Should a Flat from a Brand-New Telescope Look Like This?

Well- these look like typical dust motes.  You can be as meticulous as you want in cleaning things, but dust will find a way to get on camera windows, lenses. and filters.  It doesn't take much for dust to stick like a magnet and often they are not that easy to see.  Your image looks find and the flats are doing what they are supposed to do.   That is the most important thing.  My flats do not exhibit as many dust motes as yours, but my setup is in an enclosed observatory.  Despite that, I still get some.  I would not be overly concerned unless your images display the dust motes.

Hi Jerry, I've see this a lot on my ASI2600, though not to the significance of what you have there. I had to open it and blow off the sensor and clean the glass. I can't tell if this is a problem from the ZWO build or something that happened to my camera over time. Either way, cleaning it helped a ton.

The image looks good though. What filter are you using with this? I assume you ran in fine on your AM5? 

In theory we should get some more clear nights later this week and you can get back at it

Jerry Gerber:
Optolong L-Pro 2" filter

good choice. I've been using that on broadband targets with my asi2600mc and it seems to work pretty well to cut most of the light pollution. glad to hear the guiding went well on the AM5. If get enough clear nights I can try mine on the NYX and we can compare notes. 

yes, i did unscrew the last cover to expose the actual sensor. I felt the same way, but this is where the dust particles actually were located on mine. All I had to with mine was blow the inside cover the UV glass. 

I would suggest you try this in the cleanest space you can though so it doesn't introduce more. I used PVC and some 2mil painters plastic to create an impromptu enclosure. I'm not actually sure if it helped but why not, right

Jerry Gerber:

Nick Grundy:

Jerry Gerber:
Optolong L-Pro 2" filter

good choice. I've been using that on broadband targets with my asi2600mc and it seems to work pretty well to cut most of the light pollution. glad to hear the guiding went well on the AM5. If get enough clear nights I can try mine on the NYX and we can compare notes. 

yes, i did unscrew the last cover to expose the actual sensor. I felt the same way, but this is where the dust particles actually were located on mine. All I had to with mine was blow the inside cover the UV glass. 

I would suggest you try this in the cleanest space you can though so it doesn't introduce more. I used PVC and some 2mil painters plastic to create an impromptu enclosure. I'm not actually sure if it helped but why not, right

If I feel confident enough to remove the UV clear glass over the sensor I will try it, I'd better read up on it before I do.  I think the dust may be on the inside of that cover, but not sure.  In any event, at least for now, the flats are taking care of it.

Don`t go there , calibration takes them out so why worry?
rog.

The first rule of optics cleaning is don't clean the optics
Personally I am more concerned about the color gradients present in your master flat, than the dust shadows.

This is a flat i have from the 130PHQ + Radian Triad QuadBand + ASI2600mc Pro just to compare

I don't recall if this was an artifical light source or not. I also don't claim to be amongst the flat-taking experts we have around here. 

interesting tool

those dust particles are 2mm from the sensor

fortunately, the flats have been taking care of it. Though it was fun the once, I don't feel like constructing anymore clean rooms to open it up :-)

Dust on your sensor is far more defined… sharp and nearly black. As you move away from the sensor, the particles become larger and more diffuse and out-of-focus. Kind of what happens to your stars as you move out of focus… they go from pinpoint (hopefully ) to large circles (or donuts if you're using a scope with a central obstruction).

Chances are exceptionally good those dust particles are on the glass covering the sensor… which is about 12mm or so if memory serves. If not there, then the next most likely place would be on a filter. You mentioned the Optolong. I'd check it as well if you're going to clean things.

In any case, one of the very reasons for taking flats is to calibrate that out of your data. It's also why flats need to be taken with the imaging train unaltered.

If it calibrates well, probably best to leave as is. Flats always look horrible (at least mine usually do) and yours actually don't look that bad at all. Everyone's risk aversion will be different, but I would not open the UV-filter glass from the sensor chamber for this.

If you're often opening the imaging train when swapping it around between scopes, frequently rack the focuser all the way in for storage and back out again for imaging which sucks air in as it extends, or just shoot from an extremely dusty location, then trying to keep things totally clean might be a bit hard. But I've been getting extremely good results with fairly minimal upkeep with my setup by using those APS-C sensor swabs you can find on amazon. They cost about 50 cents each, though I bet you could find them cheaper on ebay or aliexpress if you don't mind waiting a month.

