Thoughts on a mini EAA set up!

[PatG]

Hi Everyone

I'm thinking about repurposing an old Meade LX90 Alt-Az fork go to mount and installing a WO 72mm refractor on it in conjunction with a lodestar x2 mono for some EAA.

I'm contemplating if this can be set up without a tripod (i.e the fork mount fixed to a concrete base) as a sort of micro observatory, within a small  lift of enclosure and controlled via laptop from indoors. 

I have done some EAA previously but with the de-forked 200mm SCT on an EQ mount and wanted to find a way of having a quick / semi permanent EAA exclusive set up. 

Curious to understand how well Alt Az works for EAA, what length exposures can be achieved? and also if anyone else is using such a small refractor for EAA and what sort of results are you getting?  

Be interested to hear some feedback & peoples thoughts on this idea.

Pat

 

 

 

 

 

 

[AKB]

Greetings!

Try this memorable and extensive thread to see what is possible…

Looking forward to seeing and hearing your experience with your planned setup.

Tony

 

 

[PatG]

Thanks Tony - What a great thread !

[Martin Meredith]

Alt-az works fine for EAA even at longish focal lengths. I've only ever used alt-az (at 800mm) for the last 8 years and can expose for up to 30s without problems, although it depends which part of the sky I'm pointed at. Nowadays I tend to limit my exposures to the range 5-20s.

A different issue you might have with your proposed scope/camera combination is undersampling. I've used a ED77mm Borg refractor at its native f6 with a Lodestar X2 mono and much of the time (ie when seeing was moderate or better) was getting blocky stars due to undersampling. I'm planning to try again with the smaller pixels of my ASI 290MM soon.

Martin

 

[catburglar]

Field rotation (which is the issue for alt-az imaging) is independent of focal length. For any given ‘acceptable’ amount of field rotation the max exposure time increases as the object moves away from the N-S meridian and reduces as it gains altitude…There’s a table in the book Astrophotography on the go that does the calculations for observers at different latitudes…the take home is that in the UK you should be able to get 30sec exposures without appreciable field rotation in any direction below about 60degrees in altitude and 15second exposures up to about 80degrees.

I’d agree with Martin- the scope/camera combo will be significantly under sampled at about 4arcsec per pixel…so you might be better off looking at a smaller pixel CMOS camera like the ASI178 (probably bin x2) or the 183 if you want a larger FoV or the 290 which is about 1.5arcsec per pixel unbinned 

[PatG]

Thanks for the advice chaps. I had not given much thought to the camera pixel size / telescope combo.

Presumably smaller telescope requires a camera with a smaller pixel size - What is the optimum arc seconds per pixel ?

Be interested in seeing some examples of results with the ASI 290 / 183 cameras. 

I will do some research.   

 

 

 

 

 

 

 

 

[catburglar]

The optimum sampling frequency depends on your local seeing conditions…For seeing at 2” then you’d want to sample somewhere around 2/1.6= 1.25 arcsec per pixel and about 2.5 arcsec per pixel if you’ve got 4” seeing. 
 

I’d probably aim for the latter because with a relatively small aperture you want to maximise your SNR so you can see faint things and @ 2.5 arcsec per pixel your system is approx 4x faster than at 1.25.