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Coriorda

Focal length limit for an imaging novice?

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Hi, 

My first forum post, so “Hi” everyone!

Just wondered if there is a typical Focal length to start at for complete beginners.

I, like most of the world it seems, am interested in the Redcat 51 at 250mm. Which I’m hoping will be forgiving in terms of alignment accuracy and focussing. Though having a few failed attempts with a zoom lens I’m not so keen on the helical focuser.

So the WO ZS61 caught my eye but at 360mm with my crop sensor DSLR am I pushing things too soon and likely to end up disappointed.?

Any advice greatly appreciated 

 

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Hello! I shouldn't think you'll be disappointed with that set up. It will fit in targets such as the Heart Nebula really nicely. Get a Bahtinov Mask to aid focusing and your away 👍 Your right though, I really long focal length will be a bit harder to deal with but your way down at 360, I started at 650 without any trouble!

Edited by Rustang
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6 minutes ago, Rustang said:

Hello! I shouldn't think you'll be disappointed with that set up. It will fit in targets such as the Heart Nebula really nicely. Get a Bahtinov Mask to aid focusing and your away 👍 Your right though, I really long focal length will be a bit harder to deal with but your way down at 360, I started at 650!.

Thank you ! Much appreciated. That’s a brave start at 650mm. I thought my first light was 300mm but only after the session did I notice that I forgot to zoom in and took the subs at 70mm 🤪

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You should be fine at 250mm, it will be very forgiving when it comes to guiding/tracking. It only gets especially tricky at 1000mm and beyond.

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I started with an SCT at 2000mm, and I must say it was difficult ;) I think either of those scopes would be great, you should be able to get excellent images with those short focal lengths.

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1 minute ago, HunterHarling said:

I started with an SCT at 2000mm, and I must say it was difficult ;) I think either of those scopes would be great, you should be able to get excellent images with those short focal lengths.

lol... 2 metres... you'd have to be stark raving bonkers to start at that FL :D:D:D

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I started out with a decent mount which is really one of the most important bits of kit so it can handle the 650fl absolutely fine. 

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2 minutes ago, Uranium235 said:

You should be fine at 250mm, it will be very forgiving when it comes to guiding/tracking. It only gets especially tricky at 1000mm and beyond.

Thank you, I think 1000mm will take some time and deep pockets ! I see you are local .. is the WAS meeting again do you know ?

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Just now, Uranium235 said:

lol... 2 metres... you'd have to be stark raving bonkers to start at that FL :D:D:D

Pretty much :) To my credit though, I had originally gotten the C8 for observing, but I quickly became interested in imaging.

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1 minute ago, Coriorda said:

Thank you, I think 1000mm will take some time and deep pockets ! I see you are local .. is the WAS meeting again do you know ?

I dont attend any local astro society meetings (work commitments etc), but @michaelmorris (I beleive) is still involved.

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3 minutes ago, Rustang said:

I started out with a decent mount which is really one of the most important bits of kit so it can handle the 650fl absolutely fine. 

I think something simple like the Star Adventurer or Skyguider Pro should start me off ok 

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1 minute ago, HunterHarling said:

Pretty much :) To my credit though, I had originally gotten the C8 for observing, but I quickly became interested in imaging.

Same here, didn't take me very long at all before going onto imaging, I now very much enjoy both Imaging and visual. 

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1 minute ago, Coriorda said:

I think something simple like the Star Adventurer or Skyguider Pro should start me off ok 

Sure, just check the max weight capabilities and you should be fine. 

Edited by Rustang
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I had the same decisions that you are going through but I went with the William Optics Zenithstar 61 over the Red/Space cat. After spending extra on the Flat61A and now the Flat61R I could have gone with the Red/Space cat as it does not need the extra field flattener. With the extra glass I would expect the colours to be better than I am getting with the ZS61 but I am pleased with what I went for and the ZS61 has a larger aperture than the Red/Space cat.

If I were in the same position starting again I would be tempted by the ASKAR FRA400 with the FRA72RD as this has a really decent aperture of 72mm and would give you the option of 281mm with the reducer or 400mm without.

I have found the with my ZS61 I have had to get the reducer as I need a wider focal length to fit the targets in. This is also due to the 1" sensor on the ASI183MC that I am using but I have looked at other cameras and the pixel size means under sampling when used with low focal length telescopes.

I use FLO Astronomy Tools CCD Suitability to check this.

