Louis D

This image pretty clearly shows how much light in general the internal Canon cyan filter is cutting from 550nm to 700nm:

How about using a 1.25" Herschel wedge? It should have a shorter light path, but I'm not sure about its thermal dissipation capacity with a 120mm scope. I use mine with a 90mm refractor with zero noticeable heating.

For straight through correct image viewing, there used to be erecting porro prism units sold by Celestron in particular. Technically, you could substitute high reflectivity mirrors for each prism reflecting surface. IIRC, if you have enough back focus, you can use two right angle mirror diagonals in sequence and then view looking in from the side of the scope. Perhaps if you adjust the angles as with the Matsumoto unit, you could view from behind the scope. I haven't played with this concept in years, so my memory might be a bit faulty. About 25 years ago, I was trying to make a correct image, compact viewfinder scope using 2" clear aperture, surplus right angle prisms bonded at right angles to each other. It worked rather well, but it was insanely heavy due to all the glass.

Sounds like a new thread in the making. Ugly or Not Ugly for various eyepiece lines. The Delos have been likened to chicken drumsticks. Whether that is ugly or not could go either way depending on how you like the looks of drumsticks. 😁

@DaveG64 Are you asking about a field derotator? They're normally used with alt-az mounted scopes. If you're using a RA-Dec aligned mount, you shouldn't need one. The built-in rotator is there for framing purposes. If you're alt-az mounted, it could still be used for this purpose, with the derotator following it somewhere in the rest of the focuser chain.

Looks a lot like the GSO 1.25" 90-deg Erect Image Amici Prism Diagonal. You might search for reviews on it.

I was thinking the same thing. The Svbony 18mm UFF version, SV190, is only $81+tax here in the US. That's way cheaper than the 18.2mm DeLite ($269+tax). The spread in the UK is probably even wider.

You could probably only detect the vignetting of that combination by taking a photograph of an evenly lit sky or light colored wall through it and comparing it to one taken through a full 46mm or larger clear aperture diagonal.

I didn't think harmonic drives were all that picky about balance. Sure, when balanced, they have a higher load capacity, but it's not required at lighter loads.

Have you used the time to make a shroud and align the RDF? How about checking collimation? What about wrapping the focuser threads in Teflon tape? Just throwing out ways to constructively pass the time while waiting for clear skies.

Here's a composite image of the image through various telescopes of mine taken on the same night: They are all shown at the same resolution from my smartphone camera. I didn't both to 2x the ST152 achromat because the 1x image was so mushy. Ignore the brown coloration. It's an artifact of the eyepiece I was imaging with. Here's a composite showing various filters applied ahead of the camera when using the ST152: You can improve sharpness by filtering out more and more poorly focused wavelengths of light. Green is the sweet spot for achromats. Longer f-ratio achromats will have fewer issues, but then you'll have to deal with a really long tube. Think f/15 to f/25 to get closer to ED and APO performance out of an achromat.

I took a look at the list of the 50 objects in that book, and it's odd that its missing some obvious (or at least easy) crowd pleasers like M27 (Dumbbell Nebula), M17 (Omega Nebula), Epsilon Lyrae (Double Double), and any of the M34/M35/M36/M37/M38 open clusters. Many of the objects listed also require dark skies to make out, so not very good choices for the average suburban observer. If you really want endless observing possibilities, look for volumes 1 and 2 of The Night Sky Observer's Guide by Kepple and Sanner. Sky & Telescope's store has new copies of volume 2.

On axis sharpness sometimes is sacrificed for wider apparent field of view in budget wide fields. If you had compared the 32mm Plossl to a 24mm Panoptic and found the latter to have more bloated stars on axis in comparison to the former, then I would be concerned.

Check the far corners to see if the artificial star shape become more rounded at smaller f-stops.

Or even a negative profile adapter. Although I don't know if it would be wide enough to accommodate the knurled zoom collar, or if you could still grab enough of it to turn it..

To see the aberrations, you really need darkness. That means either testing at night or in a dark or near dark room of sufficient length. I would recommend a black nickel ball bearing. The chrome ones seem to reflect too much stray light from the light source, making it difficult to see aberrations. I've yet to perfect an indoor technique after trying clear Christmas tree light filaments, chrome and black ball bearings illuminated by a tactical LED flashlight (torch), or a backlit foil pinhole using various light sources. Good luck and let us know what works for you.

The other day I was trying to look at the moon while the sky was still bright in my 6" f/5 GSO Newt using a 40mm eyepiece to center it. Man, was I fighting the secondary obstruction. A quick calculation in my head of 40/5 = 8mm and my likely 2mm entrance pupil explained to myself why. After dark, even with the 40mm, I wasn't noticing the issue partly because of the dark background making it easy to shift the shadow to the side of the moon's image by not keeping it centered.

