"Long Exposure" vs Short Exposure and flats

[nkchri2]

I have an Orion Starblast 4.5 and I just found a cheap Canon Rebel XS so I can dabble in astrophotography a little. I plan on just starting with some wide fields by piggy backing my camera on the scope so I can use the EQ mount. I realize this is a cheap EQ mount... buts its what I have. Then I was considering finding a used 900nc to use on the scope as a "guide cam" to help with polar alignment using vega and to help with manual tracking. So here is the question...

I've been reading alot of tutorials on the net, and came across this DIY to make a homemade piggyback mount:

http://www.astronomyhints.com/piggyback.html

He talks about not having an electric drive and using the manual controls for tracking at the bottom of the page. He states "With the manual controls, I cap the lens of the shutter-locked camera whenever I make a pointing adjustment, then uncap the lens when finished. This ensures that any vibrations I cause or mistakes I make in adjusting do not affect my photograph."

Will this actually result in a better photograph? Will "capping the lens" like this during adjustment so you can take a 2-3 minute exposure result in better quality than just taking 15-20 second exposures and stacking them in DSS?

I also read a post about taking good flats. He said the best/easiest way he had found to take good flats was to just to maximize notepad on his laptop and hold his screen directly up to the lens. Would this actually work well? If so would you just hold the screen directly in front of the lens or a little back with a white cloth stretched over?

Thanks in advance for the help

- Kyle

[Demonperformer]

I would think it would need to be a very stable setup or you would get vibration every time you took the cap off? - which would rather defeat the object.

[ollypenrice]

This all sounds very difficult to me but the easiest way to cap the scope is simply to hold an umbrella in front of it while standing behind. I have never done it but have heard of the trick from a guest. It seems pretty smart!

I have used the laptop for flats. The spectrum was imperfect but it did work. You can also do sky flats with a T shirt over the lens.

Olly

[Sparrow]

I've nothing against DIY piggyback mounting - in fact I made my own bracketing for my guidescope which is piggybacked - as long as it's sturdy enough it should be fine. Mine was made from an old piece of aluminium from our old conservatory which has been drilled so it can fix to the scope rings of the main scope and then the guidescope bolts to it - previously I'd used a piece of wood but I was getting some flexure.

On the subject of flats, I'd be very wary of the laptop screen method - you are relying on the even illumination of the laptop screen which cannot be guaranteed. Olly's suggestion of pointing skyward with a white t-shirt is a far better solution. I actually put a piece of ordinary white A4 over the front of the scope and point skywards on a bright day which so far has seemed to work pretty well.

As for capping the lense - the best solution would be one which doesn;t involve actually touching the camera as each time you do that, the camera vibrations ned to settle down.

At the end of the day, experimentation is the key word - if it works, then great. If it doesn;t then try something else:) Either way, have fun - that's what it's all about.

Best wishes

John

[steppenwolf]

Capping the lens used to be called 'top-hatting' and was and still is one way of limiting the shake caused by the 'clunk' of the mirror in a reflex camera. There is no reason why it shouldn't be used even with a modern camera but I don't see a how it would help with re-centring the mount on an objects because the chances of getting perfect registration within a single frame would be slight - far better to take lots of images to stack and by all means 'top hat' each one to minimize camera shake.

The 'note-book' flats idea will work but as Olly has said.

[nkchri2]

Capping the lens used to be called 'top-hatting' and was and still is one way of limiting the shake caused by the 'clunk' of the mirror in a reflex camera. There is no reason why it shouldn't be used even with a modern camera but I don't see a how it would help with re-centring the mount on an objects because the chances of getting perfect registration within a single frame would be slight - far better to take lots of images to stack and by all means 'top hat' each one to minimize camera shake.

The 'note-book' flats idea will work but as Olly has said.

I don't know why I never thought about it that way... Yea if its going into the same exposure for one picture, even if you think you've tracked well manually there's no way your going to get it perfectly aligned from every tracking adjustment. Might as well just stack.

Thanks for all the input guys, much appreciated.