Widefield scope recommendations?

[Andy01]

Hi everyone,

I'm thinking of adding a widefield 'scope to my arsenal. I'd like to shoot large dso's like Rho ophiucus, Vela SNR, The spagetti nebula, even Eta Carina in full but my current setup limits me to 1.67 x1.30* (William Optics FLT 110, .8reducer/flattener & QSI 8300 wsg8 on an EQ6.)

Options appear to include the W/O 71, or 81, Borg, televue etc. I doubt I could afford a tak with my budget of around 1k so what would you recommend?
NB: I do a lot of narrowband so faster is better!

Cheers
Andy

[Amaranthus]

Another possibility is to use lenses for the shorter FL. These can range anywhere from 40 to 300 mm prime (you can go shorter, but it is more difficult). That is the route I will be taking. There are adapters to connect a lens to your T2 threads on the CCD, and if you are careful, you can get the backfocus (flange to focal distance) correct even with filters. Let me know if you want more info on this and I can give you all the gory details I've uncovered. I'm only waiting on the part from TS to be back in stock!

[sjastro]

I second the recommendation of using a lens as an alternative.

I use a 300mm Pentax f/4 which is a wonderful lens.
It's not designed for a 35mm full frame but would be a good match for the 8300 chip.
The image quality is great with the lens wide open with a 75mm aperture.

Here are some shots. on an ST-X10ME.

http://members.iinet.net.au/~sjastro/ic4701c.html
http://members.iinet.net.au/~sjastro/IC4628c.html
http://members.iinet.net.au/~sjastro/rcw102Ha.html
http://members.iinet.net.au/~sjastro/rcw133c.html

Steven

[gbeal]

The Pentax 67 lens idea is a valid one, I used in the past a nice 200mm f4, and always wondered about the 300mm f4. I currently use the 75mm Pentax.
A fellow forum member has both (200 and 300mm), and is looking to sell them at some stage, if you decide to go that way. Send me a PM if you do.
Gary

[Andy01]

Ok thanks Barry, that sounds pretty good, please pm the info :)



Thanks Steven - your shots look great! Good idea to go with the Pentax lenses - there is an adapter at Bintel, expensive at $449 but I guess all Ap stuff is... The lenses are under $350 though at least!

http://www.bintel.com.au/Astrophotography/QSI-CCD-Cameras/QSI-Pentax-6x7-Lens-Adapter/1784/productview.aspx

http://cameraexchange.com.au/index.php?route=product/product&path=81_83&product_id=743

http://cameraexchange.com.au/index.php?route=product/product&path=81_83&product_id=742

I need to work out the FOV to decide which lens though, because your pic of ic4628 is not a great deal wider FOV than my current setup gave me last weekend. (comparison pix attached)
The 165mm 2.8 might be a better option than the 300. How long is your 'scope and which chip was that taken on?
Thanks for your input :)



Thanks Gary, doing research atm but it sounds like a very good idea, thanks :thumbsup:

[Andy01]

er- Can any of you tech wizards figure out the FOV calculation for my QSI 8300 chip with the Pentax 6x7 165mm 2.8 lens?
I think it's 6.24 x 4.86* but as it's a medium format lens so I'm not sure ... :question:

Cheers
Andy

[sjastro]

I use an ST-X10ME CCD. The Pentax lens has a length of 260mm including the lens hood.
Your 8300 chip has a surface area 1.6X larger than my chip.
I selected the 300mm f/4 as I felt the 75mm clear aperture was a minimum requirement for "high resolution" work.

BTW your IC4628 image is excellent.

Regards Steven

[theodog]

That's what I get Andy.
Using Guide 9 to calculate with a QSI583ws (8300 chip) on 165mm f.l., it gives 374.2' x 281.72'.:thumbsup:

P.S. I'm currently using a 200mm 2.8 Canon on a 7D for wide field. Equivalent to 320mm full frame.

[Camelopardalis]

WO Star71? Although even with that focal length you might only just eek Eta Carinae into the frame :D

[Amaranthus]

Andy, in brief, I worked out that I can actually do it all with Canon EF lenses, which have the advantage over the Pentax Takumars in that they are still being made! (Note: if you go the Pentax route, you will still need a M42x1.0 to M42x0.75 thread adapter).

I have on my list to initially get a Canon-EF 40 and 135 mm lens, which bridges the gap to my 384mm ED80T (with FR). I'm also considering an 85mm to fit in between, and keeping my eye out for a wide-angle lens in the 14-28mm range (must be EF, not EF-S or EF-M).

Anyhow, it works out like this:
-- CCD = 15.5 mm backfocus (I'm using a QHY22)
-- TS-Canon-EF-to-T2 adapter, short = 10mm
-- TS Filter slider = 15mm
Total backfocus = 40.5mm

With a 3.5mm spacer (I have a 3mm and a 0.5mm ring, or I might find that the filter adds about 0.5mm BF itself), I have the perfect match to the Canon-EF FFD of 44 mm.

I can also add a Nikon-F to Canon-EF adapter to this system and still be within the 46.5mm FFD of the Nikon, so that gives even more flexibility with possible lenses.

