Louis D

I've found that cutting all violet and blue as with a true Wratten #8 or K2 filter (same as a Baader 495) gets you 95% of the way to the sharpest image possible in fast achromats on all but Venus. Venus really needs a deep red cut filter like a cyan filter to get all the way there because the violet/blue cut filter only gets you about 80% of the way on Venus. The unfocused deep red is really strong on Venus once you filter out the violet/blue. I don't know why, but the other planets, the moon, the sun, and Sirius really don't improve all that much with an additional red cut filter like Venus does. Dimmer objects often don't seem to improve at all with violet/blue cut filtering. If anything, they sometimes get too dim to observe. Basically, try various filtering options on each object with an achromat to see which yields the best view to your eye. It may not be the same as when imaging the object.

I could see where that works because most of the scattered light of a blue sky is blue to teal blue to shades of bright green while the Ha (C line below) has much less scattered light:

Of course, the drawback of not filtering is that it leaves unfocused light flooding across the image reducing contrast and sharpness. For instance, I was out looking at Venus in my ST80 last night comparing various filters, including a few new ones. Without violet and far red filtration, it was difficult to discern its phase. With both in place, it was quite clear what phase it was in. However, Venus is sort of the acid test for a fast achromat, thus making the filtering challenge that much more fun for me to research. I have reflectors, a Mak, an ED, and an APO I could have used, but I was interested in finding out how much can be teased out of a fast achromat by properly filtering unfocused wavelengths. I'm not going to put a wanted ad on CN to try and secure a used Chromacor for the price of a nice APO to fix the chromatic focus of a fast achromat. What would be the point of that given my scope collection? Instead, I want to be able to make recommendations to beginners on how to get the most out of their fast achromats since they are so commonly bought by them as starter scopes. You are right that filtering out certain wavelengths for low contrast objects will make it more difficult, if not impossible, to see certain features. In those cases, it may take using a series of complementary filters to tease out various features, refocusing for each filter.

Try it on the full moon to see if features are easier to discern thanks to the dimming effect. I find binoviewing really helps with observing the full moon thanks to putting two eyes on the target instead on one on the target and one eye closed seeing blackness.

Check the crafting section of Hobby Lobby or Michaels or the countertop section of Home Depot or Lowes and look for thin, opaque, flexible plastic-like material to bend around (or inside) the struts. Once cut to size, the ends can be fastened with self-adhesive strips of Velcro along the length of the seam. For truss Dobsonian secondary cages, folks have used Kydex, Formica, or even Protostar Flocking Board to bridge between the upper and lower rings.

Unfortunately, I don't think Messier ever published his list on a web page; and if he did, it is probably long since defunct. The Internet Archive only goes back to 1996, not 1774, so we're out of luck there as well. 😉

Anyone know why this scope was designed to barely work with a mirror 1.25" diagonal? Most of my refractors come to focus with the focus tube halfway out using a 2" diagonal, and even further out with a 1.25" diagonal. The lone exception is my 23 year old ST80 which doesn't have enough in-travel to accommodate a 1.25" Herschel wedge. I'm betting the OP's scope wouldn't, either. With such a gently sloping cone of light at f/12.5, I can't imagine it was done to avoid vignetting.

Sounds a bit like the aforementioned Baader TurboFilm.

If you have a big scope (8" minimum), and if you can get it on target, big/nearby globular clusters will resolve at high powers (>200x) even under moderately light polluted skies (Bortle 5 to 7). They aren't as spectacular as under dark skies, but they are certainly visible as more than just a smudge. In smaller scopes, they generally just remain smudges due to resolving power issues. Compact open clusters also resolve nicely at higher powers in smaller scopes under moderately light polluted skies as has been mentioned above. Smaller, more distant OCs will require more aperture and higher powers to resolve as with GCs. Even brighter nebula benefit from using line filters in light polluted skies. The Veil Nebula goes from invisible to wow with a premium OIII filter in my 15" Dob under Bortle 5 conditions.

I'd have loved to have had a peek at the moon last night, but the mosquitos are swarming thanks to a very wet spring here. I really need to move to a more arid climate to enjoy astronomy better.

If you can come up with a way to replace that tall 1.25" adapter with the filter wheel, it should be doable. I can't do that with the low profile focuser on my Dob, though. I have only 25mm of backfocus from the top of the focuser housing.

