Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Prime focus wins my favor

  • Please log in to reply
6 replies to this topic

#1 GSBass

GSBass

    Apollo

  • -----
  • topic starter
  • Posts: 1,162
  • Joined: 21 May 2020
  • Loc: South Carolina

Posted 03 October 2020 - 10:11 AM

After having a few months under my belt doing planetary photography I am starting to understand the benefits of shooting at prime focus, specifically having a long focal length mak. I know people take outstanding photos using Barlow lenses and I make an effort to take a set of barlowed photos everytime I go out. But here is the rub, I rarely see more detail in a barlowed image versus a resized/cropped prime focus image. My frame rate is much faster at prime focus,x2 or better, Also better creative control as far as being able to crop to the right scale to minimize bad seeing or focus issues. Having the option to include moons or not etc..... Also noting that because my frame rate is faster I can have more frames to stack before rotation becomes an issue... able to keep the moons from elongating and still have enough frames for a clean image to apply wavlets to.  I may try barlowing some more if I ever get a night of exceptional seeing but I am not yet convinced spreading the disc over more pixels is going to overcome the inherent disadvantages of a Barlow. ( I know this opinion mainly applies to long focal scopes, others don’t really have any choice on whether to Barlow or not, but it’s definitely dropped off my required list of ensuring I get the best images during my session.

——-

just adding a photo from last night, seeing was not great so not much detail but it is much better than the barlowed image I took mainly because of frame rate and a suspicion that during bad seeing the stacking programs deal with motion better on a smaller disc (speculation)

Attached Thumbnails

  • 2D6A97AF-17DE-4059-B25C-1E0EFF982A13.jpeg

  • cometcatcher, Kenny V., WarmWeatherGuy and 4 others like this

#2 Kiwi Paul

Kiwi Paul

    Viking 1

  • -----
  • Posts: 636
  • Joined: 13 Jul 2020
  • Loc: Carterton, New Zealand

Posted 03 October 2020 - 12:56 PM

Hi.
I generally agree with you. In my view the Barlow is only necessary to help match the f ratio of your telescope to the pixel size of your camera. I have a C8 f/10 SCT and my camera is a QHY5III-178C with pixels of 2.4 microns. Using the 5x rule, 2.4 x 5 = f/12. So the SCT at f/10 was undersampling and could see it at times. One tome I could see features in the GRS competing with the pixelation which suggested I needed to present a larger image to the camera. I have a 2x Barlow and started using that with good results but with a confirm f/20 this is a bit high for the reasons you point out. Andrew (Tulloch) put me onto a place that sell 1.5x barlows which for me sounds like a good solution. F/15 is a little larger than f/12 but I think it will be close to spot on. The rule of 5x is the base value. It can be a bit higher as discussed in this forum.
Cheers Paul

#3 GSBass

GSBass

    Apollo

  • -----
  • topic starter
  • Posts: 1,162
  • Joined: 21 May 2020
  • Loc: South Carolina

Posted 03 October 2020 - 03:53 PM

My 462 has 2.9 microns so I am pretty close to ideal at prime, have been pleased so far.... feel fortunate I researched first because I imagine lots of people buy cameras without realizing the importance of matching them to their scope

Hi.
I generally agree with you. In my view the Barlow is only necessary to help match the f ratio of your telescope to the pixel size of your camera. I have a C8 f/10 SCT and my camera is a QHY5III-178C with pixels of 2.4 microns. Using the 5x rule, 2.4 x 5 = f/12. So the SCT at f/10 was undersampling and could see it at times. One tome I could see features in the GRS competing with the pixelation which suggested I needed to present a larger image to the camera. I have a 2x Barlow and started using that with good results but with a confirm f/20 this is a bit high for the reasons you point out. Andrew (Tulloch) put me onto a place that sell 1.5x barlows which for me sounds like a good solution. F/15 is a little larger than f/12 but I think it will be close to spot on. The rule of 5x is the base value. It can be a bit higher as discussed in this forum.
Cheers Paul



#4 Kiwi Paul

Kiwi Paul

    Viking 1

  • -----
  • Posts: 636
  • Joined: 13 Jul 2020
  • Loc: Carterton, New Zealand

Posted 03 October 2020 - 04:13 PM

Yes. I bought the QHY because it has a very large chip (from memory 3400x2300 something like that) and so if you start off with the largest ROI then it is quite easy to locate your target. Before I bought this camera I had started off using a converted web cam with a chip size of 400x300 or something like that and it was often a trial just to get onto your target. After target acquisition it is very easy then to close down the ROI as much as your tracking will allow to get the highest frame rates.
Cheers Paul

#5 Kiwi Paul

Kiwi Paul

    Viking 1

  • -----
  • Posts: 636
  • Joined: 13 Jul 2020
  • Loc: Carterton, New Zealand

Posted 03 October 2020 - 04:15 PM

If you are close to ideal with 2.9 microns then there is no need for the Barlow because you should be able capture all the detail presented to the camera.
Paul
  • GSBass likes this

#6 GSBass

GSBass

    Apollo

  • -----
  • topic starter
  • Posts: 1,162
  • Joined: 21 May 2020
  • Loc: South Carolina

Posted 03 October 2020 - 04:39 PM

Yes... when I first started I got some bad advice , it just took some experience to realize, at the time it made sense to me that you might get more detail by spreading an image over more pixels , people give advice with good intentions but in the end it was a good learning experience, camera matching is critical, you have a little leeway but if you want to shoot at different focal lengths then you better have multiple cameras

If you are close to ideal with 2.9 microns then there is no need for the Barlow because you should be able capture all the detail presented to the camera.
Paul



#7 GSBass

GSBass

    Apollo

  • -----
  • topic starter
  • Posts: 1,162
  • Joined: 21 May 2020
  • Loc: South Carolina

Posted 04 October 2020 - 10:58 AM

My sensor is pretty small, it’s just a hd chip, although I could crop to increase frame rate I generally have been shooting full frame because I don’t see that dramatic of frame rate increases til I drop down to like 800x600 or something like that.... my full frame average is normally around 100fps on Jupiter... Saturn is more challenging to get it above 50fps, target acquisition has not been a issue because I chose to get a mirror flip box and the largest reticle I could find.... 20mm, so it’s literally easy to get the target back on chip when I lose it,  precise focusing remains my biggest challenge

Yes. I bought the QHY because it has a very large chip (from memory 3400x2300 something like that) and so if you start off with the largest ROI then it is quite easy to locate your target. Before I bought this camera I had started off using a converted web cam with a chip size of 400x300 or something like that and it was often a trial just to get onto your target. After target acquisition it is very easy then to close down the ROI as much as your tracking will allow to get the highest frame rates.
Cheers Paul




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics