I've been thinking how good some extra focal length would be. Rather than buying a SCT or RC, I was thinking about adding a 2x Powermate to my 1270mm f/5 Newtonian. I have a reducer for my Esprit which gives me 550mm and 375mm from the same OTA and I was think that 1270mm and 2540mm from the Newt would be good (obviously the down side would be f/10). This would be for imaging and not visual. Does anyone have any experience in a similar combination, or perhaps suggestions how to do it better?
Kevin,
I’ve had a full set of Powermates and adapters for a quite a few years now which I use primarily for visual in my 12” Goto dob and for Planetary imaging in my newts.
I did some experimenting ( DSO imaging ) during 2020 with a 2 x Powermate and my 8” newt under dark skies
I imaged a few brighter DSO targets namely Carina and Running Chicken nebula with 2 PM x 1000mm fl or 2000mm with 5 minute subs using my 2600MC at Highest Dynamic Range or Gain 0
I must admit the longer focal length using the PM at the same aperture (200mm ) effectively making my scope an f10 didn’t work that well. At 5min subs the frames were quite dim and after stacking an hour or so worth and processing , detail was evident but they had low contrast and brightness. I don’t think 10 min subs would have helped either , just puts more unnecessary risk on your sub quality in regards to tracking , guiding , passing cloud and satellite trails etc....
So I guess from my experimenting it didn’t prove very successful
Maybe a 10” newt with more aperture may prove better but it will still be a slow scope
I haven’t pursued it any further.
PM’s are excellent for visual and Planetary work , that’s what they were designed for
I'd thought about that. Most Barlows are 1.25 inch although a few 2 inch ones are available and most seem to be for visual. I wasn't sure how they'd go for imaging. Do you have any brand suggestions?
Most barlows one step up from the el super cheapos [ say, $75 or more]are quite good for AP, barlows being simple lenses. If you have the odds and ends
needed, you could try a 1.25"one, which you might already have. If reasonably happy with the results, you could step up to a 2" if you wanted to.
You could try eyepiece projection, and then you would only need an
eyepiece projection adaptor, and you could use EPs between 20 and 40mm,
so you would have control of your focal ratio.
Here are a couple of EP projection single frames I took many years ago with an 8" Newt;
forgotten which eyepiece.
raymo
Most barlows one step up from the el super cheapos [ say, $75 or more]are quite good for AP, barlows being simple lenses. If you have the odds and ends
needed, you could try a 1.25"one, which you might already have. If reasonably happy with the results, you could step up to a 2" if you wanted to.
You could try eyepiece projection, and then you would only need an
eyepiece projection adaptor, and you could use EPs between 20 and 40mm,
so you would have control of your focal ratio.
Here are a couple of EP projection single frames I took many years ago with an 8" Newt;
forgotten which eyepiece.
raymo
I think Kevin was thinking more about DSO imaging than planetary. As I mentioned DSO imaging with a 2x didn’t work well for me. You don’t get enough photons landing on each pixel due to longer focal ratio at same aperture.
Nice EP projection images too !!
Martin, that was why I suggested a 1.5x, his 10"scope would still be significantly faster than any f/10 SCT not fitted with a reducer, and
slightly faster than the f/8 models. Worth a try if not too much money has to be expended in the process.
raymo
Last edited by raymo; 12-04-2022 at 10:13 AM.
Reason: more text
Martin, that was why I suggested a 1.5x, his 10"scope would still be significantly faster than any f/10 SCT not fitted with a reducer, and
slightly faster than the f/8 models. Worth a try if not too much money has to be expended in the process.
raymo
Martin, that was why I suggested a 1.5x, his 10"scope would still be significantly faster than any f/10 SCT not fitted with a reducer, and
slightly faster than the f/8 models. Worth a try if not too much money has to be expended in the process.
raymo
Quote:
Originally Posted by Startrek
Yep worth a try !
Thanks for the advice guys. I do have a cheapo 1.25" Barlow that was thrown in for free when I bought the Esprit. I might have a 1.25" - 2" adapter somewhere so will give it a try. I was definitely thinking deep sky so 1900mm focal length at f/7.5 would probably be good.
Another question though. Would I need to leave the coma corrector in place or should I leave it out. Accurate back focus distance might be difficult with coma corrector, Barlow and adapter. I'm guessing that off-axis coma is less of an issue with the longer focal length and smaller field of view?
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Quote:
Originally Posted by kosborn
Thanks for the advice guys. I do have a cheapo 1.25" Barlow that was thrown in for free when I bought the Esprit. I might have a 1.25" - 2" adapter somewhere so will give it a try. I was definitely thinking deep sky so 1900mm focal length at f/7.5 would probably be good.
Another question though. Would I need to leave the coma corrector in place or should I leave it out. Accurate back focus distance might be difficult with coma corrector, Barlow and adapter. I'm guessing that off-axis coma is less of an issue with the longer focal length and smaller field of view?
Kevin
At f7.5, most commercial coma correctors designed for f3.5-f5 will over correct. Yes leave it out especially if you have a smaller sensor, ie APS C or smaller.
Kevin is on the money; coma will be negligible at f/7.5, and any you do pick
up will be right at the edge of the frame, so would be easily cropped out.
raymo
