Telescope vs lens for astrophotography

[Celestron4]

I am looking at options for upgrading my astrophotography setup and am unsure what my next steps should be. I am currently using a Tamron 18-400mm zoom lens attached directly via a dovetail to my Celestron AVX mount. I have got some good images with it but have found it limiting with the lack of guiding and effectively being restricted to only one nights worth of imaging due to the adjustable focal length. 

Example image: https://www.astrobin.com/574wgj/

I have a couple of questions and would be grateful for some advice.

1) Is it worth sticking with the current setup and looking into a dual bar setup to allow for autoguiding or am I better off replacing it with a prime lens or small refractor?

2) I have an Altair 60mm guidescope which I have been using with my Celestron C6 SCT. Could I use this for guiding the lens of would it cause additional issues?

3) If I was to look at upgrading the lens, would I be better off getting a Samyang 135mm F2 lens or a small refractor (Altair 60 EDF, Skywatcher Evolux 62, etc.)?

[Elp]

That example image is good. It just needs more time. Could you not tape the zoom ring down once focussed and carry on doing multiple sessions?

Autoguiding will be beneficial but introduces more technical issues which once sorted generally don't bother you again.

The guidescope is fine, you just need to be mindful of getting declination balance once everything is rigged up.

The SY135 is one of the best pieces of optical equipment for astro imaging. You may get a duff lens as a lot of people say, but I've got two and no issues. Tilt is the only main issue you tend to get as the lens is quite heavy so your mounting solution will need to be looked at.

A small refractor is also good, but cannot achieve the wide FOV you get with the 135, and no where near the speed. FYI the front glass of the 135 is 60mm diameter so its not loosing out to a small refractor in light gathering capability, you just normally have to stop it down to F2.8 or so.

[M40]

A samyang plus a few extras just popped up in the for sale section if you are interested, timing is everything 🤔

[Celestron4]

I've been using some electrical tape to hold the zoom - I can't lock it so it's necessary to get any images at all. Even with it taped it still shifts a bit between imaging sessions which have been a bit sporadic recently due to bad weather. 

If I was to future-proof myself and go for a small refractor (probably 300mm ish - I would like to be able to photograph M31, NGC7000 etc without needing to worry about mosaics if I can) are there any in particular you think I should be looking at?

[Elp]

I use a WO Z61, I like this one as I can use it for visual and imaging plus I can change the FOV and speed via different reducers. The general only issue is it's a doublet so the inevitable blue tinge around stars is slightly visible when imaging long exposure but it's no where near the level you get with an acromat, it produces excellent images and has never let me down.

Their redcat is highly regarded.

Any Askar will be good, the FMA is their better range I believe. There's also the new Starfield 60, probably too new for any reviews. The Skywatcher EDs are also decent.

Edited December 1, 2023 by Elp

[PhilB61]

I originally imaged with a Nikon 200-500mm zoom lens but found it difficult to get consistently good focusing, too much backlash in the manual focuser, moving to a Skywatcher 72ED was a revelation in both ease of focusing and overall image quality, and at a fraction of the cost. M31 is nicely framed at the native 420mm focal length, I use a flattener but not a reducer.