Although I am still confused when I see people using 3.76micron sensors and getting really great images. I am still learning all the time.

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13 hours ago, Grant Fribbens said:

I had the same decisions that you are going through but I went with the William Optics Zenithstar 61 over the Red/Space cat. After spending extra on the Flat61A and now the Flat61R I could have gone with the Red/Space cat as it does not need the extra field flattener. With the extra glass I would expect the colours to be better than I am getting with the ZS61 but I am pleased with what I went for and the ZS61 has a larger aperture than the Red/Space cat.

If I were in the same position starting again I would be tempted by the ASKAR FRA400 with the FRA72RD as this has a really decent aperture of 72mm and would give you the option of 281mm with the reducer or 400mm without.

I have found the with my ZS61 I have had to get the reducer as I need a wider focal length to fit the targets in. This is also due to the 1" sensor on the ASI183MC that I am using but I have looked at other cameras and the pixel size means under sampling when used with low focal length telescopes.

I use FLO Astronomy Tools CCD Suitability to check this.

Although I am still confused when I see people using 3.76micron sensors and getting really great images. I am still learning all the time.

Thank you, an excellent and thought provoking response . Really useful to learn from your experience. I had not considered the Askar and reducer combination.  I wonder if it would prove too heavy to a simple Skyguider / star Adventurer Mount ?

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The Askar is 2.56kg on its own. Depending on the extras you may be adding such as reducer, guide scope, camera adapter to focuser you will get to the 5kg limit of the Star Adventurer pro quite quickly and many say that you should only go to half the amount of the max payload when imaging. I actually started out with the Star Adventurer pro myself and that is also why I went down the Red/Space cat vs ZS61 route, but soon found the lure of a Goto mount so bought an EQ-5 second hand and was then hooked especially when I made my own EQMOD cable and got into the world of computer control. I have now upgraded from the EQ5 to the EQ6-R Pro but do think that the HEQ5 is a bit lighter and manageable if you have to keep taking the whole rig up and down but availability is an issue at the moment.

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On 12/10/2020 at 22:34, Coriorda said:

Hi, 

My first forum post, so “Hi” everyone!

Just wondered if there is a typical Focal length to start at for complete beginners.

Hi Coriorda (is that you real name?),

Welcome at the forum!

I do not think there is a typical starters focal length, started myself with a 1050mm f/7 apochromat. It is true that controlling a scope with a larger focal length is less forgiving than one with a shorter one, and generally needs deeper pockets as the scopes are more expensive and the mounts needs to be larger. What you will find out when imaging at larger focal lengths is that larger objects may not fit the image size and needs mosaicking (for me the Pleiades is one of those objects as I have a small chip-sized ZWO ASI1600MM Cool camera). On the other hand there are objects that are too small for the RedCat like Stephan's Quintet (even on the small side for my apo). What you can and cannot image in a single frame can easily be checked using free software like Stellarium. In it you can define your scope(s) and camera(s), select an object and then see what the framing will be. At some time in future you will therefore, just like me, find yourself considering getting a second camera and/or scope for objects that are otherwise difficult to image (for me this would be a SkyWatcher Esprit 80ED with a focal length of 400mm).

Nicolàs

 

Edited by inFINNity Deck
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11 hours ago, inFINNity Deck said:

Hi Coriorda (is that you real name?),

Welcome at the forum!

I do not think there is a typical starters focal length, started myself with a 1050mm f/7 apochromat. It is true that controlling a scope with a larger focal length is less forgiving than one with a shorter one, and generally needs deeper pockets as the scopes are more expensive and the mounts needs to be larger. What you will find out when imaging at larger focal lengths is that larger objects may not fit the image size and needs mosaicking (for me the Pleiades is one of those objects as I have a small chip-sized ZWO ASI1600MM Cool camera). On the other hand there are objects that are too small for the RedCat like Stephan's Quintet (even on the small side for my apo). What you can and cannot image in a single frame can easily be checked using free software like Stellarium. In it you can define your scope(s) and camera(s), select an object and then see what the framing will be. At some time in future you will therefore, just like me, find yourself considering getting a second camera and/or scope for objects that are otherwise difficult to image (for me this would be a SkyWatcher Esprit 80ED with a focal length of 400mm).

