what would happen if i switched to an MH lamp from an HPS, one week before harvest? would i gain crystals or anything?
My completely newby guess -
your plant would possibly stretch and take away energy from flowering a little to look for the red light spectrum it loves in flowering that you so evily stole away and replaced with more of a blue spectrum light.
from this link
OpenDNS
200 - 280 nm UVC ultraviolet range which is extremely harmful to plants because it is highly toxic.
280 - 315 nm Includes harmful UVB ultraviolet light which causes plants colors to fade.
315 - 380 nm Range of UVA ultraviolet light which is neither harmful nor beneficial to plant growth.
380 - 400 nm Start of visible light spectrum. Process of chlorophyll absorption begins. UV protected plastics ideally block out any light below this range.
400 - 520 nm This range includes violet, blue, and green bands. Peak absorption by chlorophyll occurs, and a strong influence on photosynthesis. (promotes vegetative growth)
520 - 610 nm This range includes the green, yellow, and orange bands and has less absorption by pigments
610 - 720 nm This is the red band. Large amount of absorption by chlorophyll occurs, and most significant influence on photosynthesis. (promotes flowering and budding)
720 - 1000 nm There is little absorption by chlorophyll here. Flowering and germination is influenced. At the high end of the band is infrared, which is heat.
Although I agree with the post before min ein that using both lights would be best but I am assuming you're using one ballast that is switchable and this is not an option.