Camera Lenses - Testing Methodology & Maintenance

[Xiga]

Hi guys

I’m looking to dabble in some widefield imaging after picking up a vintage Tamron 135mm F2.8 lens. As this will be my first time using a lens, I thought I’d ask for some advice from you fine folk in the SGL community while I wait for these never-ending clouds to clear.

The rig itself is very simple, it’s just a Celestron Universal (Vixen) Mounting Plate, with a finder-guider on one end and the DSLR (D5300) & Lens on the other (connected via  Manfrotto 234 tilt head). The lens has a small metal hood and I’ll be attaching my 2” IDAS-D1 filter on the end, followed by some step-down rings. The full aperture of the lens is 48mm (F2.8), so I’m going to test it using a 37mm SR Ring (F3.65) and a 30mm SR Ring (F4.5). I’m hoping, with just an APS-C sensor, that I will be able to get away with the 37mm one, but I’ll have to let the images decide that. I’ll be focusing manually using a Bahtinov mask and Bahtinov Grabber. Acquisition will be via SGP.

I’d like to give the lens a proper test under the stars, which means looking for edge sharpness, chromatic aberration, and reflections. With this in mind, can anyone suggest some good targets to test for these issues? I’m thinking targets with a fairly high elevation, to minimise atmospheric interference.
M45 seems like an obvious one for testing for reflections, unless anyone can think of a better one? And can anyone think of an area of the sky fairly high-up and with a dense starfield for testing for CA and edge sharpness? All suggestions welcome!

As for taking the test subs, I’ll be using ISO 200 (as always) and will test a few diff exposures to see what works best, i.e not over-exposing too many stars. Is there anything else I need to consider? Once I’ve taken the necessary test subs, if I’m lucky and get some clear skies then I’ll probably try and capture either the Seagull Nebula or M42 & the HH Nebula.

And finally, when it comes to scrutinising the subs, I presume I only need to look at the raw individual subs themselves, and not a stack? I can use APP to neutralise the background and remove any gradients in individual subs to make like easier. Have I missed anything else out?

Ps – I have a slight concern that my 5Kg CW (even if pushed up to the top of the CW bar) will be too heavy for the rig (I can’t test this at the mo as I keep my astro gear out at my parents house in the country). I obviously want to balance things as best as I can, just to reduce wear and tear on the mount. The Celestron Universal Mounting Plate comes with lots of ready-made holes, can anyone think of a simple DIY solution to add some variable weight to the rig? Something cheap that could be attached easily and won’t move around. I’ve been scratching my head thinking of something but I’m not getting anywhere with it, lol.

Pps – Do I need to take any precautions as regards bringing the lens back inside to a warm house from near-freezing temps? What’s the best practice for lens maintenance?

Thanks guys!

[happy-kat]

I'd use a lens shield, mine are made from flower pots and flocked. Unless you dislike defraction spikes I'd still test the lens wide open as well.