For someone starting out in visual astronomy on your apparent budget, I would recommend the GSO 6 inch DOB Telescope. It's a slow mirror, so figure and collimation aren't so critical. There is no CA at all. You can get by without a coma corrector as well at its f-ratio. The Dob mount is rock solid stable compared to any budget Alt-Az or EQ mount. The viewing comfort is nice with the right adjustable observing chair. The only reason to not recommend it is its storage and transport size. If you plan to take it on mass transit to get to your observing site, I would instead recommend a 102mm or 127mm Mak on an alt-az mount to avoid counterweights.

Sounds like a plan as long as Monica is aware of the assembled weight and bulk. 10" Dobs are starting to get on the heavy/bulky side of things. I think it breaks down into two major pieces fairly easily for transport.

It is. I can't quite figure out what the extra $100 for the Orion Skyline version buys you over the Apertura AD8 version. Both have free shipping and charge sales tax. The accessories are basically identical except that the AD8 includes a cheap moon filter.

Don't overlook the Explore FirstLight series. Their altitude trunnions alone make them very worthwhile.

And all of those stars that you can see are strictly within the Milky Way galaxy, our home galaxy. On top of that, only about 3000 to 5000 of the billions of stars in the MW are visible to the naked eye, and most of them are relatively near to us.

I would think a pure blue filter like that used for RGB color separation coupled with a variable ND filter would be ideal for experimentation.

That's why I bought a Dob as my first scope. I went to two public outreach star parties in the late 90s. There were about 40 to 50 scopes on the field each time. Each time, I was most impressed with the views through the mid to large sized Dobs as compared to the views through the A-P APOs and SCTs on the field. I won't lie, those A-P APOs sure looked pretty, but 10" to 18" Dobs totally blew them away on nebula, globs, and planetary detail under our Texas skies. The larger SCTs all had mushier views than the equivalent sized Dobs. They all couldn't have had bad collimation. It was only with the recent arrival of the Celestron EdgeHD SCTs that I thought planetary views rivaled their Dob counterparts. On the same night of good seeing at these parties, I can easily pick out more detail aperture for aperture on planets in custom Dobs having hand figured mirrors over commercial mirrors. That, and the mirror cell, focuser, and motion mechanics are so much better. You would think the commercial suppliers would be trying to up their game after 25+ years of making Dobs, but they're not. I think the mirrors have improved in quality, but not the mechanics. They're still using heavy materials with poor sticktion/friction balance. Where are their premium offerings to compete with the custom makers? Toyota created Lexus to successfully compete with the luxury car brands. You would think the Asian telescope makers would follow suit with a luxury astro brand to take away market share.

That's wild that it needs 20 pounds of counterweights, especially when the lower tube assembly already weighs 50 pounds. And what's up with the weight of the groundplate assembly? That's crazy heavy. And here I thought using oak instead of aluminum was a heavy material choice. Why use steel? Because it's cheap and strong?

Why are commercial truss goto scopes so heavy? Starmaster's old Sky Tracker system only added 11 pounds to the rocker box (including gel cell).

And here I thought my 15" Tectron truss Dob was heavy at a little over 100 pounds.

Good find. Basically, you just need to find a hard, nubby surface that is glossy. The finer the nubbiness, the higher the sticktion (more contact points to overcome static friction) and the smoother the motion (like driving on a finely rutted road vs. a potholed road). The bearing surface area of the Teflon blocks also plays into the whole motion equation as well. Larger blocks will play better with larger nubs by spanning multiple at once. There's definitely an art to it based on materials availability and matching them up properly. For instance, nylon furniture glides may work better than Teflon blocks on certain surfaces. You just have to experiment to find out what works best.

Try surfacing the bearing surface with Fiberglass Reinforced Wall Panel in a nubby texture. It should be glossy to have low friction, but fairly coarse to reduce stiction. You can run a piece of Teflon across it to get an idea of the feel of it. I won't lie, it can sound a bit like rumble strips on a highway when rotating the scope, but it works great.

Short of using a perfectly cleaved crystal surface or a vacuum sputtered crystal surface, a Herschel wedge's uncoated first surface mirror should yield just about the smoothest, lowest scatter diagonal surface possible. And, unlike a prism diagonal which has a similarly smooth surface, there is no possibility of introducing chromatic aberrations into the image. Thus, it probably would yield the highest contrast possible for ultra-bright objects short of straight through viewing. Of course, with straighter through viewing, you'd probably have to introduce some sort of dimming filtration, and that could add scatter itself. For those not familiar with the internals of a Herschel wedge, I've attached a Wikipedia diagram below:

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