Newtonian Reflector vs Ritchey-Chrétien

[Simon Dunsmore]

Hi Folks,

Looking for some wisdom from the corporate brain.

I presently have the following set up.

Skywatcher 130PDS

HEQ5 Pro goto mount

ASIAIR Pro

Canon D60 DSLR

The set up works well but I'm wanting to add a different OTA to image galaxies and DSO with a little more magnification.

I'm considering a 6inch Ritchey-Chrétien as this falls within the budget of £500.

I'm wondering what you folks think of this set up. Anyone have any experience of using both the 130PDS and the Ritchey-Chrétien? Would the weight work OK on the HEQ6 pro?

Will I notice much difference?

 

Thanks in advance for your help 

All the best 

Simon

 

[The Lazy Astronomer]

3 hours ago, Simon Dunsmore said:

Hi Folks,

Looking for some wisdom from the corporate brain.

I presently have the following set up.

Skywatcher 130PDS

HEQ5 Pro goto mount

ASIAIR Pro

Canon D60 DSLR

The set up works well but I'm wanting to add a different OTA to image galaxies and DSO with a little more magnification.

I'm considering a 6inch Ritchey-Chrétien as this falls within the budget of £500.

I'm wondering what you folks think of this set up. Anyone have any experience of using both the 130PDS and the Ritchey-Chrétien? Would the weight work OK on the HEQ6 pro?

Will I notice much difference?

 

Thanks in advance for your help 

All the best 

Simon

 

Heavily caveated by the fact that I have no first hand experience with either, but here's what should theoretically be true:

Assuming you're looking at FLOs StellaLyra brand, the RC weighs only about 1.5kg more than the 130pds, so with cameras and accessories you should be easily within your mounts weight limit for imaging - so, no problem there.

I've seen conflicting things about how 'easy' to live with RCs are - some say they're basically hassle free and hold collimation really well, others that they can be a little finicky. People with ownership experience will be better placed to advise on this. 

The next thing to consider is the sampling rate you will achieve. Your camera has 4.3um pixels; with the 130pds, that gives a sampling rate of ~1.4 arc seconds per pixel ("/px). It's generally accepted that the limit is about 1"/px due to the effects of the atmosphere, so 1.4"/px is actually a pretty good sampling rate.

With the RC (again, assuming the StellaLyra RC6), you are more that doubling your focal length, which will get you more "magnification" (cue @vlaiv to explain why using the term magnification is not correct when imaging), but it leads to a sampling rate of ~0.65"/px, which puts you below what that 1"/px that the atmosphere is likely to limit you to. 

What this means is, although your target would appear larger in the frame, you're not really going to be recording any more detail, just spreading that detail out over more pixels. 

The next downside is that when you 'over-sample' like this, you negatively impact your signal to noise ratio (SNR), by spreading light out over more pixels than is necessary to record the finest detail your sky conditions allow. You could improve the SNR by binning the image in software during processing, but this has the effect of reducing image resolution (to basically the same as if you imaged with your 130pds), so at this point, you might as well save yourself £500 and use that instead!

Next issue (a lot of downsides, aren't there?!) is at longer focal length you will much more sensitive to tracking errors. If you're guiding, you'd want to be achieving an RMS of around 0.3" for an image scale of 0.65"/px. Unfortunately, that is likey not possible, or at least not consistently possible, with your class of mount. 

In summary, there would be minimal (if any improvement) to your images in theory, and it would add an extra layer of complexity/frustration, so personally, I wouldn't go down this route. 

[Simon Dunsmore]

Hi and thanks so much for taking the time to reply in such detail.

I need to try and get my head around those equations so that I can perhaps decide where I'm best to aim for in the future. For example, do I need to think about saving up and replacing the camera as well as the ota etc.

Food for thought.

 

Thanks again 

Simon 

[Clarkey]

Everything the Lazy Astronomer is correct, but I will add my bit as a RC owner.

