Mirror lens as piggyback scope

[sharkbite]

Hi folks...

Just got my seestar, and due to its convenience have been doing more imaging in the last month than i have in the last 2 years with my old setup.

It could do with a tad more reach, and since its actually very good at tracking, i thought i might piggyback something on it....

This would have to be fairly light - so im thinking something like a DSLR mirror lens, which are (presumably due to their crapness) very cheap on the second hand market....

My question is: How crap are they?

Has anyone used something like a 500mm F8 mirror lens for imaging deep sky and what are the results like?

cheers for any advice....

[iborg (Philip)]

Here is a link that might lead you to some useful information.


I suspect direct recommendation with a specific lens of interest will be your best guide.


Philip

[AstroViking (Steve)]

I've tried using a 1000mm mirror lens for astro work.

The optical characteristics leave so much to be desired that I no longer have any interest in using it.

The stars (even at the centre of the image) come out looking like pregnant 'V's and the vignetting is insane.

Now, to be fair, the lens I'm working with must be at least 30 years old (and I can find zero information about it on the Internet) so its possible that they've evolved significantly over the years...

V.

[sharkbite]

Thank you both....

Steve - sounds like your lens is out of collimation - at least thats what my lx90 results were like until i fixed it....for the life of me i dont know how you would collimate a dslr lens though...

Vignetting would be less of an issue for me - i'd use a dedicated astro cam with a smaller sensor for that. (there is no way a seestar could hold the weight of an SLR)

Ive seen a view posts on cloudynights and dpreview - so its hard to sort which lens is worth the bother...

thanks again!

[rmuhlack (Richard)]

I have a Rubinar 500mm f8 mirror lens which was surprisingly well corrected, though I found some interesting reflections (which may have been due to the lens or maybe filter).

See here https://www.iceinspace.com.au/forum/...d.php?t=103048

[sharkbite]

"I have a Rubinar 500mm f8 mirror lens which was surprisingly well corrected, though I found some interesting reflections (which may have been due to the lens or maybe filter). "

Wow! apart from the donuts that's not bad!

No disrespect to the 400d - i loved mine back in the day - but i reckon mating that lens with a dedicated astrocam could yield some good results!

There is hope yet!

Thanks Richard!

[rmuhlack (Richard)]

Originally Posted by sharkbite

No disrespect to the 400d - i loved mine back in the day - but i reckon mating that lens with a dedicated astrocam could yield some good results!

There is hope yet!

Thanks Richard!

no worries. the 400D was the first camera I used for deep sky astro. no live view, no plate solving (or really any automation actually) - those were the days...

[sharkbite]

As i dive into this rabbit hole...

i found this:

https://www.youtube.com/watch?v=1AzQ4y_qwrM

dont let the title fool you - the video is more interesting than it implies, and has a method it it to correct some of the optics problems like overly soft images....and potentially the strange v-shaped stars issues as well.

[bojan]

Rubinars are pretty much OK..

I have 10/1000 Macro, I had to relax the main mirror a bit, now it can't be much better..
https://www.iceinspace.com.au/forum/...hlight=rubinar


https://www.strickling.net/russentonne.htm


My Rubinar 500 f8 is not so good, but still usable.


MTO-1000A is very heavy.. but OK



I also have Opteka 500 f/6.3, DG.. bad at corners but OK in the centre of the frame, see here, post #24:
https://www.cloudynights.com/topic/4...3-mirror-lens/
Maybe it is usable for solar eclipses (very light)..

and here:
https://www.iceinspace.com.au/forum/...ghlight=opteka

[By.Jove (Jove)]

Worth watching this before you buy one, the results are mixed.

https://www.youtube.com/watch?v=x2BiM7BGQMU

You might find a secondhand SCT OTA for less than some of the premium mirror lenses (Nikon/Canon).

[sharkbite]

Originally Posted by By.Jove

Worth watching this before you buy one, the results are mixed.

https://www.youtube.com/watch?v=x2BiM7BGQMU

You might find a secondhand SCT OTA for less than some of the premium mirror lenses (Nikon/Canon).

Thank you!

The video i referenced is actually part 2 of this one - part 2 shows how the problems can be fixed...

Are there SCT's as tiny or light enough to mount on the s50 though?
(thats the idea behind using a camera lens)

[By.Jove (Jove)]

Originally Posted by sharkbite

Thank you!
Are there SCT's as tiny or light enough to mount on the s50 though?

OK so that's the constraint.

The smallest SCT's were the Meade 4" f/10 (model 2045) and the Celestron C5 was rebadged as a telephoto lens under another name. But it's still a C5.

So yes it will have to be a mirror lens if you want to bring the size/weight down to comparable with a 500 f/8.

[OzEclipse (Joe Cali)]

Sounds like you need a very lightweight combination.

I had a very cheap and extremely lightweight (300g) Hanimex 300mm f5.6 mirror purchased in the very early 1980's.

https://lens-db.com/hanimex-300mm-f56-mirror/

It used a T ring adaptor. The correction was surprisingly good across the field on the example I owned which I discovered on those rare occasions I could fluke the focus right. In the film era, focusing was a challenge. I didn't have a knife edge focuser. Focus is not a problem with modern DSLR's and live view or an astrocam.

I can't vouch for how they would perform with a digital sensor. Keep in mind that film grain was 5-10 times bigger than pixels found in modern digital sensors.

They are no longer in production. If you could find a second hand version it might do the job.

There is one on eBay here:

https://www.ebay.com.au/itm/20479712...Bk9SR-7JtOaSZA

Joe

[sharkbite]

Thanks Gents,

I'll keep in mind your feedback when sourcing one...

'course now the main problem is - i sold all my old kit to fund the s50...
including a Zwo224....which would have been perfect for this task....

so i'll need to get me a lightweight astro cam as well...

[By.Jove (Jove)]

I poked around on Astrobin - several mirror lenses are listed under equipment - but couldn't find examples of images taken with them.

You might be breaking new ground by trying !