[kitfoxdrvr]

Ben:

Just thinking out of the box...

His first line in his first post was he was looking for 135mm or up. I am also a telescope owner and currently have a 4" Televue up to a 12" Meade, and I love them, but I have a hole between 200mm (Pentax) and the 550mm Televue, and for prime focus astro work, that is a big gap. It all depends on where he wants to fill in. Lots of deep sky stuff is appropriate in the 200mm up to 1000mm range and lots of scope and dedicated lens options in between. Lots of prices, too! True camera lenses have their advantages (aperture control, WP, ease of focus, compactness), but so do scopes (price, IQ, weight). I have used several of the long lenses you mention, and will likely soon have the 300mm (wish for the 400 2.8, but not going to happen, sadly), but for wildlife, not astro.

Now, on the Pentax scope, BIG +1! They are truly great optics; I got a chance to spend a few hours looking through a 105, and it is at least as good as my Televue. Some of the prettiest out of focus diffraction rings I have ever seen. It will cost you-but worth it imho. If I did not have the Televue, I would have made an offer on the spot. Seeing what Pentax can do with long focal length objectives makes me all the sadder that they will not do a fast zoom/fixed focal length long lens for cams any more. Sad...

Kit

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Last edited by kitfoxdrvr; 09-15-2011 at 12:08 PM.

[photoracer]

As a long time astrophotographer I can give you some pointers. 300mm is a fairly optimum FL for piggy-back imaging. One of my favorite lenses for this is the Tamron AD-2 300/2.8. I have a couple of heavy mounts that I usually mount my 6" APO refractor on. But the shortest I can make it's EFL even with it's dedicated 4 element (2 ED) focal reducer/field flattener is about 600mm (F4.1). Some objects like M31 in Andromeda are so large when imaged for a good amount of time that the longest lens you can use to get it all in is around 250-300mm. So I generally mount a separate camera and lens combo for anything that covers a significant part of the sky. Hardest thing to do is focus the setup. You can forget AF. Best way without a dedicated focus control system (Robofocus) is either to tether the camera to a PC and keep shooting exposures to judge the focus or use Live View in magnified mode. You can buy dedicated knife-edge focusers or objective masks to aid in focusing but most don't work that easily. Putting a clear glass focusing screen on the camera is some help.
But without having expertise or good books to guide you the best thing to get your feet wet is the Moon and planets. While you can stack images on the planets, you will find you don't need to for situations where they are close and bright.
I used to have a Pentax 75EDHF (older version). To use an FR/FF and get 350/4.7 you would need to eliminate the eyepiece. Getting a 2" camera adapter with filter threads on it will do. Or use an FR/FF that mounts just in front of the camera. William Optics and Televue have made some dedicated ones that take the place of the camera adapter entirely. You just need one designed for telescopes with FL below 600mm.
As for cameras Canons tend to rule just because they put some effort into making the cameras friendly for astro work. However right now I am inclined to favor the Pentax K-5 or the Nikon D7000 over any of the Canons due to the ability to do longer noiseless exposures at base ISO. With a Canon you might have to do 20 3 minute exposures to prevent noise. With a K-5 you could do 6 10 minute or even 4 15 minute exposures to get the same or better results. And you never do DSLR imaging without either an AC power supply or freshly charged batteries because DSLRs use power to hold the shutter open as opposed to older film cameras like the LX or F1 that did not have to do that. but that was in the days where we did long exposures of over 1 hour (longest I ever did was a 3 hour one.).
I could fill an entire forum with the info you might need, but better would be to locate one of the books on DSLR astrophotography and study it.
Kent Gittings

[Ben_Edict]

Now, on the Pentax scope, BIG +1! They are truly great optics; I got a chance to spend a few hours looking through a 105, and it is at least as good as my Televue. Some of the prettiest out of focus diffraction rings I have ever seen. It will cost you-but worth it imho. If I did not have the Televue, I would have made an offer on the spot. Seeing what Pentax can do with long focal length objectives makes me all the sadder that they will not do a fast zoom/fixed focal length long lens for cams any more. Sad...

Kit

"Sad" is the right word to describe a lot of Pentax' activities or the lack of...

The small SDHF75 is not really cheap. but also not expensive, considering the IQ and the built. If you look for it secondhand it is probably around 1000 USD in the US, depending on condition and it is worth every Penny. Also, it is very portable, as you can screw the tube apart into two small sections, each not much longer than a 200mm lens, though somewhat fatter. A friend from the astronomy club, where I have been a member for almost 30 years has taken wonderful images with the small refractor: CCD Astrophotography

His images really show, what the small scope is capable off.

Ben

[Ben_Edict]

I used to have a Pentax 75EDHF (older version). To use an FR/FF and get 350/4.7 you would need to eliminate the eyepiece. Getting a 2" camera adapter with filter threads on it will do. Or use an FR/FF that mounts just in front of the camera. William Optics and Televue have made some dedicated ones that take the place of the camera adapter entirely. You just need one designed for telescopes with FL below 600mm.

To add a reducer and other 2-inch accessories I designed my own adapter, which was much shorter than the original Pentax one (which was excessively expensive on top), which even allows to use a 2-inch diagonal with many eyepieces... it sports a 2-inch thread and takes the Lumicon FR. A field flattener is not really necessary for the Pentax scopes, as they are designed with an integrated flattener in the rear section anyway.

Ben

[DeadJohn]

Thanks for the replies. I wasn't aware of the Pentax astro scope. I searched around a bit and see it's 500/6.7. The built in field flattener is a big plus. Skywatcher has a very tempting 500/5.0 fast apochromat in their 2011 catalog but it's not available for sale yet. I wonder how well they've controlled CA in such a fast scope. There are also many good 80ED scopes on the market. The carbon fiber Stellarvue Raptor is light enough for extra usage as a handheld terrestrial scope.

Lens-wise, though, I'll keep an eye out for the Adaptall 300/2.8. A search of the marketplace found zero sales. I found some for-sales that were later cancelled and unfulfilled wanteds. I'll watch the marketplace, garage sales, etc. for fast lenses in the 135 through 300 range.

I'm in no rush to buy. I still have a lot of learning to do on the equipment I already have. My existing 2000mm + Barlow works well for planets, and focal reducer brings it to 1260/6.3 for deep sky.

P.S. Not looking to hijack my own thread, but Meade just announced a new LXD800 mount specialized for astrophotography with an integrated autoguide solution. Costly ($5999) but actually not expensive for such a heavy duty mount. P.P.S to wife: Don't worry, I'm not buying the mount. At least not until we pack up and move to much darker skies.

[Raybo]

This might be of interest to all of you, I had to sell it due to my divorce.