I have an RC8 which is a FL of 1624mm which I use binned at 2x2 to give a pixel scale around 1"/pixel. For the RC6 you could use a focal reducer to get the FL down to around the 1000mm mark which would give you a reasonable pixel scale. Personally I really like the RC as an imaging scope. Although the collimation is tricky, once set it does not seem to need much adjustment. It took me a while to get a good method for getting the collimation right - but once sorted it was not too bad. The other benefit is that although it is not the best option, it can be used as a planetary imaging scope with a barlow.

Just to check, do you have a HEQ5 or 6 as it mentions both above? The HEQ6 will easily handle it but I suspect it will be closer to the limit with an HEQ5.

[JP-S]

6 hours ago, The Lazy Astronomer said:

The next downside is that when you 'over-sample' like this, you negatively impact your signal to noise ratio (SNR), by spreading light out over more pixels than is necessary to record the finest detail your sky conditions allow. You could improve the SNR by binning the image in software during processing, but this has the effect of reducing image resolution (to basically the same as if you imaged with your 130pds), so at this point, you might as well save yourself £500 and use that instead!

This point gets made a lot but if it's true then I wonder why is it that so many people have big SCT's, RC's & various other kinds of long FL scopes for imaging? I understand that it still makes sense for planetary, cameras with large pixels and anyone with extraordinarily good seeing but the proliferation of these scopes seems to exceed the prevalence of those factors.

[Simon Dunsmore]

Hi all 

Thanks for the responses. 

Firstly, its a HEQ5, sorry for any confusion.

I have to confess I'm becoming a little confused here. I took a look at the astronomy.tools calculator and it seems that there isn't really a set up that works well with RC.

Can I ask, If i wanted to gain more detailed images (for want of a better phrase, magnified) what would be the best set up. Using my current scope and camera the galaxies I image are always pretty small in the frame.

I appreciate that I am going to need a bigger budget but it would be good to have a target.

Correct me if I'm wrong here, but it seems the longer the focal length the greater accuracy is needed when tracking which = a heq6 mount. Does this mean that I'm operating the equipment that I have at its limit? Would money be better invested in a mount upgrade and an RC.

Sorry for all the questions, as I said, I'm a little confused

Thanks

Simon

 

[alacant]

Hi

We had a rc visit before the pandemic, but IIRC...

... sticking with the title of the post, we believe our (several) nt150l outclasses a 150rc, If only for the contrast and clarity of the former. The rc needs quite a bit longer to match it.

That maybe because the Newtonian is f8 and the rc f9 but simply comparing the light path obstruction, you're left wondering how the rc gets any useful light at all!

Remember also that at f8, the Newtonian needs no corrector,

Here are our €200 Newtonian images.

Cheers and HTH.

 

      

 

Edited September 9, 2021 by alacant

[catburglar]

Although your pixels are 4.3microns with a colour chip you are sampling every 8.6 microns in each colour because of the Bayer matrix and then interpolating…so if you can guide with the required accuracy to make use of the greater image scale (approx 0.5”/pixel) then you might see some improvement from a longer FL.

The RC might be easier for your mount to handle because it’s shorter, but as Alacant has shown, you can get nice images out of a humble f8 Newtonian.

[Simon Dunsmore]

Hi,

Mummmmm! interesting. So would you consider the Skywatcher ExplorerPL NR to be an option. Focal length 1200. It comes in a lot cheaper at £270.

Thanks 

Simon

Edited September 9, 2021 by Simon Dunsmore
Error

[The Lazy Astronomer]

4 hours ago, JP-S said:

This point gets made a lot but if it's true then I wonder why is it that so many people have big SCT's, RC's & various other kinds of long FL scopes for imaging? I understand that it still makes sense for planetary, cameras with large pixels and anyone with extraordinarily good seeing but the proliferation of these scopes seems to exceed the prevalence of those factors.

