Back focus help....again

[austen4]

Hello all,

I know i will need some back focus to image. But i have options and a couple of questions.

1. Barlow lens - could someone explain how they work, i know they increase the overall focal length but i have heard they create more back focus. Could someone explain how please?

2. Move the primary mirror up the tube - with this i will need new springs and screws, but is there a guide on how to do it? Will i have to drill new holes and if so what are they for?

3. New focuser - seems as a worthy option to choose but will this create enough back focus? It would be a dual focuser which is another reason to choose it.

I will be using a DSLR for imaging by the way.

Thanks greatly to anyone who replies

[Peter Drew]

Hello Austen. A reminder of what scope you are using, some recent Newtonians are already configured to use a DSLR, these usually need an extension tube to reach focus with an eyepiece. If not one of these, you will probably have to adopt one of the options you mention. The Barlow method normally works and is the simplest option but unfortunately this will introduce unwanted magnification if you plan to image DSO's. A Barlow lens is a negative optical lens that modifies the angle of the converging beam of light from the primary mirror, a 2X Barlow will halve this angle and make it seem as though it has come from a telescope of twice the focal length hence doubling the magnification. The Barlow also moves the focal point away from the normal position, usually enough for the camera to achieve focus. Moving the primary mirror up the tube can be done by either adjustment or the use of linger springs and screws if the upward movement is relatively small. If not, and the mirror cell fits entirely within the main tube, it is possible to copy the existing retaining screws further up the tube and then refit the cell in the new position. Your third option, a low profile replacement focuser might overall be your best bet but would involve the most expense. Hope this helps.

[austen4]

Thank you excactly what i was looking for. Im hoping to use with the old sw 200p. If or when i get it i will measure how much back focus i will need to get then go from there choosing my option. Do the low profile focussers give suffient amount of back focus?

thanks again

[Peter Drew]

It will depend on how low profile the focuser is. When focused fully in, the difference in height above the main tube is the amount of extra backfocus you can gain.

[FrankieValley]

Hi Austen,

I had the SW 300P and back focus didn't turn out to be the headache I had read about. I happily used a DSLR, webcams and a Meade DSI. I even managed to use the DSI with the Starlight Xpress Filter Wheel and the SX slim OAG! I only had about 1-2mm of inward focus travel left, but I managed it. I do have to say that I was using a Baader MPCC and that 'may' have helped my situation, but as I never tried without it, I don't know. You'll probably want one (or similar field flattener) to avoid misshapen stars at the edges of images.

I would recommend SW field flattener for use with your DSLR. It is designed for SW newts and it works a treat as it comes with a nifty 2" collar that allows 2" things to slide all the way into the focus tube (the collar shipped with my 300P had a shoulder inside).

In conclusion, don't worry about back focus. In my experience it wasn't a problem with the 300P and it isn't with my 250PDS. For the latter, I expected loads more back focus, but I reckon I only got 2-3mm more inward travel than I diid with the 300P.

My experience of barlows is that they give more back focus, which is good i.e. move the focal plane away from the tube. I never had any trouble with eyepieces, barlows, webcams or DSLR (with SW coma corrector). I got close to being in trouble with the Meade DSI, but then I did want a filterwheel, an OAG and a flattener in the rig! I was able to find adaptors to do the job - no mirror moving, etc.

Also, low profile focusers for the SW are lower profile but looking at images from a review, they sit very high and you will only give yourself 1-3mm from the looks of it. For £130 that is quite expensive and my experience with the standard crayford was I din't need to change it.

[austen4]

Gret thanks for that David some really good advice there. About the flattener, does this mean that filters can be put inside the flattner. I had a quick look at the flattener and it looks quite tall. Does it fit inside the focuser or does it poke out the top of the focusser which would this create a larger problem for back focus. Sorry for the amount of questions.

Thank you so much

[FrankieValley]

Hi Austen,

This link should explain all. Scroll down for the indtructions, but let me know if you are still confused - Skywatcher coma corrector for F5 or faster Newtonians :: Adapters - Photo Visual :: Altair Astro

[austen4]

Hi Austen,

This link should explain all. Scroll down for the indtructions, but let me know if you are still confused - Skywatcher coma corrector for F5 or faster Newtonians :: Adapters - Photo Visual :: Altair Astro

Thanks that explains it really well. The corrector effectively acts as an eyepiece with a t ring.

Great thanks everyone for your help it is really appreciated

[FrankieValley]

Hi Austen,

This link should explain all. Scroll down for the indtructions, but let me know if you are still confused - Skywatcher coma corrector for F5 or faster Newtonians :: Adapters - Photo Visual :: Altair Astro

.... I should possibly say that the 'focal plane' is in (roughly and maybe exactly) the same place for the visual and the DSLR configuration. The difference is that the eyepiece looks (focusses) at the focal plane and the CCD chip sits 'on' the focal plane. That is why the visual set up is set back and this difference is true across the board. Loads of back focus for imaging (as it seems), but less for CCD's.... in reality the focal plane sits around the same point (for similar equipment that is providing the focal point).

[FrankieValley]

Thanks that explains it really well. The corrector effectively acts as an eyepiece with a t ring.

not quite. the corrector flattens a curved focal plane to become a flat one. I would liken it to a 2" nosepiece with glass in it. Take out the glass and it will still work, albeit with misshapen stars at the edges. It is really a 'clever adaptor'.

You may use the collar that comes with it (replaces the collar with a shoulder inside it) in the future. I used mine with my Baader MPCC as that behaves like a 2" nosepiece. I couldn't use the SW coma corrector, because it does not physically fit to the OAG - different thread, so I had to buy an MPCC.

That reminds me - make sure you get a T mount with M48 thread and not M42 (T thread - confusing huh? T mount but not T thread). Only the latter will connect to the SW coma corrector. Not sure if it's threaded for filters, but I use a filter that clips in the camera body - Astronomik CLS Light Pollution EOS Clip Filter | Telescope Accessories | Rother Valley Optics

[austen4]

I see, I understand it better now. Unfortuantly i do not have a canon, do they do anything similar for a pentax camera (k mount) ? Would you be able to link me to a t adapter with the m48 ring for the flattener?

Thanks again guys

[austen4]

Would it be possible to just put a filter between the comma corrector and the camera adapter? but then this would increase the amount of back focus i would need if there is not a filter clip for my camera. I did google it but could find anything. Look like i will just have to move the primary mirror up the tube slightly. Is there a diagram anywhere of the location of the springs for colimation?

Sorry for the questions again

[FrankieValley]

I see, I understand it better now. Unfortuantly i do not have a canon, do they do anything similar for a pentax camera (k mount) ? Would you be able to link me to a t adapter with the m48 ring for the flattener?

Thanks again guys

This may work to get a 2" nose down the focus tube, but check with them first for your scope - ScopeStuff

One of the very bottom two items should be what you want for your Pentax - Accessories @ Modern Astronomy

I can't find a Pentax adapter to M48 thread, so I suggest you use a Baader MPCC (M42 thread) or simply attach a 2" nosepiece first.

The distance between CCD and coma corrector is pretty critical with +/- 1mm being about all the leeway you get. See here - http://www.baader-planetarium.com/pdf/mpcc_e.pdf

Therefore, not sure if you would get a filter in. I'd move mirrors as a very last resort.