What focal length telescope should I buy?

[Spongey]

Hello all!

 

I have been interested in astrophotography for many years now, and have taken the seemingly standard route of DSLR + wide angle lens -> upgraded DSLR -> DSLR + small tracking mount (iOptron skytracker pro)-> DSLR + telephoto lens + small tracking mount. This has allowed me to capture some pretty good wide field images, and I have recently started imaging some larger DSOs such as M31 and M42.

 

However, I am now looking to get more into the hobby and was wondering what focal length telescope I should go for. I have decided on a refractor for their relative simplicity and optical quality compared to a fast newt, however I am not made of money... The two scopes I have been looking at are the SW 80ED and the SW 100ED, most likely on an HEQ5 pro mount. Considering that my telephoto lens maxes out at 400mm f/6.3, I am unsure that the 80ED (with flattener) at 510mm f/6.37 would be much of a step up. The 100ED has a more generous 765mm focal length (with flattener), but is much larger. I find the 400mm lens to be quite sharp even wide open with only some slight coma at the edges of the frame. For reference I am shooting on an unmodified Canon 6D, and intend to do so for a few years at least, before perhaps upgrading to a dedicated astrophotography camera such as an OSC. My main interests are shooting nebulae and galaxies.

 

Any advice is greatly appreciated! Thanks in advance.

 

I have attached a few images of some of my results so far that I have achieved with my current setup for those interested.

[knobby]

They're cracking images , I'd just carry on as you are ?

Maybe just get an HEQ5 and look at guiding for longer exposures on dimmer subjects ?

[Charic]

.

[Prador]

Lovely images!

Out of interest, what lenses are you currently using?

[vlaiv]

With Canon 6D, I would say that you can go up to 1000mm focal length "safely".

Mind you, increased focal length is going to bring in smaller FOV, which is good for galaxies and smaller targets.

This is based on upper limit of sampling resolution of ~1.3"/pixel. Heq5 guides ok on such resolutions (with a bit of care). If you plan to switch to dedicated astro camera, you might want to take into consideration future camera as well.

Canon 6D has quite a large pixels at 6.54um. Not many OSC CCDs (or CMOS) sensors have such large pixels, so you maybe should not jump to upper range of FLs straight away (like aim for 1000mm). If you want to be "future proof", limit yourself to about 750-800mm.

I think this would be very good combination for you to work with, but since you are budget limited - it might not be an option at all.

https://www.teleskop-express.de/shop/product_info.php/info/p10133_TS-Optics-PhotoLine-125mm-f-7-8-FPL53-Lanthanum-Apo---2-5--Focuser.html

+

https://www.teleskop-express.de/shop/product_info.php/info/p5120_Riccardi-0-75x-APO-Reducer-and-Flattener-with-M63x1-Thread.html

Weight is right for HEQ5, and focal length is very good at 975mm reduced to ~730 and F/5.85. That reducer/flattener should be able to cover very large sensor (they say up to 44mm diameter).

This is also decent scope (that can be used with above flattener):

https://www.teleskop-express.de/shop/product_info.php/info/p3041_TS-Optics-PHOTOLINE-115-mm-f-6-96-Triplet-APO-with-2-5--RP-focuser-1-11.html

 

 

[dannybgoode]

Nice photos. What lens are you using? I have a Canon 2x teleconverter which essentially doubles the FL of my lenses so would be a cheap way of getting more reach. 

Downside is that it also doubles the aperture so an f2.8 lens becomes an f5.6. Not so much of a problem, if like me you are shooting with f2.8 glass but wouldn’t be so great if you were shooting with an f5.6 lens to begin with. 

Given what you’re achieving though I’d also think spending as much on the mount now and a guiding set up would be the way to go and then look at a dedicated scope and Astro cam down the line. 

[ollypenrice]

The thing is not to treat focal length in isolation but to combine it with pixel size into the truly meaningful term which is arcseconds per pixel. This is a handy calculator. http://www.12dstring.me.uk/fovcalc.php The pixel scale of the system specifies its theoretical resolution. You then need to know what resolution your guiding and seeing will really allow, but Vlaiv's figure  of  1.3"PP is sensible. With very good kit, or more basic kit very well tuned, it is possible to go below that but you need good stable nights to do so. (Not just transparent ones, that is.)

The Canon 6D has a full frame sensor, I think?  (35mm on the long side.) Be aware that, even with field flatteners, only a very small minority of scopes will cover such a chip either in terms of illumination or in terms of flatness of field. The smaller APSc chips can be covered by a far greater number of scopes. You need a flat, illuminated field at least as large as your chip's diagonal, so 43mm. Alas some scope manufacturers are a little optimistic in their claims regarding their corrected circles. As well as reading the claims I'd try to find some images done with the proposed setup before committing to it. You could always crop the distorted edges but you'd be wastintg chip you'd paid for.

Regarding field of view, remember that you do not get a bigger image of a small target by having a smaller chip. The size of the small object on screen is determined only by the pixel scale/resolution. This is why the term 'crop factor' is essentially nonsensical. It implies that you get a longer focal length by using a smaller chip which is, of course, plain wrong.

Olly

[Spongey]

Thanks for all your feedback everyone.

I think I will first be looking into getting a solid mount and guiding system setup before going for a scope based on what I have read. 

The lenses that I used for the above photos are a Canon 24-105 IS STM and a Tamron 100-400 respectively. I have attached another image of the Rho Ophiuchi cloud complex that I reprocessed recently with some new techniques that I have learnt over the last few months

Olly, you raise a good point regarding field illumination and it is one that I will need to think about going forward; it does seem that not many scopes will cover such a wide imaging circle. I think I will be sticking with the 6D for now, and might have to live with cropping the edges of my frames before upgrading to a dedicated astrophotography camera in the future.