There's nothing wrong with a bit of over sampling per se (provided your mount can track accurately enough for a given resolution), it just means you'd need more integration time in your images to achieve the same SNR as someone else who sampled 'correctly' (for lack of a better word).

Deconvolution may be able to help to recover some of the detail smeared out by the atmosphere in slightly oversampled images too, but as an example, I image at 1.7"/px and to be honest even that is probably oversampling for my sky conditions most of the time. (☹)

1 hour ago, Simon Dunsmore said:

Hi all 

Thanks for the responses. 

Firstly, its a HEQ5, sorry for any confusion.

I have to confess I'm becoming a little confused here. I took a look at the astronomy.tools calculator and it seems that there isn't really a set up that works well with RC.

Can I ask, If i wanted to gain more detailed images (for want of a better phrase, magnified) what would be the best set up. Using my current scope and camera the galaxies I image are always pretty small in the frame.

I appreciate that I am going to need a bigger budget but it would be good to have a target.

Correct me if I'm wrong here, but it seems the longer the focal length the greater accuracy is needed when tracking which = a heq6 mount. Does this mean that I'm operating the equipment that I have at its limit? Would money be better invested in a mount upgrade and an RC.

Sorry for all the questions, as I said, I'm a little confused

Thanks

Simon

 

Most galaxies are tens to hundreds of millions of light years away, so they are pretty small to us.

There's two ways to increase your imaging scale: increase the focal length or decrease the pixel size.  Both would involve buying new equipment (which is part of the fun of this hobby isn't it? 😁), but either way, l think you'd be better off focusing initially on a better mount or upgrading/tuning your existing one. You'd want to be tracking with an accuracy of approximately half your image scale to attempt to take advantage of any increased resolution. If we accept 1"/px as the limit, then you know you need your mount to be consistently guiding with an RMS of 0.5". Once you can achieve that, then you can look at options for increasing your imaging scale. 

And now, to throw a spanner in the works, you may want to consider looking into "lucky" DSO imaging, as recently highlighted by astrobiscuit (aka rorymultistorey). This may allow you to image at higher resolutions than traditionally accepted, but would likely need a dedicated astro cam with a high quantum efficiency to be able to capture enough detail in the short exposures used (~5ish seconds).

[alacant]

1 hour ago, Simon Dunsmore said:

ExplorerPL NR to be an option

It would involve quite a bit more work. The Bresser version is of superior build quality with a rigid, light alloy tube and proper 2" r-p focuser.

Don't get too hung up on the theory, If we did, there'd be very few if any amateur images produced with long focal length telescopes north of Bilbao. On the nights when you don't have the seeing, bin or just image with a shorter telescope. Even better, have a go anyway. It's supposed to be fun!

Cheers

Edited September 9, 2021 by alacant

[Simon Dunsmore]

10 hours ago, alacant said:

Hi

We had a rc visit before the pandemic, but IIRC...

... sticking with the title of the post, we believe our (several) nt150l outclasses a 150rc, If only for the contrast and clarity of the former. The rc needs quite a bit longer to match it.

That maybe because the Newtonian is f8 and the rc f9 but simply comparing the light path obstruction, you're left wondering how the rc gets any useful light at all!

Remember also that at f8, the Newtonian needs no corrector,

Here are our €200 Newtonian images.

Cheers and HTH.

 

      

 

Hey Alacant

Great images!! Thanks for sharing these.

Can I ask what dslr you are using? Also, is this the scope?

https://www.astroshop.eu/telescopes/bresser-telescope-n-150-1200-messier-hexafoc-ota/p,14197

Thanks again 

Simon 

[alacant]

23 minutes ago, Simon Dunsmore said:

Can I ask what dslr you are using?

Either an eos450d or an eos700d

23 minutes ago, Simon Dunsmore said:

Also, is this the scope?

No. that's the old version. The new version is better.

Cheers

Edited September 9, 2021 by alacant