[astro744]

Welcome!

Your telescope has a focal length of approx 1200mm so a 4mm eyepiece with give you 300x which is useful on less than 5% of nights, dark or suburban. Aim for 150-200x as a max power for planets and have a quality Barlow on hand for nights when seeing (steadiness of the air) permits greater magnification.

I'm not familiar with the 42mm but for a low power sweeper it should be fine and the low power will be quite nice. The exit pupil will be about 7mm and some may argue this is approaching the high end but it depends on your age and how much your eye can hangle. If you are under 30 the 7mm is quite OK and in any case for the purpose you are using it (finder/sweeping) the exit pupil is not critical. (Exit pupil is the diameter of the light entering your eye and is equal to focal length of eyepiece / focal ratio of telescope).

Note too low power low cost eyepieces generally all suffer from field curvature and other abberations at the edge making their useable fields only about 70% in diameter. This is where only a lot more money will buy a sharper edge of field such as a 41mm Panoptic or 31mm Nagler. Again if you are young, the eye can handle field curvature much better and re-focus quicker as needed (straining the eye though).

One may argue that having the outer 30% sharp is not necessary but if you are searching for faint galaxies it could mean the difference between seeing a galaxy or not seeing one when at the edge.

Perhaps someone with a 42mm and an 8" f6 can comment if the combination is OK. I would recommend a good quality Plossl such as the Tele Vue 32mm Plossl that will give you 1.3 degrees and when combined with your Telrad should be sufficient. Also the 32mm is 1.25" and you only need a 1.25" UHC filter too. An alternative would be the 24mm Panoptic which will give you the same true field at a higher power but a nice wide and sharp apparent field albeit at a higher cost (also 1.25"). The 24mm Pan is a very nice eyepiece.

Not sure about the Pentax but I would think the quality is high. A 12mm focal length will give you 100x which is fine for most DSO's and the 2mm exit pupil will give you very good contrast.

For the cost of the three eyepieces you mentioned I would go the 24mm Panoptic and you can always save for more later. The 24mm Panoptic is a keeper and can be used and performs well on any telescope and the views are extremely pleasing.