Query regarding image circle and Canon 6D mk1

[brettnjan (Brett)]

Hi - we are new to this forum and are wanting to dip our toes into the world of astrophotography. We have a Canon 6D Mk1 that we use for travel and landscape photography and are now looking to purchase a suitable refractor to pair with it. The Canon sensor is 43.04mm across the diagonal. A couple of the scopes I am considering are the Sky Rover ULT 115 ED APO triplet or the Sky Rover Reference Series ULT 102 ED APO triplet. These scopes have a 3" and 3.5" focuser respectively, so presumably without a reducer flattener and using 48mm T-piece there will be no vignetting issues. However, the dedicated flatteners for these scopes are also 0.8 reducers and yield an image circle of 42mm. Presumably this means the image will not fully cover my sensor i.e. the extreme corners will be black? Also are there likely to be vignetting issues even within the 42mm image circle that would further reduce the usable portion of the frame? Just not sure if this combination will be slightly compromised or whether this is a completely normal set-up. Many thanks for any information to help me understand this better.

[Camelopardalis (Dunk)]

Brett, welcome to Ice in Space

It will really depend on the specs of the flattener/reducer. If the reducer presents an M42 thread to the camera, it will almost certainly result in some vignetting. Whether or not it’s manageable is difficult to answer.

Most flattener/reducers designed to cope with a full frame DSLR will be on the larger side and present an M48 thread. T-rings for full frame cameras screw on to this.

The other complication is whether or not optical issues with the scope or flattener muck up the stars towards the corners. If so, then the vignetting is likely moot and you can just crop away the ugly bits.

[casstony]

Hi Brett, do you already have a mount or do you have one in mind? Are you familiar with guiding?

For starting out in astrophotography a shorter focal length will make it easier to get good results.

[Atmos (Colin)]

With a 42mm imaging circular as long as you have a M48 connection on the flattener/reduced you won’t get a lot of vignetting. Whether it’s 42/43/44mm it doesn’t make any real difference. The 6D has quite large pixels compared to most cameras these days which makes it quite forgiving so I think you’ll be happy with whatever telescope you go with that corrects for a full frame camera.

[xelasnave]

Hi Brett
Welcome to Icenspace.
I would also ask if you have a mount or one in mind.
Selecting a good mount is I believe perhaps more important than selecting a scope...although given that I am getting away with very short exposures I my focus on a good mount is not as intense as it once was.
However getting a good mount and having it work at its best perhaps is the first goal one should make.
Further once you get your mount take some time to work with lenes that you may already have for your camera...even a short FL will do but something like a variable lens would be ideal so you can say start at 79 mm FL and go up to 200 mm.
No doubt you will go for auto guide but perhaps take time to seek results with your mount unguided simply so you can get a feel for the way it works and how it performs in various situations and what balance it likes..if any..mine loves a lot of weight in the West for example.
Again welcome and please share your thoughts on your mount selection.
Alex

[brettnjan (Brett)]

Many thanks for the helpful replies. In answer to the queries raised:
- the dedicated reducer/flattener has a 48mm thread
- we do not yet have a mount but plan to go for an equatorial "go to" mount
- I am familiar with the concept of guiding but unsure whether I need it from the outset - would I be correct in assuming it depends somewhat on the exposure length used?

Am probably still a month or two away from purchasing kit, but this is helping me firm up some choices.

[brettnjan (Brett)]

Hi Alex - just responded to other replies before I saw yours. Yes I definitely need a good mount. A Saxon EQ 3 or 5 has been recommended.
I have been doing some work with my existing lenses (200mm f2.8, 28-70 f2.8, 17-40 f4, and 50mm f1.4), based on the 500 rule and playing around with various ISO settings. I will certainly continue to use my existing kit in addition to whatever OTA I get, and am definitely looking forward to being able to increase my exposure times.

[xelasnave]

Hi Brett
May I suggest looking at a HEQ 5 ...and I think it has an additional "pro"..This is a great mount and has far surpassed my expectations.
I use mine with a 80mm refractor but it has carried a eight inch success fully.
And those lens..you will not believe the results you can achieve with each of them on a good mount.
Alex

[casstony]

With longer focal length, heavier refractors you need to get a fairly accurate polar alignment of the mount and preferably guide the mount to get images where the stars are not streaks.

I'd suggest you start with a shorter focal length refractor (or a camera lens), along with the HEQ5 Pro mount as Alex suggested, to make things easy on yourself.

With one of the following scopes you could get away without guiding for a start, still get reasonable images and not have to spend a fortune:
1. William Optics Redcat 51 (250mm focal length) - here's some images I took with that scope:
http://www.iceinspace.com.au/forum/s...d.php?t=176779
http://www.iceinspace.com.au/forum/s...d.php?t=176809

2. Sharpstar 61EDPH (270mm focal length) - here are some images through that scope:
http://www.iceinspace.com.au/forum/s...d.php?t=177184

While both scopes are built with full frame cameras in mind, my camera is only APSc, so you might need to look elsewhere to see how they perform for full frame.

While you build your skills with the smaller refractor or camera lens, you could keep an eye on the classifieds to see if you can pick up a suitable larger refractor at ~2/3 of new price. The two you mention in post #1 would be ok as would an Esprit 100 and various other 4" refractors. The Esprit 100 comes with everything needed to image, including a flattener.

[brettnjan (Brett)]

Alex - many thanks for the tip on the HEQ5. Not vastly more expensive than the std EQ5, but definitely looks to have more features and upgraded internals coming from the EQ6. Will seriously consider this.
Tony - loved the images you provided, very impressive results. I should add a little more background which may explain the FL choice a bit more. I am the one who is most interested in astrophotography, whereas my wife leans more towards visual astronomy hence looking at longer FL's. I have fast DSLR lenses covering FL up to 280mm so we are looking for something to give a bit more reach but still be suitable for AP and visual. A refactor of around 100mm and ~f7 seems a good fit without compromising too much in either area. Having said that - we welcome other opinions on the subject.

[casstony]

I think you're on the right track then Brett. The Sky Rover 115mm triplet is the same scope as my TS115 and also the same as the Meade Series 6000 115. The Meade offers better value than the Sky Rover with extra included accessories for the same price and might have easier warranty support. There are two Melbourne Meade dealers who would supply the scope (Ozscopes and Optics Central) but it's not listed on their websites. Here it is at Astro Anarchy (1/2 way down page): http://www.astroanarchy.com.au/opticalTubes.html
…. includes the reducer, finder, case and diagonal as well as the scope/rings/dovetail.
If you price it at the other shops specify that the reducer is included in the $3500 price (since it's listed as a bonus item).

United Optics in China makes these refractors and sells them to a host of re-branders.

Here are some images I've taken through my TS115:
http://www.iceinspace.com.au/forum/s...d.php?t=176184
http://www.iceinspace.com.au/forum/s...d.php?t=176438
http://www.iceinspace.com.au/forum/s...d.php?t=176418
http://www.iceinspace.com.au/forum/s...d.php?t=176435

The 115 gives a little more reach for visual in addition to being a good imaging scope.

[brettnjan (Brett)]

Tony - many thanks for the suggested options. I'm not ready to purchase immediately (wife's birthday in 3 months!) so have time to fully explore them all, although your images are making me impatient to get started.