It's good to mention that if it's really dusty inside, you should definitely air-dust with a squeeze-duster first before going to the swabs in case there are any large particles that can get dragged along and scratch things, and you should only really take one swipe per side of the swab too as an extra precaution.

I'll admit that I am lazy with getting out new swabs every couple swipes though, and have a trick that works well to get more use out of each swab and makes them slightly more effective too. If you pinch the plastic packaging and pull the fresh swab out quickly, it picks up the slightest bit of static charge that sucks up loose dust really well without having to press the swab right against the surface to grab the dust. Afterwards, a quick flick on the plastic handle will knock off anything loose, and I can usually swipe it again with good success. I've found that I only really get one shot with the static trick though, it doesn't seem to work again a second time. Also if it's rather humid out, the static won't last more than a few seconds either so it's something to do swiftly and precisely lol.

Cleaning it out entirely sounds time consuming but once it's clean I never spend more than 2-3 minutes any time I open/close the system again(which isn't very often). If I'm taking off my reducer from the OAG/filter wheel, then I'll just take a peek with my mini flashlight to see if there's anything obvious to blow out before it falls inside. If I'm opening the filter wheel or going any deeper than that, before I close it up I'll get the flashlight again(I use a streamlight microstream usb) to do a thorough check and air-dusting, and maybe some light swipes with a fresh swab if needed.

An extremely helpful thing that might get overlooked is after you've used a fresh swab on any critical/optical surfaces, take 5 more seconds to swipe it over any other non-optical surfaces inside the filterwheel housing/cover before tossing it. This took care of 99% of the random migrating dust I was shooting new flats to correct for - the kind that seems to appear out of nowhere even though the system hasn't been opened recently, then disappears/migrates again a few hours or nights later. If I wanted exact flats for each image set to correct for loose dust moving around, I'd probably end up spending more time shooting new flats than it probably takes to manage the dust in the first place. The only time I really need to shoot new flats now is when I rotate the camera assembly to prevent that embossed ring artifact, but even then I'll usually be lazy first and try my previous flats again and see if DBE can take care of any differences before I'll go shoot new flats lol.

A last side-note, I originally had been using a small bottle of cleaning solution that came with a sensor cleaning kit, but found that it left an extremely faint residue when it evaporated. I figure that since I'm (hopefully) not getting oily fingerprints or sticky debris in the imaging system, the solution was doing more harm than good and have since stopped using any solution entirely.

Clear skies, and clean flats 
-Mike H.

I don't have any examples from my own system at the ready, but I'll take a look tomorrow and see if I can find any to show you. The ones I'm describing aren't small/localized artifacts, they'll be like one large gradient-shaded-ring towards the edge of the frame, centered on the projected image circle(not necessarily the center of the sensor).

My guess is that the light falloff wasn't exactly centered in the flat, and then when the sensor rotates and the image gets corrected, the difference appears as a shaded ring around the optical center with over/under correction where it doesn't match up. I've seen them on my GT81 after I rotate significantly from the position where I took the flats, and they'll vary depending on the filter as well with some being more pronounced than others. I haven't used the GT81 in a few months since I've been fine tuning my C6, but I don't remember it being as much of an issue after I tightened up the focuser tube slides, so I think it might have been related to that.

I'm pretty sure I know the ones you're talking about though. They'll look as if something shifted and the flats no longer lined up so they made these little crater rim rings/shadows where the correction didn't match up. If your dust bunny is taking a ride on the filters and the wheel doesn't go back to the exact same position that it was in when you shot the flats, then you could definitely get slight misalignments in dust vs flat frame correction.

For example, say that the wheel made a CW approach for each R G B filter when you shot the flats, but the wheel switched directions at some point during your imaging sequence. Maybe the sequence was set up to shoot G B R instead that might go 2x CW for G and B, and then backtrack -3 positions CCW for R.

I know with my ZWO EFW there is an option to enable rotation in one direction to help with this problem. It takes a bit longer to change filters since it often needs to take the long way around to get to the next filter, but I'd suggest trying to enable that and see if you have the same problem. If you don't have that mode then you could try setting up your image sequence where the filter wheel only ever moves forward to the next slot in the wheel, or just shoot one flat and one filter each night if you're planning on spending a lot of time on a target where you won't be doing it all in one night anyway.