I am currently leaning towards the 100ED for it's larger focal length despite it's comparatively slower optics, although this may change after I get a mount and guiding setup sorted. On this point, does anyone have any recommended guiding setups for an HEQ5-pro that will be future-proof when upgrading to a ~900mm scope in the future?

Thanks again,

Sponge

[vlaiv]

Here is what you should consider with Heq5 for high resolution work (there are possibly other things, but for these I'm sure they contribute):

- Tuning mount / belt modding. Tuning can be done at very small expense if you do it yourself. Taking mount apart and replacing bearings, cleaning and lubricating again, and then adjusting everything. Don't know if it voids warranty (probably so). Belt modding is a bit more expensive because you need a kit to do it. There are pre made kits that preserve proper ratio of gears and can be used with hand controller. There are "cheap" DIY things that you can do - most of parts that you need are readily available online for use with robotics and such. This style of belt mod can only be used with computer and EQMod software (ASCOM driver for HEQ5 and other Skywatcher mounts). It has option to set custom gear ratio which you will need.

- look into changing mount saddle plate. Default one is simple two screw one, but you can upgrade it to accept both vixen and losmandy dovetails and have surface clamping for better stability.

- Do a strong pier mounting of change stock tripod for better one.

So consider all of the above. You don't need to do them straight away, and can decide on "upgrades" as you see fit depending on your mount's performance (there are sample to sample variations).

As for future proof and adequate guiding - I think OAG is the best solution. You will not have to worry about guide precision as it will always use same focal length and the same or smaller pixels. It can easily be swapped between scopes without need for external mounting. Only thing that you want to check if backspacing requirements, but there are slim OAG options available that don't take away much of your optical path.

[Adam J]

On 20/01/2019 at 22:05, vlaiv said:

With Canon 6D, I would say that you can go up to 1000mm focal length "safely".

Mind you, increased focal length is going to bring in smaller FOV, which is good for galaxies and smaller targets.

This is based on upper limit of sampling resolution of ~1.3"/pixel. Heq5 guides ok on such resolutions (with a bit of care). If you plan to switch to dedicated astro camera, you might want to take into consideration future camera as well.

Canon 6D has quite a large pixels at 6.54um. Not many OSC CCDs (or CMOS) sensors have such large pixels, so you maybe should not jump to upper range of FLs straight away (like aim for 1000mm). If you want to be "future proof", limit yourself to about 750-800mm.

I think this would be very good combination for you to work with, but since you are budget limited - it might not be an option at all.

https://www.teleskop-express.de/shop/product_info.php/info/p10133_TS-Optics-PhotoLine-125mm-f-7-8-FPL53-Lanthanum-Apo---2-5--Focuser.html

+

https://www.teleskop-express.de/shop/product_info.php/info/p5120_Riccardi-0-75x-APO-Reducer-and-Flattener-with-M63x1-Thread.html

Weight is right for HEQ5, and focal length is very good at 975mm reduced to ~730 and F/5.85. That reducer/flattener should be able to cover very large sensor (they say up to 44mm diameter).

This is also decent scope (that can be used with above flattener):

https://www.teleskop-express.de/shop/product_info.php/info/p3041_TS-Optics-PHOTOLINE-115-mm-f-6-96-Triplet-APO-with-2-5--RP-focuser-1-11.html

 

 

Can you really expect a HEQ5pro to handle a 125mm doublet from a weight perspective? Its 7.6kg before you add anything else. 

[vlaiv]

10 minutes ago, Adam J said:

Can you really expect a HEQ5pro to handle a 125mm doublet from a weight perspective? Its 7.6kg before you add anything else. 

I don't think weight is going to be issue - but length might. I'm happy with HEQ5 performance with RC8" and all bits on it. That is easily over 10Kg of weight (bit less than 9Kg for ota alone).

With that scope (125mm frac), setup is likely to be over meter long and that is quite arm momentum, so it could be a bit problematic in wind, but I don't have direct experience with such setup. I did mount 8" F/6 Newtonian that is over meter long, and total weight was certainly in 13Kg range (Ota is about 11kg and I used guide scope instead of OAG then - needed x3 5Kg CW to balance it), so it's doable but I would not call the performance reliable (I would not recommend anyone to take such combination seriously).

[ollypenrice]

ED80 or ED100? 2.2"PP versus 1.5"PP. There will be nights on which that extra resolution will materialize in the image and nights on which it won't. You'll always get more FOV with the 80, of course.

Olly

[MartinB]

Some lovely images Spongey and you are, clearly, already a competent astroimager!  You said you want to image galaxies and nebulae.  Certainly, if you wish to image smaller galaxies than M31 or galaxy clusters then you will benefit from a longer focal length.  I don't know what your local sky conditions are  but looking at your images it does look like you have access to some dark skies so galaxies and other broad band targets (reflection and dark nebulae) should be on your list.  If you  are predominantly imaging in light polluted skies however, you will always be struggling with broad band targets.  The most popular targets for those of us with sky glow are emission nebulae using narrow band filters.  Unlike galaxies, there are many large targets available and your current range of focal lengths will give you plenty to go at.  Unfortunately, as you know, your unmodded camera will hold you back when it comes to capturing Ha.  Your M42 and HH images are excellent efforts with an unmodded camera but a lot of photons have been blocked by the IR filter.  So, for galaxies I think you would get good value from upping your focal length to an ED100, with your camera you will very rarely have your resolution limited by atomospheric conditions.  On the other hand,  if you are plagued by LP then a modded DSLR makes more sense than upping your focal length.  Either way, I think an HEQ5 would be a great investment.  There is no such thing as future proofing in astroimaging, but even if you went along a continuing pathway of bigger and longer you will still have a great mount for shorter focal length imaging.