Hi - the mirror in my Durst 138 enlarger is deteriorating to the point where I would like to replace it. Is it a special kind of mirror/glass or will any reasonable kind of mirror be good enough? The heat absorbing glass is in tact in the housing FYI. If it is a special kind of mirror is there a generic name for it that I could give to my local glass/mirror guy? Appreciate any advice, tips or your experience in a similar situation. Thanks.