I see there are lots of questions about candling. Good to see people asking questions; that's how we learn & even keep learning from hearing new questions that we hadn't considered. Relax & put your anxious energy into researching your questions (use the Search box above on this page to see what others have to say).
The hens get off of the nests & the eggs start to cool some until they get back. So just candle as little as possible & quickly, then put the lid back on. They can take a little brief temp drop. I hand turn my eggs & candle while turning 3 different days before "lockdown." The temp drops during the turning & doesn't seem to hurt. I like to fumble as little as possible when candling & just do them all at once briskly but carefully, as not to drop an egg. Taking the lid off & on is like fumbling for me, too risky, too much handling.
I like to grasp my handheld sized 9 Led flashlight (cost under $10) with my left hand with light facing up, pinch my thumb & first finger together to form a circle, lay an egg sideways in this circle with the wide air cell end to the left, then look for the 3 main things. Air cell on left, dark mass &/or veining in the middle after the air cell, then the egg white liquid on the right pointed end. It is so interesting to see each of these 3 areas change as the next candling day comes. With the air cell on the left, just tilt the flashlight towards the air cell end a little to see it better in the early stages.
I candle at day 7 or 10 (try not to freak out about seeing blood rings or whatever, I used to see them all of the time when they weren't even there, be patient & wait till next candling), then at day 14 (eases the curiosity & weed out the obviously clear/unfertile eggs), then again at day 18 (which is your last chance to candle, remove yucky eggs, raise humidity & the "lockdown" of incubator, not to be opened until all chicks hatch). In my December hatch on day 7 & day 10 there were 3 of the 6 eggs that seemed much more developed with more dark mass, but the others caught up later when all 6 hatched; didn't know that could happen.
See this great site on egg progression, allthough my small 9 Led flashlight does not pick up all of these details or all movement of the chick, it gives you the idea of what stages there are & what to look for. Don't give up too early on the possible chick inside by cracking them open around day 7 or 10, wait till day 14 or so & give it a chance in case you didn't see it right. The egg is an amazing miracle!
*Candling (see #6)
https://www.backyardchickens.com/forum/viewtopic.php?pid=3364955#p3364955
*Egg progression (see #1)
https://www.backyardchickens.com/forum/viewtopic.php?id=261876&p=1
*Air cell
http://www.poultryclub.org/VHIncubation.htm
Madcap621: I don't toss eggs at day 7 & wait till day 14 or so. At that point it's 2 wks that the eggs has been under heated conditions, so my dogs don't get to eat them; but I do crack open every one over a papertowel at the trash can to see if anything different is in there to learn by. Also, I got my "Springfield digital indoor thermometer with hygrometer" on
ebay for under $15, you can calibrate the hygrometer at home easily.