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Your teleacope has a focal length of 1524mm and ratio of f5.
I would recommed the 32mm Tele Vue Plossl over the 40mm for the following reasons:
1. The 40mm (38x mag.) and 32mm (47x) both have 27mm field stop diameters thereby giving exactly the same true field. ie. you will see the same amount of sky with either but the view in the 32 will be a little bigger and with a darker backround (more contrast). To achieve the same true field the apparent field of the 40mm is only 43deg and the 32mm is 50deg. The apparent field is the window you look through and 50 deg is more pleasing as around 40 and and lower can appear like looking through tunnel.
2. The exit pupil of the 40mm will be 40/5 = 8mm whereby the 32 will be 32/5 = 6.4mm. If your eye cannot open to greater than 6.4mm you are in essence not utilising the full diameter of the primary mirror. However this is not as significant a problem that some believe but contrast too is better at the higher power/smaller exit pupil.
3. The 40mm and 32mm both have good eye relief for eye glass wearers and you are correct in choosing a Powermate over a Barlow since the Barlow will throw the exit pupil out further making it difficult to hold your eye at the correct position over the eyepiece. However the Powermate is heavy and you mention having the 15 and 8mm Plossls, I would recommend buying the 11mm Plossl as it will give you similar power to 40/4 with the Powermate. In fact the 4x Powermate will give you unuseable powers when combined with the 8mm and 15mm eyepiece on your telescope for 99% of the time. Work on a maximum power of around 300-400x which you can get with your 8mm eyepiece and 2x Powermate or 2x Barlow. Even powers between 200x and 300x will be rare for planets due to seeing but when the air is steady a bigger image certainly is nice to have, just dont expect the sky to co-operate that often. The 4x Powermate is very heavy too and will cause balance broblems on your dob that will require counterbalancing.
If you want to maintain Plossls (lower cost) then my suggestion to you is to purchase the remaining Tele Vue Plossls to complete the set, ie, 8mm, 11mm, 15mm, 20mm, 25mm, 32mm but not the 40 or 55mm (2").
This will give you Powers of 190x, 138x, 101x, 76x, 60x, 47x. The 32mm will give you a true field of 1deg at 47x. This is ideal for Messier hunting and general scanning the sky.
You can also add a 2x Barlow or Powermate to double each of the above powers. All the eyepieces will Barlow OK although the 2x Powernmate is probably a better match for the 32mm Plossol but it will still Barlow quite well. and in any case you already have 15mm so 32/2=16mm is not essential.
At a later time you can invest in say a 35mm Panoptic which will give you 43x ar 1.5deg. or a better combination is a 41mm Panoptic that would give 1.7deg at 37x. and a 27mm Panoptic 1.1deg at 56x. The 41 and 27mm Panoptics are parfocal and the 35mm would not be needed. Yes the 41mm Pan give a large exit pupil but you are using it for low power wide field star hopping and contrast is not critical. Note these are 2" eyepiece and any filters you buy for the 1.25" series will not fit.
To summarise I would recommend that you get the remaining Tele Vue Plossls and a quality 2x Tele Vue Barlow to give you plenty of range in magnification and true field. You could skip the 11mm and use a 20mm with 2x Barlow. Likewise the 15mm with 2x Barlow is close to your 8mm and 32mm with 2x Barlow is close to your 15mm. The advantage of having the Barlow is to give you 4mm(8) and 5.5mm(11) should you need it on a night when the air is steady. Buy the Barlow last to keep initial costs down or just buy the 32mm, 20mm, 2x Barlow now to give you a nice spread of powers. The are many options as you can see.
If you only buy one get the 32mm first as you will absolutely love the low power views it gives. Follow this with the 20mm Plossl as this will give you a nice 4mm exit pupil giving very good conrast for deep sky objects.
Whatever you choose, enjoy and welcome to the forum!
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