Is a Dew Shield needed on an F8 8" RC OTA and is a Focal Reducer required

[Jkulin]

Thanks to a very kind gentlemen on here I am now the owner of an Altair Astro (GSO) F8 8" Ritchey Chretien OTA :-)

Just wondered if I needed a Dew Shield or even a Dew Heater (Guess I will for winter) as there is no glass at the front to mist up, will the primary be affected on cold nights.

Also do I need a focal reducer or do I only need that if I want faster optics?

Thanks in anticipation.

[LightBucket]

There is no need for a dew heater, unless it's at the end where the primary mirror is....and you can get heaters for those, like on a Newtonian scope...as for a dew shield, well I guess it wouldn't hurt...

as for the focal reducer, you use one lower the f ratio or speed up the Optics as you say, but whether you need one is up to you, are you an imager of just visual..??

edit: I see by your sig,  you do image, so if you put an 0.5x focal reducer on your scope it would be an f4 which in theory means twice as fast so half the time needed for the same exposure....that said there is much science behind this, so it's not quite as simple as that...  and others will be able to answer you in better detail...

[Jkulin]

Thanks LB, I bought an Astro Heater and after purchasing it wondered if it was needed, is there anything wrong with using it at the primary end?

So is there any benefit of using a dew shield, no point buying one if it will be a waste of time?

Yep I think you are right get a focal reducer to reduce the exposure time.

[LightBucket]

The dew shield will only be a benefit if using your scope in the higher zenith positions where dew can fall on the motor....no real use otherwise..

[Jkulin]

Ringing around today, some say you should run without a FR, others say it might be useful but not convinced as the RC does have a flat field, but no one actually says yes it is a must if you want to image properly.

I do have LP but not bad, but I do run a 2" Astronomik CLS-CCD filter to stop this, the extra F stop would be useful for imaging but is it critical?

Has anyone on here got an RC and use a FR?

BTW Checking with Altair, no dew shield required, little or no benefit to a Primary dew heater, some benefit to using the Kendrick Secondary dew heater, but...How Much? £85 delivered

Thanks.

[pete_l]

When you look at FRs, check the size of the image circle they produce.
Many FRs have such small "outputs" that if you have a large-ish sensor in your camera that you actually get a smaller image than without it.

[Jkulin]

16 minutes ago, pete_l said:

When you look at FRs, check the size of the image circle they produce.
Many FRs have such small "outputs" that if you have a large-ish sensor in your camera that you actually get a smaller image than without it.

Thanks, As I'm using a full sized DSLR (Nikon D800) and when I switch to using a CCD then will almost certainly go full size, it makes me wonder whether it will defeat the whole object of the exercise by going for a FR.

[Peter Drew]

I would continue "as is" until a need arises.  

[Jkulin]

Thanks Pete, yep will have some playing before I spend anything more.

[Commanderfish]

Whilst I don't know much about Richey Chretiens, I do know that on a standard newt there is an advantage of using a dew shield, and that is blocking stray light from entering the tube.  This helps with contrast, aiding a nice dark background.  As dew shields are cheap (or even home made), I think this is a worthwhile addition to any scope that doesn't already have one built in.  I home made one for my 16" dob using a black camping matt and some stick on velcro.  Cost about £7.

[Jkulin]

Thanks CammanderFish, it's not down to price just trying to keep weight to a minimum.

The argument supplied as to why no dew shield is required is that the tube is totally open at the secondary and thus no dew can settle, but there are some recommendations for a dew heater.

I'll have a look at some camping matt anyway, thanks.

[Jessun]

Hi! Yes many use a reducer on RCs. F8 is generally considered slow. Well at least sluggish. A common reducer for the 8" is one made by Astro-Physics. It's a 0.67 which would bring your RC down to f5.36. This will make your exposures 2.2 times faster.

Why is it faster? Well because your RC - like many designs of course - will collect light and concentrate that to a circle of focused light at the back where you put the camera. If you held a piece of paper there if would look like a circle, bright in the middle, fading towards the edges. The reduced sits in front of the camera and it will catch light at the periphery that would otherwise have missed the camera. More photons will hit the camera, but ONLY because you're now gathering light from a bigger patch of the sky.

So you're not exposing anything quicker per se, you're just looking at more sky. Your objects/targets will be smaller too which may not be what you desire but it's all a trade off. On the plus side is that if your sky quality is bad then this will appear as a grainy busy background around your target. Unwanted signal basically in a somewhat random pattern. It helps to have many exposures to even this unwanted signal when you stack in order to get rid of it. The more exposures the better. Some swear to see improvement up to 100 exposures or 'subs', whereas the maths suggest that the gain of more sub exposures fall rather quickly.

As you process the images, stacking etc, personally I found it easier to work with a bigger number of exposures. And easier equals more fun.

As to dew heater, it really depends on the ambient air. It can't be ruled out that one night you would have benefited from one. Many high end RCs have this as an option. But save the money I'd say.

A dew shield on a closed tube will help, but again it all depends on what the air is like. And how often it's damp enough to deposit. It's a cheaper solution but then makes the setup more prone to wobble if there's a wind....

It's an eternal game of compromise!

/Jessun

 

 

[Jkulin]

Thanks Jessun, that really does help.

I've just spent a couple of hours going through images taken on Astrobin and probably less than 2% use a FR, I use one on my ED80 but as the reports say that the images are exceptionally flat on the RC then as you say the benefit is for gaining speed.

Looking at Astrobin the two recommended are as you suggested the Astro Physics CCDT67 0.67x and the Kepler 0.75 RC, the AP is €240 which I may have to delay or risk a divorce!

I think I'll knock a Dew Shield up and if I think it a risk then will use it as long as the wind is reduced.

I had decided to get a Kendrick Dew Heater, just a little concerned that the power leads may distort the defraction spikes.

Thanks once again.

[MattJenko]

I have found my 6" primary has frozen over on a couple of cold winter nights without a dew shield, which would have given it a bit more protection for the higher in the sky objects during those evenings.

[Commanderfish]

Last night I tried pulling off the dewshield on my 16" Dob whilst viewing the M13 Hercules Glob.  Clear difference in contrast with far more resolved pin point stars with the dew shield on.  Loss of detail and definition with the dewshield removed.  

The drawtube on a Newt is close to the aperture, whereas on an RC it's at the far end from the aperture.  So perhaps the effect might not be as pronounced on an RC?  Or maybe there will be an equal effect as less stray light is entering your tube when you have a dewshield....

[Jkulin]

Typically we have had a couple of clear nights and the mount is not back from being hypertuned yet, so I can't even try the RC out yet, not that it is needed yet but still waiting for the FR to arrive from teleskop-express.de

I've had problems trying to find some 1mm ABS plastic to know up a dew heater from as most don't do lengths long enough, it needs to be about 80cm long, can anyone point me in the right direction?

I've also decided that even if it is an overkill, I'm going to use a dew heater on the primary as well as the secondary and when it isn't windy then use a dew shield as well, better safe that sorry.

Thanks for eveyones advice, much appreciated.