keep dew shield on for flats?

[alcol620]

Hi folks

Clarification required. When using my 8SE the dew shield used is quite large and obviously moved around when dismantling the set up. I can't take flats with the set up in situ and have to take the scope off the mount. The dew shield is likely to move before the flats are taken. 

My question is: Is it necessary to keep the dew shield on the scope when taking flats?

 

Thanks for advice

[MattJenko]

In theory no, as the dew shield does not impinge on the light entering the scope which hits the sensor.

[perfrej]

I'd say yes, in order to mimic the imaging conditions as closely as possible

[ollypenrice]

Are you using panel flats? If you are, then the light reflecting off the inside of the dewshield is likely to be quite different from anything it was reflecting out in the dark during the shoot. I take Per's point about mimicking the conditions but I wonder if a bright light source scattering off the sides of the shield might not intoroduce more differences than not using it at all. Assuming its well flocked or blackened it should be OK. In the end try both and see which is best.

Olly 

[alcol620]

Thanks everyone. I have the dew shield flocked although it's beginning to look a bit worse for wear. I use a light box.  Will try both next time. It will clearly get rid of any dust motes, I am thinking that the vignette that might be affected?

[Stub Mandrel]

Surely its effect will be no different from the effect the tube has on flats taken with a reflector?

[alcol620]

The difficulty is making sure that the large dew shield is in the same position when taking the flats off the mount as it is when imaging.

[BlueAstra]

I think the aperture stop for a Schmitt-Cassegrain is at the front corrector plate, so in theory the addition of a long dew shield will affect the off-axis illumination. Whether it is significant or not I'm not sure, but taking two sets of flats as suggested above (one with and one without the shield) should show whether there is a measurable effect.

[alcol620]

Thanks everyone.

I have now taken flats, with and without the dew shield. Using Pixinsight what is the best process for comparing the two master flats produced? Not too technical please.

 

 

 

[Stub Mandrel]

I would suggest a full histogram stretch.

Local contrast enhancement (as in astra) or local equalisation (photoshop) will exaggerate any areas where there are boundaries.

[alcol620]

I have compared the with and without dew shield flats and to my un-trained "eye and technique" there appears to only be marginal differences. I have applied them to calibrate a small set of lights and once integrated there appeared little difference between the images, at least to the best of my knowledge and inexperience. I will take any future flats without trying to refit the dew shield as I am satisfied with the end result without it.

Thanks to everyone for your helpful inputs

Regards

Alec