Thinking of buying a better telescope for looking at moon's craters and I need suggestions

[Cosmic Geoff]

16 hours ago, erseavetir said:

I was thinking of buying this 1.5x to 2x Barlow for my father to use the 1.5x portion of that Barlow with his 17.5mm eyepiece to test out near 12mm eyepiece views:
https://www.teleskop-express.de/shop/product_info.php/info/p53_TS-Optics-Optics-TSB21-1-5x-and-2x-achromatic-Barlow-Lens---1-25-inch---fully-multicoated.html

It ought to work either way. At x2 with 17.5mm it should yield around the maximum usable magnification with the Mak.  I don't use a Barlow with my Mak though - I prefer to use an 8mm eyepiece or zoom.

 

16 hours ago, erseavetir said:

Also, I was thinking of buying this diagonal to replace the one that came with the OTA or our "Celestron 90° Star Diagonal (1.25")" that we had:
https://www.teleskop-express.de/shop/product_info.php/info/p604_T2-diagonal-prism-90----T2---1-25--connection-on-both-sides.html

I did upgrade the diagonal on my Mak to a prism diagonal in hopes of improving its performance on planets.  However note that upgrading a diagonal generally gets you a better quality product rather than yielding any improvement visible through the eyepiece.  You won't for instance see an improvement of a few percentage points in light transmission without using measuring instruments.

 

16 hours ago, erseavetir said:

And for the final piece of the puzzle, I was thinking of buying this helical focuser:
https://www.teleskop-express.de/shop/product_info.php/language/en/info/p1751_Baader-Focusing-Eyepiece-Holder-and-Adapter-from-T2-to-1-25-.html

I am satisfied with the focusing action on my Mak and have no plans for an upgrade.

I did fit a helical focuser on my Helios Newtonian to give a fine focus in the same way, but that telescope is f5 and had a single acting rack and pinion focuser so it really needed a finer action for the short focal length eyepiece combos.

[erseavetir]

I see, thank you so much for letting me know about this Cosmic Geoff, I guess any other upgrades won't be necessary for my father's mak as well. other than I guess maybe buying different eyepieces like the ES 25mm eyepiece or a 10mm eyepiece later on.

[LukeSkywatcher]

I don't think a 17.5mm will be much good for observing the Moon. Yes it will be visible but the craters won't be very big. I think he would be better off with a 8-10mm Plossl. They cost 30-40 Pounds. 

[Louis D]

12 minutes ago, LukeSkywatcher said:

I don't think a 17.5mm will be much good for observing the Moon. Yes it will be visible but the craters won't be very big. I think he would be better off with a 8-10mm Plossl. They cost 30-40 Pounds. 

At 88x and an exit pupil of 1.4mm, he'll be able to see decently sized craters at a very good exit pupil for maximized resolution and contrast.  Going to an 8mm plossl will yield 193x and a 0.66mm exit pupil which will be pushing the limits of that scope.  I rarely push my 127 Mak above 150x to 170x because the image starts to break down at those tiny exit pupils.  Yes, the craters are larger, but they're grainier looking.  I would recommend 9mm or 10mm as the absolute lowest focal length eyepiece for that scope.