What @Zermelo said. Long and short of it, give the Svbony SV135 7mm to 21mm zoom a try. I haven't picked one up yet, but independent eyepiece testing and user reports agree it is very sharp. It's also very affordable.

You have to be very careful with all laser pointers, especially the cheap green ones outputting at 432nm. They often output way more than the rated 1 mW to 5 mW in green. Even worse, they often output 100 mW or more in the infrared because the cheap ones omit the IR blocking filter. You won't even realize your retina is being fried if flashed by one. They are great for pointing out things in the sky, but you have to be really careful to scan for any aircraft in the area to avoid dazzling and temporarily blinding a pilot. If that happens, the authorities will definitely hunt you down. The biggest issue I have with them is that little kids try to jump up and grab it out of your hand because they want to wave it around to pretend it's a light saber. They pay no attention to what you're pointing to in the sky. It's a huge distraction for everyone involved. Now, let's say you're done pointing out things with it. Now you have to secure it away, so little hands won't grab it when you're not looking. Can you be 100% certain little hands can't access it? Some high end laser pointers come with keys to lock and unlock them to secure them properly. Removing and safely storing the battery or batteries is another option to safe them. They've been such an issue in Australia that you have to be a trained member of a recognized astronomy club to be allowed to point them to the sky at outreach events.

Well, I guess we'll just have to get someone to buy 10 XLs in each of the US and UK from various dealer stocks, and then run extensive tests on them to determine if the differences in viewing experiences are due to unit to unit variations. 😏

I just checked US online retailers today, and the Pentax XL zoom goes for ~$329 at most retailers. The Baader BHZIV goes for $309 at pretty much every retailer. I recall the XL costing that much 20 years ago. For comparison, the APM Super Zoom doesn't seem to have a US retailer since Don closed up shop. However, direct from China via AliExpress under the Sky Rover brand it is available for as little as $329. For the XL and BHZ, you'd more than likely need to add local sales tax. The SZ would be tax/tariff free when ordered via AE.

I came across the term didymium as relates to neodymium and think it might account for the difference between the Baader Neodymium and generic M&SG filters. It turns out that didymium is a mixture of neodymium and praseodymium which happen to be right next to each in the period table of elements. The two elements are almost always found together in nature. Didymium glass has been in use for years by glass blowers and some blacksmiths to cut down the glow from hot sodium to a bearable level visually. Neodymium accounts for most of the blocking, but praseodymium accounts for some as well. It's this cutting of sodium emission lines that accounts for it being a decent light pollution filter when sodium lamps are the dominant LP source. My supposition is that Baader uses pure neodymium glass while the Chinese made M&SG generic filters use the cheaper didymium glass. As such, the latter blocks a bit more light thanks to the contribution of praseodymium. I think didymium is probably cheaper because it takes extra processing steps to separate neodymium from praseodymium. The neodymium glass has higher overall transmission than the didymium (M&SG) glass, so it would be better for faint DSOs. However, for making a poor man's Contrast Booster in combination with a cheap yellow filter to use while observing planets, the extra sodium line absorption of didymium (M&SG) filters might actually be advantageous. It also turns out that photographers have been using didymium filters for photographing fall foliage to make it look more vibrant by cutting some of the orange/yellow part of the visible spectrum to create a band-gap to reduce a certain muddiness to the image. This is basically what the Contrast Booster is doing, but additionally it cuts violet light with a yellow filtration to further increase perceived contrast visually.

Yes, @John mentioned that possibility above. However, the glass disc must be plane flat and free of bubbles or other flaws. It must also be multicoated to avoid a fairly serious loss of light due to reflection. I would think centering has to be done at the factory as with catadioptrics. There are hybrid scopes such as Mak-Newts that use spherical curves for everything but the flat secondary to avoid figuring a parabolic primary. There's also a Schmidt-Newt, but I know very little about what's going on with it. I would also think that glass secondary mounts would be size limited at a certain point, just like refractor primaries, so probably not the best choice for giant Dobs.

It depends on the thickness of the spider vanes and their number. Curved vanes distribute the spike around the entire field of view, slightly decreasing contrast by increasing background brightness. The main time diffraction spikes can be a pain is if you're trying to find a faint companion to a bright star, and it happens to be right behind a diffraction spike. If the tube is rotatable, you can reorient the spikes to possibly reveal the faint companion by putting it between spikes. I would take diffraction spikes in a fast Newt any day over the unfocused light at either end of the spectrum in a fast achromatic refractor of similar aperture.

Not sure what your point is, but if it is that a particular channel needs a different focus to be sharp than the other channels, then so be it. However, it could also be that due to design choices, the blue channel is less well corrected and doesn't ever reach a sharp focus relative to the other channels.