A query for Coronado PST owners

[lukebl]

Hi all.

The cloudy weather's making me want to spend some money, and I'm tempted by a Coronado PST.

I have read that inward focus is a problem when imaging, and just wondered if anyone can tell me if I will be able to focus using, say, a 2x Tal Barlow? I will be imaging with a DFK or a QHY IMG132e, or maybe my QHY5.

[Freddie]

With a 2x barlow you should be fine. Using a mono cam is definately the best option.

[Ant]

Mono cam is the way to do it.

i have a DMK21 and use it quite happily with the PST without a Barlow.

this is the one that you need - http://www.firstlightoptics.com/adaptors/low-profile-125-nosepiece-c-threads.html

Cheers

Ant

[Freddie]

I also have a DMK and that nosepiece and can't quite get focus!!

There have been a few posts on here and it looks like there are slight differences between scopes so some can achieve focus, some can't.

[Ant]

My thread here - - shows the image scale of three different optical trains.

dmk21

dmk21+2xbarlow (just lens section screwed into 1.25"adapter)

dmk21+2.5x teleview Barlow.

Cheers

ant

[lukebl]

Thanks for the info, guys.

I can see myself entering a new nightmare world of imaging here. Given the quantity of bits and bobs I keep buying through boredom due to the weather, I'm not sure I want to spend even more on adaptors, though!

[Freddie]

If you use the barlow, you shouldn't need the adaptor. You may also find, as I did, even if you get one, you may still not be able to focus with just the cam on the PST.

[JamesF]

The problem with the PST is that focusing doesn't actually change the physical configuration of the main parts of the telescope. It's done by increasing or decreasing the length of the light path by shifting the pentaprism back and forth. That doesn't actually move the image plane very far at all and because of the design some cameras just can't get anywhere near it. The adaptor allows the DMK/DFK to reach the image plane. I think you could leave it on all the time if you wanted to use the camera elsewhere.

I can't recall which DFK you have, but the issue with using the barlow is that the smaller sensors won't get full image on-chip even without a barlow (something the size of the DMK41 is required for that), so if you wanted a full disc image you'd be doing a lot of mosaic work if you used a barlow and DFK21.

James

[riklaunim]

Non-collar nosepieces should allow focusing without a Barlow in a Lunt 35 or a PST. The standard ones have them and you can't focus the image by a small amount. I used/use a 1.5x Barlow to bump the focus point and resolution by using a GSO 2x lens threaded to a short nosepiece of the camera.

[Freddie]

But not every PST will reach focus with a "non-collar nosepiece" and a DMK. Mine doesn't, so getting the adaptor is not a guarantee for success.

[Merlin66]

Freddie,

Agreed.

Please remember the PST was basically designed back in 2003 for visual observing, not imaging....so there is only limited focus adjustment to accommodate standard eyepieces.

99% of the time the short, shoulderless DMK adaptors will get you to focus. For the other cameras, DSLR etc a barlow lens mounted at the front of the nosepiece seems to work for the majority of users.

The final fall back is to use an eyepiece and eyepiece/ camera adaptor.