5 replies to this topic

[Ridgesoar]

In my ignorance I have a question.. I have  been using a "rich sky" telescope (RASA) with my Nikon. Great with large nebs etc. but I have to blow up small galaxies  etc so much that I loose detail.  Focal length is about 600.

would a refractor with a FL of 800 give me more of a  chance at the smaller DSOs. Thanks.

[Joe1950]

I don’t do any astrophotography, but going to a scope with an 800mm FL would increase your image size 33%.

 

It would probably be more economical to increase the focal length on your 600mm set-up to 1200mm with a 2x Barlow in line. That would double the image size of objects.

 

I’m sure those with more knowledge on the subject will give their opinion.

 

Enjoy!

[sg6]

A little but not much, 600 to 800 is not a great deal. Image would simply be 33% bigger at the sensor. However that is bigger.

 

Going to sound obvious but small galaxies are (well) small.

 

It is a bit like wanting a bigger aperture, mostly they are a little brighter or more apparent but still small, focal lengths do not keep match often with aperture increments. When people go from say 8" to 16" the focal ratio does not also get maintained for the image size to increase.

[drd715]

Galaxies come in all sizes from Andromeda size to micro miniature. Going longer than 500 or 600 mm will require much better tracking, probably autoguiding and sharp color free optics. As in the tolarances are more critical. This inevitably means more costly.

Consider tracking first and figure you need sub arc second tracking to retain details of small galaxies and other smaller subjects. I would suggest you work on getting a good over sized mount and setup an auto guiding system, maybe PHD. Set it up and spend some time just figuring out how to make it work properly.

After that consider longer optics. Size up the objects you wish to imagine and scale your focal length to frame them within reason. When you get past 1000 mm things get quite demanding.

Time to read up on pixel size to resolution of your optics. And the resolution in relation to your "seeing" - sky clarity. Dark sky sites are a huge improvement in getting a better image. If your seeing resolution is 2 arc seconds or better and your tracking is 1 arc second then you can try some to imaging of the small stuff - but it will be challenging.

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[StarBurger]

Doubling your FL ( e.g.to 1200 mm for example) will indeed give you a larger image scale (x2) but at the expense of 1/4 of the intensity of light falling on your sensor so it will take  X4 exposure length to capture the same intensity of image as at 600 mm.

Easily done with a X 2 Barlow but be aware that increasing the FL will double the difficulty of tracking or guiding and with mount drive errors,wind,balance, and polar alignment will give you double the trouble of those small errors you may see at 600 mm.

I would say to exhaust the possibilities of the larger DSO's with what you have now, and there are many of them coming around every month.

I don't believe that  moving from 600 mm to 800 mm will satisfy you (it wouldn't for me !).

Those tasty little galaxies need at least 1200 mm and $$$$$'s!

[Ridgesoar]

Thank you all. Great help!