Nicolàs

 

Thank you, lots for me to think about there. I’m thinking to try and hone my skills at lower focal lengths and more portable kit before getting any more adventurous, though 400mm does sound tempting either via skywatcher or Askar FRA that grant mentions above. I really appreciate you taking the time to respond and am grateful for you highlighting areas I had not even considered !

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12 hours ago, Grant Fribbens said:

The Askar is 2.56kg on its own. Depending on the extras you may be adding such as reducer, guide scope, camera adapter to focuser you will get to the 5kg limit of the Star Adventurer pro quite quickly and many say that you should only go to half the amount of the max payload when imaging. I actually started out with the Star Adventurer pro myself and that is also why I went down the Red/Space cat vs ZS61 route, but soon found the lure of a Goto mount so bought an EQ-5 second hand and was then hooked especially when I made my own EQMOD cable and got into the world of computer control. I have now upgraded from the EQ5 to the EQ6-R Pro but do think that the HEQ5 is a bit lighter and manageable if you have to keep taking the whole rig up and down but availability is an issue at the moment.

Thanks Grant, yes sounds like that might be pushing the boundaries of the SA with the Askar, perhaps I’ll keep my eye out on the second hand market. Though with Xmas fast approaching decision time is drawing near am am leaning toward ZS61 over Redcar. Both appear to have excellent reviews, but the conventional focuser is my favoured route I think 

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It might be best to get your head round the notion of resolution now rather than later. Forget, entirely, the notion of 'crop factor.' It has no meaning whatever in astrophotography. It carries with it the implication that you increase the resolution with a smaller sensor, but you don't. Nor is resolution (of detail) governed only by focal length. It's just as much under the control of pixel size.

What you need to know to define your resolution is how large an area of sky lands on each pixel. The more sky per pixel, the lower your resolution. Increasing pixel size and/or reducing focal length will lower your resolution and make life easier at the expense of detail. Increasing focal length and/or reducing pixel size will increase your resolution but will need better tracking, better seeing, better focus etc. The key unit is arcseconds per pixel which can be found using an assortment of online calculators such as this: http://www.12dstring.me.uk/fovcalc.php  As a rule of thumb, amateur images of small targets in high resolution will be taken at around 1 arcsc per pixel or a little less. Below this atmospheric turbulence imposes its own limits. This is the tricky end of imaging!  Very widefield telescopic images may be at 3.5"PP. Beyond 3.5"PP your stars risk looking 'blocky' when shown at full size but you can get attractive images beyond 3.5"PP if you don't show them full size.

Chip size affects only how much sky you can frame. Having an object fill a small chip does not 'zoom you in' as crop factor implies. Your resolution is defined exclusively as in the previous paragraph.

Olly

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And of course the rabbit hole goes deeper than that -- small sensors tend to have smaller photosites in order to keep the pixel count up, which tends to increase noise, other things being equal.

The really good answers have all been given, but to summarize:

  • Get an idea of what you can frame with your DSLR at various focal lengths with Stellarium or Telescopius.com or something similar
  • Find out the pixel size, in micrometers, of your particular sensor so you can run the image-scale calculations
  • Know that the classic error of the beginning imager is to put too long a scope on too small a mount.
  • Pick up one of the several good books on the subject to get a handle on these interrelated concepts
  • Mount, mount, mount, mount. Much more important than optics, for anything over 100mm

Making Every Photon Count is very well-recommended, my personal favorite and my introduction to this art/science/obsession is The Deep-Sky Imaging Primer. You will learn not just about the techniques, but why they matter, in a structured and sensible way. 

Doing deep-sky work means that you have to learn to do a lot of things well, many of which aren't very intuitive (e.g., scope aperture matters way less than in visual astronomy; there's an amazing number of targets suitable to APS-C sensors and 200-500mm focal length). Since exposures have to be long, tiny imperfections in tracking the sky can ruin the image. We are talking single digits' worth of arcseconds, here, aka 1/3600th of a degree. Here, for example, is a shot from my attempt at Andromeda the other night; the error here is on the order of 7-9 arcseconds over a half-minute interval. And that's at a mere 360mm F/L.

So again, the mount is absolutely crucial, and you should literally spend the majority of your budget on that. If the mount is wobbly, ain't nothin' gonna fix that. If the scope is short, you just image big things for awhile.

All hail the people who started with big scopes and made it work for them. Me, I have frustrations enough with a short one!

 

Screen Shot 2020-10-15 at 11.37.13 AM.png

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