At least this is the theory :) I need to wait until TS restock the Canon-EF-to-T2 adapter before I can actually prove it all out!

[Andy01]

Hi Barry,

I think the Pentax with the QSI adapter route is looking ok, particularly as the image circle from a medium format lens will give me a flat field at close to wide open.

It's a 2.8 lens so probably working at f4.0 is ok - the lenses are in plentiful supply as almost no-one uses Pentax 6x7's professionally anymore.

Anyway- food for thought, see how you go and thanks for the input!

[gregbradley]

As already posted a 165 F2.8 Pentax 6x 7 lens has an aperture of 59mm and with a KAF 8300 camera covers 285 x 377 arc minutes at 6.73 arc seconds per pixel which is heavily undersampled which does not seem to matter much with really widefield images.

It is an excellent lens that is sharp wide open at F2.8. I have used it several times.

I have used several camera lenses over the years. The Pentax 6 x 7 have the advantage of lots of backfocus. I am not sure Canon EF has as much backfocus. There was a comparison table somewhere and Pentax 6 x 7 were up at the top.

These are what I have used:

1. Pentax 6x7 165 2.8 a top notch performer, the best. Quite cheap. Probably under $200 or so.
2. Pentax 6x7 55mm hmmm not so good but cleans up around F4.Also not bad binned 2x2 (another way round aberrations of some lenses).
3. Pentax 67 75mm F3.5 - lovely, very sharp. These lenses are under $100
4. Pentax 6 x 7 300mm F4 quite good a bit of chromatic aberration. About $350 on Ebay.
5. Canon FD 85 so so. Quite a bit of chromatic aberration. Not worth much at all.
6. Nikon 50mm F1.8D and also the D version - great.
7. Nikon 105 F2.5 Not bad, a bit of chromatic aberration but good.
8. Nikon 180mm F2.8 ED. One of the better lenses. Some chromatic aberration but stop down to F4 its nice. Around $400
9. Nikon 85mm F1.8G quite nice but F2.8 onwards needed. new these are around $600 or so.
10. Samyang 24 F1.4 one of the better lenses from F2 on. About $400 or $500. Maybe more.
You can sometimes get the Pentax 67 300mm F4 EDIF which is the one to get. It often goes for around $1000. Marco Lorenzi used one for quite a while and those images were stunning. That was with a Proline 16803 which is many times larger than the KAF8300 but you get the idea.

KAF 8300 is micro 4/3rds sized so equivalent full frame aperture is double. So 300mm on a full frame is 600mm equivalent with the 8300.
Basically you would get 2.6X wider field of view than your FLT110.

That is enough to capture the entire Eta Carina nebula with some space around it. A nice FOV really. You don't want it too wide unless you want to image the Milky Way.

Greg.

[Andy01]

Wow, That's a comprehensive list thanks Greg :)

Many many thanks - that speeds up the process enormously thank you. I'll go the 165mm Pentax with the QSI adapter thingy.

Just need to find the right timing to convince SWMBO what a great deal this is and how much money I'm saving by not buying another 'scope and all will be well as Rho Ophiuchi becons ...

[Spookyer]

I tried to use my Nikon lenses with my SBIG STT8300 with the correct adapters but could not get satisfactory images with any of them really which was quite disappointing.

Brett

[sjastro]

In addition to Greg's post one of the unfortunate aspects of using camera lenses is that while they may perform satisfactorily on terrestrial subjects, the off axis aberrations show up with a vengeance on astroimages even on small sized chips.

I have no issues with my Pentax 300mm and 70mm lens on an STX-10ME, but a recently purchased Pentax 43mm f/1.9 is an absolute disaster even when stopped down to f/4.
According to the Photozone review it should be a "star" performer.

I sent the lens back which was checked against the ISO12233 test chart (see attachment), the conclusion being there was nothing wrong with the lens.
In response I sent them a test image of a star field taken with a Pentax k-r, which they refused to except it has valid evidence.

Currently the issue is unresolved.......

Steven

[gbeal]

And therein lies part of the beauty of the likes of the Pentax 67 lenses, or most any medium format lenses. You are usually only using the centre sweet spot of the optics.
Gary

[gregbradley]

I would generally steer away from fast camera lenses as a general rule with a few exceptions.

Digital cameras these days are relying more and more on lens profiling and corrections done in camera with the firmware.

Lens designers are actually taking that into account when designing new lenses in many instances.

Fast lenses usually mean poor chromatic aberrations and coma wide open.

So you waste the first couple of F stops as they are designed for low light terrestrial or for increased background blur not sharpness, not low vignetting nor chromatic aberration.

The designer has several factors they have to play with and some of these are conflicting so its usually a compromise.

The only decent fast lenses I have used are:

Samyang 24 1.4 (really F2 onwards and really F2.8)
Nikon 85mm F1.8G (same, F2.8 onwards)
Fuji XF 35 1.4.

So you're better off getting F2.8 lenses and then try them wide open and be prepared to go to F4 or F5.6 to get them performing reasonably.

I agree about the Pentax 67's. Long back focus and able to use the centre sweet spot of the optics.

Greg.

Greg.