781 bringing the total to 103 objects. Astronomy writer Camille Flammarion – also a Frenchman – added object number 104 from Messier’s notes. Finally, some astronomers published a revised version in 1967, bringing the total up to 110 Messier objects.

If they are not very interested, impossible to share :). I brought the telescope last summer to one of my close friends' place who claimed was very interested in Astronomy and buying his own telescope. I showed him the Dumbbell Nebula, it was really nicely visible and quite high. His comment: "That's it? A puff of smoke?" At least he spent the night observing Jupiter. DSO work is really not for everybody, even people who otherwise enjoy the Moon and the Planets :). He still hasn't bought one.

Back in August 2022 I got my first telescope and of course started observing my first Messier objects. Soon after that, I got my hands on the entire catalog, printed a nice A3 sheet and started planning and observing them. Why did it take 21 months if some of you can do it in a single night during the Messier marathon? Let me break down some of the lessons learned to help others on the same journey: Time flexibility - With two pre-school kids and a job, I couldn't pick and choose the times/dates of when I could observe. There were limited days/nights when I could go out for hours at a time. Some of these are visible only at certain months of the year, so planning was a must. It also didn't help that half of this time my telescope was at our summer house where we would go only once a month. Location, location, location - Before we even discuss equipment, the location you are in will determine IF you will cover all the Messier objects.. and IF you do, it will also determine HOW FAST you can locate/observe them. There were times with bad transparency in the freezing cold where it would take me half an hour for a single galaxy.. This would take a lot faster if I was at a lower latitude (the galaxy would be higher in the sky there) with less light pollution and warmer climate. Some of these objects are very low at the horizon and are visible only for few hours at specific months of the year. I live in a city of half a million and taking a 15 minute drive to the outskirts, allowed for locations where even the faintest Messier objects are doable.. I'd say Bortle 4-5 is doable.The lower you can get, the better it gets of course. It is recommended to drive south.. but I live in the north of the city so it was much closer to drive North. This however compromised the objects low on the southern horizon. The very last object I did was a star cluster, M19. Magnitude 5.59, easily visible but over the big city at an altitude of 8 degrees with a bit of clouds at the horizon.. it was tough! but I did it an wrapped it up last night. Weather, weather, weather - Where I live, it is quite normal this year to have a whole month of cloudy weather. Your average joe may say the weather outside is nice and clear but to hunt down some of these faint galaxies, you need to have really clear skies. Some of these high altitude clouds, visible only on radar and the charts on Meteoblue or Clearoutside can really wreck an entire night of observing and dramatically increase the difficulty of observing some of the faint galaxies. The Moon! - Once the top three points align, the Moon must not be in the sky, otherwise again it becomes very difficult observing some of the faint galaxies or nebulas. So having all these four points align has been quite the challenge and I was more then happy when they finally did. Telescope - I had an 8" manual DOB and I would say it was the bare minimum that I needed to observe the catalog under my conditions. If you can get under a dark enough/clear enough sky, I am pretty sure it is doable with a 4"-6" as well. Eyepieces & Filters - The key to my observing were a 2" 70degree 40mm and 32mm for the initial location of the target and then a nice 68 degree 15mm. For the galaxies, the Zoom eyepiece was really crucial. I could fine tune the magnification where the faint fuzzy was most visible. The UHC filters helped quite a lot on some of the Nebulas which were not easily visible without it. Starhopping - I did the whole catalog fully manual. The whole process was simplified by having three key pieces of equipment: Stellarium Plus - I did 90% of the catalog with the regular version before I bought the plus. The Plus is so much better that it can simulate the eyepiece view allowing for quick recognition of where one is. Cheap inclinometer for like 10$ - I would get the altitude of the object from Stellarium, get the telescope to the right altitude and then just sweep left and right in the general vicinity of where the object is in the constellation. I would quickly point the telescope in the general vicinity with a laser finder.. and if there were aircraft in the vicinity, I could use the RDF. I have a combo finder. The optical finder has been dismounted for over a year from my telescope as it was literally just dead weight. EXPERIENCE - Observing is definitely a skill. Had I begun with the Virgo cluster of galaxies as my first objects I would have surely failed, especially in the freezing cold when they are most visible around here. With experience one is able to know what to roughly look for just by looking at the size and magnitude of the object in stellarium. An inexperienced observer could have the galaxy right in the middle of the eyepiece and would see absolutely nothing. Experience really helped me locate and observe some of the faintest Messier objects, especially under not ideal conditions with a bit of high altitude clouds dimming them even further. It was really a great experience and some of these objects will stay in my mind forever. Apart from the famous well known ones like M13, M92, M31, M51, Orion etc.. The one that made the most lasting impression on me, and clearly underrated in my view was the Leo Triplet in my 26,32mm 2" eyepiece. I could look at these beautiful three galaxies, fitting in the same view for an hour if it wasn't for the freezing cold. This is just my opinion but I really think observing the entire Messier catalog visually is the best the whole of Astronomy has to offer. It is both a trip down the history of Astronomy as it is in visual splendor. It has definitely brought me the most life lasting impressions. I will be taking a bit of break now from tough DSOs and focusing on building my EQ platform and doing some simple imaging of the Moon and Sun from the comfort outside of my apartment under much better weather conditions with an occasional look at the most beautiful DSOs easily visible even from the Bortle 6 where we sit.

Yes it is really easy and there is almost no downside to cleaning a modern mirror with modern coatings regularly the right way. Some people do it 4 times per year for many years with no side effects, even on big 12" , 16" , 20" telescopes. I do mine once on average for every 20-30 observing nights. The claims that dust does not make much of a difference are simply wrong and not backed by any scientific studies, quite the contrary is the truth. I cover that below as well that dust DOES make an impact and it takes little of dust to do so. Check the video I made to avoid a couple of key bad advice that seems to be everywhere, such as cotton swabs:

If you want to dig in real deep and understand the physics/biology behind the whole '2mm exit pupil is best' , have a look here: The human eye (telescope-optics.net) If you are happy with some general rules of thumb, I would go with: 1) If you want to enjoy a nice, sharp, maximum resolution image - choose a 2mm eye pupil. 2) For dimmer objects, the bigger the exit pupil, the better as long as the object doesn't get too small. For galaxies it is worth having a Zoom eyepiece as you can dial in the optimum magnification/brightness = exit pupil for a certain object, under certain conditions. 3) Exit pupils above 6mm , or even above 8mm and even up to 10mm can be useful. Say you have a 16" F/4.5 telescope and you want to use a 2" 40mm as a finder. Even though you will be losing some potential light, you are getting a nice field of view instead. The secondary obstruction and shadow is usually not an issue unless you are looking at the Moon, which normally you wouldn't on such a telescope with a 40mm. 4) Exit pupils below 1mm start to run into a lot of problems due to the design of our eyes.. floaters, blurry image etc. I very rarely go below 1mm but on some very rare exceptions I have found Saturn in an exit pupil of only 0.3mm quite pleasant to look at. I was quite surprised as the whole reason why I put in a 3x Barlow with a 7mm was to show my friend that such magnification is useless... I was wrong. It was 3 AM and the seeing was exceptional. Amazing view.