Popular 60mm finders -limited capability?

[Merlin66 (Ken)]

You've seen the Ads on eBay and Orion etc. for these 60mm finders with helical focusers.....look like a great idea for finder/ guiders - right?

Well maybe - I bought one a couple of years ago to mount on my C11 as an electronic finder (replacing a 135mm telelens set-up). No major issues and it still works very well with my DMK51. (One issue - these finders definitely need a UV-IR filter fitted to the camera or you'll get bloated images. Not so easy to achieve with T thread adaptors).

I also picked up a "dud" 60mm finder last year - which appeared to give "carp images". Well, I've finally tracked down the problem. I star tested the finder and used the Ronchi grating on it - not perfect but I've seen a hellava lot worse.
I put the QHY5Lii on it and disaster - soft carp images even at (what I thought was the best focus). ??????
After many iterations, I finally traced the issue to the length of the camera nosepiece (gets worse when you fit a UV/IR filter) v's the available length of the 1.25" section of the helical focuser!!! You can check this easily by feeling inside with your finger - there's a "shoulder" about 30mm inside which blocks the nosepiece. What this means, is you think you're focusing but the camera is "fixed" by the shoulder and your adjustments are achieving nothing.

The end result: not capable of properly focusing cameras with long nosepieces but can be made to work with cameras using the T thread attachment.

Caution!!!

[Wavytone]

Hi Ken, many years ago I recycled two camera lenses from my SLR days.

These old camera lenses can still be found, ranging from 135 to 1000mm and make a very nice super-finder for a larger telescope, cheaper than buying a good refractor.

However ... they are not sharp at high power and unsuitable for imaging - a modern sensor like yours has pixels far too small.

Even the recent canon full frame sensor with 19micron pixels is probably not going to work well.

[Atmos (Colin)]

As the previous owner of that “dud” 60mm, was the issue mostly that I was using a 1.25” noisepiece AND without a UV/IR Filter?

[glend (Glen)]

There is a new generation of 50-60mm ED finder/guide scope/ wide field refractors that can provide a multi-purpose scope that fullfills various roles. Skywatcher has a new Evo ED 50mm guidescope, in fact you can buy it from Tasco retailers now, price is around $379 AUD I believe. Andrews can get it in. There is a thread about it on Cloudy Nights, related to turning it into a proper wide field astrograph. Teleskop-Services, and Astrotech have their versions of the WO Z61, featuring FPL-53 objectives and proper 10:1 focusers capable of supporting imaging gear:

https://www.astronomics.com/astro-te...et_p20522.aspx

The Astrotech 60mm is $369 USD, and supports full frame cameras, there is an available flattener. I have one of these.

Here are the details of the Skywatcher Evo Guide 50:

https://www.firstlightoptics.com/gui...uidescope.html

[Merlin66 (Ken)]

Glen,
Yes, these new additions look very interesting.

If you just need a finder/guider then bang per buck those 50 or 60mm eBay finders are very good value.

[Merlin66 (Ken)]

Colin,
Yes, I think the nosepiece and UV-IR was an issue.
(However, I also found a defect in the helical focuser which limited it's travel - since repaired - this would certainly have aggravated the situation.)

Drop me a PM.

[Merlin66 (Ken)]

Wavytone,

Yes, I've made use of all my ol' Zuiko lenses. The 135 f2.8 and the 200 f4 have made great finders in the past (I also use them on the spectroscopes - the performance is pretty good....)

The only issue, a bit of a PITA, is getting/ making a suitable adaptor to use either T thread cameras or 1.25" nosepieces with these lenses. I've successfully modified Olympus extension tubes for these adaptors.