Thanks for the clarification Chris... I wasn't sure about that, just I knew that one of the M-series was using it.

My point, though, was that someone might also need a software upgrade.... especially with the new CR3 RAW file format (if they shoot RAW).

I had some experience with that. When I upgraded from 8MP Canon 30D to 15MP 50D - which also involved a revision of the CR2 file from 12 bit to 14 bit - my post-processing software would no longer work, so I had to upgrade that too (approx. another $400 cost on top of the purchase of two cameras). Then I found my computer operating system was incompatible with the new software. A new OS also needed more RAM, and CPU upgrade was strongly advised.... I ended up buying a new computer.... maybe another $1000 by the time I added graphics card, a network card, installed RAM and some other things. In the end, what started out as a simple camera upgrade ended up being a LOT more complex and expensive than I expected!

The transition from Digic 7 & CR2 to Digic 8 & CR3 concerns me. I know my software, which I really don't want to upgrade right now, couldn't handle it (no problem, since I'm not in the market for either EOS R or M50).

I just wanted to give the original poster a heads up to watch out for this, since his or her budget seems fairly tight.

It's interesting all the different suggestions here... Virtually any relatively recent DSLR or mirrorless would be a significant improvement in high ISO capability compared to a 9 or 10 year old T1i (I don't know how old the OP's Pentax is). And, sure, a recent full frame camera would be even better. But with limited budget, changing systems or "going full frame" sounds like it would be out of the question. Folks recommending $1000 and more expensive Nikon DSLR bodies (for example) and others who are insisting that a full frame upgrade is the only solution really aren't taking into account that to change systems or do a FF upgrade would just be too costly. The OP would need to replace lenses and all, too. Those cameras might work great, but would end up costing far more than the $1000 budget.

Among the alternatives I suggested, Canon SL2 body only is on sale right now for $500... but if they want a more advanced current model T7i is $700, 77D is $750 and 80D is $1000. 80D is also available refurbished from Canon USA for $800. Recent models such as T6i and T6s are also available at reasonable cost, both new and refurbished. OP also mentions some concern about size and weight (hence ruling out full frame), which is why I mentioned Canon M-series mirrorless as a possibility. For example, there's a seriously discounted sale price on the top-of-the-line M5 right now... $579 body only (regularly $979)... or $700 in kit with EF-M 15-45mm STM lens... or $900 with the EF-M 18-140mm STM lens. (With M-series, OP would also need to get an adapter to use any existing EF/EF-S lenses on the camera.)

The best solution would be to pick one or the other system between what they already have, and upgrade that with a newer APS-C camera. This would give them improved high ISO performance, plus allow them to continue using lenses, etc. that they already have. I know the Canon system and based my recommendations on that. I don't know the Pentax system very well, but I bet a similar upgrade may be possible with that.

Whichever they choose, it would probably make sense to later sell off the other system and, if they wish, put any gains toward expanding the system they keep... it's expensive using multiple systems!

While I understand the importance of a fast lens, the use of a tripod, and that we've discussed many times the advantages of a "Full Frame" sensor when it comes to overall low light performance, I am pretty much stuck in a crop sensor world for my photography hobby. This is primarily because of the price differences, but it is also due to the overall, physical size of the FF, DSLR's that I have had the pleasure to handle personally, with say the Canon 6D being about as big as I would ever care to go. While neither of my DLSR's are noted for their low light performance, I do okay with my Pentax K3 and/or my backup Canon T1i. However, low light performance has always been a big disappointment for me with either camera. While the K3 has a much higher ISO range than the T1i, it also appears to bring in more noise at comparable ISO settings and frankly, neither is really acceptable to me above say ISO 1600. I guess going higher with the ISO is better than not getting a shot, but even after extensive efforts in post processing, the noise levels are bothersome to me and I have to wonder if there isn't a crop sensor DLSR or Mirrorless body out there that could be a real improvement in overall low light performance without having to move up to a FF sensor.

Therefore, I am interested in discussion on which crop sensor body might have the best overall low light performance (not just how high I can set the ISO, but more about the results when I do use a high setting). I'd also be interested in what I should be looking for as I research this area of performance in today's offerings, as well as any other less equipment related tips you might be willing to share. Before you ask, I am on a fixed income and my photography budget is a lot lower than I like it to be. Therefore my personal, yet optomistic, budget would have to be under $1000.00 and I would prefer to find something used and save every dollar I possibly can. I'd be very willing to do without things like GPS and WiFi as long as there is real improvement in the low light performance. Thanks and good shooting to all.

The size of the sensor does not determine the amount of light a camera takes in. Its the shutter speed and the aperture that determain. Now the quality of the sensor does determine how light sensitive you ISO can go up to. But, ISO is only electronic gain and that is what causes noise. So the same lens on a full frame and on a crop sensor let in the same amount of light. So when buying a new camera look for better ISO clarity and low light sensitivity to get better exposure in low light situations. Image stabilization can help you get one or two extra stops in low light situations too. I have a Nikon D500 and love it. I can shoot in virtually no light and get an image. On a D850 full frame can beat it. Just barely.