dew shield and flats

[iapa]

I am of the opinion that I should take my flats with any dew shields attached as this also reduce incidence of reflections due to the extended tube.

Does anyone disagree? and why?

[AKB]

In general, then with dew shield will also accommodate any extra vignetting that it might involve.  However, I must say, I’ve struggled myself with doing successful flats for a Hyperstar configuration with a sheild, so it may well depend on what type of 'scope you’re using.

[iapa]

4 minutes ago, AKB said:

In general, then with dew shield will also accommodate any extra vignetting that it might involve.  However, I must say, I’ve struggled myself with doing successful flats for a Hyperstar configuration with a sheild, so it may well depend on what type of 'scope you’re using.

10" reflector, 8" SCT and reflector.

Not got the funds for Hyperstar on the SCT, yet.....

[Waldemar]

It depends on what kind of dew shield you're using. The flexible ones tend to deform depending on the position the scope is in, under the influence of gravity, so flats may introduce problems instead of solving them, but in case of using a hard shell dew shield, yes... it should stay on when making flats. The point of flats is to compensate for all the artefacts in the optical train, including the dew shield as long as it is a reproducible problem, which is not the case with a flexible shield.

[Demonperformer]

I have recently been wrestling with the same question for my 8SE. Until now I have been using a chunk of 3mm yoga matt stuck on a (floppy) cardboard tube and not bothering with flats. However, I have decided the time has arrived to change that policy and so have just ordered one of these. Not particularly cheap, but looking at alternatives (heavy [solid] cardboard tubes, etc) it seemed not to bad for something built to do the job.

HTH

[Waldemar]

This is a flexible shield as well and the chances are it will sag a bit and get deformed by gravity while on the scope. 

The proof of that will be in the shape of the vignetting signature in your flats. If not evenly round, then that will be caused by the mentioned effect.

EDIT: the best ones are the aluminium dew shields, but they cost....

[MartinB]

You are likely to get quite a lot of reflected light unless the shield is well flocked

[Waldemar]

All aluminium dew shields are flocked or blackened....

Edit:  as an example:  https://www.teleskop-express.de/shop/product_info.php/language/en/info/p10057_Astrozap-Alu-Taukappe-fuer-9-25--Celestron-CPC.html  or: https://www.teleskop-express.de/shop/product_info.php/info/p5998_Astrozap-Aluminium-Dew-Shield-for-Celestron-8--SCT-and-EHD-Telescope.html

[iapa]

I am using an ASTROZAP for my 10" reflector which is flocked on the inner side.

That is one reason I got it - the flocking was intended to reduce reflections, with the heaters I have on mires I don't have a condensation issue (that is visible :))

Also wanted to reduce cahngeces of stray light hitting the inside of the tube (not yet flocked, but skywatcher 'black').

 

[HunterHarling]

On 12/21/2017 at 05:27, AKB said:

In general, then with dew shield will also accommodate any extra vignetting that it might involve.  However, I must say, I’ve struggled myself with doing successful flats for a Hyperstar configuration with a sheild, so it may well depend on what type of 'scope you’re using.

Tell me about it, I have never been able to take flats with hyperstar

[Johanvk]

These flats pose an urgent problem for Hyperstar. Since two years I operate a(n older) C14 with a Hyperstar, and spend quite some observing time last year, on getting decent flats. All customary advice (T-shirt, domeflat, lightbox, flatscreen, etc.) did not work. The only reasonable results were obtained by sky-flats. But the timeslot to get them is only about a quater of an hour in twilight. Otherwise the image is either saturated or too many stars turn up (at the F2). I don't understand why this is not reported wider. Either I'm missing something, or I'm clumsy. As it stands I suspect de F2 bundle catches too much internal reflections to allow the usual flats to be taken.

The speed of the Hyperstar is a great advantage for me. The flats however limit photometric use and mosaiking.

I'm stuck - please comment!

 

[Waldemar]

For my RASA I use a flocked alu dewcap and a Gerd Neumann EL panel for flats. Since the RASA is also very fast (f 2,2) I needed to dimm the EL panel with special foil. These are the items I use:

Dewcap:  https://www.teleskop-express.de/shop/product_info.php/info/p7784_Astrozap-starre-Taukappe-fuer-Celestron-EHD-1100--Hyperstar-und-RASA-11-.html

EL panel: http://www.gerdneumann.net/english/astrofotografie-parts-astrophotography/aurora-flatfield-panels/uebersicht-aurora-flatfield-panels-overview/aurora-flatfield-leuchtfolie-d315mm-mit-12v-inverter.html  as well as the mentioned dimming foil at the bottom of this page.

Geen probleem, Johan!

[Johanvk]

OK, I'll try.